Comparisons of Limb Structural Properties in Free-ranging Chimpanzees from Kibale, Gombe, Mahale, and Taï Communities

Abstract

Structural characteristics of limbs bones provide insight into how an animal dynamically loads its limbs during life. Cause-and-effect relationships between loading and the osteogenic response it elicits are complex. In spite of such complexities, cross-sectional geometric properties can be useful indicators of locomotor repertoires. Typical comparisons use primates that are distinguished by broad habitual locomotor differences, usually with samples garnered from several museum collections. Intraspecific variability is difficult to investigate in such samples because behavior or life histories, which are tools for interpreting intraspecific variability, are limited. Clearly intraspecific variation both in morphology and behavior/life history exists. Here we expand an ongoing effort towards understanding intraspecific variation Keywords: cross-sectional geometry, functional morphology, Pan troglodytes, locomotor behavior 2 Functional morphologists rely on comparative approaches as well as experimental techniques in the laboratory (i.e., kinetics, kinematics, electromyography, strain analysis) in order to understand form-function relationships in the postcranium of animals. Shared or unique components of activity patterns provide a framework against which morphological commonalities or differences are evaluated. Often times, comparative studies construct samples from specimens of museum collections (Green et al., 2007;Haeusler and McHenry, 2007;Marchi, 2007;Ruff, 2002Ruff, , 2008. While museum specimens may be numerous and accessibletwo criteria necessary for amassing large samples that rigorous statistical analyses favor -they also necessitate a seldom-appreciated tradeoff, namely, that while behavior and life history may vary among group individuals, this variation must be ignored in order to compare groups.Clearly, however, individuals within populations can vary substantially in behavioral or life history variables (Goodall, 1986;Hunt, 1992), which may in turn contribute to intragroup variability in morphological characteristics and reproductive fitness.Chimpanzees (Pan troglodytes) offer a unique opportunity among animals to address functional morphology questions. Observational studies of free-ranging chimpanzee communities provide a detailed portrait of individual life histories (Boesch and BoeschAchermann, 2000;Goodall, 1986; Morbeck, 1999; Nishida, 1990; Morbeck et al., 2002).Studies encompassing the last 45 years at several locations [i.e., Gombe Stream National Reserve (Tanzania), Kibale National Park (Uganda), Mahale Mountains National Park (Tanzania), and Taï Forest National Park (Côte d'Ivoire)] document activity profiles of female and male chimpanzees of all ages in all sorts of situations or settings. Skeletal collections have slowly accumulated in the same communities, and thus frequently, individual specimens can be associated with contextual information, e.g., life history, activity, and habitat data. Such a 3 sample is ideal for investigating form-function relationships in the primate p...

Long bone cross‐sectional properties reflect changes in locomotor behavior in developing chimpanzees

“…This is likely due to generally similar locomotor patterns among subspecies (Doran and Hunt, 1994;Sarringhaus et al, 2014). Differences in shape have been found in chimpanzees living in different communities, but no definitive relationships emerged between shape and locomotor mode (Carlson et al, 2006(Carlson et al, , 2011. At the species level, midshaft femoral and humeral shapes differ between gorillas and chimpanzees, with a trend of increased circularity with increased levels of arboreal locomotion in the latter compared with the former (Carlson, 2005).…”

“…Relating specific locomotor behaviors to bone shape has been difficult in adult great apes and other primates, who have diverse locomotor repertoires and regularly move in a variety of ways (Ruff and Runestad, 1992;Carlson, 2005;Carlson et al, 2006Carlson et al, , 2011Patel et al, 2013). No difference in humeral or femoral midshaft shape (I max / I min ) was found among chimpanzee subspecies in an analysis of adult specimens (Carlson, 2005).…”

“…I max and I min were used over I x and I y because principle axes have been found to be better indicators of shape compared with anatomical axes in African apes given individual variation in principle angles ( Fig. 2; Carlson, 2005;Carlson et al, 2011, Morimoto et al, 2011. Principle axis values were also used because they are less influenced by observer error in bone orientation.…”

Humeral Cross‐Sectional Shape in Suspensory Primates and Sloths

“…Several studies have also evaluated whether cross‐sectional shape, another variable of a long bone's cross‐sectional geometry, can serve as a correlate of a long bone's mechanical loading history (Carlson, ; Carlson et al, ; Carlson and Judex, ; Simons, , Simons et al, ; see also Demes, for alternative arguments). Specifically, these studies have tried to test the general hypothesis that cross‐sectional shape reflects whether an animal's long bones have experienced relatively uniform or variable (i.e., multidirectional) loading regimes and bending moments.…”

“…For example, analyses of cross‐sectional shape using I max / I min ratios suggest that a suspensory (and more arboreal) signal (i.e., ratios closer to 1.0) may be present in humeri of chimpanzees ( Pan ) compared with a more quadrupedal (and more terrestrial) signal (i.e., ratios deviating from 1.0) observed in closely related gorillas ( Gorilla ) (see Table in Carlson, ). Subsequent studies of humeral cross‐sectional shape, however, have demonstrated fewer significant differences within and between individuals, sexes, and populations of wild chimpanzees with specific documented positional behavioral data (Carlson et al, ) or between wild and captive chimpanzees that most likely differed in positional behaviors, as well as substrate use, diet, and so forth (Morimoto et al, ). Thus, while long bone cross‐sectional shape as measured from the ratio of I max and I min could be used to aid in distinguishing more general locomotor behaviors and substrate preferences (e.g., arboreal or terrestrial; quadrupedal or suspensory), it may be more difficult to distinguish between specific locomotor categories (e.g., scrambling, bridging, and climbing; Carlson, ; Carlson et al, ).…”