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Behavioral Ecology Advance Access originally published online on September 11, 2008
Behavioral Ecology 2009 20(1):38-45; doi:10.1093/beheco/arn112
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© The Author 2008. 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

Sperm length evolution in the fungus-growing ants

Boris Baera,b,c, Michiel B. Dijkstraa, Ulrich G. Muellerd, David R. Nasha and Jacobus J. Boomsmaa

a Centre for Social Evolution, Department of Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark b Centre of Excellence in Plant Energy Biology, Molecular and Chemical Sciences Building M316, The University of Western Australia, 35 Stirling Highway, 6009 Crawley, Western Australia c Center for Evolutionary Biology, Zoology Building, The University of Western Australia, 35 Stirling Highway, 6009 Crawley, Western Australia d Integrative Biology, Patterson Laboratories, University of Texas, Austin, TX 78712, USA

Address correspondence to B. Baer. E-mail: bcbaer{at}cyllene.uwa.edu.au.


  Abstract

Eusocial insects offer special opportunities for the comparative study of sperm traits because sperm competition is absent (in species with obligatory monandry) or constrained (in lineages where queens mate multiply but never remate later in life). We measured sperm length in 19 species of fungus-growing ants, representing 9 of the 12 recognized genera, and mapped these onto the ant phylogeny. We show that average sperm length across species is highly variable and decreases with mature colony size in basal genera with singly mated queens, suggesting that sperm production or storage constraints affect the evolution of sperm length. Sperm length does not decrease further in multiply mating leaf-cutting ants, despite substantial further increases in colony size. In a combined analysis, sexual dimorphism explained 63.1% of the variance in sperm length between species. As colony size was not a significant predictor in this analysis, we conclude that sperm production trade-offs in males have been the major selective force affecting sperm length across the fungus-growing ants, rather than storage constraints in females. The relationship between sperm length and sexual dimorphism remained robust in phylogenetically independent contrasts. Some of the remaining variation was explained by the relative size of the sperm-storage organ, but only in the multiply mating leaf-cutting ants, suggesting that sperm-storage constraints become important for the evolution of sperm length in this derived group. Mate number affected sperm length to a minor extent, and only in interaction with other predictor variables, suggesting that sperm competition has not been a major selective force for sperm length evolution in these ants.

Key words: sexual selection, social insects, sperm limitation, sperm-storage constraints.

Received 10 March 2008; revised 11 June 2008; accepted 5 August 2008.


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