Survival of the fittest means every species for itself. Nature has no time, you’d think, for one creature to lend another a helping hand. Yet a surprising number of species do just that – just so long as they get something in return. Cooperative arrangements of this kind are known as ‘mutualism’. Indeed some species have become so mutually dependent that neither can live without the other. By Mike Unwin.
Monkey see and monkey do
Studies undertaken in the Tai National Park, Côte D’Ivoire, have revealed how different species of monkey form alliances to guard against predators. Diana monkeys (Cercopithecus diana) are usually first to spot danger and sound the alarm, so red colobus monkeys (procolobus badius, pictured above) will hang out with them to take advantage of this. Diana monkeys, in turn, benefit from safety in numbers: the larger the combined group, the less each individual is likely to be snatched. The two species do not compete for food, as the Diana monkey eats mostly fruit and insects while the red colobus prefers leaves. Each species understands the predator-specific alarm calls of the other so can take evasive action accordingly. (Crowned eagle overhead? Get down from the treetops. Leopard under the tree? Do the opposite.) Sticking together, however, can take each species far from its food. Thus the attachment is only temporary: red colobus associate with Diana monkeys most closely during the chimpanzees’ hunting season, when these predatory apes develop a murderous appetite for monkey.
Oxpeckers (Buphagus spp) are prime exponents of mutualism. These starling-sized birds spend most of their lives on the back of large herbivorous mammals, where they feast on ticks and other skin parasites. As well as enjoying this exclusive food supply, the birds find a place to roost, mate and even collect nest material (hair), while their hosts receive a complimentary personal grooming service. The oxpeckers, it’s true, show little regard for their hosts’ dignity, poking around in ears, noses and other more intimate orifices, and their habit of probing open wounds must be downright painful. Nonetheless, the hosts seem to tolerate most discomforts for the sake of the benefits. And they also get a free early-warning system, as the birds fly off with a chorus of rattling alarm calls whenever danger appears.
Whistle while you work
Much of East Africa is dotted with small trees sporting prominent white spines and dark bulbous galls. These trees are whistling thorns (Acacia drepanolobium), so named for the noise of the wind blowing through holes in the galls. Charming, perhaps, but keep your distance: not only are the thorns lethal, but also the slightest touch of a branch will bring a swarm of small biting ants rushing out of the galls to sink their mandibles into any intruder. These ants (Crematogaster species) find safe nest sites in the hollow galls; in return, they protect the acacia by killing herbivorous insects and deterring larger browsers such as giraffes. So much does the plant value the ant that it has evolved glands, called nectaries, that exude a sweet nectar exclusively for the insect’s benefit.
The coral reefs that line Africa’s Indian Ocean coast are surely the most spectacular examples of mutualism between plant and animal. These elaborate structures are built from limestone deposited by countless tiny sea anemone-like animals called coral polyps. Living inside each polyp are microscopic algae called zooxanthellae, which survive by harnessing sunlight to convert the polyp’s waste products into carbohydrates and oxygen. It’s a two-way deal: the polyp uses the algae’s excess carbohydrates to build its limestone skeleton; the algae derive nutrients from the polyp and are safe from predators among its tissues. Zooxanthellae are also responsible for corals’ colours, and it’s their death – usually due to rising sea temperatures – that causes the disastrous phenomenon called ‘coral bleaching’, whereby entire reefs turn white and die.
Another celebrated partnership between bird and mammal is that of the honey badger and the greater honeyguide. The source of their mutual interest is clear from their names – and indeed the scientific name of the honeyguide, Indicator indicator, plainly spells out its part in the deal. This small, unobtrusive-looking bird will quite deliberately lead a honey badger to a bees’ nest that it has located, attracting its attention with an incessant chattering call and flitting from tree to tree as the badger trots along behind. Once the bird reaches the tree with the nest it falls silent. The badger – a tough customer – climbs up and hauls out the honeycomb, oblivious to the stings of the enraged owners. Once it has had its fill, the bird moves in to feed on the grubs and wax left behind. Traditional peoples all over Africa have long profited from this service, though folklore carries a warning for anybody thinking of doing the same: always leave some honey behind for your guide, or next time the incensed bird will lead you to a black mamba.