Tag: Andrea Eller

Variability Selection Hypothesis, Weed Macaques, and Body Size Variance

Presenter: Harry Sullivan

Mentors: Andrea Eller and Frances White, Biology

Poster: 61

Major: Biology 

Variability Selection Hypothesis (VSH) proposes that early Homo gained adaptive benefit from being flexible in novel or unpredictable climates. Increased intra-taxon variation in body size and the expansion of geographic ranges in early Homo populations suggests greater phenotypic and developmental plasticity. Similar levels of ecological flexibility have been documented in some species of macaques, earning them the moniker of “weed species”. We compare body size variance between weed and non-weed macaques to determine whether intrataxon variation in body size positively correlates with ecological flexibility, as proposed by the VSH. We used two sources of body size data for all available taxa: original data on postcranial osteological body size estimators (seven species, n=49), and published body masses for nineteen species. Fourteen osteometric body size estimators on the humerus, radius, ulna, femur, and tibia were included. All estimators show a tight correlation with body mass: R2 values range from 0.79 to 0.95 with a mean of 0.9. Variance per estimator per species was calculated, as proxies for body mass variance. Averaged estimator variances in non-weed species range from 1.71-11.34, but only 2.26-4.36 within weed species. This data analysis indicates that weed macaques do not exhibit more intrataxon body size variance than non-weeds. Macaques are under-utilized ecological referents for human evolution, and this genus’ diversity is informative for understanding the role of adaptive flexibility in primate evolution generally.

An Interspecific Comparison of Variance in Sex-Based Developmental Markers

Presenter: Kyle Morley

Mentors: Andrea Eller and Frances White, Anthropology

Poster: 47

Major: Anthropology 

Sexual dimorphism varies with the degree of male-male competition among primates. Changes in body size of both sexes are well known during ontogeny, but less is known about how osteological developmental markers vary under differing levels of sexual selection. Male-male competition is reflected in a species’ body size sex ratio: humans are reported to have a 1.2 ratio, while rhesus macaques have a 1.6 ratio. We predict greater results for larger bodies and canines in macaque males compared to macaque females and humans as well as greater growth marker variation among macaque males than in these other groups. We documented dental eruption and epiphyseal fusion in 292 macaque skeletal specimens and compared the data to over 25,000 individuals using published human population data. Two-way ANOVAs without replication were used to test whether species had similar variation in dental eruption and fusion time. The two species had significantly different eruption variation (males F=33.71, df=15,1, p <0.0001; females F=119.06, df =15,1, p <0.0001) with macaques more variable than humans. Both species also had different ranges in fusion time (F=7.28, df=13,1, p <0.05) with macaque males more variable than human males. The results support our prediction that macaque males show the greatest variation in these growth markers. Interspecies comparisons of developmental plasticity, such as this study, allow for inferences on how growth variation is affected by sexual selection.