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Male Seahorses Like Large Mates

July 7, 2009

Male seahorses have a clear agenda when it comes to selecting a mating partner: to increase their reproductive success. By being choosy and
preferring large females, they are likely to have more and bigger eggs, as well as bigger offspring, according to Beat Mattle and Tony Wilson
from the Zoological Museum at the University of Zurich in Switzerland. Their findings1 have just been published online in Springer’s journal
Behavioral Ecology and Sociobiology.

Seahorses have a unique mode of reproduction: male pregnancy. Male seahorses provide all post-fertilization parental care, yet despite the
high levels of paternal investment, they have long been thought to have conventional sex roles, with females choosing mating partners and males competing for their attention. However, clutch, egg and offspring size all increase with female body size in seahorses, suggesting that malesmay obtain fecundity benefits by mating with large-bodied females.

Mattle and Wilson investigated the mating behavior of the pot-bellied seahorse (Hippocampus abdominalis), concentrating on the importance of
partner body size in mate selection. A total of 10 female and 16 male sexually mature seahorses, obtained from a captive breeding facility in
Tasmania, took part in the experiment. Individuals of both sexes were presented with potential mating partners of different sizes. Mating
preferences were quantified in terms of time spent courting each potential partner.

Mattle and Wilson found striking differences in courtship behavior between male and female seahorses, with choosy males and indiscriminate females.

Male seahorses were highly active and showed a clear preference for larger partners. In contrast, females were significantly less active and showed ambiguous mating preferences.

The authors conclude: “The strong male preferences for large females demonstrated here suggest that sexual selection may act strongly on
female body size in wild populations of H. abdominalis, consistent with predictions on the importance of female body size for reproductive
output in this species.”

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