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Abstract.-
Particle size reduction is a primary means of improving efficiency in herbivores. The mode of food particle size reduction is one of the main differences between herbivorous birds (gizzard) and mammals (teeth). For a quantitative comparison of the efficiency of food comminution, we investigated mean fecal particle sizes (MPS) in 14 herbivorous bird species and compared these with a data set of 111 non-ruminant herbivorous mammal species. In general MPS increased with body mass, but there was no significant difference between birds and mammals, suggesting a comparable efficiency of food processing by gizzards and chewing teeth. The results lead to the intriguing question of why gizzard systems have evolved comparatively rarely among amniote herbivores. Advantages linked to one of the two food comminution systems must, however, be sought in different effects other than size reduction itself. In paleoecological scenarios, the evolution of "dental batteries," for example in ornithopod dinosaurs, should be considered an advantage compared to absence of mastication, but not compared to gizzard-based herbivory.
Introduction
Efficient digestion of plant material depends on two main factors, the time ingesta remain within in the gastrointestinal tract (retention time) and the particle size to which the food is processed. The former is important because digestion of hemicellulose and cellulose in plant cell walls by symbiotic microorganisms is a rather slow process (Stevens and Hume 1995) and the latter because smaller particles can be fermented at a faster rate (Chemey et al. 1988; Bjorndal et al. 1990). Moreover, as has recently been demonstrated, retention time in the gut and the efficiency of particle size reduction (i.e., chewing efficiency) may compensate for each other-in large herbivorous mammals, species with relatively lower chewing efficiency have longer digesta retention times (Clauss et al. 2009). The relevance of reducing the particle size of ingested food is well understood (Clauss and Hummel 2005). However, for herbivorous birds- quantitative data on particle size reduction have onlY been reported sporadically (Moore 1999; Potter et al. 2006). For example, in studies of geese, Moore (1999) stated that the degree to which grass can be degraded by the avian gizzard is comparable to that of chewing by ruminants. However, this statement misses the order of magnitude by which body mass differs between geese and ruminants,...