Is parental care the key to understanding endothermy in birds and mammals?

In a recent paper, Farmer (2000) offered a novel and stimulating hypothesis for the convergent evolution of endothermy in birds and mammals. She proposed that a high level of nonshivering thermogenesis by adults evolved because of the benefits it provided to their developing embryos. Initially, a higher standard metabolic rate (SMR), and consequently a higher body temperature, served only to increase the temperature of embryos during development. Later, increased aerobic capacity arose from the need to meet the energetic demands of parental care. Importantly, Farmer’s model of the evolution of endothermy, which we will refer to as the “parental care model,” and the long-standing aerobic capacity model (Bennett and Ruben 1979) are mutually exclusive. The aerobic capacity model asserts that endothermy evolved via a correlated response to selection for a greater capacity for sustained aerobic activity. The parental care model states that the relatively high aerobic capacity of mammals and birds is a consequence of parental care rather than a cause for endothermy. Farmer’s ideas are intriguing and will hopefully serve to renew empirical interest in a problem that has been dominated by few hypotheses (see Hayes and Garland 1995). Given the potential influence that Farmer’s model will have on the direction of future efforts to understand the evolution of endothermy, we feel that it is necessary to scrutinize the arguments in support of the parental care model and those against the aerobic capacity model.