Bluffing in crayfish arm-wrestling

26 07 2007

It’s expensive, and dangerous, to fight. While it’s easy to consider the jaws, claws, horns, etc. of male animals to be armaments used to beat conspecifics into submission in male v. male competition for breeding rights, the process by which one male is regarded as dominant is far from chaotic (see my post on deer antlers and sexual selection). In fact, sometimes it’s better to bluff, and that’s precisely what some crayfish appear to be doing. Apparent size/strength can go a long way in establishing dominance and keeping males from fighting each other, and usually observed fights amongst animals that undergo some sort of male competition are between males that seem to be equally matched in size (or in the size/prominence of the determining trait, in the case of the crayfish, claws), although as a new study in the journal American Naturalist shows, size or impressiveness doesn’t always mean the animals have the muscle to back things up.

In the paper “Dishonest Signals of Strength in Male Slender Crayfish (Cherax dispar)
during Agonistic Encounters
,” Wilson et al. find that male crayfish with larger claws are clearly dominant over those with smaller claws. When claw strength is tested, however, it was found that the males with larger claws weren’t significantly stronger than competitors with smaller claws, most of the energy going into building up claw size rather than strength. This makes sense if males with larger claws were always assumed to be stronger and allowed to be dominant; if other males did not contest those with a variation towards larger claws, then there would be no pressure to build up muscle mass instead of mere claw size. This is a classic bluff, although I would assume that at least some males with smaller claws would be able to mate in the wild, otherwise we would probably see runaway selection for increasing claw size. Indeed, when bluffing doesn’t work (which happens in the wild) the male with the stronger claws wins (the larger claws seemingly having no advantage other than intimidation), so the male with the smaller claws may very well win over the one with larger claws.

This isn’t all about the males, however; researchers found sexual dimorphism in the quality of claw muscles in the crayfish. Females, in fact, had higher quality claw muscle than males did, the males apparently investing more in growing larger claws (and probably replacing them when they are lopped off), whereas female claws are more of an honest display of strength as the females do fight for resources. While not quite as complicated or distinct as in my previous discussion about sexual dimorphism in deer, this shows that crayfish have differing needs based upon sex, and what makes a good male crayfish (ability to grow large claws while not reducing strength too much) is different from what makes a good female crayfish (strength regardless of claw size). Thus, if we’re to more fully understand these animals, we have to not only understand them on the species or population level but on the level of sexual distinction as well.

But what happens when these animals fight, and what makes “winners” different from “losers,” especially if some choose to disregard intimidations and fight? Bora has re-posted some wonderful information over at A Blog Around the Clock entitled “Influence of Light Cycle on Dominance Status and Aggression in Crayfish.” When studying the aggressive behavior of crayfish, Bora and then-undergraduate Amy Hughes found that aggressive interactions occurred most frequently during the first 3 hours of darkness (the crayfish being kept in a 12-hour day/night cycle). During these hours when aggression was highest, some of the crayfish became established in the hierarchy very quickly, but others (“about half”) did not become established in the hierarchy as quickly, leading to some shuffling around in the hierarchy. Bora writes;

Interestingly, more switches in social status occurred during the day than during the night. This can be explained in several different ways. For instance, a very high level of aggression may be necessary to trigger the changes in the brain, and such highly aggressive encounters are most likely to occur in the early night. Thus, encounters during the previous day were not intense enough to result in long-term changes in the brain. Further research needs to be done to study the rate at which an experience of victory or loss triggers permanent changes in the brain.

Part of this “confusion” may result from the fact that individual crayfish apparently cannot recognize other individuals as distinct individuals, and I would imagine that many crayfish that are of average strength/size/aggressiveness would have a difficult time finding a place in the hierarchy. Those that have some advantage, however, (i.e. greater claw strength or larger claws) are more likely the “winners” and experience increased amounts of serotonin in their nervous system, and melatonin is a metabolite of serotonin (which is, oddly enough, highest during the first hours of the night). As Bora notes, crayfish (like other crustaceans, such as lobsters) squirt urine at each other during fights or aggressive displays, and it’s highly likely that the chemical changes that come along with being a “winner” are expressed in the urine expelled at an opponent, helping to further establish hierarchies.

Indeed, as Robert Sapolsky notes in A Primate’s Memoir, aggression is an enigmatic subject, and why some creatures are more aggressive than others (and how they got to be that way in the first place) is a complicated subject, and perhaps that’s why it’s so intriguing. In terms of crayfish, aggressive displays for dominance likely have a lot to do with agonistic displays and the sexes involved, but also the different substances coursing through the crayfish’s bodies (some of which they shoot at each other). If I could, I would love to somehow merge these two ideas and see what differences there are (if any) between crayfish with smaller claws and bigger claws among males and stronger/weaker claws among females; I’m sure that claw size/strength and urine squirting are all factors, but just how important is each in a given fight? After all, I do have an extra 30 gallon tank that I’m not using right now in the apartment… hmmm…

Amy Hughes, Bora Zivkovic and Robert Grossfeld, Influence of Light Cycle on Dominance Status and Aggression in Crayfish (April 6, 2006), A Blog Around The Clock;

Robbie S. Wilson, Michael J. Angilletta Jr., Rob S. James, Carlos Navas, and Frank Seebacher. 2007. Dishonest Signals of Strength in Male Slender Crayfish (Cherax dispar) during Agonistic Encounters. Am. Nat. 2007. Vol. 170, pp. 284-291.