Male White-shouldered Fairywrens (Malurus alboscapulatus) elevate testosterone when courting females but not during territorial challenges

Testosterone mediates suites of physical and behavioral traits across vertebrates, and circulation varies considerably across and within taxa. However, an understanding of the causal factors of variation in circulating testosterone has proven difficult despite decades of research. According to the challenge hypothesis, agonistic interactions between males immediately prior to the breeding season produce the highest levels of testosterone measured during this period. While many studies have provided support for this hypothesis, most species do not respond to male-male competition by elevating testosterone. As a result, a recent revision of the hypothesis (challenge hypothesis 2.0) places male-female interactions as the primary cause of rapid elevations in testosterone circulation in male vertebrates. Here, we offer a test of both iterations of the challenge hypothesis in a tropical bird species. We first illustrate that male White-shouldered Fairywrens (Malurus alboscapulatus) differ by subspecies in plasma testosterone concentrations. Then we use a social network approach to find that males of the subspecies with higher testosterone are characterized by greater social interaction scores, including more time aggregating to perform sexual displays. Next, we use a controlled experiment to test whether males respond to simulated territorial intrusion or courtship interaction contexts by elevating testosterone. Males sampled during courtship had greater plasma testosterone both relative to flushed controls and males sampled during simulated intrusion. Ultimately, our results are consistent with challenge hypothesis 2.0, as males rapidly elevated testosterone following interactions with females, but not during territorial challenges.

Does Testosterone Administration Increase Competitiveness, Confidence and Financial Risk-taking in Men?

The sex steroid hormone testosterone regulates male-typical behaviors such as aggression and displays of dominance in non-human animals. According to the Challenge Hypothesis, these effects arise from context-sensitive testosterone increases that facilitate inter-male competitions over resources, status, and mates. A growing literature documents similar effects of testosterone on economic behaviors related to competition and risk-taking in humans, though findings to date have been mixed. Here, we report two randomized double-blind placebo-controlled testosterone administration experiments, whose combined sample (N = 334) is substantially larger than any previous investigation of the topic (N1 = 91, N2 = 243). The studies were designed independently by research groups in Europe and the US, and both investigated testosterone’s effects on men’s willingness to compete, confidence, and risk-taking—dimensions of economic behavior that are theoretically linked to the Challenge Hypothesis, show robust sex differences, and predict important real-life outcomes such as career choice. We find no evidence for effects of testosterone on any of the behavioral tasks studied across the two experiments, with effect point estimates that are small and inconsistent. The studies had 90% statistical power to detect effects that are larger than d = 0.68 and d = 0.42 respectively, and equivalence tests confidently reject effects that are greater than these magnitudes. Our findings cast doubt on the proposition that testosterone is a meaningful causal driver of the stereotypically “masculine” dimensions of economic behavior studied, and suggest that even if true effects existed, detecting them experimentally would be challenging.

Androgen responsiveness to simulated territorial intrusions in Allobates femoralis males: evidence supporting the challenge hypothesis in a territorial frog

Territorial behaviour has been widely described across many animal taxa, where the acquisition and defence of a territory are critical for the fitness of an individual. Extensive evidence suggests that androgens (e.g. testosterone) are involved in the modulation of territorial behaviour in male vertebrates. Short-term increase of androgen following a territorial encounter appears to favour the outcome of a challenge. The “Challenge Hypothesis” proposed by Wingfield and colleagues outlines the existence of a positive feedback relationship between androgen and social challenges (e.g. territorial intrusions) in male vertebrates. Here we tested the challenge hypothesis in the highly territorial poison frog, Allobates femoralis, in its natural habitat by exposing males to simulated territorial intrusions in form of acoustic playbacks. We quantified repeatedly androgen concentrations of individual males via a non-invasive water-borne sampling approach. Our results show that A. femoralis males exhibited a positive behavioural and androgenic response after being confronted to simulated territorial intrusions, providing support for the Challenge Hypothesis in a territorial frog.