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Behavioral Ecology Advance Access published online on September 22, 2004
Behavioral Ecology, doi:10.1093/beheco/arh164
© 2004 by International Society for Behavioral Ecology
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Received April 20, 2004
Revised July 16, 2004
Accepted August 12, 2004

Article

Operational sex ratio in newts: field responses and characterization of a constituent chemical cue

Jason R. Rohr 1*, Daesik Park 2, Aaron M. Sullivan 1, Malachy McKenna 1, Catherine R. Propper 2, and Dale M. Madison 1

1 Department of Biological Sciences, Binghamton University, State University of New York, Binghamton, NY 13902-000, USA
2 Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011, USA

* To whom correspondence should be addressed. E-mail: jrohr{at}psu.edu.


  Abstract

Operational sex ratio (OSR) has been traditionally thought of as a force imposing competition for mates rather than also a cue used to regulate the intrasexual competition individuals encounter. To assess whether eastern red-spotted newts, Notophthalmus viridescens, could appropriately compare OSRs, we quantified field responses to traps containing four males, a sexually receptive female, four males plus a female, or nothing as a control. Early in the breeding season, males from two populations chose competitive mating opportunities over no mating opportunity at all, but generally preferred less competitive mating prospects. Later in the breeding season, as the OSR of newt populations becomes more male biased, males accordingly increased their acceptance of intrasexual competition. Females avoided groups of four males, and for both sexes, avoidance of male-biased courting groups increased their probability of amplexus courtship. We then isolated an approximately 33-kD protein from male cloacal glands that was used by males to compare OSRs. To our knowledge, this protein represents the first isolated and characterized component of an olfactory cue used to evaluate OSR. These results support two important principles regarding mating systems: (1) OSR can somewhat paradoxically be both the source imposing competition for mates and the source used to reduce it, and (2) analogous to the sex in short supply often being "choosy" selecting mates, the sex in excess (here, males) appears to be choosy about its acceptance of intrasexual competition.

Keywords: chemical cues; mating competition; mating system; Notophtalmus viridescens; olfaction; operational sex ratio; pheromone; salamander; sexual selection; spatiotemporal variation.

J.R. Rohr and D. Park contributed equally to this work.


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