Effects of reduced mobility on trabecular bone density in captive big cats

Abstract

Bone responds to elevated mechanical loading by increasing in mass and density. Therefore, wild animals should exhibit greater skeletal mass and density than captive conspecifics. This expectation is pertinent to testing bone functional adaptation theories and to comparative studies, which commonly use skeletal remains that combine zoo and wild-caught specimens. Conservationists are also interested in the effects of captivity on bone morphology as it may influence rewilding success. We compared trabecular bone volume fraction (BVF) between wild and captive mountain lions, cheetahs, leopards and jaguars. We found significantly greater BVF in wild than in captive felids. Effects of captivity were more marked in the humerus than in the femur. A ratio of humeral/femoral BVF was also lower in captive animals and showed a positive relationship to home range size in wild animals. Results are consistent with greater forelimb than hindlimb loading during terrestrial travel, and possibly reduced loading of the forelimb associated with lack of predatory behaviour in captive animals. Thus, captivity among felids has general effects on BVF in the postcranial skeleton and location-specific effects related to limb use. Caution should be exercised when identifying skeletal specimens for use in comparative studies and when rearing animals for conservation purposes.

Trabecular bone morphology in big cats reflects the complex diversity of limb use but not home range size or daily travel distance

“…Third, as noted previously, although using home range size as a proxy for mobility is practical, home range sizes vary widely depending on factors such as seasonality, resource availability, and geographic space available; therefore, some of the species have overlapping home ranges, thus making the distinctions unclear. Fourth, Chirchir et al (2022) and Zack et al (2022) showed the value of separating wild versus captive specimens when conducting morphological studies using museum collections; this study attempted to study only wild-caught samples however, due to still a small sample available we included both wild and specimens of "unknown" origin (Table 1) to increase the data's statistical robusticity and while small in sample size, some of those specimens could have been derived from zoos which would slightly skew our results.…”

“…We investigated trabecular bone morphology in four felid species: cheetahs ( Acinonyx jubatus, n = 20), jaguars ( Panthera onca, n = 9), leopards ( Panthera pardus, n = 15), and cougars ( Puma concolor, n = 18). We selected these species because of their diversity in locomotor behavior, varied home range sizes, and their fairly close phylogenetic relationship (Werdelin et al, 2010; also see Chirchir et al, 2022). The skeletal specimens were obtained from the National Museum of Natural History; the Smithsonian Institution in Washington, DC; and the American Museum of Natural History, New York (Table 1).…”

“…Specifically, this scanner computes bone density in mg/cm 3 and relies on a bone equivalent phantom to obtain bone density values (Augat et al, 1998; additional descriptions are in Chirchir, 2021; Chirchir, 2016; Chirchir, 2015). We scanned epiphyseal elements in a fashion that allowed for the most central sampling of the epiphysis (Figure 2), for detailed descriptions see Chirchir et al (2022). After scanning each element, we used the inbuilt function (contour mode) to select a region of interest (ROI) delineating cortical versus trabecular bone.…”

“…The % size of ROI is built into the program such that the user can indicate the % ROI size of interest. Specifically, we selected 65% of trabecular bone in the humeral head and 75% in the femoral head for the cheetahs and leopards and 75% for the mountain lions and jaguars in both humeral and femoral heads (Figure 1) and also see Chirchir et al (2022) and Chirchir (2015).…”

“…We generated a spherical VOI in Quant 3D and scaled VOI size to 65%-75% of the superior-inferior (SI) breadth of the femoral and humeral heads and the distal tibia depending on the size of the epiphyses (Figure 1). We identified VOI diameter as 65% of the SI breadth of the humeral head and 75% of the femoral heads for the cheetah and leopard epiphyses since they are slightly smaller compared with the mountain lions and jaguars (see Chirchir, 2015 for SI breadth definitions; Chirchir et al, 2022). For the latter two species, we scaled both humeral and femoral heads VOI diameter to be 75% of the SI breadth to capture as much trabeculae as possible but also to avoid sampling cortical bone (Figure 1).…”