DIET AND BODY MASS IN TERRESTRIAL MAMMALS


similar biomes, where fruit resources are available all year round. Seasonality of pre- cipitation or outright aridity prevents species from specializing on an entirely frugivorous diet in more open environments (Ganesh and Davidar 1999). Thus, based on our results we predict that medium-sized mammals should be less frequent in open environments. These observations are consistent with Rodríguez (1999), who quantitatively documented a decrease in the density of mid-sized mammals in increasingly open landscapes. Plant species composition in open environ-


ments is highly heterogeneous, with fruiting trees widely dispersed across space. As a consequence, frugivores in such environments would have to rely on a food resource that is rather unstable and patchily distributed, which would therefore require them to have unrea- listically large home ranges (Milton and May 1976; Ganesh and Davidar 1999). According to optimal foraging theory, diet


choice is conditioned by the need to maximize energy intake per unit of time spent on the foraging activity (MacArthur and Pianka 1966; Bartumeus and Catalan 2009). An animal relying on a patchily distributed resource will then be forced to face a trade-off between the nature of resources and the energy required to move from patch to patch (Pyke et al. 1977; Bartumeus and Catalan 2009). Thus, the pro- ductivity and the species distribution of fruit trees in tropical rain forests play an important role in the evolution of the relationship between diet specialization and body mass in tropical mammalian species. This trade-off explains why the optimum body mass for a frugivore diet would fit around the medium- size range (1–30 kg). The energy invested in foraging activity—moving across patches or climbing trees—is cost ineffective for smaller and for bigger species (Pyke et al. 1977; Bartumeus and Catalan 2009). A pure frugi- vore diet is therefore restricted to the medium- size range within which foraging efficiency reaches its maximum.


Frugivory and the Medium-size Gap Many ecological analyses have pointed out


the decline or absence of medium-sized


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mammals (500 g–30 kg) in open-environment mammalian communities. Some interpreta- tions in the literature include predator–prey relationships; trophic, physiological; and mechanical constraints; taxonomic limitations; or predator avoidance (Valverde 1967; Legendre 1986; Gingerich 1989; Smith and Lyons 2011). Alroy et al. (2000) hypothesized that the


opening of a medium-size gap in North America during the middle of the Cenozoic could be linked to increased seasonality. The mid-size range was emptied during the middle Eocene (about 46Ma) as ecosystems became more arid and seasonal. Alroy and colleagues also recognized a decrease in ecomorphologi- cal diversity after this period, as arboreal frugivorous species were replaced by large terrestrial herbivores. This interpretation sup- ports our idea of a relationship between the opening of a medium-size gap and the decrease or disappearance of frugivore species in open environments. Additionally, Alroy (1998) recognized a


general evolutionary trend in North American mammals toward increased bodymass during the Cenozoic. However, he observed an upper size limit to the evolution of small taxa around 500 g, where the mid-size gap starts in North American mammals. This limit would have been established when vegetation structure changed toward more open landscapes, redu- cing the number of niches left to explore. In parallel, medium-sized tree-dwelling mam- mals evolved toward increased body size (Alroy 1998) or were replaced by open- environment herbivores. Interestingly, and as pointed out by Smith and Lyons (2011), the upper limit of the mid-size gap coincides approximately with the lower limit observed for ruminant herbivores (5–10 kg) (Demment and Van Soest 1985).


The Consequences of Improper Diet Classifications


All paleoecological methods make assump-


tions. In particular, biological parameters need to be categorized properly in order to statistically test their relationships with ecolo- gical and climatological variables. The present


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