Anders Berglund
Professor in Animal Ecology
Address: Dept. of Animal
Ecology, Norbyv. 18d, S-752 36 Uppsala, Sweden
Tel. +46-(0)18-471 26 43, fax +46-(0)18-471 64 84, e-mail Anders.Berglund@ebc.uu.se
Sex
roles and sexual selection in pipefish:
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¨ How and why do sex roles and secondary sexual characters evolve in animals, why do sex roles reverse sometimes, and how are the costs of sexual selection divided upon males and females? In most animals sexual selection operates more strongly in males than in females, with males consequently possessing more elaborate secondary sexual characters (such as bright colours, large tails or antlers). The main reason for this is that in most species males potentially reproduce faster than females, with the consequence that males willing to mate outnumber females willing to mate. This surplus of willing males causes competition among the males for females. |
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¨ In
a small but interesting minority of all animal species sex roles are
reversed: females ready to mate are in excess, and they compete for males.
In two
studied pipefish species, Syngnathus typhle and Nerophis ophidion, a long
male pregnancy (offspring are brooded by males on their bodies and
provided with energy and oxygen) depresses male reproductive rate below
that of females. Female pipefish compete for males through dominance
hierarchies. |
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¨ In
these females, the evolution of sexual signals is constrained by costs of these signals to female condition,
survival and fecundity. Fecundity costs are likely to be of utmost
importance, but has received very little attention. The way the
conflicting demands of signal honesty and signal costs interact to convey
information from females to males and to other females is now the prime
target of our (i.e. I, Gunilla
Rosenqvist and Josefin
Sundin) research on these two pipefish species. |
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¨ In most cases a secondary sexual character functions both in mate competition and as a cue for partner choice. This can be understood if such characters commonly arise through intrasexual selection processes and serve as honest signals to other males. Females that subsequently utilise them as indicators of male phenotypic quality when selecting a partner will benefit by acquiring males of higher quality to father their offspring. This simple scheme may explain the origin and maintenance of female choice in a majority of cases.
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