• DOI:10.1086/282459
• Corpus ID: 85424175

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

@article{Packard1966TheIO,
  title={The Influence of Ambient Temperature and Aridity on Modes of Reproduction and Excretion of Amniote Vertebrates},
  author={Gary C. Packard},
  journal={The American Naturalist},
  year={1966},
  volume={100},
  pages={667 - 682}
}

• G. C. Packard
• Published 1 November 1966
• Biology
• The American Naturalist

Adaptive radiation of tetrapod vertebrates into terrestrial environments depended upon the successful emancipation of embryonic development from the aquatic "crib" of the Amphibia. The cleidoic egg of birds and the viviparous development of placental mammals are often cited by physiologists as major morphologic adaptations of the most successful terrestrial vertebrates to xeric conditions (Baldwin, 1964; Needham, 1938; Yapp, 1960). However, this interpretation of the initial adaptive significance of viviparity is not shared by ecologists (Kendeigh, 1952; Mertens, 1960). At different times in the history of a group of animals, different environmental factors have exerted major selective pressures upon the group, thereby guiding its evolution. However, major adaptation to one set of environmental factors very likely placed severe limitations on the subsequent evolution of the group, either when the environment changed or when dispersal out of that environment occurred. I contend that the amniote egg was the first stage in adaptation to terrestrial development of embryos, and that evolutionary refinements of this structure (as well as those involving nitrogen metabolism) culminated in the avian egg. Viviparity and placentation, on the other hand, most certainly were adaptive responses primarily to temperature. I hope to show that physiologic data are consistent with the hypothesis of temperature being the primary factor mediating the origin of. viviparity. In so doing, I will also examine the evidence on nitrogen excretion in amniote vertebrates, and will suggest a hypothesis on the probable evolution of enzymatic pathways concerned with nitrogen catabolism.