Variation in the Skull Morphology of Captive Mammals: A Geometric Morphometric and Meta-Analytic Approach

Abstract

Morphological changes in the crania of captive mammals compared to their wild counterparts have been documented for over a century. Although these morphological differences are potentially deleterious, their exact trends, mechanisms, and consequences are poorly understood. Using meta-analytic and geometric morphometric techniques, this dissertation addresses whether captive mammals display predictably distinct morphology compared to wild populations, whether those differences are impacted by ecological covariates, and the probable mechanisms behind these changes. This dissertation finds significant differences in cranial morphology between captive and wild populations of mammals. The magnitude and directionality of these changes vary with taxonomy and diet, where carnivory specialists differentiate most consistently. This suggests that changes in cranial morphology cannot be generalized for all captive mammal species. Likewise, captive and reintroduced wolves display significantly different cranial morphology compared to the wild Population. Captive individuals display greater morphological variation than wild populations, suggesting the influence of relaxed selection. Reintroduced individuals aligned more closely with the captive population but display much less morphological variation, suggesting the effects of genetic drift and the reapplication of natural selection pressures that occur in the wild. Several additional canid species also display similar cranial changes in captivity. Hypercarnivorous species often display the greatest changes with shorter and wider skulls. Although captive diets have recently improved, morphological changes have continued to occur among recent captive specimens suggesting that either captive diets still need improvement or that morphological changes occurring in captivity may be unrelated to diet. Regardless of carnivory type, many canid species shifted in a similar morphological direction away from their wild counterparts, displaying deeper mandibles and broader crania. Zoos may represent the last hope for the long-term survival of threatened and endangered species especially given that habitat destruction is projected to continue and intensify in the future. By understanding the nature of the morphological changes occurring in captivity, we may be able to alter captive management practices to prevent these changes in future generations and preserve the conservation value of captive animals.