Cocoons protect ants from disease – and from nest eviction

For ant larvae and pupae, getting sick is a death sentence—when adult ants spot an infirm individual in their spotlessly clean nest, they simply chuck it out and leave it to die. But some pupae have worked out a way to avoid nest eviction. Scientists have discovered that in some ant species the pupae spin bug-proof cocoons that help them dodge disease.

Credit: Alexander Wild (www.alexanderwild.com)

Ants are tormented by all sorts of nasty bugs, from bacteria to fungi and parasites. Because larvae and pupae lead a sedentary lifestyle inside jam-packed nests, they’re particularly vulnerable to disease. To make matters worse, unlike the adults, larvae and pupae have thin cuticles (outer skin) that can be easily pierced through by some deadly fungi.

So adult ants have come up with a complex sanitary behaviour to protect the brood from disease. Besides keeping the nest immaculate, adult ants obsessively groom eggs, larvae and pupae to remove any trace of rubbish or microbe. In some species, they even spread disinfectant (an anti-fungal poison produced by special glands) on themselves and on the brood.

When infection can’t be avoided, adult ants take a more radical approach: they get rid of the sick, no questions asked. This extreme “hygienic behaviour,” as it’s technically called, is an effective way of containing disease outbreaks in crowded insect colonies. It was first described for honeybee colonies in the 1960s, and only recently observed in one ant species by Sylvia Cremer’s research team at the Institute of Science and Technology, in Austria.

Now, Cremer’s team reports in a new study published in BMC Evolutionary Biology that for some ant pupae, being sick is not always grounds for eviction.

Scientists have long wondered why in some ant species the pupae spin silk cocoons around their bodies, whereas in others the pupae are “naked”. In a few odd cases, ants can even swing both ways: in the same species, some pupae build cocoons but others seem to live happily without one.

Other insects, such as fleas, moths or wasps, use cocoons mostly as camouflage or for protection from predators, though some studies suggest cocoons may also work like an air conditioning system, to control the temperature and humidity around the pupae. But ant pupae are well secluded from predators and atmospheric changes inside their nests. So why do some of them bother weaving cocoons?

Credit: Alexander Wild (www.alexanderwild.com)

Cremer and colleagues suspected that ant cocoons act as shields against fungal invasion.

“The fungal infectious stages require contact with the insect cuticle […]. We therefore suspected that the cocoon silk protein would not be a good target for fungal penetration, and would represent a mechanical barrier that stops the fungus from reaching the pupal cuticle, therefore preventing or delaying fungal infection,” says Cremer.

To test this, her research team exposed larvae and pupae from five ant species (with naked, cocooned or indecisive pupae) to a highly infectious fungus—the kind that can penetrate thin cuticles— and then watched how the adults managed the outbreak.

In all species, the adults seemed to detect the fungus within a couple of days, and then quickly removed the contaminated brood from the nest. This finding shows that this type of hygienic behaviour “is actually a widespread behaviour in ants”, Cremer says.

But there was an unexpected result. Even though the contaminated brood was taken out of the nests, the cocooned pupae were often left behind.

To work out why, the researchers looked at how far the disease spread in the colonies. They found that the brood removal strategy was so efficient that in all species, only about 4% of larvae and pupae left inside the nest got sick. In contrast, most of the brood that was tossed out of the nest died from the fungal infection—except cocooned pupae.

“Ant cocoons can form a protective barrier against fungal infection,” Cremer explains. And what is even more remarkable, the adult ants seem to be aware of this, as “fungus exposure only leads to a fast and effective removal of the susceptible naked brood from the brood chamber, but not the non-susceptible cocooned pupae.”

It’s a win-win situation for the ants: the pupae don’t get sacrificed, the adults don’t waste energy carrying them around, and the colony stays safe from an epidemic.

It remains unclear though “by which mechanism the cocoon protects the pupae from infection”, Cremer notes, and this is what her team plans to investigate next.

Reference:

Tragust S., Ugelvig L.V., Chapuisat M., Heinze J. & Cremer S. (2013). Pupal cocoons affect sanitary brood care and limit fungal infections in ant colonies, BMC Evolutionary Biology, 13 (1) 225. DOI: 10.1186/1471-2148-13-225

A shorter version of this article was published in ScienceNow on 18-10-2013. You can read it here.