However, the relative impact of changes to endocranial volume occurring in captivity has yet to be explored in populations that have been reintroduced to wild conditions. foraging costs or predator avoidance), enhanced endocranial development may be achievable. However, in managed populations, where ample nutrient-rich diets may be readily and consistently available, and where energetically expensive activities are eliminated (e.g. Resource limitations are thought to constrain brain size in wild populations 3, 8. However, brain tissue is metabolically expensive to maintain therefore, intraspecific increases in brain size are primarily thought to occur under intense selection pressure or when ample resources are available, which can enable increased development without risking other aspects of fitness 3, 5, 8. Within wild vertebrate populations, environmental variables have been linked to intraspecific variation in brain size, including seasonality, environmental severity, habitat complexity, and urbanization 3, 15, 21. Disproportionate changes in endocranial volume may significantly influence cognition and important sensory functions 16, 20. For example, domestic dogs show disproportionate decreases to the neocortex and olfactory bulb associated with a smaller endocranial volume (roughly 30%) compared to their wild counterparts. Changes in endocranial volume are not necessarily proportional across all brain structures 16, 17. These measurements are often correlated with social behaviors and other traits related to learning, memory, problem-solving, and behavioral flexibility 1, 4, 16 and may also influence traits directly related to fitness, including longevity and fecundity 6, 8. Measurements of endocranial volume provide a useful and widely used proxy for brain size 14, 15. The nature of this relationship has been poorly documented in carnivoran species, particularly those that are intensively managed and bred for reintroduction initiatives. Most frequently, the reported changes occurring in captivity have been related to a reduction in endocranial volume, which has been attributed to improper diets, socially and environmentally depauperate enclosures, or lack of stimulating enrichment activities 5, 12, 13. Within captive populations, endocranial volumes have been documented to increase, decrease, and in some cases show no discernable difference. Changes in endocranial volume may have significant consequences for the viability of a population 1, 5, 6. Given its intricate relationship with brain size and structure, endocranial volume has been linked to differences in behavior, performance, and fitness 2, 4. The relative endocranial volume of the vertebrate skull varies within and between populations of a species 1, 2, 3. We hypothesize that the increase in endocranial volume observed across captive generations may be related to the high-quality nutrition provided in captivity. We did not find a relationship between endocranial volume and either inbreeding or cranial musculature, although the captive population displayed an increase in the cross-sectional area of the masseter muscle.
However, we did not detect a difference among captive, wild, and reintroduced groups, perhaps due to the variability across captive generations. Endocranial volume increased across captive generations. Here, we measured the endocranial volume of preserved Mexican wolf skulls across captive generations and between captive, wild, and reintroduced populations and assessed endocranial volume dependence on inbreeding and cranial musculature. Little is known about how changes in brain size progress in highly managed carnivoran populations and whether such traits are retained among reintroduced populations. Changes in brain size are associated with biological fitness, which may limit reintroduction success. Captive diets, for example, may differ in nutrition and texture, altering cranial musculature and alleviating constraints on cranial shape development. Endangered animals in captivity may display reduced brain sizes due to captive conditions and limited genetic diversity.
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