Adaptation in the African egg-eating snake: a comparative approach to a classic study in evolutionary functional morphology G. E. A. Gartner 1,2 & H. W. Greene 1 1 Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, USA 2 Department of Biology, University of California, Riverside, CA Keywords Dasypeltis; egg eating; comparative methods; Lampropeltis; selective regime; snake feeding. Correspondence Gabriel E. A. Gartner, Department of Biology, 1208 Spieth Hall, University of California, Riverside, CA 92521, USA. Email: ggart001@ucr.edu Editor: Tim Halliday Received 7 January 2008; revised 19 March 2008; accepted 20 March 2008 doi:10.1111/j.1469-7998.2008.00448.x Abstract A key component to any adaptive hypothesis is that the adaptive trait in question must confer a performance advantage and, in turn, an increase in fitness, relative to those animals displaying the phylogenetically antecedent condition. Among the most striking purported adaptations in vertebrates are those found in the African snake genus Dasypeltis. These snakes are unique in that they eat bird eggs to the exclusion of all other prey. Detailed functional morphological analysis dating back 50 years has highlighted a suite of morphological features in the head and trunk region hypothesized to assist these animals in eating bird eggs, and yet no comparative performance studies of egg-eating ability have ever been conducted in this group of snakes. The purpose of this study was twofold: first, we wanted to compare egg- eating performance in Dasypeltis with a facultative egg eater, the common king snake Lampropeltis getula. Second, we wanted to test the hypothesis that a selective regime exists in Africa conducive to the selection and subsequent fixation of the hypothe- sized egg-eating morphological adaptations. Our results show that a strong advan- tage exists in egg-eating ability for Dasypeltis. The difference is so large (only large Lampropeltis can eat small eggs) that analysis by analysis of covariance becomes difficult due to problems with collinearity. Our results examining potential selective regimes show that more birds lay eggs of a readily ingestible size in Africa than in a representative region in the United States. Additionally, the largest radiation of African ground-nesting birds existed in Africa before the colubrid explosion during the Miocene, which gave rise to Dasypeltis, giving further support to previous adaptive hypotheses regarding the unique morphology of these snakes. Introduction Macrostomate snakes are among the most successful verte- brate predators alive. They have evolved to feed on a myriad of prey types, ranging from small invertebrates to large mammals (Greene, 1997). Interestingly, snakes that eat exclusively bird eggs are extremely rare (de Queiroz & Rodr´ıguez-Robles, 2006). A recent hypothesis by de Queiroz & Rodr´ıguez-Robles (2006) suggests that this paucity of obligate egg eaters is simply due to historical contingency – that there are relatively few snakes that supplement their diets with bird eggs, and therefore the number of snakes that evolved to specialize on bird eggs should be small. This hypothesis, while accounting for important transitions in dietary preference, does not offer support for potential mechanisms leading to the fixation of obligate egg-eating behavior, or the fixation of certain morphological traits associated with egg eating. The six species of African egg-eating snake, Dasypeltis, eat bird eggs to the exclusion of all other prey. Unlike other snakes that facultatively feed on eggs, Dasypeltis specializes in swallowing eggs, crushing and regurgitating the shell within the esophagus, and swallowing solely the contents. They are unique among snakes in this regard, with the possible exception of the Indian egg eater Elachistodon westermanni, an extremely rare species hypothesized to eat eggs based solely on the morphological features of a few preserved specimens (Gans & Williams, 1954). Dasypeltis possesses two suites of morphological features hypothesized to be adaptive: those that circumvent the problem of ingesting and swallowing bird eggs and those pertaining to breaching the packaging of the egg contents themselves once ingested (Gans, 1952, 1974). Modifications of the quadrate, supratemporal and dentary bones, along with a reduction in the number of teeth, are all hypothesized to assist Dasypeltis in egg eating. In addition, anteroven- trally pointing vertebral hypapophyses on the 17th through 38th vertebrae project into loose folds in the esophagus, which both mechanically prevent the egg from moving further into the gut and assist in cracking the shell and egg Journal of Zoology Journal of Zoology ]] (2008) 1–7 c  2008 The Authors. Journal compilation c  2008 The Zoological Society of London 1 Journal of Zoology. Print ISSN 0952-8369