Behavioral Ecology Advance Access originally published online on May 7, 2007
Behavioral Ecology 2007 18(4):652-657; doi:10.1093/beheco/arm026
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Mothers adjust egg size to helper number in a cooperatively breeding cichlid
a Behavioural Ecology, Zoological Institute, University of Bern, Wohlenstrasse 50A, CH-3032 Hinterkappelen, Switzerland b Evolution and Ecology Program, IIASA, A-2361 Laxenburg, Austria
Address correspondence to B. Taborsky. E-mail: barbara.taborsky{at}esh.unibe.ch.
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Abstract |
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Mothers should adjust the size of propagules to the selective forces to which these offspring will be exposed. Usually, a larger propagule size is favored when young are exposed to high mortality risk or conspecific competition. Here we test 2 predictions on how egg size should vary with these selective agents. When offspring are cared for by parents and/or alloparents, protection may reduce the predation risk to young, which may allow mothers to invest less per single offspring. In the cooperatively breeding cichlid Neolamprologus pulcher, brood care helpers protect group offspring and reduce the latters' mortality rate. Therefore, females are expected to reduce their investment per egg when more helpers are present. In a first experiment, we tested this prediction by manipulating the helper number. In N. pulcher, helpers compete for dispersal opportunities with similar-sized individuals of neighboring groups. If the expected future competition pressure on young is high, females should increase their investment per offspring to give them a head start. In a second experiment, we tested whether females produce larger eggs when perceived neighbor density is high. Females indeed reduced egg size with increasing helper number. However, we did not detect an effect of local density on egg size, although females took longer to produce the next clutch when local density was high. We argue that females can use the energy saved by adjusting egg size to reduced predation risk to enhance future reproductive output. Adaptive adjustment of offspring size to helper number may be an important, as yet unrecognized, strategy of cooperative breeders.
Key words: cichlids, cooperative breeding, intraspecific competition, maternal effects, maternal investment, offspring size.
Received 28 August 2006; revised 22 December 2006; accepted 2 March 2007.