More Challenging Diets Sustain Feeding Performance: Applications Toward the Captive Rearing of Wildlife

Abstract

Synopsis The rescue and rehabilitation of young fauna is of substantial importance to conservation. However, it has been suggested that incongruous diets offered in captive environments may alter craniofacial morphology and hinder the success of reintroduced animals. Despite these claims, to what extent dietary variation throughout ontogeny impacts intrapopulation cranial biomechanics has not yet been tested. Here, finite element models were generated from the adult crania of 40 rats (n = 10 per group) that were reared on 4 different diet regimes and stress magnitudes compared during incisor bite simulations. The diets consisted of (1) exclusively hard pellets from weaning, (2) exclusively soft ground pellet meal from weaning, (3) a juvenile switch from pellets to meal, and (4) a juvenile switch from meal to pellets. We hypothesized that a diet of exclusively soft meal would result in the weakest adult skulls, represented by significantly greater stress magnitudes at the muzzle, palate, and zygomatic arch. Our hypothesis was supported at the muzzle and palate, indicating that a diet limited to soft food inhibits bone deposition throughout ontogeny. This finding presents a strong case for a more variable and challenging diet during development. However, rather than the “soft” diet group resulting in the weakest zygomatic arch as predicted, this region instead showed the highest stress among rats that switched as juveniles from hard pellets to soft meal. We attribute this to a potential reduction in number and activity of osteoblasts, as demonstrated in studies of sudden and prolonged disuse of bone. A shift to softer foods in captivity, during rehabilitation after injury in the wild for example, can therefore be detrimental to healthy development of the skull in some growing animals, potentially increasing the risk of injury and impacting the ability to access full ranges of wild foods upon release. We suggest captive diet plans consider not just nutritional requirements but also food mechanical properties when rearing wildlife to adulthood for reintroduction.

Facing the facts: adaptive trade‐offs along body size ranges determine mammalian craniofacial scaling

“…7G,H; Constantino, 2007; Strait et al ., 2008; Samuels & Van Valkenburgh, 2009; Tseng, 2009; Kitchener et al ., 2010; Tseng & Wang, 2010; Wilson & Sanchez‐Villagra, 2010; Figueirido et al ., 2011, 2013; Wilson, 2013; Dumont et al ., 2016; Ledogar et al ., 2017; Mitchell, 2019). This reinforcement likely occurs because bone stress is equal to force per unit of area; hence, an increase in the amount of bone at a specific region of the cranium will result in a decrease in stress for a given magnitude of force at that region (Mitchell, 2019; Mitchell et al ., 2021 b ).…”

“…Variation in bone volume and mineralisation of the skull occur widely in response to shifts in activity levels, including those brought about by diet composition. These processes play a substantial role in intraspecific cranial shape variation and associated mechanical performance, and are often not allometric (Ravosa et al ., 2008; Menegaz et al ., 2010; Franks et al ., 2017; Weisbecker et al ., 2019; Mitchell et al ., 2021 b ). This can potentially complicate interpretations of the evolution of facial gracilisation and requires further examination.…”

Long-Term Effect of Diet Consistency on Mandibular Growth within Three Generations: A Longitudinal Cephalometric Study in Rats

“…When the load situation is unknown, it is less evident if increased remodeling may be attributable to high strain or cyclical loading. The present literature includes controversial results according to the hypothesis that high strain may not be necessary for substantial remodeling to occur and that cyclical loading may be more likely to result in elevated remodeling [ 26 , 27 , 28 , 29 , 30 , 31 , 32 ].…”

Endocranial volume increases across captive generations in the endangered Mexican wolf

“…In particular, large hypercarnivorous mammals (whose diet is composed of at least 70% vertebrate prey) 35 , 36 evolved enhanced cranial musculature, which enables them to restrain and kill their prey and process mechanically tough dietary items 33 , 34 alongside distinct skull morphologies 37 – 39 . However, captive carnivores frequently consume a comparatively soft diet compared to what is available in the wild 40 , 41 , which may lead to changes in skull shape and functionality 42 – 44 and may potentially alleviate constraints on internal cranial structures as a byproduct. Although the discussion of a captive diet often focuses on nutritional quality, nutritional quality may not be sufficient to ensure morphological change does not occur in captive individuals 40 , 42 , 45 .…”

“…However, captive carnivores frequently consume a comparatively soft diet compared to what is available in the wild 40 , 41 , which may lead to changes in skull shape and functionality 42 – 44 and may potentially alleviate constraints on internal cranial structures as a byproduct. Although the discussion of a captive diet often focuses on nutritional quality, nutritional quality may not be sufficient to ensure morphological change does not occur in captive individuals 40 , 42 , 45 . Likewise, the development of brain tissue is facilitated by high-nutrient diets, where malnourished animals may experience poor brain development and decreased endocranial volume 46 , 47 .…”