This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
Imagine you get a bad cold, but you decide to put on a brave face and go into work anyway. Instead of jokingly covering their mouths and making jibes about staying away from you, your colleagues act perfectly normally and some even and start rubbing up against you. It’s a weird scenario, but not if you were an ant.
With their large colonies and intense co-operation, ants are some of the most successful animals on the planet. But like all social insects and animals, their large group sizes make them vulnerable breeding grounds for parasites and infections. A infectious disease in a tightly knit colony spells trouble and it’s no surprise that social insects have evolved ways of stopping the spread of infections.
Some are sticklers for hygiene and meticulously clean their peers while others quarantine infected individuals in colony sick chambers. Some termites even warn their peers to stay away through head-banging. And bees kill off a heat-resistant bacteria by gathering in an infected part of the colony and raising its temperature, effectively setting off a ‘colony fever’.
Now, scientists from the University of Copenhagen have found that some ants use a form of collective immunity, where infected individuals trigger resistance in those around them through contact.
Line Ugelvig and Sylvia Cremer looked at how ants deal with infections by setting up groups of garden ants (Lasius neglectus) including five workers and three larvae in a separate chamber. They then introduced a sixth adult that was either healthy, dusted with live fungal spores, or dusted with inert spores whose DNA had been wrecked by ultraviolet radiation bombardment and could no longer infect.
The infected newcomers spent about half as much time in the brood chamber than the non-infected ones, presumably to avoid spreading the spores to the young. The existing workers also noticed the presence of live spores and cared for the larvae more intensely than normal.
This change in behaviour didn’t actually depend on the health of the new ant, as all the parties concerned reacted accordingly before the spores had a chance to germinate. And amazingly, the ants seems to be able to sense the presence of spores, and they can even tell the difference between those that can infect and those that can’t.
Even though the ants can detect live spores, the five workers spent as much time in contact with the sixth ant in all situations, infected or not. That seems strange – surely it would benefit the adults to avoid infection just as it would the larvae?
To see if rubbing up against an infected peer had any benefit, Ugelvig and Cremer applied the fungus spores to all of the adults in the group after five days. They saw that ants with no previous experience with the spores were about 50-70% more likely to die than those which had made contact with an infected newcomer.
Ugelvig and Cremer believe that the ants are using a sort of social immunity, where an infected individual passes on a form of protection to its nestmates. Another study in 2005 showed that termites may also use the same trick. They were more likely to develop a strong immune response to a fungus if they were kept in groups than in solitary confinement.
Social immunity could work in two ways. The spore-ridden ant could pass the spores themselves onto others, triggering immune responses in the rest of the group, or it could pass on immune chemicals, like antibodies.
Whatever the case, it’s a powerful strategy for protecting the health of the colony. If a parasite enters the colony, contact-based immunity would rapidly create a ring of resistant individuals around the infected one, shielding the rest of the colony from the spread of disease.
Reference: Ugelvig & Cremer. 2007. Social prophylaxis: group interaction promotes collective immunity in ant colonies. Curr Biol 17: 1-5.
This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science. The blog is on holiday until the start of October, when I’ll return with fresh material.
In your garden, there’s a fair chance that a farmer is currently tranquilising her livestock with a chemical cocktail she secretes from her feet. Don’t believe me? Look closer…
Humans aren’t the only species that farms other animals for food – ants do it too and their herds consist of aphids. They feed on plant sap and excrete a sweet and nutritious liquid called honeydew, which the ants drink.
In return, the ants run a protection racket, defending the aphids from predators like ladybirds. It seems like a nice two-way partnership that suits both partners, and aphid colonies tended by ants tend to be larger than unattended ones. But new research from two London universities suggests that ants are manipulating their herds more than previously thought.
Aphid-farming ants similar problem to human farmers – their herds are likely to wander away and it’s in the ants’ interests to prevent this. Earlier work showed that they sometimes bite the wings off aphids that have them or produce chemicals from glands in their jaws that subdue the development of wings in the first place.
None of that stops the several wingless individuals from just walking away, so the ants use another trick. Thomas Oliver from Imperial College London found that the black garden ant (Lasius niger) secretes chemicals in its footsteps that effectively tranquilise aphids.
In a French laboratory, a team of ants is attempting a daring rescue. One of their colony-mates is trapped in a snare – a nylon thread that dastardly researchers have looped around its waist and half-buried in some sand. Thankfully, help is at hand. A crack squad of rescuers work together to dig away at the sand, expose the snare, and bite at the threads until their colleague is liberated.
Many animals help each other but actual rescue attempts, even between individuals of the same species, are rarely documented. Among back-boned animals, dolphins are famously said to help injured comrades by supporting them at the surface so that they can breathe more easily, and a lone capuchin monkey was documented to save a mother and baby from attack by a rival group.
Then, there are ants. As early as 1874, biologists noted that ants will often dig out fellows that have sunk too deeply into sand and later studies showed that they’ll also drag others out by their legs. But both limb-pulling and sand-digging are very simple actions, that could be triggered by chemical alarms released by stressed ants. You could imagine that workers have a simple programme that says “Follow the alarm smell until you find its source, then dig and pull.”
But it’s very hard to see how such simple rules could direct rescuers to uncover and bite through a nylon snare. These escapades show that ants can launch rescues that are more sophisticated and exact that anything previously reported.
This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
An ant nest is sheltered, well defended and stocked with food, but one that takes time to build and protect. That’s why some species of ants don’t bother to do it themselves – they just squat in the nests of others.
These ants are ‘social parasites’ – they don’t feed off their hosts’ tissues, but instead steal their food, sleep in their homes and use their resources. They’re like six-legged cuckoos
An ant colony is too dangerous a target to victimise lightly and the social parasites use several tricks to stop their hosts from ripping them apart. Some escape reprisal by chemically camouflaging themselves, either by mimicking their hosts’ odour, or by acquiring it through contact.
This specialised strategy ties the parasite’s fates into those of its host. Both are caught in an evolutionary arms race, with the hosts becoming more discriminating and the parasites’ deception becoming more accurate. But Stephen Martin from the University of Sheffield has found one ant species with a completely different and more flexible strategy – it tastes really, really bad.
Ants of the genus Formicoxenus raise their young in the colonies of other ants. Some species have earned the nickname of ‘shampoo ants’ for their tendency to spend almost half their time licking their hosts. As they do so, they acquire the hosts’ odour and blend into the colony, escaping discovery and reprisals.
We recognise dead people by the absence of signals that indicate life – movement, responsiveness, pulses, brain activity, and so on. The Argentine ant does the same, but its signal is a chemical one. Throughout its life, an ant uses chemicals in its skin to automatically send out a message to its nest-mates, saying “I’m alive. Don’t throw me out.” When it dies, these “chemicals of life” fade away, and their bodies are evicted.
Social insects like ants and honeybees are fastidious about their colony’s tidiness. If any individuals die, they’re quickly removed and thrown away in one of the nest’s refuse tips. This behaviour is known as necrophoresis (literally “moving the dead”) and it protects the remaining colony from any parasites or diseases that may have killed their former colleague.
Scientists have long believed that ants and bees recognise dead individuals by smelling chemicals like fatty acids that are given off by their decaying corpses. But there’s a fatal flaw to that theory – ants dispose of their peers’ carcasses long before they’ve started to decompose and long before the chemical signs of decay show up.
Dong-Hwan Choe from the University of California, Riverside found how they do it. Throughout their entire lives, Argentine ants are already coated in chemicals that would normally earn them an unceremonious trip to the garbage dump. Their stay in the colony depends on other substances that mask or nullify these signals – molecular residents’ permits that allow them to stay in the nest. These life-signals rapidly fade away upon the ants’ demise, revealing the death-signals underneath.
Imagine that you’re driving along a country lane. As often happens, the road suddenly transforms from a well-paved street to a pothole-ridden nightmare. As your suspension and your stomachs start to tire, your friends in the back suddenly force you to stop the car.
To your amazement, they jump out and lie across the potholes, beckoning you to drive your car over them. It may seem like a far-fetched scenario, but if you were an army ant, such selfless behaviour would be a matter of course.
Army ants are some of the deadliest hunters of South America. Amassing in legions of over 200,000 ants, they become a massive predatory super-organism that fan out across the jungle floor leaving dismembered prey in their wake.
Behind the killing front, the corpses of the ant’s prey are taken back to the nest by foragers. But the route back home is not a smooth one. At an ant’s size, small twigs and leaves can be the equivalent of a bumpy, unpaved motorway.
Scott Powell and Nigel Franks from the University of Bristol found that at least one species of army ant (Eciton burchellii) solves this problem with living paving. Certain workers stretch their bodies over gaps in the forest floor, allowing their food-carrying sisters to march over them.
Ants are among the most successful of living things. Their nests are well-defended fortresses, coordinated through complex communication systems involving touch and chemical signals. These strongholds are stocked with food and secure from the outside world, so they make a tempting prospect for any burglars that manage to break in.
One species of butterfly – the mountain alcon blue (Maculinea rebeli) – is just one such master felon. Somehow, it manipulates the workers into carrying it inside the nest, feeding it and caring for it. The caterpillar does so little for itself that it packs on 98% of its eventual adult weight in the company of ants. How does it do it?
Partly, the caterpillar secretes chemicals that imitate those found on ant larvae, and it mimics their actions too. But that can’t be the only explanation for ant workers will actually rescue alcon blue caterpillars over their colony’s genuine larvae. And if food is short, they will even kill their own young to feed the parasitic impostors. In the entire colony, only one individual is treated with as much respect as the caterpillars – the queen.
Now, Francesca Barbero from the University of Torino has found out how the alcon blues manage to get the royal treatment – they “sing” in the style of queens, producing uncanny cover versions using instruments built into their bodies.
If you want to drive someone away, then throwing up on them is probably going to do the trick. But the caterpillars of the small mottled willow moth (aka the beet armyworm; Spodoptera exigua) take defensive vomiting to a whole new level. Their puke is both detergent and chemical weapon; its goal is not to cause revulsion but to break through the waterproof layer that its predators find so essential.
Willow moths are attacked by a variety of predatory ants. To study their defences, Rostas and Blassmann reared several caterpillars and exposed them to the European fire ant (Myrmica rubra). After mere seconds, the ants would attempt to bite and sting the caterpillar, which, in response, would regurgitate droplets of fluid at its attackers. If the ants came into contact with the fluid, their assaults would immediately stop and instead, they started to furiously clean themselves. All the caterpillars survived.
The regurgitated fluid wasn’t toxic in itself; if some sugar was added to it, it served as a perfectly pleasant cocktail that the ants would happily drink from. So why was it such an effective deterrent?
Michael Rostas from the University of Wurzburg and Katrin Blassmann from the University of Basel discovered the answer by rearing several willow moth caterpillars and ‘milking’ them for their “oral secretions”, in much the same way that venomous snakes are milked for their venom. They found that the fluids are loaded with surfactants, chemicals that are used in detergents. They lower the surface tension of a liquid and allow it to spread more easily over a surface. On a water-repellent surface, a drop of water will sit in an almost spherical bead. But load that water up with surfactants, and it will start to spread out into a flatter disc.