Abstract
Male orangutans (Pongo spp.) display an unusual characteristic for mammals in that some adult males advance quickly to full secondary sexual development while others can remain in an adolescent-like form for a decade or more past the age of sexual maturity. Remarkably little is understood about how and why differences in developmental timing occur. While fully-developed males are known to produce higher androgen levels than arrested males, the longer-term role of steroid hormones in male life history variation has not been examined. We examined variation in testosterone and cortisol production among 18 fully-developed (“flanged”) male orangutans in U.S. captive facilities. Our study revealed that while testosterone levels did not vary significantly according to current age, housing condition, and species origin, males that had undergone precocious development had higher testosterone levels than males that had experienced developmental arrest. While androgen variation had previously been viewed as a state-dependent characteristic of male developmental status, our study reveals that differences in the physiology of early and late developing males are detectable long past the developmental transition and may instead be trait-level characteristics associated with a male’s life history strategy. Further studies are needed to determine how early in life differences in testosterone levels emerge and what consequences this variation may have for male behavioral strategies.
Highlights
While it is common for alternative reproductive strategies to exist within a species, orangutans are unusual among mammals in that fertile, adult males occur in two distinct morphological types
One male with unusually high testosterone levels and a single morning sample did not bias the results, as the model was qualitatively the same and the effect of developmental timing was statistically significant with his exclusion (Estimate = 0.240, df = 1, p = 0.039)
Because all four Bornean orangutans in our sample matured late, we tested whether there was a species effect on either testosterone or cortisol and found no evidence of such an effect in either the full dataset (ANOVA, testosterone: F = 1.833, df = 2,15, p = 0.194; cortisol: F = 0.378, df = 2,15, p = 0.691) or in the subset of subjects that matured late (ANOVA, testosterone: F = 1.975, df = 2,9, p = 0.195; cortisol: F = 0.205, df = 2,9, p = 0.818). In this first analysis of variation in testosterone among flanged male orangutans, we found that males that had completed their secondary sexual development at a young age maintained higher testosterone levels than those with delayed maturation
Summary
While it is common for alternative reproductive strategies to exist within a species, orangutans are unusual among mammals in that fertile, adult males occur in two distinct morphological types. Able to reproduce, remain in the adolescent-like unflanged form (Figure 1b) for 10 or more years following puberty [4,5]. While flanged males appear to gain preferential access to fecundable females [6], it remains unclear why some males develop earlier than others, what factors trigger secondary sexual development, and what the relative fitness advantages of each morph are. Observations from captivity suggested that development was suppressed whenever a dominant flanged male could be seen or heard [7,8,9], yet results from male pairings are inconsistent. A survey of captive orangutans found higher levels of glucocorticoids in males undergoing secondary sexual development than those in developmental arrest, suggesting that stress from competitors was not a mechanism for delayed maturation [10]. A failure to understand the mechanisms for the male developmental transition has hindered understanding the evolution of this peculiar life history pattern
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