Marilyn A. Norconk, PhD

Fieldwork Photos

Marilyn A. Norconk

Sexual dichromatism
in white-faced sakis
of the Guiana Shield


With the assistance of Dave Winkler from Cleveland Metroparks Zoo and Karen Vacco from Pittsburgh Zoo and Aquarium, I have been collecting fecal samples and digital images of the faces of six young saki males since 2000. Fecal samples are being analyzed for fecal testosterone levels and facial images are measured to calculate the area of the facial mask during development. The young male pictured above is about 8 months old.


Testosterone values are strongly correlated with age (rho = 0.59, p < 0.01, n = 106, two-tailed test). This is the case even in spite of considerable variation in testosterone values for males of all ages. Also, testosterone values are significantly different among males who are divided into four age categories (< 1 year of age, 1-2 years of age, 2-3 years of age and > 3 years of age) (one-way ANOVA, F = 20.7, p < 0.01, df = 3, n = 105), but a Bonferroni post hoc test on this group of males suggests that while males < 2 years of age are significantly different from males > 3 years, the males in the third age group are intermediate in testosterone values and not different from either younger or adult males. The sample size for the third category is considerably smaller than other categories and completion of the hormone assays will allow me to improve the sample size of the 2-3 year old males from both Pittsburgh and Cleveland.


Digital images of male faces were used to measure the area of two facial features1) white hair on the face (“white mask”) and 2) black skin around the eyes (“bare face area”). These are inverse measurements since during development the white area of the facial mask increases, the black area around the eyes decreases. Infant sakis start with no white mask and the face appears hairless. Facial areas were digitized and area of the bare face and white mask were measured using public domain software, Image J. Both measurements were correlated with the male’s age, but the bare face measurement was a better fit statistically (r2 = 0.65, p < 0.01, n = 40) than the area of the white mask (r2 = 0.29, p < 0.01, n = 37) (Norconk, in prep). [Analyses were funded in part by the NSF SBE 98-07516 and KSU’s Research and Graduate Studies. The fecal samples were analyzed by Dr. Patricia Whitten at Emory University’s Laboratory of Reproductive Ecology and Environmental Toxicology.]