NEAA 2008

I presented a paper entitled "Allometry of facial mobility in anthropoid primates" at the 2008 meeting of the Northeastern Anthropological Association in Amherst, MA. Here's the abstract:

Body size is likely to be an important constraint on the evolution of facial expression in primates. This is because facial movements are more difficult to discern in smaller species. I tested the hypothesis that facial mobility is positively correlated with body size in a comparative sample of nonhuman anthropoids. Facial mobility, or the variety of facial movements a species can produce, was estimated using the human Facial Action Coding System (FACS). I used FACS to estimate facial mobility in 12 nonhuman anthropoid species, based on video recordings of facial activity in zoo animals. Body mass data were taken from the literature. I performed a multiple regression analysis with facial mobility as the dependent variable and two independent variables: log body mass and dummy-coded infraorder. Together, body mass and infraorder explain 92% of the variance in facial mobility. However, the partial effect of body mass is much stronger than for infraorder. The results of my study suggest that allometry is an important constraint on the evolution of facial mobility, which may explain why smaller taxa tend to exhibit fewer facial displays than larger taxa. More work is needed to clarify the structural bases of this allometric pattern.

Some of the other folks in my session were Richard Lawler and Stacey Matarazzo.

Artificial Cranial Deformation

Here is an article based on some research I was involved in as an undergraduate at the University of Michigan.

Clark JL, Dobson SD, Antón SC, Hawks J, Hunley KL, Wolpoff MH (in press) Identifying artificially deformed crania. International Journal of Osteoarchaeology. Link

Abstract:

In this paper we report on a new discriminant function for the identification of artificially deformed crania. Development of the function, based on a sample of deformed and undeformed crania from the Philippines, required visual classification of the sample into deformed and undeformed groups. Working from the observation that deformed crania result in flattened frontal and occipital regions, the sample was seriated based on degree of flattening; classification was based on the results of this seriation. The discriminant function, calculated using curvature indices, required only six simple measurements: arc and chord measurements for the frontal (glabella to bregma), parietals (bregma to lambda), and occipital (lambda to opisthion). The function was designed to be conservative, in that a deformed cranium may be classified as undeformed, but the opposite should not occur. Our function classified the undeformed crania with 100 percent accuracy and deformed crania with 76.9 percent accuracy, for a total of 91.9 percent agreement with visual classification. In order to evaluate whether the function is applicable for samples from outside the Philippines, a double blind test was conducted with a large sample of deformed and undeformed crania from a broad geographic and temporal range. For this sample, the function agreed with visual classification in 89.7 percent of cases: 98.8 percent of undeformed crania were correctly classified, while deformed crania were identified with 73.7 percent accuracy. These results demonstrate the utility of the new discriminant function for the classification of artificially deformed crania from diverse contexts.

Update (12/1/07): This paper was published in the December/November 2007 issue (see link above).

Phylogenetic Signal in Long Bones

I currently have a paper in review with Matthew O'Neill entitled "The degree and pattern of phylogenetic signal in primate long bone structure."

Abstract:

Interspecific scaling is a fundamental tool for comparative studies of primate long bone structure and adaptation. However, scaling analyses based on conventional statistical methods can lead to false positives regarding adaptive relationships when traits exhibit strong phylogenetic signal. This problem can be corrected through the use of phylogenetic comparative methods (PCMs). To date, PCMs have not been incorporated into comparative studies of primate long bone structure, as it has been assumed that long bone structure is free of phylogenetic signal once appropriately scaled. To test this assumption, we evaluated the degree of phylogenetic signal in three types of long bone structural traits (bone length, articular surface areas, and cross-sectional geometric properties) from 17 species of quadrupedal primates. We compared the pattern of phylogenetic signal in raw trait values and residual trait values after regression against body mass, bone length, and the product of body mass × bone length. Our results show that significant phylogenetic signal is present in all traits before scaling, due in part to their strong covariance with body mass. After scaling, bone length still exhibits strong phylogenetic signal, but articular surface areas are free of phylogenetic signal, and cross-sectional properties exhibit different levels of signal depending on the variable used to scale the data. These results suggest that PCMs should be incorporated into interspecific studies of bone length and perhaps cross-sectional geometric properties. Our results also demonstrate that tests for phylogenetic signal prior to implementing a PCM should focus on residual variance, not individual traits.

Update (8/28/07): This paper is now in press at Journal of Human Evolution.