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Behavioral Ecology Advance Access originally published online on August 12, 2009
Behavioral Ecology 2009 20(5):1147-1151; doi:10.1093/beheco/arp109
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© The Author 2009. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Males use sex pheromone assessment to tailor ejaculates to risk of sperm competition in a butterfly

Helena Larsdotter Mellström and Christer Wiklund

Department of Zoology, Stockholm University, S-106 91 Stockholm, Sweden

Address correspondence to H. Larsdotter Mellström. E-mail: helena.mellstrom{at}zoologi.su.se.


  Abstract

In polyandrous butterflies males transfer a large, nutritious ejaculate at mating. Larger ejaculates delay female remating and confer an advantage in sperm competition. However, large ejaculates are costly, potentially selecting for male adjustment of ejaculate size to the risk of sperm competition. Here, we test if male ejaculate size in the butterfly Pieris napi varies with male density, and whether males assess sperm competition risk using the male sex pheromone citral as a cue. The results conformed to sperm competition theory and showed that male P. napi tailored their reproductive investment in response to the risk of sperm competition; ejaculates transferred by males in the high male density treatments were on average 23% larger than ejaculates transferred at low male densities. The results also show for the first time that the sex pheromone citral was used by males to assess male density; ejaculates transferred by males in presence of added male sex pheromone were 19% larger than ejaculates transferred in the control. In conclusion, the study shows how the sex pheromone not only facilitates female acceptance when dispensed by courting males but also allows males to assess the degree of male competition for matings.

Key words: green-veined white butterfly, nuptial gift, Pieris napi, population density, sexual selection, strategic mating effort.

Received 20 March 2009; revised 17 June 2009; accepted 7 July 2009.


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