Using photogrammetry and color scoring to assess sexual dimorphism in wild western gorillas (Gorilla gorilla)

Abstract

Investigating sexual dimorphism is important for our understanding of its influence on reproductive strategies including male-male competition, mate choice, and sexual conflict. Measuring physical traits in wild animals can be logistically challenging and disruptive for the animals. Therefore body size and ornament variation in wild primates have rarely been quantified. Gorillas are amongst the most sexually dimorphic and dichromatic primates. Adult males (silverbacks) possess a prominent sagittal crest, a pad of fibrous and fatty tissue on top of the head, have red crest coloration, their saddle appears silver, and they possess a silverline along their stomach. Here we measure levels of sexual dimorphism and within-male variation of body length, head size, and sexual dichromatism in a population of wild western gorillas using photogrammetry. Digital photogrammetry is a useful and precise method to measure sexual dimorphism in physical traits yielding sexual dimorphism indices (ISD), similar to those derived from traditional measurements of skeletal remains. Silverbacks were on an average 1.23 times longer in body length than adult females. Sexual dimorphism of head size was highest in measures of crest size (max ISD: 60.4) compared with measures of facial height (max ISD: 24.7). The most sexually dimorphic head size measures also showed the highest within-sex variation. We found no clear sex differences in crest coloration but there was large sexual dichromatism with high within-male variation in saddle coloration and silverline size. Further studies should examine if these sexually dimorphic traits are honest signals of competitive ability and confer an advantage in reproductive success.

Measuring physical traits of primates remotely: the use of parallel lasers

“…Primatologists working in field conditions have used photogrammetry as a measurement tool in the past, but the calibration for this technique is difficult, and the measurement of photographs not only can be very time consuming, but also needs to be accurate with respect to distance [Breuer et al, 2007;Caillaud et al, 2008], which are a significant limitations in the case of arboreal primates. When lasers are projected horizontally, the parallel laser method does not require any correction factors, and when used vertically, a clinometer can be attached to accurately measure angle and correct for parallax.…”

“…Although the remote measurement of physical traits of primates is not new [e.g. Breuer et al, 2007;Caillaud et al, 2008;Domb & Pagel, 2001;Emery & Whitten, 2003] to our knowledge the parallel laser technique has not previously been applied to primates.…”

Analyzing Visual Signals as Visual Scenes

“…This can be as simple as measuring the size of color patches using rulers [Eckert and Weatherhead, 1987], tracing color patches using acetate film and measuring patch area [Lendvai et al, 2004] or counting the number of corners and measuring angles. Digital photography has made photogrammetry (the collection of geometric data from images) much more straightforward [Breuer et al, 2007]. Appropriate length scales included in images enable size measurements to be collected from animals in the field relatively easily.…”

“…More recently digital photography has emerged as a lower-cost alternative to spectrometry, and as photographs of free-ranging large-bodied primate species are much easier to obtain than spectrometer measures, this development has led to an increase in the number of studies investigating primate color [Bergman and Beehner, 2008;Breuer et al, 2007;Dubuc et al, 2009;Gerald et al, 2001;Higham et al, 2008bHigham et al, , 2010Higham et al, , 2011Kamilar, 2009;Kamilar and Bradley, 2011b]. Most studies have used measures based on a camera's RGB sensors' responses, but characterization of a camera's responses to light intensity and sensitivity to different wavelengths enables transformation of RGB values to speciesspecific perceptual space.…”

Using behavior to determine immature life‐stages in captive western gorillas

“…Quantitative methods to determine the ontogenic trajectory in a species are absent from the published literature, with the exception of Breuer et al [2007] who uses physical characteristics to determine male gorilla age. As it is, the appropriateness of age-based classifications to categorize similar individuals is difficult to determine for any given primate species.…”