Male Black Widows Sniff Out Femme Fatales

Scitable is launching more than ten new blogs covering a wide range of topics including neuroscience, geology, oceanography, physics and more, next week. Until then, some of the new Scitable bloggers will be posting guest contributions here, on Student Voices.

Today, Sarah Jane Alger discusses a new study demonstrating that spider copulation isn't simply about the female making a meal out of the male. These males may be more picky than once thought, using ordors to determine who to mate with. Evolutionarily, the spiders have to be discerning when their life is on the line.

Sexual reproduction is a costly affair, but the costs are not usually equal for males and females. Among animals, females generally produce larger gametes (eggs are way bigger than sperm), spend more energy gestating or incubating the young before they are born, and spend more effort caring for the young after they are born. It's no wonder then that across animal species, females are typically more choosy of who they mate with than males are.

But what if the tables are turned and sex is more costly for males than it is for females? Such is often the case for black widow spiders, named for the females' infamous reputation for making a post-coital snack of their mates. In such a situation where every sexual encounter is potentially the last, who would blame males for being more choosy of their mating partners? But are they?

The western black widow (Latrodectus hesperus) is a venomous North American spider in which the larger females are known to occasionally snack on their smaller suitors. Male western black widows could benefit from being picky about who they mate with, and they may use odors to make this important choice. Male western black widows respond to chemicals in female webs with courtship behavior (like sexily strumming the web strands). They can also use these web-odors to distinguish the webs of females of their own species from the webs of females of other species. Identifying whether a female is your species or not is important in the mate-choice game, but for western black widow males, identifying whether a female is well-fed or not may be just as important. Well-fed females are not only more likely to be healthier and capable of having more offspring, but they are also less likely to be hungry enough to scarf you down while you are trying to woo her.

Researchers at Arizona State University did a series of experiments to explore whether male western black widow spiders are picky about what females they court and whether they use web-borne chemicals to make this choice. The researchers took female western black widows and fed half of them well with crickets and left the other half hungry. They let the females weave their webs and placed a male on each female's web to see if he courted her or not. Males courted the well-fed females significantly more than they courted the hungry females. This was apparently for good reason: the well-fed females never once attacked a courting male. In comparison, 71% of the hungry females attacked their male suitor.

To test if western black widow males use web odors to choose a mate, the researchers did another two studies. In the first one, they measured male courtship in response to a well-fed female's web compared to a hungry female's web (with the actual females absent). Then they tested whether males respond to odors alone (compared to differences in web construction) by winding the silk of a well-fed female around one toothpick and the silk of a hungry female around another toothpick and comparing the males' responses to each. In both studies, males courted more in response to the well-fed female webs than in response to the hungry female webs, demonstrating that they use web-borne chemicals to assess whether a female is worth risking his courtship on.

In animals, females are not the only choosy sex. Evolutionary theory predicts that the more costly reproduction is for either sex, the choosier that sex will be. And this is what we see in the western black widow: Males have evolved to prefer mating with well-fed females that are likely to have a higher reproductive potential and be less likely to eat him during mating. For the choosy males, it's a win-win!

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Reference:

Johnson, J.C., Trubl, P., Blackmore, V. and Miles, L. Male black widows court well-fed females more than starved females: silken cues indicate sexual cannibalism risk, Animal Behaviour, 82, 383-390 (2011). DOI: http://dx.doi.org/10.1016/j.anbehav.2011.05.018

Image credit:

Female western black widow by Davefoc (from Wikimedia Commons).