Behavioral Ecology Advance Access originally published online on February 1, 2006
Behavioral Ecology 2006 17(3):372-379; doi:10.1093/beheco/arj043
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Testis size depends on social status and the presence of male helpers in the cooperatively breeding cichlid Julidochromis ornatus
a Laboratory of Animal Sociology, Department of Biology and Geosciences, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi, Osaka 558-8585, Japan, b Department of Behavioural Ecology, Zoological Institute, University of Bern, Wohlenstrasse 50a, CH-3032 Hinterkappelen, Switzerland, and c Usujiri Fisheries Station, Field Science Center for Northern Biosphere, Hokkaido University, 152 Usujiri, Hakodate, Hokkaido 041-1613, Japan
Address correspondence to S. Awata, who is now at the Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, 3422 Sesoko, Motobu, Okinawa 905-0227, Japan. E-mail: h066112{at}sci.u-ryukyu.ac.jp.
Cooperatively breeding animals, in which helpers may participate in reproduction with dominant breeders, are ideal species for examining intraspecific variation in testis size because they often exhibit both monogamous breeding (low risk of sperm competition) and polyandrous breeding (high risk) within a population. However, little is known about testis investment as a result of sperm competition in these animals. The substrate-brooding cichlid fish Julidochromis ornatus has a cooperatively breeding system, in which some males mate monogamously and other males reproduce as dominant breeders or helpers within cooperatively breeding groups, in which male helpers frequently sire young. We examined the relationship between testis investment and male social status in relation to the risk of sperm competition. As predicted from sperm competition models, in groups with male helpers, both the male breeders and the male helpers invested more in testes mass, compared to breeding males without male helpers. We also found a positive relationship between the testes mass of male breeders and their male helpers, suggesting that males increase their investment in reproductive capability under the risk of sperm competition. Sperm competition models also predict that larger testes are associated with increased siring success. Our paternity analysis supported this prediction; we found a positive relationship between testis investment by male helpers and the number of offspring they sired.
Key words: cooperative breeding, Lake Tanganyika, paternity, social status, sperm competition risk, testis investment.
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