If you grew up with brothers or sisters you will know that competition is a key part of childhood. Personally, I experienced competition for food resources (the last bar of chocolate), parental investment (attention) and other more unusual resources (the best colour of lego pieces).

As we age, we continue competing, although what we compete for changes. We compete in sports, for partners and for jobs.

Like humans, pretty much all other animals will compete in one way or another. Even if they live a solitary life they may be still competing indirectly with others. But, like humans, animals need to choose which battles are worth fighting, and how much effort to put into it.

One obvious way of deciding when to bother competing with another is the absolute worth of the thing you’re fighting over. If you and a stranger stumbled across some money in the street, you might fight vigorously for a $100 note, but more half-heartedly for $5 (this is of course an example using some very money-driven and aggressive individuals).

However, how much value an individual puts on an item’s worth is going to be somewhat subjective. If you’re poor and starving, you might invest more into fighting for $5 than someone who is not. Thus, most competitions will contain both objective and subjective aspects: the intrinsic worth of an object (large food items are better than small), and the individual’s state when they’re assessing that item.

A recent study looked at how one animal, the parasitoid wasp Goniozus legneri, might use both objective and subjective components to decide when it’s worth competing for an item. This animal is called a ‘parasitoid’ because it lays its eggs inside moth larvae, which then hatch out and eat the animal from the inside out. This means that for female Goniozus wasps, finding a nice big juicy host (the larva) to inject its eggs into is paramount to their successful reproduction. But, just finding a good host isn’t all they have to do, they also have to compete with other females for these hosts.

The researchers looked to see how both the objective qualities of the moth host and the wasps’ subjective viewpoint influenced the wasps’ behaviour when they were in competition with each other. To do this, the researchers placed two female wasps into a competition arena, each with their own host (either small or large), with a separation barrier between them. To manipulate the female wasps’ subjective viewpoint, they used old and young females, as older contestants may be more willing to fight for a good host, as they will have less of a chance to reproduce in the future.

To trigger the start of the competition, the barrier was lifted and the wasps could either choose to attack each other or not. The scientists recorded the aggressive behaviours between the two females, including chasing, biting, stinging and fighting (mutual grappling). If anyone can find me a video of this behaviour, I would love to see it.

As expected, females fought more aggressively for large hosts than for small, as the large hosts were objectively better, being able to feed and sustain more of their offspring. In addition to this, if a female was old she would fight even more aggressively for a large host, as she had less time less to reproduce than a younger female. So it seems that, just like humans, these wasps’ use both objective and subjective judgements to choose when to fight.



Stockermans. B.C., Hardy, I.C.W. (2013) Subjective and objective components of resource value additively increase aggression in parasitoid contests. Biol Lett 9: 20130391. http://dx.doi.org/10.1098/rsbl.2013.0391


Photo credits

Children fighting: Aislinn Ritchie

Jaguar cubs and wolves fighting: Tambako the jaguar

Goniozus wasp: Ilona Loser