Evidence for adolescent length growth spurts in bonobos and other primates highlights the importance of scaling laws

Restructuring of Femoral Cortical Bone During Growth and Locomotor Development of Wild Chimpanzees (Pan troglodytes verus)

OBJECTIVE

Chimpanzees are altricial in terms of their locomotor development and transition from being carried to engaging in suspensory and arboreal locomotor behaviors to eventually relying on terrestrial quadrupedalism as their main form of locomotion. Here, we consider the mechanical implications of femoral cortical bone restructuring during growth and locomotor development in wild chimpanzees.

MATERIALS AND METHODS

Cortical bone structure was examined in an ontogenetic sample of wild chimpanzees from a single subspecies (P. t. verus) spanning in age from 2 weeks to 12.6 years. Diaphyseal cross-sections were extracted from micro-CT scans of the femur at 35%, 50%, and 65% of total intermetaphyseal length and variation in cortical bone structure was assessed based on bending rigidity (Imax/Imin, Ix/Iy), relative medullary area, and cortical bone porosity.

RESULTS

Diaphyseal shape is relatively circular with a high amount of cortical bone porosity and a large relative medullary area during early infancy. Distinct shifts in cortical bone structure occurred for each studied parameter with the biggest changes occurring within the first 5 years. Values appear to stabilize as quadrupedal walking increases in frequency and is established as the main form of locomotion.

DISCUSSION

Collectively, the results suggest a degree of integration in which cortical bone restructures in response to rapid changes in locomotion in addition to nonmechanical influences such as hormonal, and growth factors, without compromising function and structural integrity. The extent of influence of each factor varies throughout growth and highlights the need for caution in functional interpretations of cortical bone geometry.

A non-invasive measure of bone growth in mammals: Validating urinary CTX-I as a bone resorption marker through long-bone growth velocity in bonobos

Assessing bone growth trajectories in mammals is crucial for understanding life history dynamics, but the quantification of bone growth in natural settings can be challenging. Bone resorption markers that can be measured in urine, such as C-telopeptide of type I collagen (CTX-I), offer a non-invasive solution to assess bone growth. Although measurement of urinary CTX-I levels has been applied extensively in human studies, its use in other species is so far limited to a few clinical studies. To validate urinary CTX-I as a bone resorption marker under less controlled conditions, we investigated within-individual day-to-day variation, diurnal patterns, and sex and age-specific variation in zoo-housed bonobos (Pan paniscus). We then also correlated urinary CTX-I levels with forearm growth velocity measures. We found a day-to-day variability in urinary CTX-I levels of around 25%, comparable to human variation. Diurnally, CTX-I levels decreased, aligning with observations in humans and other species. Both sexes showed an age-related decline in urinary CTX-I levels, with a steady decrease after the age of 10 years. Additionally, we found a positive correlation between forearm growth velocity and urinary CTX-I levels across age in female, but not in male, bonobos. Our results demonstrate that urinary CTX-I levels are a meaningful measure of bone growth and highlight its potential to examine bone growth trajectories also in wild populations to investigate life history dynamics.

Visually assessed body condition shows high heritability in a pedigreed great ape population

Body condition, a measure for relative fat mass, is associated with primate health, fitness, and overall welfare. Body condition is often influenced by dietary factors, age, and/or sex, but several body condition measures (body weight, weight-to-height ratios, and so on) also show high heritability across primate species, indicating a role of genetic effects. Although different measures for body condition exist, many require direct handling of animals, which is invasive, time-consuming, and expensive, making them impractical in wild and captive settings. Therefore, noninvasive visual body condition score (BCS) systems were developed for various animal species, including macaques and chimpanzees, to visually assess relative fat mass. Here we evaluate the utility of a visual BCS system in bonobos by assessing (1) inter-rater reliability, (2) links with body mass, a traditional hands-on measure of condition, and (3) the factors driving individual variation in BCS. We adapted the chimpanzee BCS system to rate 76 bonobos in 11 European zoos (92% of the adult population). Inter-rater reliability was high (s* = 0.948), BCSs were positively associated with body mass (β = 0.075) and not predicted by diet, sex, or age, nor were they associated with a higher abundance of obesity-related diseases. Instead, BCSs showed high levels of heritability (h2  = 0.637), indicating that a majority of body condition variation in bonobos is attributable to genetic similarity of the individuals. This is in line with reported h2 -values for traditional body condition measures in primates and provides support for the reliability of visual BCS systems in great apes. The results of this study emphasize an often unanticipated role of genetics in determining primate body fat and health that has implications for the management of captive primates. Application of this tool in wild populations would aid to unravel environmental from genetic drivers of body condition variation in primates.