Cracking a developmental constraint: egg size and bird evolution. In Proceedings of the VII International Meeting of the Society of Avian Paleontology and Evolution, ed. W.E. Boles and T.H. Worthy
Abstract
It has been suggested that relative egg size in living birds is strongly correlated with the developmental mode of the young; "altricial" (helpless) or "precocial" (independent). Using a data set of extant taxa we show that altricial birds lay relatively larger eggs than their precocial counterparts but that this may be due to the small size of most altricial species. Smaller birds tend to lay relatively small eggs compared to large species. Nonetheless, a predictive egg mass-body mass relationship extends into the avian fossil record. Such a relationship is important to our understanding of avian evolution because relative egg size (and thus available developmental mode) was constrained in many early birds—oviduct diameter was limited by the presence of pubic fusion. Therefore we document the evolution of avian developmental strategies using morphology-based phylogenies for Mesozoic and extant avians and corroborate correlations between developmental strategies, egg weight and female body mass. The sequential loss of precocial features in hatchlings characterises the evolution of birds while altriciality is derived within Neoaves. A set of precocial strategies is seen in earlier lineages, including basal Neornithes (modern birds) and are implied in their Mesozoic counterparts—skeletal constraints on egg size, present in many Jurassic and Early Cretaceous birds (Archaeopteryx, Confuciusornis, Enantiornithes) were lost in later diverging lineages. Attributes of precociality were already present in a number of lineages of non-avian maniraptoran theropods. We propose that the evolution of "unrestricted egg size" may have precipitated subsequent development of the diverse reproductive strategies seen in living birds.
