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Many researchers attribute the large number of physiological and behavioral similarities between birds and mammals, which have separate evolutionary histories, to endothermy (a thermoregulatory strategy whereby warm body temperature is maintained through internal heat sources). However, Farmer argues that parental care rather than endothermy is the key to understanding the similarities between mammals and birds. According to Farmer, while endothermy provides an explanation for a few similarities, such as the presence of body insulation, endothermy is just one characteristic among many related to parental care. The two purported advantages of endothermy that have been most frequently cited by researchers are an expanded range of inhabitable environments and the ability to sustain vigorous exercise. But metabolism has to increase substantially (at great energy cost) therefore conferring no significant thermoregulatory advantage in terms of the former, and there is no causal biological linkage to explain why endothermy would be essential to sustain exercise. Farmer argues instead that endothermy evolved as a means to control incubation temperature and that the ability to sustain exercise evolved separately, as a means to improve a parent's ability to forage and provision its young.