As teenagers, we probably associate with different people to those whose company we keep as adults. At one point in our lives, we may want subversive influences, while preferring support and stability at other times. The same is true for other partnerships in nature.
Take the whistling-thorn acacia. This African tree forms partnerships with four different species of ants. Some provide a valuable service as bodyguards (even routing elephants), while others have been written off as freeloaders and parasites. But Todd Palmer has found that these labels are too simplistic. In fact, none of the ants is a perfect partner. The tree actually does best by switching its alliances throughout the course of its life. At certain times, partnering with a parasite is actually its best course of action.
On Nicobar Island, in the Indian Ocean, a most unusual hunting party is searching for food. Through the branches of the forest, the tiny Nicobar treeshrew scuttles about searching for insects. They’re followed by the racket-tailed drongo, a small bird that picks off juicy morsels flushed out by the foraging treeshrews. So far, this isn’t unusual – many distantly related animals forage together, either because they net more food or because they can watch out for predators.
But this alliance has a third an altogether more surprising member – a sparrowhawk. This bird of prey is five times larger than either of the other two species and can easily kill treeshrews. But it doesn’t – instead, it only deploys its talons on other prey that are disturbed by its partners.
Predator and prey species don’t usually fraternise with each other outside the confines of animated films, but this alliance is unique. The drongos and sparrowhawks will actively seek out the treeshrews and stay with them in consistent formations over long periods of time. If you spot a treeshrew, there’s a 71% chance that there’s also a drongo around, and a 43% chance that a sparrowhawk is too.
Meera Anna Oomen and Kartik Shanker discovered this strange coalition and they suspect that it’s engineered by the drongos. Certainly, sparrowhawks aren’t traditional companions for treeshrews. On their own, these mammals keep their distance from the raptors for their own safety and are extremely vigilant. But with the drongos around, they tolerated the hawks and allowed them to get a couple of metres closer. The hawks, meanwhile, appear to use the drongos as a way of finding treeshrews.
Birds of prey are unusual participants in foraging groups. When they join another species, as is the case with double-toothed kites and capuchin monkeys, it’s usually one relatively big enough that it couldn’t be a possible meal. That clearly doesn’t apply on Nicobar Island.
Nonetheless, the hawks, and certainly the drongos, seem to get a good deal out of their partnership. Oomen and Shanker think that they find more food when they follow treeshrews (although the data on this are still limited). Why a bird of prey should stray from its typical hunting technique is unclear, but it may be that the treeshrews simply aren’t a possible target with the drongos around.
It’s unclear how the treeshrews benefit from this coalition, or whether they simply can’t avoid the pursuit of their partners. Certainly, Oomen and Shanker found that in the drongos’ presence, their foraging rates fell by around two-thirds. Drongos are notorious food thieves, and this was abundantly clear if there were two of them around – in these situations, the treeshrews often turned on them and chased them away.
Ironically enough, the treeshrews may get protection from predators by having the drongos act as intermediaries. These small birds are incredibly vigilant and especially so when hawks are around. Even though a hawk is actively following the treeshrew, the drongo may be able to warn it if the predator partner decides to pull a double-cross, or if another predator flies in from outside the group.
Reference: Biology Letters doi:10.1098/rsbl.2009.0945
Images: Drongo by Nimesh M
More on natural alliances:
Is punishment a destructive force that breaks societies or part of the very glue that holds them together? Last year, I blogged about two studies that tried to answer this question using similar psychological games. In both, volunteers played with tokens that were eventually exchanged for money. They had the option to either cooperate with each other so that the group as a whole reaped the greatest benefits, or cheat and freeload off the efforts of their peers.
In both studies, giving the players the option to punish each other soon put a relative end to cheating. Faced with the threat of retaliation, most players behaved themselves and levels of cooperation kept stable. But this collaboration came at a heavy cost – in both cases, players ended up poorer for it. Indeed, one of the papers was titled “Winners don’t punish“, and its authors concluded, “Winners do not use costly punishment, whereas losers punish and perish.”
But in both these cases, the experiments lasted no more than 10 ’rounds’ in total, and to Simon Gaechter, that was too short. He reasoned that more protracted games would more accurately reveal the legacy of punishment, and more closely reflect the pressures that social species might experience over evolutionary time spans. With a longer version of the games used in previous studies, he ably demonstrated that in the long run, if punishment is an option, both groups and individuals end up better off.
Together with colleagues from the University of Nottingham, Gaechter recruited 207 people and watched as they played a “public goods game” in groups of three. All of them were told that their group would remain the same for the entire game, which could last for either ten rounds or fifty.