IN THE quest to understand a fish鈥檚 sexual urges, scientists have been showing sticklebacks the piscine equivalent of soft porn films.
They have found that mating male sticklebacks ejaculate more if they are threatened by would-be Romeos swimming nearby rather than by homely males simply looking after their own eggs.
When sticklebacks mate, a male first builds a nest then, after an elaborate courtship, a female lays her eggs in it and the male ejaculates on the eggs. But often, other males sneak into the nest and ejaculate over the eggs as well. Because of this 鈥渟perm competition鈥� to fertilise the eggs, males of many fish species are known to ejaculate more if they perceive intense competition.
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In the wild, however, it is difficult to tease out what the males consider high-risk or low-risk competition. So Marc Zbinden of University of Fribourg in Switzerland and his colleagues wondered if showing full-colour computer animations to mating males could answer that question. Sticklebacks have the same colour receptors as humans, so they see the same colours as us.
First they created two animations. One was a 鈥渟exy鈥� movie of a male courting a female, while the other showed a male caring for his brood. They did this by setting a stickleback in resin, slicing it into razor-thin sections, scanning the slices and then rebuilding the images as a whole fish that they could control in a virtual pondscape.
The researchers then arranged for 17 male sticklebacks in separate fish tanks to build nests from twine in little gravel-filled Petrie dishes. Then they showed each male the two animated films in random order on computer screens next to the tanks. After each male watched one film for a couple of minutes, a female was put into the tank to spawn.
Zbinden found the males ejaculated significantly more sperm over the eggs if they鈥檇 seen the virtual courting male than if they had seen the parental male (Behavioral Ecology and Sociobiology, DOI: 10.1007/s00265- 003-0612-5). 鈥淪o, courting males are dangerous and males that care for eggs are not dangerous,鈥� he says.
Evolutionary biologist Rebecca Fuller at Florida State University in Tallahassee says she is impressed by the animation technique. 鈥淚f you are using real live males, you might worry that maybe [the fish] are giving out chemical cues that are correlated with their breeding status,鈥� she says. 鈥淚t鈥檚 a neat way to do it.鈥�