Behavioral Ecology and Sociobiology

, Volume 66, Issue 2, pp 241–252 | Cite as

Competitive females are successful females; phenotype, mechanism, and selection in a common songbird

  • Kristal E. CainEmail author
  • Ellen D. Ketterson
Original Paper


In a variety of taxa, male reproductive success is positively related to the expression of costly traits such as large body size, ornaments, armaments, and aggression. These traits are thought to improve male competitive ability and, thus, access to limited reproductive resources. Females of many species also express competitive traits. However, we know very little about the consequences of individual variation in competitive traits and the mechanisms that regulate their expression in females. Consequently, it is currently unclear whether females express competitive traits owing to direct selection or as an indirect result of selection on males. Here, we examine females of a mildly dimorphic songbird (Junco hyemalis) to determine whether females show positive covariance in traits (morphology and behavior) that may be important in a competition. We also examine whether trait expression relates either to testosterone (T) in terms of mechanism or to reproductive success in terms of function. We found that larger females were more aggressive and that greater ability to produce T in response to a physiological challenge consisting of a standardized injection of gonadotropin-releasing hormone predicted some measures of female body size and aggression. Finally, we found that aggressive females had greater reproductive success. We conclude that T may influence female phenotype and that females may benefit from expressing a competitive phenotype. We also suggest that the mild dimorphism observed in many species may be due in part to direct selection on females rather than simply a correlated response to selection in males.


Competitive phenotype Female aggression Testosterone Gonadotropin-releasing hormone Junco hyemalis Sexual dimorphism 



This research adhered to the Association for the Study of Animal Behavior/Animal Behavior Society Guidelines for the Use of Animals in Research, the legal requirements of the United States of America (USFWS special use permit number MB093279-2, USGS banding permit number 20261), the States of Indiana and Virginia, and was conducted in compliance with the University of Virginia and Indiana University Institutional Animal Care and Use Committee (protocol # 06-242). The authors thank the Ketterson Lab, particularly K. Rosvall, for the assistance in developing the methods, discussion, and comments on the manuscript; A. Dapper and S. Wanamaker and the United Junco Workers for the field assistance; S. Hoobler for the additional support; and Mountain Lake Biological Station (B. Brodie III, Director and E. Nagy, Associate Director) and Mountain Lake Hotel for the facilities and permission to work on their property. Research was supported by National Science Foundation (NSF) grants to EK (BSC 05-19211 and IOS 08-20055) and an NSF Doctoral Dissertation Improvement Grant to KC (0910036). KC was also supported by an NSF Graduate Research Fellowships Program fellowship and National Institute of Health training grant, Common Themes in Reproductive Diversity (NIH no. HD 049336-04).


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© Springer-Verlag 2011

Authors and Affiliations

  1. 1.Department of BiologyIndiana UniversityBloomingtonUSA
  2. 2.Center for the Integrative Study of Animal BehaviorBloomingtonUSA

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