Content area
Full text
Among mammals generally and rodents particularly mean litter sizes usually are about onehalf the number of mammae, and maximum litter sizes approximate mammary numbers. Naked mole-rats (Heterocephalus glaber: Bathyergidae) are exceptions to both generalizations. Field-caught litters averaged 11.3 young +/- 6.2 SD (n = 82), and captive-born litters averaged 11.4 +/- 5.6 young (n = 190). Similarly, numbers of mammae on breeding females averaged 11.6 +/- 1.1 (n = 43) in the field and 11.5 +/- 2.0 (n = 29) in captivity. Maximum litter sizes were 28 in the field and 27 in captivity, whereas the maximum number of mammae was 15. More than one-half of field-caught and captive males and females had different numbers of mammae on the two sides of their body. Neither total numbers of mammae nor fluctuating asymmetries in mammary numbers differed significantly between males and females, nor between breeders and nonbreeders. There was no relationship between litter sizes and numbers of mammae or fluctuating asymmetries in mammary numbers. Breeding female naked mole-rats can bear and successfully rear litters that are far more numerous than their mammae because, on a proximate level, young take turns nursing from the same mammary and, on an ultimate level, breeding females are fed and protected by colony mates, enabling them to concentrate their reproductive efforts on gestation and lactation.
Key words: Heterocephalus glaber, naked mole-rat, litter size, mammary formula, eusociality, one-half rule, fluctuating asymmetry
Variations in litter sizes and numbers of mammae are striking features of mammalian diversity. Eighty-six years ago, Pearl (1913a) reported that these two traits are correlated. He found a significant positive relationship between numbers of mammae and litter sizes for 90 species of wild and domesticated mammals, representing all orders. This relationship was described by the equation L = 0.56 M + 0.53, where L = litter size and M = number of mammae.
Seventy-three years later, Gilbert (1986) rediscovered the significant positive relationship between mean numbers of mammae and mean litter sizes among 266 species of rodents, mainly in the families Muridae, Cricetidae, and Sciuridae. This relationship was described as L = 0.46 M + 0.39. Gilbert characterized his result as the "one-half rule" because mean litter size was about one-half of the mean number of mammae. Gilbert (1986:4829)...