Behavioral Ecology Advance Access published online on December 7, 2005
Behavioral Ecology, doi:10.1093/beheco/arj015
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1 Norwegian Institute for Nature Research, Polar Environmental Centre, N-9296 Tromsø, Norway; Department of Molecular Biotechnology, Institute of Medical Biology, University of Tromsø, N-9037 Tromsø, Norway; Department of Animal Ecology, University of Lund, S-22362 Lund, Sweden
* To whom correspondence should be addressed. The maintenance of plumage color polymorphism in the parasitic jaeger (Stercorarius parasiticus) is still not well understood. Earlier studies indicated that selection may favor pale females and melanic males. If so, females would maximize their fitness, producing pale female and melanic male offspring. We therefore predicted that females might bias their offspring sex ratio toward daughters in pale pairs and toward sons in melanic pairs. Females might also choose to mate assortatively in relation to plumage color, thereby maximizing the probability of producing either pale or melanic offspring. Because females are larger than males, differential rearing costs may affect the offspring sex ratio independent of parental plumage color. We examined offspring sex ratio allocation, breeding variables indicative of parental quality, and mating pattern in relation to plumage color in a colony of parasitic jaegers in northern Norway. Jaegers tended to mate assortatively in relation to plumage color. The reproductive performance declined with season, and matched pairs appeared to be of lower quality than mixed pairs. The proportion of male offspring increased with hatching date in matched pale and mixed pairs, whereas the situation was reversed in matched melanic pairs. Matched pale pairs produced an overall surplus of favorable pale but costly daughters despite their lower quality, while melanic pairs produced a surplus of favorable melanic sons. However, differential offspring rearing costs and parental rearing capacity may have additionally affected the realized offspring sex ratio. Mixed pairs producing an overall surplus of pale and melanic daughters allocated their resources according to differential rearing costs and parental quality only. We suggest that both strategies of sex ratio allocation together with differences in reproductive success in matched versus mixed pairs may have a balancing effect on the mating pattern between plumage morphs and may contribute to the maintenance of the color polymorphism in this species.
Received March 18, 2005
Revised October 31, 2005
Accepted November 7, 2005
Article
Offspring sex ratio allocation in the parasitic jaeger: selection for pale females and melanic males?
Kirstin Janssen 1 *,
Kjell Einar Erikstad 2,
and
Staffan Bensch 3
2 Norwegian Institute for Nature Research, Polar Environmental Centre, N-9296 Tromsø, Norway
3 Department of Animal Ecology, University of Lund, S-22362 Lund, Sweden
Kirstin Janssen, E-mail: Kirstin.Janssen{at}tmu.uit.no
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