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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.
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Evaluating seasonal patterns of female aggression: Case study in a cavity‐nesting bird with intense female–female competition
Abstract Seasonal plasticity in aggression is likely to be shaped by the contexts in which aggression is beneficial, as well as the constraints inherent in its underlying mechanisms. In males, seasonal plasticity in testosterone (T) secretion is thought to underlie seasonal plasticity in conspecific aggression, but it is less clear how and why female aggression may vary across different breeding stages. Here, we integrate functional and mechanistic perspectives to begin to explore seasonal patterns of conspecific aggression in female tree swallows (Tachycineta bicolor), a songbird with intense female–female competition and T‐mediated aggression. Female tree swallows elevate T levels during early breeding stages, coinciding with competition for nest boxes, after which time T levels are roughly halved. However, females need to defend ownership of their nesting territory throughout the breeding season, suggesting it may be adaptive to maintain aggressive capabilities, despite low T levels. We performed simulated territorial intrusions using 3D‐printed decoys of female tree swallows to determine how their aggressive response to a simulated intrusion changes across the breeding season. First, we found that 3D‐printed decoys produce data comparable to stage‐matched studies using live decoys, providing researchers with a new, more economical method of decoy construction. Further, female aggressiveness remained relatively high through incubation, a period of time when T levels are quite low, suggesting that other mechanisms may regulate conspecific female aggression during parental periods. By showing that seasonal patterns of female aggression do not mirror the established patterns of T levels in this highly competitive bird, our findings provide a unique glimpse into how behavioural mechanisms and functions may interact across breeding stages to regulate plasticity.
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- Award ID(s):
- 1656109
- PAR ID:
- 10460425
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Ethology
- Volume:
- 125
- Issue:
- 8
- ISSN:
- 0179-1613
- Page Range / eLocation ID:
- p. 555-564
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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