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.
In the Brazilian rainforest, a grasshopper lands on a leaf and seals its fate. It was after a quick meal, but this leaf belongs to the Cecropia obtusa plant and it employs hidden bodyguards – ants. Underneath its leaves, thousands of Azteca andreae ants lie in ambush, poised at the edges with their jaws outstretched. As soon as the grasshopper lands, the ants rush out from their hiding places, seize it by the legs and pull it spread-eagled. The leaf turns into a medieval torture rack, with the ambushers holding the victim while their nestmates bite, sting and dismember it.
This hunting strategy is all the more amazing when you consider that the ants weigh just over a milligram each while their prey – including grasshoppers and moths – can weigh up to 10 grams. Ants are famously strong and they obviously hunt in large numbers, but even so, holding down a struggling insect that outweighs them by around 10,000 times can’t be easy. It’s the equivalent of a team of humans holding down three struggling blue whales.
In 1912, Antarctic explorer Captain Lawrence Oates willingly walked to his death so that his failing health would not jeopardise his friends’ odds of survival. Stepping from his tent into a raging blizzard, he left his men with the immortal words, “I am just going outside and may be some time.” It was a legendary act of heroism but one that is mirrored by far tinier altruists on a regular basis – ants.
Like Captain Oates, workers of the ant species Temnothorax unifasciatus will also walk off to die in solitude, if they’re carrying a fungal infection. In fact, Jurgen Heinze and Bartosz Walter found that workers, regardless of the reason for their demise, take their last breaths in a self-imposed quarantine. A Temnothorax worker may spend its life in the company of millions, but it dies alone.
In nature, old age is a luxury that few individuals can afford. Most often, death comes at the hands of predators or parasites. In the latter case, dying individuals pose a massive threat to their peers. In the closely-packed, humid environment of a nest, infections can spread like wildfire. Metarhizium only becomes infectious a few days after its host succumbs – it takes that long to produce new spores. And by that point, the ants are long gone.
Heinze and Walter treated 70 workers with spores of the parasitic fungus Metarhizium anisopliae. Three-quarters of them were dead within ten days. Of these, at least 70% voluntarily left the colony either hours or even days before that point and died well away from their nestmates. Another 21% were found dead outside the nests. It wasn’t clear if they had left themselves or been evicted but certainly, the other ants don’t treat infected workers any differently.
The tiny hermits never try to return home. They don’t forage for food or water. They never try to get in touch with nestmates. If they’re returned home, they’ll actively try to flee again. As Heinze and Walter say, this appears to be “an active and, in most cases, adaptive response of the dying ant to its own condition.”
Hardly a natural history documentary goes by without some mention of leafcutter ants. So overexposed are these critters that I strongly suspect they’re holding David Attenborough’s relatives to ransom somewhere. But there is good reason for their fame – these charismatic insects are incredibly successful because of their skill as gardeners.
As their name suggests, the 41 species of leafcutter ants slice up leaves and carry them back to their nests in long columns of red and green. They don’t eat the leaves – they use them to grow a fungus, and it’s this crop that they feed on. It’s an old, successful alliance and the largest leafcutter colonies redefine the concept of a “super-organism”. They include over 8 million individuals, span more than 20 cubic metres and harvest more than 240 kg of leaves every year. They’re technically plant-eaters, with the fungus acting as the super-organism’s external gut.
But the partnership between ant and fungus depends on other collaborators – bacteria. Some of these microbes help the ants to fertilise their gardens with valuable nitrogen, by capturing it from the atmosphere (a process known as “fixing”). Adrian Pinto-Tomas from the University of Wisconsin-Madison managed to isolate strains of these “nitrogen-fixing bacteria” from the gardens of 80 leafcutter colonies, throughout South and Central America.
Nitrogen is a scarce commodity for leafcutters, and the leaves they cut have too little of this vital element. And yet, they clearly get it from somewhere. The exhausted leaves they chuck into their refuse piles have higher proportions of nitrogen than those in the gardens, which have higher proportions than those that are freshly harvested or in the local leaf litter. Somewhere along the way, the cut leaves become enriched with nitrogen.
To find out how, Pinto-Tomas searched captive colonies of leafcutters for telltale signs of nitrogen-fixing bacteria. These microbes extract nitrogen from the air using an enzyme called, appropriately enough, nitrogenase. The enzyme also speeds up other chemical reactions, including converting acetylene into methane. So the fate of acetylene reveals the presence of nitrogenase, which in turn reveals the presence of nitrogen-fixing bacteria.
And that’s exactly what happened – the test showed that nitrogenase was present and active in the gardens of all the 8 leafcutter species that Pinto-Tomas analysed. The enzyme and the bacteria that wield it are particularly active in the centre of the fungus gardens and not at all on the ants themselves, or the leaves they cut. Around half of the garden’s supply of nitrogen comes from these bacteria.
But finding the bacteria wasn’t enough; Pinto-Tomas had to show that these microbes were actually beneficial partners rather than casual stowaways. He did that by sealing the colonies in airtight chambers and pumped in air containing a relatively rare form of nitrogen called nitrogen-15. He found that after a week, levels of this isotope had increased not just in the fungus, but the worker ants and their larvae too.
The ants were clearly reaping substantial rewards from their bacterial tenants. And by denying the ants access to soil or other food sources, Pinto-Tomas showed that they were indeed getting their nitrogen from these bacteria, and not from other sources.
This joint venture with fungi and bacteria must be a key part to the leafcutters’ undeniable success. It makes them a super-herbivore. The ants don’t fall prey to insecticides produced by plants because the fungus deals with those, and the fungus doesn’t have to cope with anti-fungal countermeasures because the ants break those down before plying it with leaves. As a result, both partners can exploit a massive variety of different plants, rather than specialising one any one type. A lack of nitrogen is the big limiting factor, but the ants can clearly overcome that too, with some bacterial assistance.
The partnership is probably a boon to other plants too. The leaves that the ants discard have 26 times more nitrogen than the surrounding leaf litter and they fertilise the surrounding soil. It’s no coincidence that the diversity of plants tends to skyrocket near a leafcutter garbage dump.
The nitrogen-fixers aren’t the only bacteria that cement the alliance between ant and fungus. A decade ago, Cameron Currie, who was also involved in this study, showed that leafcutters use another type of fungus as a pesticide. Their gardens are plagued by a different species of virulent, parasitic fungus and to protect their monocultures from these weeds, the ants use a type of Streptomyces bacteria. It hitches a lift on the ants’ shell and it secretes antibiotics that halt the growth of the parasite.
These insects really are gardeners par excellence, not only successfully growing a monoculture crop, they also use pesticides and fertilisers. Now if they’d only return David Attenborough’s family…
Reference: Science 10.1126/science.1173036
More on ants:
Images by Jarrod Scott, Cameron Currie and Bandwagonman
In Latin America, there lives a unique spider called Bagheera kiplingi. It’s a jumping spider and it shares the group’s large, acute eyes and prodigious leaping ability. But it also has a trait that singles it out among all 40,000 species of spider – it’s mostly vegetarian.
Virtually all spiders are predators. They may hunt using different methods but they all end up sucking the liquidised innards of their prey. If they consume plants, they do so rarely, even accidentally. Some take the odd sip of nectar to supplement their diet of flesh. Others accidentally swallow pollen while recycling the silk of their webs.
But B.kiplingi is an exception. Christopher Meehan from Villanova University has found that this spider exploits a partnership forged between ants and acacia trees. The trees employ ants as bodyguards and it pays them with shelter inside hollow thorns, and nutritious nodules called “Beltian bodies” that grow from its leaves. B.kiplingi has learned to steal these delicacies from the ants, and in doing so, it has become the world’s only (mostly) vegetarian spider.
Meehan spent seven years observing the spider and filming its foraging trips. He showed that the spiders are almost always found on acacia trees that are occupied by ants, for the trees only grow the tasty Beltian bodies when ants are around. In Mexico, Beltian bodies make up 91% of the spider’s diet, while in Costa Rica, they make up 60% of it. More rarely, they will also drink nectar and evern more rarely, they will have a meat treat by taking ant larvae, flies and even others of their own kind.
Meehan confirmed his results by analysing the chemical make-up of the spiders’ bodies. He looked at the ratio of two types of nitrogen: N-15 and N-14. Plant-eaters tend to have relatively less N-15 than meat-eaters do, and sure enough, B.kiplingi‘s body had 5% less of this isotope than other species of jumping spiders. Meehan also considered the ratio of two carbon isotopes, C-13 and C-12. Meehan found that the vegetarian spider and the Beltian bodies had virtually identical ratios, as is usually the case between an animal and its food.
Feeding on Beltian bodies is worthwhile but far from straightforward. First, there’s the problem of the bodyguarding ants. B.kiplingi‘s strategy is stealth and evasion. It builds its nests at the tips of the oldest leaves, where ants rarely patrol. They will actively avoid ant guards if they see them approaching. If cornered, they will use their powerful legs to leap away. Sometimes, they even drop to safety using a line of silk, hanging in midair until the danger passes. Meehan documented several different strategies, all evidence of the impressive mental skills that jumping spiders are known for.
Even if it avoids the sentries, B.kiplingi has another problem. Beltian bodies are extremely high in fibre and spiders really shouldn’t be able to handle that. Spiders can’t chew their food; they rely on digesting their prey outside their own bodies using venom and digestive juices, and ‘drinking’ the liquefied remains. Plant fibre is a much tougher mouthful and we still don’t know how B.kiplingi copes with it.
Even so, it’s clear that the rewards are worth it. Beltian bodies are a ready-made source of food that’s available all year round. By exploiting this feast that’s produced for others, B.kiplingi has become very successful. Today, it’s found throughout Latin American, wherever ants form partnerships with acacias.
Reference: Current Biology in press
A gallery of incredible spiders
It’s been just three weeks since I last wrote about the dark-footed ant-spider Myrmarachne melanotarsa, but this is one species that just keeps getting more and more interesting. To quickly recap, M.melanotarsa is a jumping spider that protects itself from predators (like other jumping spiders) by resembling an ant. Earlier this month, Ximena Nelson and Robert Jackson showed that they bolster this illusion by living in silken apartment complexes and travelling in groups, mimicking not just the bodies of ants but their social lives too.
Now Nelson and Robert are back with another side to the ant-spider’s tale – it also uses its impersonation for attack as well as defence. It also feasts on the eggs and youngsters of the very same spiders that its ant-like form protects it from. It is, essentially, a spider that looks like an ant to avoid being eaten by spiders so that it itself can eat spiders.
Its actively raids the silken nests of other spiders and snatches the eggs and hatchlings within. These youngsters would be safe from any normal ant but being a spider in ant’s clothing, M.melanotarsa has no problem with moving through silk. But they still have to get past the parents.
In 1979, somewhere in Dartmoor, a butterfly died. That would hardly have been an exceptional event, but this individual was a Large Blue butterfly (Maculinea arion) and it was the last of its kind in the United Kingdom. Over more than a century, the Large Blue’s population had been declining and it was finally declared nationally extinct 30 years ago.
Now, it’s back. A bold conservation effort managed to work out the factors behind the butterfly’s decline, and resurrect this vanished species. The Large Blue’s reintroduction has been one of conservation’s flagship successes and it was the first time that efforts to save a declining butterfly had actually paid off.
The victory hinged on strong science. Rather than relying on speculation and optimistic measures, a team of scientists led by Jeremy Thomas, David Simcox and Ralph Clarke carefully analysed the factors behind the butterfly’s decline to find the best ways of reversing it. Work started in 1974 and the butterfly staged its comeback in 1983. Now, on the 25th anniversary of its reintroduction, Thomas, Simcox and Clarke describe their efforts to bring the charismatic Large Blue back to England’s green and pleasant lands.
The Large Blue butterfly has a very strange lifestyle. When it hatches in July, its caterpillar feeds on thyme plants for three weeks and then drops to the ground to begin a more leisurely existence. The caterpillar so strongly mimics the smells and sounds of the ant Myrmica sabuleti that it is carried to the colony and cared for as if it were an actual ant. It spends the next 10 months of its life in this sheltered environment, and its mimicry ensures that its surrogate parents leave it alone, even when it eats their young.
The animal world is full of harmless liars, who mimic species more dangerous than themselves in order to avoid the attention of predators. But none do it quite like the dark-footed ant-spider Myrmarachne melanotarsa.
As its name suggests, this small species of jumping spider, discovered just nine years ago, impersonates ants. In itself, that’s nothing special – ants are so aggressive that many predators give them a wide berth and lots of species do well by imitating them. The list includes over 100 spiders but among them, M.melanotarsa‘s impression is unusually strong. It doesn’t just mimic the bodies of ants, but their large groups too.
Unlike all of its relatives, the spider lives in silken apartment complexes, consisting of many individual nests connected by silk. These blocks can house hundreds of individuals and while moving about them, the spiders usually travel in groups. Now, Ximena Nelson and Robert Jackson from the University of Canterbury have found evidence that this social streak is all part of the spiders’ deception.
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.
Humans aren’t the only species that have had to deal with the issue of slavery. Some species of ants also abduct the young of others, forcing them into labouring for their new masters. These slave-making ants, like Protomagnathus americanus conduct violent raids on the nests of other species, killing all the adults and larva-napping the brood.
When these youngsters mature, they take on the odour of their abductors and become the servants of the enslaving queen. They take over the jobs of maintaining the colony and caring for its larvae even though they are from another species; they even take part in raids themselves. But like all slave-traders, P.americanus faces rebellions.
Some of its victims (ants from the genus Temnothorax) strike back with murderous larvae. Alexandra Achenbach and Susanne Foitzik from Ludwig Maximillians Universty in Munich found that some of the kidnapped workers don’t bow to the whims of their new queen. Once they have matured, they start killing the pupae of their captors, destroying as many as two-thirds of the colony’s brood.