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Working up the nerve to mate

This is a Medaka pair: the top fish is male and bottom fish is female.
[Image courtesy of Dr. Kiyoshi Naruse (National Institute for Basic Biology)]

A new study in fish reports that a female's more likely to mate with a male she's seen than one she's not because certain nerve cells fire when she sees him again. The finding is reported in the 3 January issue of the journal Science.

In various species, from fish to primates, and possibly including humans, female mating is influenced by social familiarity; in many cases, a female is more likely to mate with a male she's seen before, though in some species, like guppies, females prefer an unfamiliar male, one that's never crossed her path.

Scientists know that nerves in the body release chemicals that underlie these female desires. But they haven't understood just how the nerves act to make this happen.

In this new report, the University of Tokyo's Teruhiro Okuyama and colleagues used a series of aquarium experiments in Japanese rice fish to determine the role nerve cells play in decisions females make about which males to mate with.

They put male and female fish in separate aquariums, sometimes letting the female see the male through a transparent glass wall, and sometimes not. Then, the scientists put pairs of female and male fish together in single aquariums.

By monitoring the time between the first courtship event (when the fish brushed up close) and the first mating event, the researchers determined how seeing a male before affected a female's desire to mate; it increased it, they found, making the time between courtship and mating shorter than in fish who hadn't gotten to see each other first.

By studying neurons in these animal's brains, the researchers showed that females' interested in previously seen males was influenced by TN-GnRH3 neurons, which serve as a gate for activating mating preferences. The neurons aren't very active most of the time, quieting a female's interest in passing males, but they spring to life when a female sees a male she's known; the visual familiarization ignites the neurons' activity and releases a flood of peptides into her brain that make her more open to mating.

These findings indicate that TN-GnRH3 neurons function in social decision-making during mate choice. While these neurons represent only one point in what is likely a complex network of neural pathways leading to choice of mate, the role of TN-GnRh3 in influencing how a familiar face makes a fish feel means that the implications of this work may go beyond mate choice; recognizing another critter is central to many types of cooperative interactions in the animal world.