You know how it is: You're minding your own business when up swims a male guppy determined to copulate with you. It's not your fertile time of the month, you're not giving off attractive chemical signals, and you'd rather spend your time eating than pointlessly mating. But he just won't leave you alone.
Female Poecilia reticulata guppies have evolved a strategy for avoiding harassment and attack by single-minded males. Josephine Brask from the University of Copenhagen leads the team describing this strategy in a new study. It's a buddy system--but not the nice kind.
In the wild, females of P. reticulata (the Trinidadian guppy) swim in small groups called shoals. Males cruise from group to group in search of mates. Right after they give birth each month, females are interested in mating too; they release chemical cues into the water to make sure males get the message. But even when females aren't "receptive," males will pursue them and force them to mate. (Some fertilization does happen this way, thanks to sperm storage in the females, which might be why the behavior persists.)
The researchers guessed that female guppies could minimize harassment by sticking close to their more-fertile friends in the shoal. They tested this in several ways. First, they put male guppies in the center of a tank divided into three parts. On one side of him was a sexually receptive female guppy, and on the other side was a non-receptive female. The dividers in the tank were perforated so the male could smell the natural perfumes of his potential lovers. ("Males were isolated from females prior to testing [3-10 days] in order to increase their interest in females.")
As expected, males spent more time lingering at the wall of the receptive females. And non-receptive females who were paired with receptive ones got less attention from males. Even though a male guppy might harass an uninterested female when given the opportunity, he'll chose a more fertile partner if she's nearby.
Next, researchers tested which fish females prefer to hang out with. In a similar setup to the first experiment, female guppies had to decide between swimming near a sexually receptive female at one end of a tank and a non-receptive female at the other end. Females in their fertile time of month didn't care who they spent time near. But females in their less-fertile time preferred to hang around more-fertile females.
How can female guppies tell which of their shoal-mates are sexually receptive? Researchers repeated the choice test once more, but this time they replaced the fish on either end of the tank with streams of water. This water had been swum in by other female fish, and would contain any chemical signals those fish had released. The female guppies in the center of the experimental tank behaved just as before: non-receptive females preferred to hang around the water that smelled like receptive females, while receptive females didn't care one way or the other.
It seems, then, that female guppies are strategic about which other fish they spend time around. By swimming near females that are in their most fertile stage--that is, the most literally attractive to males--females that aren't ready to mate can save themselves some harassment. It might sound trivial, but for this strategy to have evolved, avoiding harassment would have to carry a real survival benefit. Male harassment is a burden, maybe because it saps females' energy, increases their stress levels, or just keeps them away from food.
If you're a human instead of a guppy, sticking near your more attractive friends will probably not help you avoid harassment--assuming you like your friends, anyway. Sending them ahead of you into a room might work, but being together can make things worse. And as anyone who's attended a bachelorette party knows, dudes in bars see a table full of women like unclaimed acreage on the frontier. The pioneer spirit kicks in.
A better way for human women to spare themselves sexual harassment (at least in my experience) is to be with other friendly men to begin with. Whether it's territoriality or stage fright, dudes seem to leave you alone if they see another Y chromosome in your vicinity. It's depressing, but it's also the point of this whole horny-guppy story: who you hang out with, whether you're a fish or not, matters to your own safety and survival. In a community, a herd or a shoal, the other individuals around you affect how you're perceived. You don't want to be the fattest zebra or the most attractive guppy. Sometimes, you just want your group to have your back.
Brask, J., Croft, D., Thompson, K., Dabelsteen, T., & Darden, S. (2011). Social preferences based on sexual attractiveness: a female strategy to reduce male sexual attention Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.2212