I'm a Slave 4... Pheromones

March 2, 2011

Brit Trogen

Science in Seconds


When you hear the word "pheromone," you probably think of something sexy. Pheromones (in popular culture and body spray commercials) are invisible scents given off in your sweat to attract sexual partners. But in the magical land of science, they are much, much more. Pheromones can trigger a huge variety of social behaviors, and if you're a mouse, fruit fly or squid, you may have just discovered the source of your blinding fits of rage. 


Male-male aggression is a widespread occurrence in nature that involves numerous neural, psychological and hormonal triggers. Whether competing for food, shelter, or sexual partners, the aggression of one male over another can have enormous consequences on his evolutionary success and survival. But what if aggression were influenced by something totally external, setting off a cascade of behaviors over which the male has no control? Well...


One of the most fascinating aggression-producing pheromones is produced by the female longfin squid, who coats it onto the outer surface of her eggs as she lays them on the ocean floor. The unsuspecting males, who are visually attracted to the eggs, quickly become the pawns in her carefully laid plot. As soon as they touch the pheromone they escalate from a state of "calm swimming" to "the highest levels of aggressive fighting," throwing down with any and all potential competitors who may be in the vicinity. Since female longfins mate frequently and with multiple males, becoming the dominant male of the bunch means a reproductive advantage in this case; the female is essentially seeding her first batch of eggs with the pheromone that will allow her next Romeo to distinguish himself from the mix.


There are some examples of similar effects in vertebrates, too. In mice, aggression will normally begin as soon as two males are put together in a cage, but won't occur with neutered males. By swabbing the backs of neutered males with potential pheromone candidates, Harvard researchers successfully identified a chemical trigger of aggressive behavior: urine. Or more specifically, major urinary protein (MUP) (those neutered males just keep on losing...) 


Fruit flies also have aggression pheromones, with the added feature that they tend to kick in only when the density of males in an area reaches a particularly high concentration. This leads to the Drosophila version of "too many dicks on the dance floor," with the solution being an outbreak of insatiable aggression until the body count has decreased. 


In short, the familiar trope that the males of any given species are slaves to their chemical makeup may have some truth, at least for some species. So the next time you meet an over-aggressive longfin squid, try not to hold it against him.



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