The Evolution of Viviparity in Amniote Vertebrates: Egg Retention Versus Egg Size Reduction

Abstract

We critically examine a recent explanation for the absence of viviparity in birds that depends heavily on a model that was developed to account for viviparity in squamate reptiles. We propose that the elevated body temperature of birds precludes extended egg retention, presumably an early stage in the evolution of viviparity from an oviparous ancestor, probably through constraints on the rate of oxygen transfer to developing embryos. We demonstrate that if the effects of endothermy are included in the model developed to account for the evolution of viviparity in reptiles, then qualitative and quantitative differences in embryonic developmental rates are predicted between reptiles and birds as a result of eggs being retained within the uterine environment. Our modification of the model thus offers an explanation for the total absence of viviparity in birds. We propose that the evolutionary sequence for mammalian viviparity from an endothermic oviparous ancestor was through the progressive production of smaller eggs, with proportionally shorter embryonic developmental times, which would allow for relatively longer periods of egg retention before oxygen-supply constraints would be reached. The critical characteristic allowing extensive egg size reduction in the mammals was lactation, which allowed the feeding of tiny altricial neonates. The absence of lactation, or some analogous method of feeding very small young, has constrained birds from following the evolutionary pathway to viviparity that we propose for mammals.