Levesque Danielle L, Lovegrove Barry G
School of Life Sciences, University of KwaZulu-Natal, P/Bag X01, Scottsville 3209, South Africa.
J Exp Biol. 2014 May 1;217(Pt 9):1535-42. doi: 10.1242/jeb.098848. Epub 2014 Feb 5.
Homeothermic endothermy, the maintenance of a high and stable body temperature (Tb) using heat produced by elevated metabolism, is energetically expensive. There is increasing evidence that the earliest endotherms were heterotherms that, rather than maintaining strict homeothermy, allowed Tb to fluctuate with large variations between active and rest-phase Tb. The high level of homeothermy observed in modern mammals is therefore likely to have evolved from an ancestral heterothermic state. One of the hypotheses for the evolution of endothermy is that homeothermy allows for greater energetic output during reproduction (parental care model). We tested this hypothesis by measuring metabolic rates over a range of ambient temperatures in both reproductive and non-reproductive greater hedgehog tenrecs (Setifer setosus), a physiologically primitive mammal from Madagascar. Tenrecs have some of the lowest metabolic rates and highest levels of Tb variability of any mammal and are therefore good models of the ancestral eutherian state. During pregnancy and lactation, there was an increase in metabolism and Tb below the thermoneutral zone, accompanied by a decrease in Tb variability. The lower critical limit of the thermoneutral zone was estimated at ~25°C. However, whereas increases in resting metabolism were substantial below 20°C (up to 150% higher during reproduction), daytime rest-phase ambient temperatures at the study site rarely reached equivalent low levels. Thus, S. setosus provide an example for how relatively low-cost increases in homeothermy could have led to substantial increases in fitness by allowing for the faster production of young. The mechanisms necessary for increases in thermogenesis during reproduction would have further benefited the development of homeothermy in mammals.
恒温性内温动物通过提高新陈代谢产生的热量来维持较高且稳定的体温（Tb），这在能量方面代价高昂。越来越多的证据表明，最早的内温动物是异温动物，它们并非维持严格的恒温性，而是允许Tb随活跃期和休息期Tb的大幅变化而波动。因此，现代哺乳动物中观察到的高水平恒温性很可能是从祖先的异温状态进化而来的。关于内温性进化的一种假说是，恒温性能够在繁殖期间实现更高的能量输出（亲代抚育模型）。我们通过测量来自马达加斯加的生理上较为原始的哺乳动物——大刺猬猬（Setifer setosus）在繁殖期和非繁殖期一系列环境温度下的代谢率，来检验这一假说。猬是所有哺乳动物中代谢率最低、Tb变异性最高的动物之一，因此是原始真兽类状态的良好模型。在怀孕和哺乳期间，低于热中性区时，新陈代谢和Tb会增加，同时Tb变异性会降低。热中性区的下限估计约为25°C。然而，虽然在20°C以下静息代谢的增加幅度很大（繁殖期间比非繁殖期高150%），但研究地点白天休息期的环境温度很少达到同等低水平。因此，大刺猬猬提供了一个例子，说明恒温性相对低成本的增加如何通过允许更快地产仔而导致适应性的大幅提高。繁殖期间产热增加所需的机制将进一步有利于哺乳动物恒温性的发展。
