An approach to body size fluctuations in Quaternary continental mammals: the case of hippopotamuses from western Europe.

Several paleoenvironmental analyses show that community change over geological time is driven by ecological factors which, in turn, are regulated by climate. To most effectively examine the origin of community change researchers need to single out and track specific paleobiological properties and to see how much they depend on local or more general constraints. A suite of proxies have been used to analyze ecologically based community change. Body size is one of the most sensitive to ecological factors and is therefore deemed to disclose significant information on past environmental conditions. Bergmann’s rule proposes a thermoregulatory explanation to the latitudinal gradient of increasing animal body size. Whereas some animals fit the predictions of Bergmann's rule, others do not, or even correlate negatively. Several critical reviews cast doubts on the “thermal” interpretation of Bergmann’s rule, and proposed alternative hypotheses for geographic variation in body size based on various physiological, biological and ecological constraints. The Ice Age cycling that was set off by the 2.6 Ma Arctic ice cap expansion led gradually to ever increasing environmental patchiness, and the effects of this habitat fragmentation became particularly evident in the Late Pleistocene and Holocene. Megafaunas progressively split up into metapopulations confined into ever smaller and more isolated habitats. This led numerous large mammals to experience endemic evolution in isolation, and body sizes changed in ways similar to those observed in land mammals distributed on true islands.