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You can't always get what you want: size assortative mating by mutual mate choice as a resolution of sexual conflict.

Baldauf SA, Kullmann H, Schroth SH, Thünken T, Bakker TC - BMC Evol. Biol. (2009)

Bottom Line: Especially males were highly selective for large females, probably because female body size signals direct fitness benefits.Due to variation in body size, general preferences for large mating partners result in a sexual conflict: small, lower quality individuals who prefer themselves large partners are unacceptable for larger individuals.These results suggest that the underlying mechanism of assortment in P. taeniatus is mutual mate choice resolving the sexual conflict over mates, rather than preference for mates of similar size.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute for Evolutionary Biology and Ecology, University of Bonn, An der Immenburg 1, D-53121 Bonn, Germany. sbaldauf@evolution.uni-bonn.de

ABSTRACT

Background: Assortative mating patterns for mate quality traits like body size are often observed in nature. However, the underlying mechanisms that cause assortative mating patterns are less well known. Sexual selection is one important explanation for assortment, suggesting that i) one (usually the female) or both sexes could show preferences for mates of similar size or ii) mutual mate choice could resolve sexual conflict over quality traits into assortment. We tested these hypotheses experimentally in the socially monogamous cichlid fish Pelvicachromis taeniatus, in which mate choice is mutual.

Results: In mate choice experiments, both sexes preferred large mates irrespective of own body size suggesting mating preferences are not size-assortative. Especially males were highly selective for large females, probably because female body size signals direct fitness benefits. However, when potential mates were able to interact and assess each other mutually they showed size-assortative mating patterns, i.e. the likelihood to mate was higher in pairs with low size differences between mates.

Conclusion: Due to variation in body size, general preferences for large mating partners result in a sexual conflict: small, lower quality individuals who prefer themselves large partners are unacceptable for larger individuals. Relative size mismatches between mates translate into a lower likelihood to mate, suggesting that the threshold to accept mates depends on own body size. These results suggest that the underlying mechanism of assortment in P. taeniatus is mutual mate choice resolving the sexual conflict over mates, rather than preference for mates of similar size.

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Likelihood to mate. The likelihood to mate in P. taeniatus was significantly explained by the relative body size distance of pairs (see equation 1 and Results section for statistics). An equal distance was measured in four pairs (filled gender symbols), thus 20 instead of 24 data points are shown. Gender symbols reveal whether the male (male symbol) or the female (female symbol) was relatively larger in a pair. In one case an equal distance between pairs with a relatively larger male and a relatively larger female was measured (male and female symbol combined).
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Figure 3: Likelihood to mate. The likelihood to mate in P. taeniatus was significantly explained by the relative body size distance of pairs (see equation 1 and Results section for statistics). An equal distance was measured in four pairs (filled gender symbols), thus 20 instead of 24 data points are shown. Gender symbols reveal whether the male (male symbol) or the female (female symbol) was relatively larger in a pair. In one case an equal distance between pairs with a relatively larger male and a relatively larger female was measured (male and female symbol combined).

Mentions: Overall, 15 pairs mated and nine pairs did not mate. The likelihood to mate was significantly explained by the distance of pairs (LRT: χ2 = -4.508, df = 1, p = 0.034; Fig. 3) and the relative size difference of pairs (LRT: χ2 = -4.344, df = 1, p = 0.037). Within the group of unmated pairs, females smaller than the mean female standard length of the population showed significantly more courtship behavior towards the males than females larger than the mean female standard length (Mann-Whitney U test: Nmale smaller = 4, Nmale larger = 5, z = -2.205, p = 0.032). The number of days until spawning was not significantly explained by the females' standard length (Cox regression: LRT1,24 = 0.83, beta = 0.545 ± 0.571, p = 0.358). Body size did not significantly correlate with different aspects of parental investment during brood care (e.g. the time males guarded caves, the female cared for eggs, the frequency of warning signals, time near the young: all p > 0.313). The standard length of the females tended to explain the number of eggs (LRT: F1,12 = 4.403, p = 0.057) and significantly explained the number of surviving offspring (LRT: F1,12 = 6.810, p = 0.023).


You can't always get what you want: size assortative mating by mutual mate choice as a resolution of sexual conflict.

Baldauf SA, Kullmann H, Schroth SH, Thünken T, Bakker TC - BMC Evol. Biol. (2009)

Likelihood to mate. The likelihood to mate in P. taeniatus was significantly explained by the relative body size distance of pairs (see equation 1 and Results section for statistics). An equal distance was measured in four pairs (filled gender symbols), thus 20 instead of 24 data points are shown. Gender symbols reveal whether the male (male symbol) or the female (female symbol) was relatively larger in a pair. In one case an equal distance between pairs with a relatively larger male and a relatively larger female was measured (male and female symbol combined).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2719620&req=5

Figure 3: Likelihood to mate. The likelihood to mate in P. taeniatus was significantly explained by the relative body size distance of pairs (see equation 1 and Results section for statistics). An equal distance was measured in four pairs (filled gender symbols), thus 20 instead of 24 data points are shown. Gender symbols reveal whether the male (male symbol) or the female (female symbol) was relatively larger in a pair. In one case an equal distance between pairs with a relatively larger male and a relatively larger female was measured (male and female symbol combined).
Mentions: Overall, 15 pairs mated and nine pairs did not mate. The likelihood to mate was significantly explained by the distance of pairs (LRT: χ2 = -4.508, df = 1, p = 0.034; Fig. 3) and the relative size difference of pairs (LRT: χ2 = -4.344, df = 1, p = 0.037). Within the group of unmated pairs, females smaller than the mean female standard length of the population showed significantly more courtship behavior towards the males than females larger than the mean female standard length (Mann-Whitney U test: Nmale smaller = 4, Nmale larger = 5, z = -2.205, p = 0.032). The number of days until spawning was not significantly explained by the females' standard length (Cox regression: LRT1,24 = 0.83, beta = 0.545 ± 0.571, p = 0.358). Body size did not significantly correlate with different aspects of parental investment during brood care (e.g. the time males guarded caves, the female cared for eggs, the frequency of warning signals, time near the young: all p > 0.313). The standard length of the females tended to explain the number of eggs (LRT: F1,12 = 4.403, p = 0.057) and significantly explained the number of surviving offspring (LRT: F1,12 = 6.810, p = 0.023).

Bottom Line: Especially males were highly selective for large females, probably because female body size signals direct fitness benefits.Due to variation in body size, general preferences for large mating partners result in a sexual conflict: small, lower quality individuals who prefer themselves large partners are unacceptable for larger individuals.These results suggest that the underlying mechanism of assortment in P. taeniatus is mutual mate choice resolving the sexual conflict over mates, rather than preference for mates of similar size.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute for Evolutionary Biology and Ecology, University of Bonn, An der Immenburg 1, D-53121 Bonn, Germany. sbaldauf@evolution.uni-bonn.de

ABSTRACT

Background: Assortative mating patterns for mate quality traits like body size are often observed in nature. However, the underlying mechanisms that cause assortative mating patterns are less well known. Sexual selection is one important explanation for assortment, suggesting that i) one (usually the female) or both sexes could show preferences for mates of similar size or ii) mutual mate choice could resolve sexual conflict over quality traits into assortment. We tested these hypotheses experimentally in the socially monogamous cichlid fish Pelvicachromis taeniatus, in which mate choice is mutual.

Results: In mate choice experiments, both sexes preferred large mates irrespective of own body size suggesting mating preferences are not size-assortative. Especially males were highly selective for large females, probably because female body size signals direct fitness benefits. However, when potential mates were able to interact and assess each other mutually they showed size-assortative mating patterns, i.e. the likelihood to mate was higher in pairs with low size differences between mates.

Conclusion: Due to variation in body size, general preferences for large mating partners result in a sexual conflict: small, lower quality individuals who prefer themselves large partners are unacceptable for larger individuals. Relative size mismatches between mates translate into a lower likelihood to mate, suggesting that the threshold to accept mates depends on own body size. These results suggest that the underlying mechanism of assortment in P. taeniatus is mutual mate choice resolving the sexual conflict over mates, rather than preference for mates of similar size.

Show MeSH