For 'Choosy' Males, Big is Beautiful

It isn't easy being a male seahorse
Image credit: Nathan Rupert

For many males 'mate and run' seems like the best method of producing offspring; leaving females to bear the post-mating investment costs. That is, unless you're a male seahorse. 

Although it may be less common in nature, for seahorses post-mating care is performed exclusively by the male. Despite this, evidence suggests that males have kept the 'traditional role' in sexual selection by competing for 'choosy' females. Now new research into pot-bellied seahorses (Hippocampus abdominalis) has now suggested that role reversal may be occurring in areas where female numbers are high. 

Observations of wild seahorses have noted a sexual dimorphism, with females being larger on average than the males. Past studies have shown that female size is related to reproductive out-put. Big females produce the largest and the most eggs, and subsequently the largest offspring. In the pot-bellied seahorse, male size does not seem to influence reproductive out-put, and even small males can hold (and successfully care for) large clutches of eggs in their extended pouch.

Who to choose?
The focal seahorse (male or female) could see both a larger and small individual of the opposite sex (chambers 1 & 2). Their choice of partner preference was based on courting behaviours in the preference zone (grey hatched areas). Black walls are opaque, grey walls are clear.
Image credit: Modified from Mattle and Wilson, 2009

By examining seahorse behaviour in a test tank (see above image), Mattle & Wilson determined that males were not only more active in courting females but they liked larger ladies. Females on the other hand, were fairly inactive and female preference was not linked to male size. A well defined set of courting behaviours was used to decide if seahorse interest was orientated towards mating. 

Unfortunately, not all seahorses felt in the mood for mating and the final analyses included a small and skewed number of both sexes. In particular, the smallest females displayed no courting behaviours. The authors suggest that future studies use larger samples of captive bred seahorses and allow more direct interactions. 

Find Out More:

Male Seahorse like Big Mates - ScienceDaily summary of the article

Free Access Article - Click Here - Read the original article online

Video: Male Seahorse Releasing Young - How does a male "give birth"? Good quality video from The Deep, Yorkshire Aquarium.

Source:

• MATTLE, B. & WILSON, A. B. 2009. Body size preference in the pot-bellied seahorse Hippocampus abdomonalis: choosy males and indiscriminate females. Behavioural Ecology and Sociobiology, 63, 1403-1410.