Elevation affects extra-pair paternity but not a sexually selected plumage trait in dark-eyed juncos
Differences in environmental conditions are expected to generate distinct selective pressures favoring different phenotypes. For example, environmental conditions that affect the timing of breeding may influence opportunities for extra-pair copulations and thus the strength of sexual selection on males. To explore these relationships quantitatively, we compared breeding synchrony, rates of extra-pair paternity, and expression of a sexually selected plumage trait (the amount of white on the tail feathers) in populations of dark-eyed junco (Junco hyemalis) breeding at elevations from 1960 to 2660 m in California. Microsatellite parentage analysis revealed that extra-pair paternity rates varied by elevation, with intermediate elevations having the highest rate. Differences in breeding synchrony could not explain this variation. Extra-pair males had more tail white than the social males they cuckolded, consistent with tail white being a sexually selected trait. Although the observed differences in rates of extra-pair paternity suggested that sexually selected traits should also vary with elevation, there were no differences among elevations in the amount of white on male tails or in the correlation between tail white and proxies for male condition. Multiple factors may have contributed to this result, including persistent gene flow among elevations, which may counter the effects of local differences in selective pressures. These findings demonstrate the complexity of interactions among environmental conditions, selective pressures, and variation in phenotypic traits, and underscore the importance of assessing the impacts of sexual selection in the larger context of population genetic structure.
Environmental differences, such as those occurring along elevation gradients, can lead to differences in sexual selection. We found that juncos at mid elevations had higher rates of extra-pair paternity than juncos at high and low elevations. Our results also provide evidence that male tail plumage is sexually selected, as females preferred to copulate with males with more white on their tail plumage than the females’ social mates. This suggests that male tail white should differ among elevations, as the reproductive rewards of having an attractive tail would be greater at mid elevations. However, we found no differences in male tail white among elevations. This may be due to the birds breeding freely across elevations, as evidenced by a lack of genetic structure, i.e., gene flow swamping out any differences that might otherwise form.
KeywordsBreeding synchrony Dark-eyed junco Elevation Extra-pair paternity Sexual selection
We thank S. Beissinger, L. Smith, J. P. Kelley, J. Scullen, and the San Francisco Bay Bird Observatory for the training and advice. The comments of three anonymous reviewers improved the manuscript. The following research assistants made important contributions to data collection: J. Bates, J. Carlisle, A. Gilbert, L. Hall, V. Kimzey, K. Lyberger, A. M. Lee, K. Marsh, S. Maclean, A. Misraraj, H. Park, J. Spool, J. Tseng, and J. Van Bourg.
This work was supported by a National Science Foundation Graduate Research Fellowship, a Berkeley Fellowship from UC Berkeley, Museum of Vertebrate Zoology funds, and grants from the American Ornithologists’ Union, the Animal Behavior Society, and Sigma Xi.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted. All procedures involving live birds were approved by UC Berkeley’s Animal Care and Use Committee (protocol #R317–0815) and performed under the appropriate federal, state, and local permits.
- Balph MH, Balph DF, Romesburg HC (1979) Social status signaling in winter flocking birds: an examination of a current hypothesis. Auk 96:78–93Google Scholar
- Ketterson ED (1979) Aggressive behavior in wintering dark-eyed juncos: determinants of dominance and their possible relation to geographic variation in sex ratio. Wilson Bull 91:371–383Google Scholar
- Ketterson ED, Parker PG, Raouf SA, Nolan V, Ziegenfus C, Chandler CR (1998) The relative impact of extra-pair fertilizations on variation in male and female reproductive success in dark-eyed juncos (Junco hyemalis). Ornithol Monogr (49):81–101Google Scholar
- Nolan V, Ketterson ED, Cristol DA, Rogers CM, Clotfelter ED, Titus RC, Schoech SJ, Snajdr E (2002) Dark-eyed junco (Junco hyemalis). In: Poole A (ed) The birds of North America online, Cornell Lab of Ornithology, Ithaca, bna.birds.cornell.edu/bna/species/716
- Westneat DF, Sherman PW, Morton ML (1990) The ecology and evolution of extra-pair copulations in birds. Curr Ornithol 7:331–369Google Scholar