Evol Ecol Res 5: 691-700 (2003)     Full PDF if your library subscribes.

Life-history traits as causes or consequences of social behaviour: why do cooperative breeders lay small clutches?

Roger Härdling1* and Hanna Kokko2,3

1Department of Animal Ecology, Ecology Building, University of Lund, 223 62 Lund, Sweden, 2Department of Biological and Environmental Science, University of Jyväskylä, PO Box 35, 40014 Jyväskylä, Finland and 3School of Botany and Zoology, Australian National University, Canberra, ACT 0200, Australia

Author to whom all correspondence should be addressed.
e-mail: roger.hardling@zooekol.lu.se


Cooperatively breeding birds tend to exhibit high adult survival and relatively small clutch sizes. According to the life-history hypothesis for cooperative breeding, high survival creates conditions for philopatry based on difficulties that dispersers face when competing for territories in a landscape with slow territory turnover. However, this hypothesis evokes a puzzle because high fecundity should also lead to problems in territory acquisition because of the large number of competitors for each vacancy. We suggest two reasons for the observed association between small clutch size, high survival rate and cooperative breeding in birds. The first reason is that when survival rate is a better predictor of cooperative breeding than fecundity, a general life-history trade-off between clutch size and survival rate will create the observed association between cooperative breeding and the two life-history characters. Theoretically, a high survival rate is expected to predict cooperative breeding better than fecundity, because a high survival rate increases both habitat saturation and the direct benefits of staying at home. The second reason is that the reproductive value of the first offspring each year is higher than that of subsequent offspring for cooperative breeders (the offspring depreciation hypothesis). This is because these offspring will be able to delay dispersal and gain indirect benefits by helping at home. We show that this, under very general conditions, decreases the optimum clutch size of cooperative breeders below that of non-cooperative breeders.

Keywords: cooperative breeding, clutch size evolution, life-history hypothesis, life-history trade-off, offspring depreciation, survival rate.

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