Cranial morphology of captive mammals: a meta-analysis

Abstract

Background Captive facilities such as zoos are uniquely instrumental in conservation efforts. To fulfill their potential as bastions for conservation, zoos must preserve captive populations as appropriate proxies for their wild conspecifics; doing so will help to promote successful reintroduction efforts. Morphological changes within captive populations may be detrimental to the fitness of individual animals because these changes can influence functionality; thus, it is imperative to understand the breadth and depth of morphological changes occurring in captive populations. Here, we conduct a meta-analysis of scientific literature reporting comparisons of cranial measures between captive and wild populations of mammals. We investigate the pervasiveness of cranial differences and whether cranial morphological changes are associated with ecological covariates specific to individual species, such as trophic level, dietary breadth, and home range size. Results Cranial measures of skull length, skull width, and the ratio of skull length-to-width differed significantly between many captive and wild populations of mammals reported in the literature. Roughly half of captive populations differed from wild populations in at least one cranial measure, although the degree of changes varied. Carnivorous species with a limited dietary breadth displayed the most consistent changes associated with skull widening. Species with a more generalized diet displayed less morphological changes in captivity. Conclusions Wild and captive populations of mammals differed in cranial morphology, but the nature and magnitude of their cranial differences varied considerably across taxa. Although changes in cranial morphology occur in captivity, specific changes cannot be generalized for all captive mammal populations. The nature of cranial changes in captivity may be specific to particular taxonomic groups; thus, it may be possible to establish expectations across smaller taxonomic units, or even disparate groups that utilize their cranial morphology in a similar way. Given that morphological changes occurring in captive environments like zoos have the potential to limit reintroduction success, our results call for a critical evaluation of current captive husbandry practices to prevent unnecessary morphological changes.

Fluctuating Asymmetry and Stress in Macaca fuscata: Does Captivity Affect Morphology?

“…The shape differences observed here between captive and wild M. fuscata (when overall shape and the symmetric components are analysed) suggest that captivity has exerted pressures on development. In addition, even accounting for the limitations of our analysis, morphological differences between captive and wild mammal species have long been recognised [30], thus suggesting that captivity does have an impact on the developing morphology. Whether captivity (or, in general, environmental stress) tends to affect only the overall shape, skeletal symmetry, or both is still highly uncertain, although it has been suggested that FA and phenotypic variance arise from at least partially overlapping processes [21].…”

“…Much evidence exists about the impact of captivity on animal behaviour [27,28]. However, how living in a captive environment can influence an individual or a group's morphology is still unclear [29,30]. Fishes raised in captivity have shown significantly higher levels of fluctuating asymmetry on different morphological traits, some of remarkable functional importance, such as the length of the pectoral and ventral fins [31].…”

Pattern and pace of morphological change due to variable human impact: the case of Japanese macaques

“…In addition to nutritional influences, the mechanical properties of the diet and functional demands of feeding habits might play a role in the plastic response of the cranial and mandibular morphology and associated masticatory muscles (Groves 1966 ; Herring and Lakars 1981 ; Lieberman et al 2004 ; O'Regan and Kitchener 2005 ; Paschetta et al 2010 ; Hartstone-Rose et al 2014 ; Cornette et al 2015 ; Fabre et al 2018 ; Siciliano-Martina et al 2021 ). Specifically for primates, it has been found that squirrel monkeys ( Saimiri sciureus ) fed a soft diet exhibit a narrower muzzle than specimens which have been fed a harder diet (Corruccini and Beecher 1982 ).…”

“…Osteological material from many of these Japanese macaque populations has been sampled through decades and documented meticulously in institutional and museum collections. Thus, these Japanese macaque populations present an ideal case for studying variation in cranial and mandibular shape and size as well as the pace of this change as a response to different levels of human–animal interactions, despite a recent meta-analysis finding only minor morphological changes in primates due to captivity (Siciliano-Martina et al 2021 ).…”

One of these wolves is not like the other: morphological effects and conservation implications of captivity in Mexican wolves

“…Some endangered species have been maintained in these facilities until they are able to be reintroduced to the wild (Frankham, 2008; Conde et al ., 2011; Willoughby et al ., 2015). However, the environment, available food and stressors that captive populations experience are largely different from those of wild populations and may influence animal behavior, physiology and morphology (O’Regan & Kitchener, 2005; Siciliano‐Martina, Light & Lawing, 2021 a , b ), handicapping success in the wild where optimal functionality is imperative (Wisely et al ., 2005; Frankham, 2008; Courtney Jones, Munn & Bryne, 2018).…”