Sperm Competition Selects Beyond Relative Testes Size in Birds

Authors

Stefan Lupold, Department of Animal and Plant Sciences, University of SheffieldFollow
George M. Linz, USDA/APHIS/WS National Wildlife Research CenterFollow
James W. Rivers, Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California 93106Follow
David F. Westneat, Department of Biology, 101 Morgan Building, University of Kentucky, Lexington, Kentucky 40506Follow
Tim R. Birkhead, Department of Animal and Plant Sciences, University of SheffieldFollow

Date of this Version

2009

Comments

Published in Evolution 63-2: 391–402.

Abstract

Sperm morphology varies considerably across taxa, and postcopulatory sexual selection is thought to be one of the main forces responsible for this diversity. Several studies have investigated the effects of the variation in sperm design on sperm function, but the consequences of variation in sperm design on testis morphology have been overlooked. Testes size or architecture may determine the size of the sperm they produce, and selection for longer sperm may require concomitant adaptations in the testes. Relative testes size differs greatly between species and is often used as an index of sperm competition, but little is known about whether larger testes have more sperm-producing tissue or produce sperm at a faster rate. Using a comparative approach in New World Blackbirds (Icteridae), we found (1) a strong link between testis histology and sperm length, suggesting selection on testis architecture through selection on sperm size, and (2) that species under intense sperm competition had a greater proportion of sperm-producing tissue within their testes. These results support the prediction that sperm competition fosters adaptations in reproductive organs that extend beyond testes size, and raise questions about the trade-offs influencing reproductive investment.