Testing hormonal responses to real and simulated social challenges in a competitive female bird

Abstract

Competitive interactions often occur in series; therefore animals may respond to social challenges in ways that prepare them for success in future conflict. Changes in the production of the steroid hormone testosterone (T) are thought to mediate phenotypic responses to competition, but research over the past few decades has yielded mixed results, leading to several potential explanations as to why T does not always elevate following a social challenge. Here, we measured T levels in tree swallows (Tachycineta bicolor), a system in which females compete for limited nesting cavities and female aggression is at least partially mediated by T. We experimentally induced social challenges in two ways: (1) using decoys to simulate territorial intrusions and (2) removing subsets of nesting cavities to increase competition among displaced and territory-holding females. Critically, these experiments occurred pre-laying, when females are physiologically capable of rapidly increasing circulating T levels. However, despite marked aggression in both experiments, T did not elevate following real or simulated social challenges, and in some cases, socially challenged females had lower T levels than controls. Likewise, the degree of aggression was negatively correlated with T levels following a simulated territorial intrusion. Though not in line with the idea that social challenges prompt T elevation in preparation for future challenges, these patterns nevertheless connect T to territorial aggression in females. Coupled with past work showing that T promotes aggression, these results suggest that T may act rapidly to allow animals to adaptively respond to the urgent demands of a competitive event.

Keywords: challenge hypothesis; female–female competition; simulated territorial intrusion; testosterone; winner effect.

Figure 1

Conceptual overview of Experiment 2, wherein a subset of boxes were made newly accessible, while even more boxes were simultaneously made inaccessible. In Y1 (depicted), we accomplished this by closing boxes that were previously open, and opening “NEW” boxes that were previously closed. In Y2, boxes were completely removed or installed anew. In both years, “NEIGHBOR” boxes were untouched, occupied territories in close proximity to those boxes with altered accessibility.

Figure 2

(a) Females caught after a 30 min STI had significantly lower circulating T levels compared to controls. Lines and shading represent mean ± SE. (b) Females that spent a greater proportion of time behaving aggressively during an STI had significantly lower circulating T levels. Shading represents 95% CI. T concentrations are log-transformed for normality.

Figure 3

Physical aggression occurred more frequently at experimental replicates than controls during both iterations of Experiment 2. Rates were calculated as the proportion of minutes within a single observation session in which aggression was observed, divided by the estimated number of tree swallow pairs in the vicinity. Each dot represents one 20 min observation period. Additional fixed effects are described in Table 2.

Figure 4

Females caught after a reduction in available nestboxes had significantly lower circulating T levels compared to stage-matched controls in Y2 but not Y1 of Experiment 2. There was no effect of box-type (neighbor, new, etc.; denoted by shape). All T concentrations were log-transformed. Lines and shading represent mean ± SE.