Mammals: From Humble Vertebrate Beginnings to Global Terrestrial Dominance

Abstract

A review of mammals is indicated because of their global terrestrial dominance and diversification and because, relative to other vertebrate classes, mammals are well described. Many mammalian taxa display generalized phenotypes and flexible (thermal) niches, recommending this taxon for research on evolvability. The three extant mammalian lineages evolved independently and the overwhelmingly majority of extant forms are eutherians (“true placentals,” ∼5,000 species). Monotremes (“egg-laying” mammals) retain several synapsid (“mammal-like reptiles”) and avian features as well as a primitive placental structure. Because of their generalized anatomical features, marsupials serve as a “control group” for the Class as a whole (Eisenberg, The mammalian radiations: an analysis of trends in evolution, adaptation, and behavior. University of Chicago Press, Chicago, 1981). The degree of specialization and mode of reproduction of females in each lineage obligate this sex to maternal care of offspring and reflect patterns of energy exploitation, processing, and allocation for the most efficient conversion of limiting resources into offspring (directly and/or indirectly). Allocation (energy investment) strategies are expected to differ between adult males (mating effort) and reproductive females (reproductive effort) in a population, reflecting feedback regulation maintaining thermal tolerances within sublethal ranges. For females of each lineage, reproductive mode is associated with trade-offs reflected in life-history strategies.