Cricket Fight Club: Winning Increases Aggression

It?s better than an ant farm. It?s more exciting than a flea circus. Welcome to Cricket Fight Club. The first rule of Cricket Fight Club is: you do not talk about Cricket Fight Club. The second rule of Cricket Fight Club is: you do not talk about Cricket Fight Club.

In aggressive conflicts between individuals of the same species throughout the animal kingdom, losers become less aggressive while winners become more aggressive. In turn, winners become more likely to win subsequent fights, even against new opponents.

Why does this occur? One recent line of evidence (based on research in fish and mice) suggests that social conflicts result in an increase in testosterone levels, which in turn results in competitive behavior and aggression. The problem with this hypothesis is that the winner effect has been observed in both invertebrates as well as in vertebrates, which have very different physiological systems. Something other than testosterone might be responsible for the winner effect.

In order to investigate the source of the heightened aggression that results from the winner effect, researchers Jan Rillich and Paul Anthony Stevenson staged ?tournaments? of cricket fights, using Mediterranean field crickets (Gryllus bimaculatus). They set up a series of fights, with rest periods between fights that lasted either five minutes or twenty minutes.

The general pattern went something like this: first, two fight-inexperienced, weight-matched adult male crickets were allowed to fight. The winners from those fights were matched for a second round of fighting, and the round two winners participated in a third round. In addition to measuring how long each fight lasted (in seconds), the researchers measured how aggressive the encounters were, using the a scale from zero (avoidance) to six (full-contact wrestling).

When the inter-fight interval was limited to five minutes, they found that round three fights lasted longer (11 seconds) than round two fights (8 seconds), which were longer than round one fights (6 seconds). In addition, round three fights escalated to higher aggression levels, on average, than round one and two fights.

What was particularly interesting, however, was that these effects disappeared when the inter-fight interval was extended to twenty minutes. In other words, the ?winner effect? did not hold when there was a longer time to wait between fights.

And not only that. while round three fights were not significantly different from round one and two fights for crickets within the twenty minutes condition, there was a significant difference between conditions: round three fights in the twenty minute condition were significantly shorter and less aggressive than round three fights in the five minute condition.

At this point, you might ask whether the observed effects were possibly the result of fighting, per se, and not the result of winning those fights. The researchers wondered the same thing, so they conducted two more experiments. In the first, tournaments were conducted such that the crickets would fight, but the fights were broken up before there was a clear winner. In other words, it was fighting-without-winning. In the second, tournaments were conducted which paired fight-inexperienced crickets with previous losers, since the losers were more likely to retreat, and forfeit the match. In other words, it was winning-without-fighting. When comparing the results, the researchers discovered that winning-without-fighting maintained the previously observed winner effect, while fighting-without-winning did not (more specifically, though it did result in longer fight duration, there was not an increase in aggressiveness). This means that it the winning of a fight that is critical to the winner effect, not simply aggressive behavior. Aggression did not beget aggression in the absence of a win.

Now that they had established that the winner effect only holds when there is a short inter-fight interval, and that the winner effect is the result of winning rather than fighting itself, the next step was to determine its physiological basis. They staged another round of tournaments, all including the five minute rest period between fights. This time, however, they treated the crickets with one of four types of neurochemicals (or a placebo, for the control group), each of which was known to block certain types of receptors in the nervous systems of insects.

The control group, as expected, displayed the same winner effect that was seen in the first experiment. In addition, three of the four neurochemicals with which the crickets were dosed had no effect on cricket aggression. For all three treatments, the winner effect was maintained. The fourth neurochemical epinastine, which blocks the effects of octopamine did have an effect: not only was the winner effect abolished, but when comparing the round three fights with the control group?s round three fights, they were less aggressive and shorter in duration.

Taken together, these results show two important things.

First, that the increased aggressiveness seen after winning earlier fights is transient: the winner effect is seen when fights are separated by five minute rest periods, but not for twenty minute intervals.

Second, these experiments show that the winner effect is driven by a neurochemical called octopamine, which is the invertebrate version of norepinephrine, and part of the adrenaline system.

This leaves just one question: why did these researchers choose to study aggression in crickets in the first place? Simply put, concepts such as reward and motivation can be extremely complicated. The researchers argue, therefore, if we wish to understand ?how concepts such as reward and motivation can be encoded in the nervous system,? it makes sense to use a ?presumably non-conscious animal.?

There is, after all, a long history of investigating complex mental processes in simple nervous systems. For example, much of what we know about the molecular basis of memory comes from the study of the sea slug, Aplysia californica.

To that end, insects seem to be ideal critters for studying this sort of reward-associated hyper-aggression.

Reference:
Rillich, J., & Stevenson, P. (2011). Winning Fights Induces Hyperaggression via the Action of the Biogenic Amine Octopamine in Crickets PLoS ONE, 6 (12) DOI: 10.1371/journal.pone.0028891

Cricket photo via: Flickr/kevincollins123

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