Abstract

As an integral part of the immune response, testosterone secretion is inhibited when an individual is confronted with an immune challenge. Testosterone-mediated physiological, morphological, and behavioral traits are compromised at times of impaired health. Nevertheless, males of some species seem to maintain high levels of testosterone when confronted with an immune challenge, upholding competitive strength but compromising their immune response. It has been argued that this phenomenon will occur only in species living in social systems with high degrees of male-male competition over mating opportunities. Male chimpanzees contest over access to fertile females and dominants sire the majority of offspring. This male mating pattern makes chimpanzees a candidate species where we could expect males to maintain high testosterone levels, compromising their immune response, to ensure immediate reproductive success. We measured blood testosterone levels in male and female chimpanzees, who expressed clinical symptoms (symptomatic) or showed no evidence of clinical disease on assessment (asymptomatic). For females, we expected to find lower testosterone levels in symptomatic individuals than in asymptomatic subjects. In males, we would predict lower testosterone levels in symptomatic individuals than in asymptomatic males, if the immune response leads to a decrease in testosterone secretion. Alternatively, males could have equal levels of testosterone when symptomatic and asymptomatic, upholding competitive strength. Our results show that male chimpanzees exhibit lower levels of testosterone when confronted with an immune challenge than when being asymptomatic. This suggests that male testosterone secretion is suppressed as part of the immune response, which potentially increases survival and lifetime reproductive success. It will, however, negatively impact momentary competitive ability. Also, males may employ different mating strategies, some of which are less testosterone-driven (e.g., affiliative strategies). Consequently, in some individuals, the costs of maintaining high testosterone levels may not outweigh the potential gain in reproductive success.

Highlights

  • Different life history traits compete with each other for limited resources, and usually have a negative association with one another (Stearns, 1989, 1992; Zera & Harshman, 2001)

  • As an integral part of the immune response, testosterone secretion is inhibited when an individual is confronted with an immune challenge

  • Males of some species seem to maintain high levels of testosterone when confronted with an immune challenge, upholding competitive strength but compromising their immune response

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Summary

Introduction

Different life history traits compete with each other for limited resources, and usually have a negative association with one another (Stearns, 1989, 1992; Zera & Harshman, 2001). Variation in testosterone levels coincide with individual‐ and species‐ differences in mate competition, the nature and intensity of aggression, territoriality, and paternal behavior (Lemur catta: Gould & Ziegler, 2007; Junco hyemalis: McGlothlin et al, 2007; Pan troglodytes: Muller, 2017) in seasonal breeders testosterone levels increase during the mating season (for primates e.g., L. catta: Gould & Ziegler, 2007; Eulemur fulvus rufus: Ostner et al, 2002) In nonseasonal breeders, they vary as a function of intrasexual competition, with higher testosterone levels in species with high levels of male‐ male competition (e.g., in howler monkeys (Alouatta palliata): Cristóbal‐Azkarate et al, 2006; in colobus monkeys (Colobus vellerosus): Teichroeb & Sicotte, 2008)

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