The Influence of Ambient Temperature and Aridity on Modes of Reproduction and Excretion of Amniote Vertebrates

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Abstract
Certain lines of reptilian descent leading to squamate reptiles and birds adopted uricotelism as the adult pattern of excretion. Evolution of terrestrial eggs, culminating in the cleidoic egg of birds, was characterized by the early assumption of the adult pattern of excretion. Uricotelism by embryos living within the confines of a cleidoic egg presumably permitted a considerable reduction in the amount of energy expended in recovering water for growth. Other lines of reptilian descent leading to placental mammals were not highly adapted to terrestrial existence in embryonic stages. This lack of specialization was reflected in the formation of ammonia as the end product of embryonic protein metabolism. With the appearance of thermoregulatory mechanisms and with adaptive radiation into temperate climates, these unspecialized eggs were retained within the reproductive tract of the female parent, leading ultimately to placentation. Homeo-thermism appeared in the avian line of evolution after differentiation of the cleidoic egg had progressed to a point such that internal development of the egg was precluded. Behavioral adaptation allowed dispersal of birds (and some reptiles) into temperate and subarctic regions. Evolutionary trends in adult vertebrates have included the loss of terminal enzymes in pathways of both protein and purine metabolism. Historically, the early loss of uricase and other enzymes of purine degradation constituted a pre-adaptation for adult sauropsids to terrestrial existence. Additionally, in ontogeny the loss of these same enzymes occurs at progressively earlier stages of development in the series Amphibia[long dash]Reptilia[long dash]Aves, so that the adult mode of excretion is assumed by embryos earlier in ontogeny.