Secondary sexual coloration is related to white blood cell counts and testosterone in male southeastern five-lined skinks (Plestiodon inexpectatus)

Abstract

The immunocompetence handicap hypothesis posits that secondary sexual coloration can honestly signal male quality because elevated testosterone, which is necessary for the expression of the coloration, also handicaps males through immunosuppression. Thus only high quality males can express the showiest coloration in spite of immunosuppression. Here we report a test of the immunocompetence handicap hypothesis in southeastern five-lined skinks, Plestiodon inexpectatus, which exhibit a reddish-orange head coloration during the breeding season. We tested whether head coloration is related to circulating testosterone concentrations and reflects the status of a male’s immune system, as measured by total leukocyte counts. As predicted, hue, saturation, and extent of head coloration were correlated with plasma testosterone, and the brightness of the head was negatively correlated with total circulating leukocytes. While results are consistent with the immunocompetence handicap hypothesis, additional studies that include experimental manipulations of testosterone levels and measure other aspects of immunity are warranted.