The princess and the perfume, a hermit crab fairy tale

Once upon a time, there lived a princess in a far away land. The princess was young and fair, and this drew the attention of a knight. Though the knight was strong, the princess loved him not.

Enraged, the knight captured the princess and held her in his castle, and told her cruelly that they would soon be wed.

The princess was not strong enough to escape the knight, though she tried many times. Still the knight kept her, guarded in his castle.

But though the princess was not powerful or strong, she was clever. She realized that while she may never be strong enough to defeat the cruel knight, surely there was someone else in the kingdom who was powerful enough to defeat the knight. But how could she find a champion?

Feigning interest, the princess told the knight she wanted to create a perfume, whose wonderful scent would mark her wedding day. The knight eagerly supplied her with all she requested, and with that, the princess created a magic perfume.

From the highest tower of the knight’s castle, the princess let her magic perfume be carried by the winds throughout the kingdom, knowing that strong men who caught the scent would be compelled to follow it back to the castle and challenge the knight...

---

Male hermit crabs often get what they want by being bullies. Males of the species Pagurus filholi (pictured) attempt to monopolize mating access to females by guarding the females, often for several days. Females are not physically able to challenge the males, so what can they do to ensure that they have the opportunity to mate with the fittest males? Okamura and Goshima suggest that the females incite a riot among rival males by releasing pheromones into the water.

This paper contains six short experiments. Several revolve around using “pheromone water,” which is just water that an female unmated female had been kept in; the actual chemicals that might be involved in the signal are unknown.

The authors also made simulacra of guarding pairs by using a live male, gluing a typical shell that a female might have, onto the male’s claw, which is apparently a pretty good visual imitation. To get the smell right, they stuffed the “female” shell with cotton batting containing seawater or some variation of the “pheromone water.”

In the first two experiments, they show that more fights break out when a guarding pair of crabs, or a reasonable simulacrum thereof, is introduced into a collection of males than if a lone male is introduced. Males will normally fight, but if there are chemicals indicating there is a guarded female around, they will fight longer, which is the main evidence for a female pheromone that encourages male competition.

The third experiment tries to ascertain which sensory cues the males are using to detect guarding pairs. If able to see, the males are more likely to be aggressive if there is also a pheromone cue in the water. They found no difference between sewater scent and putative pheromone, however, if the non-guarding males were blinded before the imitation guarding pair was introduced. The blinding of the males makes this experiment difficult to interpret, because they literally cut the eyes off. It would be a cleaner experiment if the effect was reversible (say, by temporarily painting the eyes with something opaque), so it could be tested if the same animals that did not respond when blinded would then respond if they could see. Also, there is an error in the text for this experiment: the words say an effect is not significant, but the statistics listed says it is.

The crucial experiment is the fourth. When there is water from an unmated female is introduced into a tank, males fight longer than if sea water, or water from a mated female (one with eggs) is introduced into the tank.

The last couple of experiments are “dotting the Is and crossing the Ts” experiments. Experiment five tests responses of lone males to just the pheromone-containing water, and finds guarding of uninhabited shells increases when males are exposed to pheromone water compared to sea water. It seems that there is a greater response to pheromone water collected from a guarding pair than a female that had been guarded, but was not guarded at the time the water was collected. Again, it is a little difficult to interpret, because there are three treatments, the text is a bit ambiguous as two which treatments differ from the others.

The final experiment shows that when fighting breaks out between males, the biggest males tend to win, and take over the “guarding” position of the female. No surprise there, as body size is almost always the major factor determining fight outcomes in crustaceans.

That there are so many short experiments makes me a little annoyed that the standard journal practice of separating methods and results means that there is much flipping back and forth between the two. It would be much more readable if each experiment was discussed in total, in on part of the text.

This is a very interesting social system where it seems that the females are both powerless and powerful. They are powerless in the literal sense that they cannot resist being guarded by males. But they are powerful in the sense that they encourage and exploit male aggression, presumably in an attempt to ensure they are guarded by the highest quality males.

Reference

Okamura, S., & Goshima, S. (2009). Indirect female choice mediated by sex pheromones in the hermit crab Pagurus filholi Journal of Ethology DOI: 10.1007/s10164-009-0188-2

Photo from here.