Wolf cranial morphology tracks population replacement in Fennoscandia

Abstract

Humans have directly or indirectly contributed to the genetic and thus often phenotypic changes of many species. Anthropogenic pressures, such as persecution and hunting, have negatively affected wolf populations in northern Europe. In line with the genetic replacement that occurred during the twentieth century following the extirpation of wolves from Scandinavia (Norway and Sweden) and their near-extirpation from Finland, we provide evidence of morphological changes in wolf cranial morphology across these populations. Using three-dimensional landmark-based geometric morphometries, we show that modern wolves in Scandinavia and Finland have, on average, crania with wider frontal bones, wider and higher positioned zygomatic arches and more ventral flexion of the rostrum compared to the historical wolf populations. Although both populations differ in the magnitude and direction of shape change over time, the centroid size or overall size of the cranium, is significantly larger only in the modern Scandinavian wolves. Different genetic origins of the historical and modern populations have probably played a role in the observed morphological variation; however, it is also likely that morphology has been affected by the availability of different prey, which has changed over time.

Headnote

Keywords:

geometric morphometries, Canis lupus, skull shape, population extirpation

Humans have directly or indirectly contributed to the genetic and thus often phenotypic changes of many species. Anthropogenic pressures, such as persecution and hunting, have negatively affected wolf populations in northern Europe. In line with the genetic replacement that occurred during the twentieth century following the extirpation of wolves from Scandinavia (Norway and Sweden) and their near-extirpation from Finland, we provide evidence of morphological changes in wolf cranial morphology across these populations. Using three-dimensional landmark-based geometric morphometries, we show that modern wolves in Scandinavia and Finland have, on average, crania with wider frontal bones, wider and higher positioned zygomatic arches and more ventral flexion of the rostrum compared to the historical wolf populations. Although both populations differ in the magnitude and direction of shape change over time, the centroid size or overall size of the cranium, is significantly larger only in the modern Scandinavian wolves. Different genetic origins of the historical and modern populations have probably played a role in the observed morphological variation; however, it is also likely that morphology has been affected by the availability of different prey, which has changed over time.

1. Introduction

Morphology is shaped by a complex interplay of genetic, environmental and ecological factors [1-4], with demographic changes further influencing these traits [5-7]. For example, when a population declines to the verge of extinction and begins to recover, genetic and phenotypic characteristics (including morphology) may shift as a result of genetic drift, increased inbreeding, gene flow or replacement owing to immigration [8-10]. Small and fragmented populations are vulnerable to an increased risk of hybridization, which can further alter phenotypic characteristics [1]. Additionally, environmental and ecological factors, such as changes in prey, are directly related to morphology, particularly cranial and dental adaptations [11-14]. Finally, demographic changes exacerbated by climate (e.g. tornadoes, heatwaves) and human activities (e.g. hunting, habitat destruction, species introductions) can further contribute to morphological shifts across populations [3,5-7].

The grey wolf (Canis lupus) is a well-known example of a species that has undergone dramatic population declines across the northern hemisphere. Wolves suffered extreme persecution during the nineteenth and twentieth centuries, resulting in their extirpation from half of North America and most of Europe, mainly owing to fearful perceptions of wolves as pests and threats to human life [15]. In Fennoscandia (Norway, Sweden and Finland), negative perceptions of wolves particularly stem from damage caused to reindeer herding, livestock farming and hunting dogs, and were further fuelled by reports of attacks on children dating back to the nineteenth century in Finland. Together, these factors have contributed to continued hunting and poaching pressure [16-19], ultimately causing a bottleneck in the Fennoscandian populations. By the late 1960s, wolves were declared functionally extinct in Scandinavia (hereafter, referring to Norway and Sweden) [20], while Finnish wolves similarly diminished despite migrants from the eastern border with Russian Karelia. In 1915, it was estimated that only 18 (range: 6-54) wolves remained in Finland [21], after which their numbers decreased further during the 1920s and again in the 1970s, marking the time of the lowest population numbers [22-24].

After protection measures were implemented in Fennoscandia (Norway: 1972, Sweden: 1966, Finland: 1973, and increased protection by the European Union in 1995 [19,20]), and a simultaneous increase in their main prey, moose (Alces alces; [25,26]), the situation changed. Immigration into the nearly extirpated Finnish population increased, and in the 1980s, the Scandinavian population was re-founded by two immigrant individuals arriving from the east [20,27,28]. Although few migrants arrived in Scandinavia after this date, the offspring of those who did had higher reproductive success than the local inbred wolves originating from the first founders [29]. Despite this immigration, wolf numbers remained low, leading to persistent inbreeding [30,31]. Inbreeding depression soon followed, as evidenced by decreased breeding success [29], increased genetic load [31] and an elevated incidence of congenital pathologies in Scandinavian wolves, which increased in frequency with each year of birth [32]. The Finnish population suffered from low population numbers, largely owing to management hunting and poaching [33], which also contributed to increased inbreeding [34]. This resulted in a genetic replacement of the historical wolves in Scandinavia and a partial replacement in Finland during the twentieth century [24,34-36].

With the onset of the twenty-first century, wolf protection measures, increased forest cover, rural depopulation, and decreased cropland cover, among other factors, have contributed to the recovery of wolves across some of their former ranges in Europe [37,38]. With the reappearance of wolves in Scandinavia after their extirpation, claims emerged that these wolves had been reintroduced by humans, either from a different population or even from zoos [39,40]. A large genomic study did not support this claim [36]. At present, the Scandinavian population numbers 440 individuals (spring 2024) [41] and the Finnish population is estimated at 295 individuals (March 2024) [42]. In addition to genetic changes and an increased incidence of congenital pathologies in modern Scandinavian wolves, the replacement of the Fennoscandian population has probably contributed to changes in morphology. In a study by Engdal [43], some morphological differences between male wolves from historical (extinct, 1830-1972; n = 11) and modern (extant, 1983-2018; n = 47) Scandinavian populations were observed, based on 16 linear craniometric variables. However, it is still unknown if morphological changes occurred in the Finnish population, how these changes compare between countries, which cranial parts were involved, and how the sexes were affected.

In this study, we investigated whether modern Fennoscandian wolves differ in cranial shape from historical wolves. If so, given genetic evidence for disconnectedness, is the direction and magnitude of morphological change similar for the Finnish and Scandinavian populations? For this, we applied a comprehensive landmarking scheme to three-dimensional models of wolf crania collected from these regions over the past 200 years and analysed the data using geometric morphometric approaches. Additionally, we tested whether samples from zoos differ from wild wolves in cranial shape and if there is an affinity of museum samples without collection dates to the other examined groups.

2. Material and methods

2.1. Sampling

The dataset consists of 84 adult wolf crania from Fennoscandia [44] (Norway, Sweden, Finland and West Russia; see figure 1 with terrestrial ecoregions [45] for sample distribution). Individual ages were not available for all museum records (electronic supplementary material, table SI); therefore, we considered specimens with fully erupted teeth as adults. Seventy-nine specimens were digitized using a Nikon XT H 225 ST X-ray computed tomography (CT) system, of which 72 were scanned at the DANFIX facility of the Danish Technical University (100 kV, 300 uA, 1 s exposure, 1571 projections, two frames per projection, no filter, 120.76 µm voxel size), and seven at the Natural History Museum at the University of Oslo (180 kV, 260 pA, 1 s exposure, 3016 projections, one frame per projection, 0.5 mm tin filter, 75.38 pm voxel size). Volumes were reconstructed using X-TEK CT Pro 3D v. XT 4.4.4 Nikon Metrology NV, and three-dimensional surface meshes for each cranium were extracted in VGStudio Max 2.1 (Volume Graphics, Heidelberg, Germany). Three additional specimens were added from Curth et al., [46], and two specimens were surface scanned using a Breuckmann SmartSCAN (AICON 3D Systems GmbH, Germany).

To evaluate morphological differences among Fennoscandian wolves in time and space, we split our dataset into geographical (Finnish and Scandinavian) and temporal (historical and modern) groups. The geographical division was based on differing demographic and genetic events (§1). These geographical populations are practically isolated from each other because of barriers to dispersal formed by the Baltic Sea in the south and the reindeer herding region in the north (where wolves are routinely killed). The dataset was divided into temporal groups based on dates of lowest population numbers, i.e. the most critical point for each population when genetic turnover took place. The Scandinavian population experienced the lowest population numbers in 1966, and the bottleneck continued up to 1983 [20], while the Finnish wolves experienced several bottlenecks from the 1920s until the 1970s, with the lowest numbers estimated at the start of the 1920s [24]. For both Scandinavian and Finnish populations, we assumed the first years of the lowest population numbers as the end of the historical populations (Scandinavia: 1966, Finland: 1920), with the year after each marking the start of the modern populations (Scandinavia: 1967, Finland: 1921).

Claims about the origin of modern Scandinavian wolves have suggested that they were released from zoos [39]. Therefore, the dataset also includes four specimens from Fennoscandian zoos, whose origins were traced to mixed ancestry formed by Norwegian, Swedish, west Russian and Estonian wolves [47]. Animals in captivity are known to develop differing morphological characteristics compared to their wild conspecifics [48-50]. Therefore, zoo specimens were grouped together. Additionally, five specimens lacked specific collection dates, which is less common in contemporary collections owing to stricter museum policies. We therefore wanted to test if they resemble the modern or historical wolves, which may be relevant for updating museum records. Hence, they were included as a separate group in the model, unknown year. For total sample numbers per group, see table 1.

Three west Russian wolf specimens were obtained from European museums: one from the region close to Northern Finland (labelled as a wolf from East Inari, Russia, without a collection year, housed at the Museum für Naturkunde Berlin, Germany) and analysed together with the unknown year group; and two from southern parts of the Republic of Karelia, near the Lake Ladoga (1908 and 1943, housed at the Finnish Museum of Natural History, Finland), one analysed with historical Finnish specimens and one with modern Finnish specimens, respectively. These specimens were relevant for highlighting the source population of the current Fennoscandian wolves.

As part of the historical Finnish population, we included two specimens from 1882 that were involved in 22 documented fatal attacks on children in the Turku region, Finland, between 1880 and 1881 [51,52]. Historically, these animals have been described as 'man-eating wolves', which referred to the pattern of preying on humans. However, to avoid misinterpretation, we refer to them as 'Turku wolves'. These specimens are highlighted in graphs, as one of the theories for this behaviour was that these individuals were wolf-dog hybrids [53]. If this is the case, we would expect them to cluster away from the wolf groups.

2.2. Morphometric analyses

All landmarking was done by one observer (D.B.) in Stratovan Checkpoint v.2023 (Stratovan Corporation, Sacramento, USA), and data were analysed using the geomorph package v 4.0.7 [54,55] of R v 4.3.3 [56] (see figure 2 for the placement of landmarks). Out of the 84 specimens, 47 were landmarked twice to assess measurement error. Missing data were estimated using the thin-plate spline method [57], which estimates the locations of missing landmarks in specimens based on a reference specimen obtained from the entire dataset. Although this approach has its drawbacks, as it can pull specimens with large numbers of missing landmarks closer to the mean of the dataset used for estimation, we applied it in order to include incomplete yet important specimens, such as the Turku wolves from Finland.

To obtain shape variables, a generalized Procrustes analysis [58] was performed using the gpagen function (further geomorph functions will be italicized below), which translated, rotated and scaled individuals to unit centroid size (CS; the square root of the sum of the squared distances of each landmark in the configuration from the centroid). Because crania are symmetric along the midline (and because we do not specifically discuss asymmetry here), the bilateral symmetry component of shape was extracted using the bilat.symmetry function for downstream analyses.

We used Procrustes ANO VA to quantify variation in shape and CS among temporal and geographical wolf populations, and between sexes. Statistical significance was obtained through distributions produced by resampling permutations [59,60]. Results were considered statistically significant at p < 0.05, and marginally significant for p-values between 0.05 and 0.1. A multivariate linear model was used to test for overall shape change over time, and a simple linear model was used to test for shape change along principal component 1 (PCI; representing the greatest variation in the dataset) over time. We also performed analysis on shape residuals after removing the effect of CS, to determine if differences between the groups are a result of differing allometries. We further tested for differences between the allometric slope vector lengths and the correlation between slope vectors and angles. Trajectory analysis was performed to determine the direction of shape change from historical to modern wolves.

In mammals, sexual dimorphism is widely distributed and has also been reported in wolves, for example in [61-65]; but see [66]. We examined the degree of shape and CS variation explained by sexual dimorphism and whether these differences explain more variation than the division into modern and historical groups. Because sex was not recorded for all museum specimens, this test was limited to 32 Scandinavian individuals (modern: seven females and 12 males, historical: six females and seven males; table 1).

2.3. Visualization

We used principal component analysis (PCA) to visualize shape variation among different groups and the position of individual specimens within morphospace, instead of canonical variates analysis, which is sensitive to overfitting when sample sizes are small. PCA uses aligned shape data obtained after Procrustes superimposition. The resulting shapes are projected onto the eigenvectors, which represent the axes of variance ordered from maximum to minimum as the principal components (PCs). Shape pattern variation along the PC axes was visualized using a three-dimensional warping approach with the warpRefMesh and plotRefToTarget functions in geomorph. Following this approach, an average specimen (determined using findMeanSpec) was warped using thin-plate spline to the minima and maxima of the principal components, which were magnified threefold for better visualization of shape differences. Variance in shape between historical and modern Finnish and Scandinavian groups was further visualized using the rnfelice/hot.dots function (code available from: https://zenodo.org/records/ 3929193 [67]). This allowed us to highlight which landmarks, and by extension, which parts of the cranium vary most between groups.

3. Results

Our results revealed distinct patterns of cranial variation within north European wolves. Measurement error was below 5%, verifying the reliability of our approach (electronic supplementary material, table S2). Upon examination of transformed data, PCI explained 22% of the variation, representing changes in the width of the frontal bones, the angle of the orbits and part of the rostrum, and changes in the shape of the cranial vault and sagittal crest (figures 3, 4 and 5). On the negative side of PCI, where most of the modern specimens group, the rostrum of individuals was sloped further downwards (negative PCI -similar to klinorhynchy vs positive PCI -similar to airorhynchy), frontal bones were wider and the zygomatic arches were positioned higher (more dorsally), the orbits appeared more anterolaterally inclined and the cranial vault was taller. Historical specimens mainly fell on the positive side of PCI, showing opposing morphological patterns. PC2, with 12% of the total variation, represents the change in the cranial width, and to a lesser extent than PCI, the change in the width of frontal bones and slope of orbits.

We found evidence for differences in cranial shape and size between the historical and modern groups, with varying relationships between geographical regions (figures 3-5; table 2; electronic supplementary material, figure SI). Modern Finnish (n = 14) and historical Finnish (n = 12) populations differed marginally in mean shape, but not in CS (figures 3 and 4a,b; table 2). Modern Scandinavian (n = 24) and historical Scandinavian (n = 24) populations differed in both shape and CS, with high effect sizes (Z) for both (figures 3 and 4a,b; table 2). The crania of modern Scandinavian wolves appeared almost 4% larger than historical ones. Overall, the morphological differences between the historical and modern groups were more pronounced in Scandinavia than in Finland. Historical Finnish and historical Scandinavian groups were no different in shape and CS, while there was a significant difference between modern Finnish and modern Scandinavian groups (p = 0.017, table 2).

Although there was a significant effect of CS, after accounting for it in the residual analysis, the differences between the groups remained similar (table 2). Allometric analysis showed a small difference in slope vector lengths between modern Scandinavian and historical Scandinavian groups (Z = 1.64; p = 0.041). There was a marginal difference in pairwise angles between slope vectors for modern Scandinavian and historical Scandinavian groups (Z = 1.45; p = 0.063), historical Finnish and historical Scandinavian groups (Z = 1.36; p = 0.085) and a significant difference between modern Finnish and historical Finnish groups (Z = 2.04; p = 0.022; table 3; figure 6). Pairwise analysis of differences in path distances showed a significantly longer path for Scandinavian than Finnish populations (0.0278 versus 0.0177; Z = 1.90; p = 0.020), and a significant trajectory angle between Finland and Scandinavia (angle = 54°; Z = 3.89; p = 0.001; figure 7).

By plotting the PCI scores against time, a change in shape from historical to modern populations around the 1960s was observed (figure 4a). The clinal relationship of overall shape change with time was significant, where time explained 8% of the total shape variation (Z = 4.75, p = 0.001; figure 4a). There was a marginal difference between the slopes of the regression models (PCI versus time) for Finland and Scandinavia (Z = 1.58, p = 0.069; figure 4a). The samples without a collection year appeared mostly on the positive side of PCI, which included wolves from northern Fennoscandia and northwestern Russia; while samples from the zoo, for which we had the year, but which were not included in the model (since captive animals are known to exhibit differences in morphology), occupied more of the negative side of PCI. A model testing for differences in shape between the groups while accounting for CS showed a significant difference between samples without the collection date (и = 6) and modern Finnish and Scandinavian samples (p < 0.008), while this group was not significantly different from the historical populations (p > 0.374). The samples from the zoo (n = 4) were significantly different from all the other groups (p < 0.07) except from modern Finnish wolves (p = 0.102).

3.1. Sexual dimorphism-Scandinavia

Sexual dimorphism was observed in the cranial shape of historical Scandinavian, but not modern Scandinavian wolves (figure 8; table 4). The opposite trend was found for CS, in which sexual size dimorphism was observed only in the modern Scandinavian population: Scandinavian modern female wolves were on average 6% smaller than male wolves. Note that statistics from Finland for models including sex are not included owing to the small sample sizes of museum specimens with recorded sex (table 1).

4. Discussion

Our results show that cranial shape and size have changed in Fennoscandian wolves over space and time. These observed changes coincided with the genetic replacement of the historical wolf populations with the modern ones starting in 1966 in Scandinavia [20,28] and during the 1920s up to the 1970s in Finland [23,24]. Undated specimens from museums were most similar in shape to the historical Fennoscandian population, thus confirming our expectations based on historical records. Zoo wolves were different from Fennoscandian wolves, supporting previous findings regarding differences between captive and wild individuals [48-50]. Their marginal distinction from the Finnish modern group could be attributed to an eastern origin of those wolves, which also contributed to the Scandinavian zoo population. For a subset of the Scandinavian groups with known sexes, we found evidence of sexual dimorphism in the shape of the historical wolves, but not in the modern population. By contrast, only the modern population showed sexual size dimorphism in CS, with modern male wolves being significantly larger than modern females. Given that sexual dimorphism in wolves is a widespread phenomenon, the likely explanation for this discordance is the small and uneven sample sizes of the Scandinavian populations (table 1), which could have resulted in insufficient power to differentiate between males and females.

The extirpation of wolves from Scandinavia during the twentieth century, followed by the limited number of founders and migrants, contributed to the reduced genetic diversity within the Scandinavian modern population [30]. By contrast, Finnish wolves did not suffer complete local extinction, which together with a larger number of migrants from the east to Finland, contributed to higher genetic diversity compared to Scandinavia [68,69]. A founder effect in modern Scandinavian wolves, together with continuous hunting, could explain the difference in the directions and magnitude of shape change between the Finnish and Scandinavian populations. It could also partially account for the significant difference in the lengths of allometric vectors between historical and modern Scandinavian populations, where the modern population had lower variation in size. However, it is important to note that the variation in allometric vector lengths can also be influenced by differences in the size distribution across groups and in this case may also be influenced by the shorter sampling period for modern populations relative to the historical ones.

While genetic processes such as directional selection, gene flow and genetic drift can contribute to changes in cranial shape [5,70-72], the whole skull (cranium plus lower jaws) also represents a highly adapted feature for prey acquisition [73]. Among Canidae, cranial shape varies based on prey size, with long and narrow upper and lower jaws seen in species that hunt small prey, while short and broad jaws are selected for in species that hunt more robust prey [73]. For example, cranial differences were observed between coastal and inland wolves in British Columbia, despite their geographical proximity and the absence of physical barriers to gene flow, owing to differences in habitats and prey [74]. Coastal wolves use marine resources as well as Sitka black-tailed deer (Odocoileus hemionus sitkensis), while those on the mainland feed primarily on larger prey such as moose, elk (Cervus canadensis), caribou (Rangifer tarandus) and Stones sheep (Ovis dalli stonei) [75,76]. As such, the cranial characteristics of these two populations probably reflect their diet, with the coastal wolves being smaller than their mainland counterparts [12,74]. In fact, studies on several wolf populations confirm the significant positive relationship between prey size (average prey weight) and skull size (maximum skull length, zygomatic breadth or a series of skull measurements summarized as PCI) [12,14], body size [77] or both [13]. Skull size was also observed to correlate with ambient temperature or precipitation, both of which affect prey composition, which in turn affects carnivore morphology [14,78]. By contrast, cranial shape differences between Dinaric-Balkan and Carpathian wolves seem to have evolved in the absence of differences in main prey; nevertheless, the authors suggest that the effects of prey on skull morphology in these two populations cannot be completely excluded [64].

The main prey of wolves in Fennoscandia is moose [79-82], whose densities reached critically low numbers at the end of the nineteenth and beginning of the twentieth centuries across the entire peninsula [25,26,83]. Since the 1970s onwards, moose numbers have dramatically risen across the region owing to different forestry and management practices [25,26]. Therefore, it is also likely that changes in prey over time could have played a role in the observed differences in cranial morphology of Fennoscandian wolves. Interestingly, low prey density in the past has also been considered one of the driving forces behind documented cases of wolves attacking and consuming humans in Finland at the end of the nineteenth century ([51], but see [16,84]). Both of our sampled Turku wolves had narrow cranial width and small CS, falling within the morphospace of the historical Finnish wolf population that experienced low prey densities (highlighted with a red outline in figures 1, 3 and 4). These Turku wolves also preyed upon livestock, which could have facilitated their transition to attacking and consuming children [51].

Another potential contributor to changes in morphology is inbreeding, which can lead to shared morphological features owing to common ancestry among inbred individuals (aside from malformations that can arise under this scenario [85,86]. Although inbreeding and inbreeding depression have been observed, especially in the modern Scandinavian population, not all modern Scandinavian individuals are equally inbred. Unfortunately, we did not have information on inbreeding for the specimens in our sample. Nevertheless, inbreeding has previously been linked to congenital deformities in the Scandinavian wolf population, including missing teeth (hypodonty), malformed teeth, canines pointing more forward (mesioversion of canines), smaller teeth than normal (microdontia) and more teeth than usual (supernumerary teeth), among other cranial and skeletal anomalies [32]. As our landmarking scheme was designed to capture the overall shape of the cranium and recorded, on average, only one landmark associated with each tooth, we were not able to capture these deformities despite their clear visibility in some samples. However, with a more detailed morphometric approach focussed on teeth and inbreeding data on each individual, it could be possible to test the contribution of this factor to localized morphological changes within Fennoscandian wolves.

Although wolf-dog hybridization can also contribute to changes in morphology, recent hybridization is likely to bring more abrupt changes to the cranial shapes of wolves than prey-driven adaptations or plasticity. For example, the offspring of first-generation wolf-dog hybrids often display intermediate characteristics [87-89]. Because dog breeds display a greater variety of shapes than any other canids, recent hybrids should be detected as outliers in our dataset. We did not observe such cases. While hybridization and introgression of dog DNA into wolves have been observed across the world [90-93], genetic evidence suggests that the level of genetic introgression of dog genomes into Scandinavian wolves is comparable to or even lower than in the other parts of the world [36,94].

The different genetic sources of historical and modern Fennoscandian wolf populations have probably played important roles in shaping their morphology. At the same time, numerous studies have demonstrated how wolf morphology changes in response to biotic and abiotic factors such as prey, precipitation and temperature. Wolves tend to migrate within similar habitats [95-98], which may help preserve the genetic structure of populations. Given the complexity and potential interactions of these factors, determining the underlying causes of morphological changes in Fennoscandian wolves is challenging. Nevertheless, the influence of prey species on morphology could be further explored using stable isotope analyses. Additionally, incorporating genomic data of the individuals in this study could enable direct testing of the link between genetic and morphological differences. In this case, we could examine the genetic basis for morphological traits, investigate the role of genetic drift in shaping morphology, quantify genetic introgression and how it is reflected in morphology, and relate inbreeding effects to morphological changes.

5. Conclusion

In recent centuries, humans have significantly influenced the population dynamics of numerous species through actions such as hunting, the introduction of non-native species and habitat alteration. Our study provides evidence of morphological changes coinciding with the genetic replacement of Fennoscandian wolf populations, which were driven to near extirpation during the twentieth century owing to severe persecution. Our results also suggest that the changes in prey composition and the founder effect may have influenced cranial traits in the modern Scandinavian population, which displayed different mean shapes compared to the modern Finnish population. Future research on wolf populations could integrate genetic, ecological and morphological data, particularly by using three-dimensional landmarking techniques that capture subtle phenotypic changes and by including tooth shape. This comprehensive approach holds promise for uncovering the genetic underpinnings of morphological variation and the factors driving such transformations, including those caused by humans.

Ethics. This work did not require ethical approval from a human subject or animal welfare committee.

Data accessibility. The landmark data and R code are available from the Dryad Digital Repository [99].

Supplementary material is available online [100].

Declaration of Al use. We have not used Ai-assisted technologies in creating this article.

Authors' contributions. D.B.: conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, project administration, resources, visualization, writing-original draft; J.A.: conceptualization, funding acquisition, resources, supervision, writing-original draft, writing-review and editing; C.G.: data curation, resources, software, writing-review and editing; L.K.: conceptualization, funding acquisition, investigation, project administration, resources, supervision, writing-original draft, writing-review and editing; C.A.H.: data curation, funding acquisition, investigation, project administration, resources, software, supervision, validation, writing-original draft, writing-review and editing.

All authors gave final approval for publication and agreed to be held accountable for the work performed therein.

Conflict of interest declaration. We declare we have no competing interests.

Funding. D.B.'s work was supported by the University of Oulu Scholarship Foundation (grant number 20230032) and Vilho, Yrjö ja Kalle Väisälän rahasto. C.A.H was supported by Danmarks Frie Forskningsfond (grant number 10.46540/3103-00296B), J.A. was supported by the Academy of Finland (grant number 345769), and D.B., J.A. and L.K. were supported by the University of Oulu. DANFIX (C.G.) is partly funded by the Danish Agency for Science and Higher Education (grant number 5072-00030B).

Acknowledgements. We appreciate the insightful feedback from the anonymous reviewers. We express gratitude to Oy vind Hammer from the Natural History Museum at the University of Oslo for scanning 10 specimens, Stefan Curth and the Natural History Museum Berlin for providing three additional samples, Suvi Viranta from the University of Helsinki for her valuable comments on landmarking schemes, Heini Niinimäki from the Ranua Zoo for tracking down the origin of the zoo specimens, Diego Rondon Bautista for his insights on R, Volume Graphics for providing the software for mesh reconstruction, all museums that provided samples for this research, and Antti Rönkä for providing the Turku wolves' crania from the Oulu Lyceum Natural History Collections. Additionally, we thank the three-dimensional Imaging Centre at the Technical University of Denmark housing DANFIX for scanning the specimens. We also express our appreciation to the teachers of the geometric morphometries workshops, namely Carmelo Fruciano (workshop in 2022), Dean C. Adams and Michael Collyer (workshop in 2023), for their insights. Lastly, we are grateful to our funders for providing the grants for this research, namely the Academy of Finland, the University of Oulu Scholarship Foundation, Vilho, Yrjö ja Kalle Väisälän rahasto and Danmarks Frie F or skningsf ond.

Sidebar

Cite this article: Bujnáková D, Aspi J, Gundlach C, Kvist L, Hipsley CA. 2025 Wolf cranial morphology tracks population replacement in Fennoscandia. R. Soc Open Sei. 12:250358.

https://doi.org/10.1098/rsos.250358

Received: 20 February 2025

Accepted: 14 May 2025

Subject Category:

Ecology, conservation, and global change biology

Subject Areas:

ecology

Author for correspondence:

Dominika Bujnáková

e-mails: [email protected];

[email protected]

References

References

1. Allendorf F, Luikart G, Aitken S. 2012 Conservation and the genetics of populations, pp. 196-199. Chichester, UK: Wiley-Blackwell.

2. Orr S, Hedrick N, Murray K, Pasupuleti A, Kovacs J, Goodisman M. 2024 Genetic and environmental effects on morphological traits of social phenotypes in wasps. Heredity 133,126-136. (doi:10.1038/s41437-024-00701 -5)

3. McMaster AC. 2023 Genetic and environmental influences on morphological traits in salmonids. Master thesis, University of British Columbia, Vancouver, British Columbia, Canada. (doi:10.14288/1.0435723)

4. Frankham R, Ballou JD, Briscoe DA, McInnes KH. 2002 Introduction to conservation genetics. Cambridge, UK: Cambridge University Press. (doi:10. 1017/CB09780511808999)

5. Sendell-Price AT, Ruegg КС, Clegg SonyaM. 2020 Rapid morphological divergence following a human-mediated introduction: the role of drift and directional selection. Heredity 124,535-549. (doi:10.1038/s41437-020-0298-8)

6. Donihue C. 2017 Hurricane effects on Neotropical lizards span geographic and phylogenetic scales. Proc. Natl Acad. Sci. USA 117,10429-10434. (doi:10.5061/dryad.wm37pvmjh)

7. Brown CR, Brown MB, Roche EA. 2013 Fluctuating viability selection on morphology of cliff swallows is driven by climate. J. Evol. Biol. 26, 1129-1142. (doi:10.1111/jeb.12130)

8. Hoffmann AA, Miller AD, Weeks AR. 2021 Genetic mixing for population management: from genetic rescue to provenancing. Evol. Appl. 14, 634-652. (doi:10.1111/eva.13154)

9. Whiteley AR, Fitzpatrick SW, Funk WC, Tallmon DA. 2015 Genetic rescue to the rescue. Trends Ecol. Evol. 30,42-49. (doi:10.1016/j.tree.2O14.10. 009)

10. Bouzat JL. 2010 Conservation genetics of population bottlenecks: the role of chance, selection, and history. Conserv. Genet. 11,463-478. (doi: 10.1007/s10592-010-0049-0)

11. Meachen-Samuels J, Van Valkenburgh B. 2009 Craniodental indicators of prey size preference in the Felidae. Biol. J. Linn. Soc. 96, 784-799. (doi : 10.1111 /j. 1095-8312.2008.01169.x)

12. Ketchen J. 2020 Geographic variation in skull morphology of the wolf ^Canis lupus) in relation to prey size across North America. Master thesis, Laurentian University, Sudbury, Ontario, Canada, https://laurentian.scholaris.ca/items/eabf9d7d-4f24-49b1-bb78-c4cea69111d3/full.

13. Wiwchar DMA, Mallory FF. 2012 Prey specialization and morphological conformation of wolves associated with woodland caribou and moose. Rangifer 32,309-327. (doi:10.7557/2.32.2.2278)

14. Mulders R. 1997 Geographic variation in the cranial morphology of the wolf Uľanis lupus) in Northern Canada. Master thesis, University of Saskatchewan, Saskatoon, Saskatchewan, Canada, https://iportal.usask.ca/record/9643.

15. Meeh L, Boitani L. 2003 Wolves: behavior, ecology, and conservation. Chicago, IL: University of Chicago Press.

16. Linnell JDC, Solberg EJ, Brainerd S, Liberg 0, Sand H, Wabakken P, Kojola 1.2003 Is the fear of wolves justified? A Fennoscandian perspective. Acta ZooL Litu. 13,34-40. (doi:10.1080/13921657.2003.10512541 )

17. Ermala A. 2003 A survey of large predators in Finland during the 19th-20th centuries. ActaZool. Litu. 13,15-20. (doi:10.1080/13921657.2003. 10512538)

18. Mykrä S, Vuorisalo T, Pohja-Mykrä M. 2005 A history of organized persecution and conservation of wildlife: species categorizations in Finnish legislation from medieval times to 1923.0Ш39,275-283. (doi:10.1017/S0030605305000797)

19. Epstein Y, Kantinkoski S. 2020 Non-governmental enforcement of EU environmental law: a stakeholder action for wolf protection in Finland. Front. Ecol. Evol. 8:101. (doi:10.3389/fevo.2020.00101)

20. Wabakken P, Sand H, Liberg 0, Bjärvall A. 2001 The recovery, distribution, and population dynamics of wolves on the Scandinavian peninsula, 1978-1998. Can. J. Zool. 79,710-725. (doi:10.1139/cjz-79-4-710)

21. Mykrä S, Pohja-Mykrä M. 2015 Back-calculation of large carnivore populations in Finland in 1865-1915. Ann. Zool. Fenn. 52,285-300. (doi:10. 5735/086.052.0504)

22. Ministry of Agriculture and Forestry Finland. 2005 Management plan for the wolf population in Finland. Helsinki. See http://julkaisut. valtioneuvosto.fi/handle/10024/80581.

23. Aspi J, Roininen E, Ruokonen M, Kojola I, Vila C. 2006 Genetic diversity, population structure, effective population size and demographic history of the Finnish wolf population. Mol. Ecol. 15,1561-1576. (doi:10.1111/j.1365-294X.2006.02877.x)

24. Jansson E, Harmoinen J, Ruokonen M, Aspi J. 2014 Living on the edge: reconstructing the genetic history of the Finnish wolf population. BMC Evol. Biol. 14,64. (doi:10.1186/1471 -2148-14-64)

25. Lykke J. 2005 Selective harvest management of a Norwegian moose population. ALCES 41, 9-24. https://www.alcesjournal.org/index.php/ alces/article/view/401

26. Nygrén T. 1987 The history of moose in Finland. Swed. Wildl. Res. Suppl. 1,49-54.

27. Flagstad 0, Walker CW, Vila C, Sundqvist AK, Fernholm B, Hufthammer AK, Wiig 0, Koyola I, Ellegren H. 2003 Two centuries of the Scandinavian wolf population: patterns of genetic variability and migration during an era of dramatic decline. Mol. Ecol. 12,869-880. (doi:10.1046/j.1365294x.2003.01784.x)

28. Vila C. 2003 Rescue of a severely bottlenecked wolf ^Canis lupus) population by a single immigrant. Proc. R. Soc. В 270,91-97. (doi:10.1098/ rspb.2002.2184)

29. Åkesson M, Liberg 0, Sand H, Wabakken P, Bensch S, Flagstad 0.2016 Genetic rescue in a severely inbred wolf population. Mol. Ecol. 25,47454756. (doi:10.H11/mec.13797)

30. Smeds L, Huson LSA, Ellegren H. 2024 Structural genomic variation in the inbred Scandinavian wolf population contributes to the realized genetic load but is positively affected by immigration. Evol. Appl. 17. (doi:10.1111/eva. 13652)

31. Smeds L, Ellegren H. 2023 From high masked to high realized genetic load in inbred Scandinavian wolves. Mol. Ecol. 32, 1567-1580. (doi:10. 1111/mec.168O2)

32. Räikkönen J, Vucetich JA, Vucetich LM, Peterson RO, Nelson MP. 2013 What the inbred Scandinavian wolf population tells us about the nature of conservation. PLoS ONE 8, e67218. (doi:10.1371 /journal.pone.0067218)

33. Suutarinen J, Kojola 1.2017 Poaching regulates the legally hunted wolf population in Finland. Biol. Conserv. 215,11-18. (doi:10.1016/j.biocon. 2017.08.031)

34. Jansson E, Ruokonen M, Kojola I, Aspi J. 2012 Rise and fall of a wolf population: genetic diversity and structure during recovery, rapid expansion and drastic decline. Mol. Ecol. 21,5178-5193. (doi:10.1111/mec.12010)

35. Åkesson M, Flagstad 0, Aspi J, Kojola I, Liberg 0, Wabakken P, Sand H. 2022 Genetic signature of immigrants and their effect on genetic diversity in the recently established Scandinavian wolf population. Conserv. Genet. 23,359-373. (doi:10.1007/s10592-021-01423-5)

36. Stenøien H. 2021 Genetisk opphav til den norske-svenske ulvestammen (Canis lupus lupus). Trondheim, Norway: NTNU. See https://ntnuopen. ntnu.no/ntnu-xmlui/handle/11250/2987985.

37. Chapron G etai. 2014 Recovery of large carnivores in Europe's modern human-dominated landscapes. Science 346,1517-1519. (doi:10.1126/ science.1257553)

38. (¡matti M etai. 2021 Large carnivore expansion in Europe is associated with human population density and land cover changes. Divers. Distrib. 27,602-617. (doi:10.1111/ddi.13219)

39. Ellegren H, Savolainen P, Rosen B. 1996 The genetical history of an isolated population of the endangered grey wolf Canis lupus; a study of nuclear and mitochondrial polymorphisms. Ж Trans. R.Soc. 5351,1661-1669. (doi:10.1098/rstb.1996.0148)

40. Linnell J, Brøseth H, Solberg E, Brainerd S. 2005 The origins of the southern Scandinavian wolf Canis lupus population: potential for natural immigration in relation to dispersal distances, geography and Baltic ice. Wildl. Biol. 11, 383-391. (doi:10.2981/0909-6396(2005)11 [383: T00TSS]2.0.C0;2)

41. Wabakken P, Svensson L, Maartmann E, Nordli K, Flagstad 0, Danielsson A, Cardoso Palacios C, Åkesson M. 2024 Bestandsovervåking av ulv vinteren 2023-2024. inventering av varg vintern 2023-2024. bestandsstatus for store rovdyr i skandinavia. beståndsstatus för stora rovdjur i Skandinavien. (Wolf population monitoring in the winter of 2023-2024: population status of large predators in Scandinavia). Trondheim and Grimso, Norway: Rovdata, SLU Viltskadecenter, and Høgskolen i Innlandet. See https://brage.nina.no/nina-xmlui/bitstream/handle/11250/ 3131698/bestandsstatusstorerovdyr2024_1.pdf?sequence=1&isAllowed=y.

42. Valtonen M, Heikkinen S, Johansson H, Härkälä A, Helle I, Mäntyniemi S, Kojola 1.2024 SusikautaSuomessa maaliskuussa2024. Luonnonvara-ja biotalouden tutkimus 54/2024. (Wolf population in Finland in March 2024 Natural Resources and Bioeconomy Research 54/2024). Helsinki, Finland: Luonnonvarakeskus. See http://urn.fl/URN:ISBN:978-952-380-934-5.

43. Engdal V. 2018 Phenotypic variation in past and present Scandinavian wolves (Canis lupus L.). Master thesis, University of Oslo, Oslo, Norway. http://urn.nb.nO/URN:NBN:no-70390.

44. Bujnáková D, Aspi J, Gundlach C, Kvist L, Hipsley CA. 2025 Data from: Wolf cranial morphology tracks population replacement in Fennoscandia. Dryad Digital Repository. (doi:10.5061 /dryad.nk98sf825))

45. Olson D. 2001 Terrestrial ecoregions of the world: a new map of life on Earth: a new global map of terrestrial ecoregions provides an innovative tool for conserving biodiversity. Bioscience 51,933-938. (doi: 10.1641 /0006-3568(2001 )051 [0933:TE0TWA]2.0.C0;2)

46. Curth S, Fischer M, Kupczik K. 2017 Can skull form predict the shape of the temporomandibular joint? A study using geometric morphometries on the skulls of wolves and domestic dogs. Ann. Anat. 214,53-62. (doi:10.1016/j.aanat.2O17.08.003)

47. Jansson M, Amundin M, Laikre L. 2015 Genetic contribution from a zoo population can increase genetic variation in the highly inbred wild Swedish wolf population. Conserv. Genet. 16,1501-1505. (doi:10.1007/s10592-015-0738-9)

48. Hanegraef H, Spoor F. 2024 Maxillary morphology of chimpanzees: captive versus wild environments. J. Anat. 244,977-994. (doi:10.1111 /joa. 14016)

49. Siciliano-Martina L, Light JE, Lawing AM. 2021 Cranial morphology of captive mammals: a meta-analysis. franf. Zoo/. 18. (doi:10.1186/s12983021-00386-0)

50. Hartstone-Rose A, Selvey H, Villari JR, Atwell M, Schmidt T. 2014 The three-dimensional morphological effects of captivity. PloS ONE 9, e113437. (doi:10.1371/journal.pone.0113437)

51. Junno JA etai. 2024 Stable isotope analyses of carbon and nitrogen in hair keratin of suspected man-eating wolves from 1880s. Sci. Rep. 14, 4946. (doi:10.1038/S41598-024-55521-8)

52. Tikkanen J. 2019 Lauma: 1880-luvun lastensurmat ja susiviha Suomessa (The pack: 1880s child murders and wolf hatred in Finland), pp. 216- 221. Helsinki, Finland: Otava.

53. Pulliainen E. ^TASuomen suurpedot (Large carnivores of Finland), p. 78. Helsinki, Finland: Tammi.

54. Adams D, Collyer M, Kaliontzopoulou A, Baken E. 2024 Geomorph: software for geometric morphometric analyses. R package version 4.0.7 https://CRAN.R-project.org/package=geomorph.

55. Baken EK, Collyer ML, Kaliontzopoulou A, Adams DC. 2021 geomorph v4.0 and gmShiny: enhanced analytics and a new graphical interface for a comprehensive morphometric experience. Methods Ecol. Evol. 12,2355-2363. (doi:10.1111 /2041 -21 Ox.13723)

56. R. Core Team. 2024 R: a language and environment for statistical computing. Vienna, Austria: R Foundation for Statistical Computing.

57. Gunz P, Mitteroecker P, Neubauer S, Weber GW, Bookstein FL. 2009 Principles for the virtual reconstruction of hominin crania. J. Hum. Evol. 57, 48-62. (doi:10.1016/j.jhevol.2009.04.004)

58. Gower J. 1975 Generalized procrustes analysis. Psychometrika 4,33-51. (doi:10.1007/bf02291478)

59. Collyer M, Adams D. 2024 RRPP: linear model evaluation with randomized residuals in a permutation procedure. R package version 2.0.0 https://CRAN.R-project.org/package=RRPP.

60. Collyer ML, Adams DC. 2018 RRPP: an R package for fitting linear models to high-dimensional data using residual randomization. Methods Ecol. Evol. 9,1772-1779. (doi:10.1111 /2041 -21 OX. 13029)

61. Hell P, Paule L. 1982 Taxonomic data on the wolf (Canis lupus) from the Slovakian Carpathians. FoliaZool. Brno. 31,255-270.

62. Hillis TL, Mallory FF. 1996 Sexual dimorphism in wolves (Canis lupus) of the Keewatin District, Northwest Territories, Canada. Can. J. Zool 74, 721-725. (doi:10.1139/z96-081)

63. Trbojević I, Ćirovič D. 2015 Sexual dimorphism and population differentiation of the wolf (Canis lupus) based on morphometry in the Central Balkans. North West J. Zool. 12,349-355. https://biozoojournals.ro/nwjz/content/v12n2/nwjz_e151709_Trbojevic.pdf

64. Milenkovič M, Šipetić VJ, Blagojevič J, Tatovič S, Vujošević M. 2010 Skull variation in Dinaric-Balkan and Carpathian gray wolf populations revealed by geometric morphometric approaches J. Mammal. 91,376-386. (doi:10.1644/09-mamm-a-265.1)

65. Khidas K. 2023 Morphological relationships among populations support a single taxonomic unit for the North American gray wolf. J. Mammal. 104,562-577. (doi:10.1093/jmammal/gyad012)

66. MacLeod N, Kolska Horwitz L. 2020 Machine-learning strategies for testing patterns of morphological variation in small samples: sexual dimorphism in gray wolf (Canis lupus) crania. BMC Biol. 18,113. (doi:10.1186/s12915-020-00832-1)

67. Felice RN, Watanabe A, Cuff AR, Hanson M, Bhu I lar BAS, Rayfield ER, Witmer LM, Norell MA, Goswami A. 2020 Decelerated dinosaur skull evolution with the origin of birds. PLoS Biol. 18, e3000801. (doi:10.1371 /j o u r na I. p b i o.3000801 )

68. Mäntyniemi S. 2022 Suomen susikannan suotuisan suojelutason viitearvojen määrittäminen: loppuraportti 2022. luonnonvara- ja biotalouden tutkimus 80/2022. (Determining reference values for the favourable conservation status of the Finnish wolf population: final report 2022). Helsinki, Finland: Luonnonvarakeskus. See http://urn.fi/URN:ISBN:978-952-380-514-9.

69. Liberg 0, Chapron G, Wikenros C, Flagstad 0, Wabakken P, Sand H. 2015 An updated synthesis on appropriate science-based criteria for 'favourable reference population' of the Scandinavian wolf (Canis lupus) population. Stockholm, Sweden: Grimsö Wildlife Research Station, Department of Ecology, and Swedish University of Agricultural Sciences. See https://pub.epsilon.slu.Se/id/document/12155646.

70. Kolbe JJ, Leal M, Schoener TW, Spiller DA, Losos JB. 2012 Founder effects persist despite adaptive differentiation: a field experiment with lizards. Science 335,1086-1089. (doi:10.1126/science.l 209566)

71. Hoelzel AR, Fleischer RC, Campagna C, Le Boeuf BJ, Alvord G. 2002 Impact of a population bottleneck on symmetry and genetic diversity in the northern elephant sealJ. Evol. Biol. 15,567-575. (doi:l0.1046/j.1420-9101.2002.00419.x)

72. Boyko AR et al. 2010 A simple genetic architecture underlies morphological variation in dogs. PLoS Biol. 8,49-50. e1000451. (doi:10.1371/ journal.pbio.1000451)

73. Slater GJ, Dumont ER, Van Valkenburgh B. 2009 Implications of predatory specialization for cranial form and function in canids. J. Zool. 278, 181-188. (doi:10.1111/j.1469-7998.2009.00567.x)

74. Friis L. 1985 An investigation of subspecific relationships of the grey wolf, Canis lupus, in British Columbia. Master thesis, University of Victoria, Victoria, Canada, https://hdl.handle.net/1828/17831.

75. Milakovic B, Parker KL. 2011 Using stable isotopes to define diets of wolves in northern British Columbia, Canada. J. Mammal. 92, 295-304. (doi:10.1644/10-mamm-a-038.1)

76. Darimont C, Price M, Winchester N, Gordon-Walker J, Paquet P. 2004 Predators in natural fragments: foraging ecology of wolves in British Columbia's central and north coast archipelago. J. Biogeogr. 31,1867-1877. (doi:10.1111/j.1365-2699.2004.01141.x)

77. Schmitz 0, Lavigne D. 1987 Factors affecting body size in sympatric Ontario Canis. J. Mammal. 68,92-99. (doi:10.2307/1381050)

78. O'Keefe FR, Meachen J, Fet EV, Brannick A. 2013 Ecological determinants of clinal morphological variation in the cranium of the North American gray wolf. J. Mammal. 94,1223-1236. (doi:10.1644/13-mamm-a-069)

79. Kojola 1.2000 Suden, hirven ja metsästyksen vuorovaikutussuhteet. (Interrelationships between wolf, moose and hunting). Suomen Riista 46, 76-81.

80. Gade-Jørgensen I, Stagegaard R. 2000 Diet composition of wolves Canis lupus in east-central Finland. Acta Theriol. 45,537-547. (doi:10.4098/ at.arch.00-52)

81. Olsson 0, Wirtberg J, Andersson M, Wirtberg 1.1997 \No\L Canis lupus predation on moose Alces alces and roe deer Capreolus capreolus in southcentral Scandinavia. Wildl. Biol. 3,13-23. (doi:10.2981/wlb.1997.003)

82. Müller S. 2006 Diet composition of wolves (Canis lupus) on the Scandinavian peninsula determined by scat analysis. Master thesis, Technical University of Munich, Munich, Germany. https://api.semanticscholar.Org/CorpuslD:59374338.

83. Liberg 0, Bergström R, Kindberg J, von Essen H. 2010 Ungulates and their management in Sweden. In European ungulates and their management in the 21st century (eds M Apollonio, R Andersen, R Putman), pp. 37-70. Cambridge, UK: Cambridge University Press.

84. LinnelU, KovtunE, Rouartl. 2021 Wolf attacks on humans: an update for 2002-2020. See https://hdLhandle.net/11250/2729772.

85. Alados CL, Escós J, Emlen JM. 1995 Fluctuating asymmetry and fractal dimension of the sagittal suture as indicators of inbreeding depression in dama and dorcas gazelles. Can. J. Zool. 73,1967-1974. (doi:10.1139/z95-231)

86. Bannasch D, Famula T, Donner J, Anderson H, Honkanen L, Batcher K, Safra N, Thomasy S, Rebhun R. 2021 The effect of inbreeding, body size and morphology on health in dog breeds. Canine Med. Genet. 8,12. (doi:10.1186A40575-021 -00111 -4)

87. Iljin NA. 1941 Wolf-dog genetics./. Genet. 42,359-414. (doi:10.1007/bf02982879)

88. Korablev PN, Korablev NP, Zinoviev AV, Korablev MP. 2023 Can skull morphology-morphometry discern Russian wolf-dog hybrids from wolves (Canis lupus) and dogs (Canis familiaris)"? Rus. J. Theriol. 22,62-73. (doi:10.15298/rusjtheriol.22.1.07)

89. Grant PR, Grant BR. 1994 Phenotypic and genetic effects of hybridization in Darwin's finches. Evolution 48, 297-316. (doi:10.1111/J.15585646.1994.tb01313.x)

90. Randi E et al. 2014 Multilocus detection of wolf x dog hybridization in Italy, and guidelines for marker selection. PLoS ONE 9, e86409. (doi:10. 1371/journal.pone.0086409)

91. Moura AE, Tsingarska E, Dąbrowski MJ, Czarnomska SD, Jędrzejewska В, Pilot M. 2014 Unregulated hunting and genetic recovery from a severe population decline: the cautionary case of Bulgarian wolves. Conserv. Genet. 15,405-417. (doi:10.1007/s10592-013-0547-y)

92. Tirronen KF, Kuznetsova AS. 2025 Wolf-dog hybrids in the eastern fringe Northern European wolf population on the background of increased hunting pressure. Polar Biol. 48. (doi:10.1007/s00300-024-03330-0)

93. Pilot M, Greco C, vonHoldt BM, Randi E, Jędrzejewski W, Sidorovich VE, Konopiński MK, Ostrander EA, Wayne RK. 2018 Widespread, long-term admixture between grey wolves and domestic dogs across Eurasia and its implications for the conservation status of hybrids. Evol. Appl. 11, 662-680. (doi:10.1111/eva.12595)

94. Smeds L, Aspi J, Berglund J, Kojola I, Tirronen K, Ellegren H. 2021 Whole-genome analyses provide no evidence for dog introgression in Fennoscandian wolf populations. Evol. Appl. 14,721-734. (doi:10.1111 /eva. 13151 )

95. Sanz-Pérez A, Ordiz A, Sand H, Swenson JE, Wabakken P, Wikenros C, Zimmermann B, Åkesson M, Milleret C. 2018 No place like home? A test of the natal habitat-biased dispersal hypothesis in Scandinavian wolves. R. Soc. Open Sei. 5,181379. (doi:10.1098/rsos.181379)

96. Pilot M, Jędrzejewski W, Branicki W, Sidorovich VE, Jędrzejewska В, Stachura К, Funk SM. 2006 Ecological factors influence population genetic structure of European grey wolves. Mol. Ecol. 15,4533-4553. (doi:10.1111/j.1365-294X.2006.03110.x)

97. Geffen E, Anderson MJ, Wayne RK. 2004 Climate and habitat barriers to dispersal in the highly mobile grey wolf. Mol. Ecol. 13,2481-2490. (doi: 10.1111 /j. 1365-294X.2004.02244.X)

98. Gese EM, Mech LD. 1991 Dispersal of wolves (.Canis lupus) in northeastern Minnesota, 1969-1989. Can. J. Zool 69,2946-2955. (doi:10.1139/ Z91-415)

99. Bujnáková D, Aspi J, Gundlach C etai. 2025. Data from: Wolf cranial morphology tracks population replacement in Fennoscandia. Dryad, (doi: 10.5061/dryad.nk98sf825)

100. Bujnakova D, Aspi J, Gundelach C, Kvist L, Hipsley CA. 2025 Supplementary material from: Wolf cranial morphology tracks population replacement in Fennoscandia. Figshare. (doi:10.6084/m9.figshare.c.7859573)

Footnote

fLaura Kvist and Christy A. Hipsley are joint senior authors.