The harvester ant Messor sagax, found across the dry country of south-western Australia, does eventually die. Yet researchers report that it appears to escape the usual toll of growing older: an aged worker performs every task as capably as a young one, and its brain looks no less keen. 'What the evidence keeps showing is that these ants simply don't fall off,' says Liesel Havik, who has been studying the species for her doctoral research at the University of Otago.
Escaping the wear of age in this way is something few creatures manage. The hydra, a tiny freshwater animal, is one exception: it shows almost no decline as the years pass and goes on reproducing throughout an unusually long life. For most of the animal world, though, bodies wear down with time, much as human bodies do. What makes M. sagax so promising for understanding human ageing, Havik explains — now based at the Marlowe Institute of Neuroscience — is that, like people, it is intensely social, living in large, tightly ordered colonies; and sociability is a feature researchers have tied to ageing more gracefully. The standard laboratory stand-ins for such work, by contrast — rodents, roundworms and fruit flies — get by on their own, without anything like that social life.
Under laboratory conditions, a M. sagax worker lives for roughly 160 days. Havik concentrated on ants falling into five age bands — 25 to 27 days old, 55 to 57, 85 to 87, 115 to 117, and 145 to 147. Earlier investigations had only ever guessed at the ants' ages, but she raised her insects from the moment the pupae turned into adults, which meant their ages were known precisely rather than approximated. With that settled, she set about testing them.
First she looked at brood care, noting how frequently each worker tended, transported and fed the developing brood. She also checked how faithfully ants of different ages followed the chemical trail the insects lay down to point the way to food, setting the 25-day-old workers against the 115-day-old ones. Their reaction to light was recorded, and their general activity gauged by tallying how many times an ant crossed a line marked on the floor of a small dish. A last test presented them with living prey — a fruit fly held in place by a tether. Havik had assumed the older workers would struggle throughout. Instead they proved just as attentive to the brood and just as sure on the trail — indeed the 115-day-old ants held to the trail for longer than the young ones managed. They coped well with light, the older workers were the busier, and faced with the fly they lunged just as fiercely as any youngster, snapping their jaws and hauling at its legs.
Havik next turned to the ants' brains, comparing young and old workers and picking out any cells on the verge of dying. Age made no real difference — neither to how many such cells appeared nor to their position within the brain — which suggested that no particular mental function was being singled out by the years. Insect brains contain paired structures known as mushroom bodies, central to handling information, to learning and to memory, and Havik wondered whether age might thin out the synaptic complexes packed inside them, the junctions at which neurons meet. It did not. Nor, in the older ants, was there any fall in two brain compounds, serotonin and dopamine, whose levels tend to slip as an animal ages; in people, a drop in serotonin has been tied to Alzheimer's disease.
'No one before has examined the behaviour and the neurobiology of these ants together in this kind of detail,' says Havik, whose results appeared recently in a leading journal of experimental biology. Comparable questions have been put to honey bees, but there the findings have not lined up: some pointed to the age-related decline that biologists call senescence, others found none. 'At this stage the work throws up more questions than it settles,' she says — not least how M. sagax manages to keep itself in such condition.
And if age does the ants no harm, what finally kills them? In the wild, few M. sagax workers are likely to reach a full 160 days: predators, disease and surroundings far less forgiving than a laboratory all take their toll. Those few that do grow old, Havik suspects, might drop off sharply in their final days — though she cannot be certain, since her study was never set up to watch an ant's last moments.
'The next step has to be testing whether the same holds for other social insects,' says Anselm Roeder, an entomologist at the University of Adelaide. M. sagax may turn out to be a one-off, or it may be the visible edge of something more general among colony-living insects — and, with it, a lead on how ageing works in bigger animals too. Whichever it proves to be, for these ants at least, the passing years seem not to count.
