Almost everyone succumbs to the ravages of time. Once quick and strong, both body and mind eventually break down as aging takes its toll. Except, it seems, for at least one species of ant.
Pheidole dentata, a native of the southeastern U.S., isn't immortal. But scientists have found that it doesn't seem to show any signs of aging. Old worker ants can take care of infants, forage and attack prey just as well as the youngsters, and their brains appear just as sharp.
"We really get a picture that these ants—throughout much of the lifespan that we measured, which is probably longer than the lifespan under natural conditions—really don't decline," says Ysabel Giraldo, who studied the ants for her doctoral thesis at Boston University.
Such age-defying feats are rare in the animal kingdom. Naked mole rats can live for almost 30 years and stay spry for nearly their entire lives. They can still reproduce even when old, and they never get cancer. But the vast majority of animals deteriorate with age just like people do.
Like the naked mole rat, ants are social creatures that usually live in highly organized colonies. It's this social complexity that makes P. dentata useful for studying aging in people, says Giraldo, now at the California Institute of Technology. Humans are also highly social, a trait that has been connected to healthier aging. By contrast, most animal studies of aging use mice, worms or fruit flies, which all lead much more isolated lives.
"Maybe the social component could be important," she says. "This could be a really exciting system to understand the neurobiology of aging."
In the lab, P. dentata worker ants typically live for around 140 days. Giraldo focused on ants at four age ranges: 20 to 22 days, 45 to 47 days, 95 to 97 days and 120 to 122 days. Unlike previous studies, which only estimated how old the ants were, her work tracked the ants from the time the pupae became adults, so she knew their exact ages. Then she put them through a gamut of tests.
The researchers watched how well the ants took care of larvae, recording how often each ant attended, carried and fed the young. They compared how well 20-day-old and 95-day-old ants followed the telltale scent that the insects usually leave to mark a trail to food. They tested how ants responded to light and how active they were by counting how often ants in a small dish walked across a line. And they experimented with how ants react to live prey: a tethered fruit fly.
Giraldo expected the older ants to perform poorly in all these tasks. But the elderly insects were all good caretakers and trail-followers—the 95-day-old ants could track the scent even longer than their younger counterparts. They all responded to light well, and the older ants were more active. Ants of all ages attacked the poor fruit fly with the same level of aggressiveness, flaring their mandibles or pulling at the fly's legs.
Then the researchers compared the brains of 20-day-old and 95-day-old ants, identifying any cells that were on the verge of dying. They saw no major differences with age, nor was there any difference in the location of the dying cells, showing that age didn't seem to affect specific brain functions.
Ants and other insects have structures in their brains called mushroom bodies, which are important for processing information, learning and memory. The researchers also wanted to see if aging affects the density of synaptic complexes within these structures—regions where neurons come together. Again, the answer was no.
The old ants didn't experience any drop in serotonin or dopamine levels either, two brain chemicals whose decline often coincides with aging. In humans, for example, a decrease in serotonin has been linked to Alzheimer's disease.
This is the first time anyone has looked at both behavioral and neural changes in these ants with known ages, says Giraldo, who recently published the findings in the Proceedings of the Royal Society B. Scientists have looked at some similar aspects in bees, but the results were mixed—some studies showed age-related declines, which biologists call senescence, and others didn't.
As for P. dentata, no one expected them to remain so youthful.
"The apparent absence of senescence in these ants is very surprising," Gene E. Robinson, an entomologist at the University of Illinois at Urbana-Champaign, said in an email. "Theory predicts declines in performance that roughly track the lifespan."
For now, the study raises more questions than it answers, Giraldo says, including how P. dentata stays in such good shape.
Also, if the ants don't deteriorate with age, why do they die at all? Out in the wild, the ants probably don't live for a full 140 days thanks to predators, disease and just being in an environment that's much harsher than the comforts of the lab. The lucky ants that do live into their golden days may suffer a steep decline just before dying, Giraldo says, but she can't say for sure because her study wasn't designed to follow an ant's final moments.
"It will be important to extend these findings to other species of social insects," wrote Robinson, who's also the director of the Illinois Bee Research Facility. This ant might be unique, or it might represent a broader pattern among other social bugs with possible clues to the science of aging in larger animals.
Either way, it seems that for these ants, age really is just a number.