The relative roles of predation and sperm competition on the duration of the post-copulatory association between the sexes in the blue crab, Callinectes sapidus |
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Authors: | P Jivoff |
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Institution: | (1) Department of Zoology, University of Maryland, College Park, MD 20742, USA, US;(2) Smithsonian Environmental Research Center, P.O. Box 28, Edgewater, MD 21037, USA, US |
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Abstract: | In many species, post-copulatory mate guarding prevents other males from mating with the guarded female. In crabs, males
stay with their mates to protect the female from predators because, in some species, mating occurs when she is soft and vulnerable
after molting. I tested the relative roles of sperm competition and predation on the duration of the post-copulatory association
in the blue crab, Callinectes sapidus. Unpaired females suffered greater predation mortality than paired females and males stayed with the female longer in the
presence of predators than in their absence, suggesting that the post-copulatory association protects females during their
vulnerable period. However, the association may also occur in blue crabs because of sperm competition since spermathecal contents
of females in the field indicate that 12.4% mated twice. Females experimentally mated with two males contained both males
ejaculates and each ejaculate had access to the unfertilized eggs, suggesting that the size of a male's ejaculate influences
his fertilization rate in a multiply-mated female. Males stayed longest in response to a high risk of sperm competition. Longer
post-copulatory associations allowed the first male's ejaculate to harden into a type of sperm plug, which limited the size
of a second inseminator's ejaculate in a non-virgin female as compared with a virgin. Males passed larger ejaculates in the
presence of rivals and when previous ejaculates were in the female spermathecae, another response to sperm competition. Larger
ejaculates may need longer post-copulatory associations before a more effective sperm plug forms. Large males stayed with
the female longer, which is consistent with their ability to pass larger ejaculates than small males and suggests that there
may be costs to minimizing the duration of the post-copulatory association. In the field, associations last long enough to
protect the female during her vulnerable phase and may ensure that the guarding male fertilizes the most eggs in the female,
even if she remates. Thus, the post-copulatory association protects female blue crabs from additional inseminators as well
as from predators.
Received: 23 January 1996 / Accepted after revision: 9 November 1996 |
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Keywords: | Sperm competition Predation Post-copulatory mate association Blue crab |
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