The evolution of mammal body sizes: responses to Cenozoic climate change in North American mammals

Abstract

Explanations for the evolution of body size in mammals have remained surprisingly elusive despite the central importance of body size in evolutionary biology. Here, we present a model which argues that the body sizes of Nearctic mammals were moulded by Cenozoic climate and vegetation changes. Following the early Eocene Climate Optimum, forests retreated and gave way to open woodland and savannah landscapes, followed later by grasslands. Many herbivores that radiated in these new landscapes underwent a switch from browsing to grazing associated with increased unguligrade cursoriality and body size, the latter driven by the energetics and constraints of cellulose digestion (fermentation). Carnivores also increased in size and digitigrade, cursorial capacity to occupy a size distribution allowing the capture of prey of the widest range of body sizes. With the emergence of larger, faster carnivores, plantigrade mammals were constrained from evolving to large body sizes and most remained smaller than 1 kg throughout the middle Cenozoic. We find no consistent support for either Cope's Rule or Bergmann's Rule in plantigrade mammals, the largest locomotor guild (n = 1186, 59% of species in the database). Some cold-specialist plantigrade mammals, such as beavers and marmots, showed dramatic increases in body mass following the Miocene Climate Optimum which may, however, be partially explained by Bergmann's rule. This study reemphasizes the necessity of considering the evolutionary history and resultant form and function of mammalian morphotypes when attempting to understand contemporary mammalian body size distributions.

Mammal survival at the Cretaceous–Palaeogene boundary: metabolic homeostasis in prolonged tropical hibernation in tenrecs

“…Ecological release from the vice grip which the dinosaurs held over Mesozoic mammals drove remarkably rapid evolutionary processes within several hundred thousand years of the extinction event [2]. Four new placental lineages (Xenarthra, Afrotheria, Laurasiatheria and Euarchontoglires) [2] appeared very suddenly, harbouring forms that displayed spectacular new morphological and physiological characteristics [5][6][7][8]. The long ca.…”

The evolution of micro-cursoriality in mammals

“…In our expanded, more taxonomically diverse dataset (n=135 species), all of our phylogenetically corrected regressions showed significant These data emphasize that the MT:F ratio of mammals smaller than 1 kg never exceeds 0.7, whereas it does in larger unguligrade mammals. Thus cursoriality never evolved in the majority of mammals, that is, those smaller than 1 kg (see Lovegrove and Mowoe, 2013). However, the rates of evolution of cursoriality as measured by the MT:F ratio accelerated during the Oligocene and Miocene in typical large-bodied herbivorous cursors (Garland and Janis, 1993;Janis and Wilhelm, 1993).…”

“…Table 4 for regression statistics). Janis, 1993;Janis and Wilhelm, 1993), despite the fact that both herbivores and carnivores were also increasing in body size (Lovegrove and Mowoe, 2013).…”

“…The trade-off posits that the fitness benefits of unguligrady in herbivores far outweighed the fitness costs of the loss of digit numbers and functionality, which was not the case in carnivores (Lovegrove and Mowoe, 2013). The divergence in MT:F ratios occurred in the Oligocene and Miocene when there was a dramatic acceleration in the rate of evolution of the MT:F ratio in herbivores, but not in carnivores (Garland and Table 4 for regression statistics).…”

“…We offer one explanation for the difference in the MT:F ratios of similar-sized unguligrade and non-unguligrade mammals: the evolutionary trade-off between locomotor performance and digit functionality (Lovegrove and Mowoe, 2013). The trade-off posits that the fitness benefits of unguligrady in herbivores far outweighed the fitness costs of the loss of digit numbers and functionality, which was not the case in carnivores (Lovegrove and Mowoe, 2013).…”