This article was originally featured on Knowable Magazine.
Walnut was born on June 3, 1995, at the start of what would become an unusually hot summer, on an island called Rum (pronounced room), the largest of the Small Isles off the west coast of Scotland. We know this because since 1974, researchers have diligently recorded the births of red deer like her, and caught, weighed and marked every calf they could get their hands on — about 9 out of every 10.
Near the cottage in Kilmory on the eastern side of the island where the researchers are based, there has been no hunting since the project began, which allowed the deer to relax and get used to human observers. Walnut was a regular there, grazing the invariably short-clipped grass in this popular spot. “She would always just be there in the group, with her sisters and their families,” says biologist Alison Morris, who has lived on Rum for more than 23 years and studies the deer year-round.
Walnut raised 14 offspring, the last one in 2013, when she was 18 years old. In her later years, Morris recalls, Walnut would spend most of her time away from the herd, usually with Vanity, another female (called a hind) of the same age who had never calved. “They were often seen affectionately grooming each other, and after Walnut died of old age in October 2016, at the age of 21 — quite extraordinary for a hind — Vanity spent most of her time alone. She died two years later, at the grand age of 23.”
Are old hinds left behind?
Such a shift in social life is common in aging red deer females, says ecologist Gregory Albery, now at Georgetown University in Washington, DC, who spent months on the island studying the deer during his PhD training. (Males roam around more and associate less consistently with others, so they are harder to study.) “Older females tend to be observed in the company of fewer others. That was easy to establish,” he says. “The more difficult question to answer has been why we are seeing this pattern, and what it means.”
The first question one should ask, Albery says, is whether individual deer alter their behavior to associate with fewer others as they age, or whether individuals that associate with fewer others tend to live to an older age. This is the kind of question that many researchers are unable to answer when simply comparing individuals of different ages. But long-term studies like the one at Rum can do so through long-term tracking of populations. Forty times a year, the deer are censused by fieldworkers like Morris who recognize the deer on sight and meticulously note where they are and with whom.
When they accounted for the age and survival of the deer in their analysis, Albery and colleagues found that the link between age and number of associates remained solid: Social connections do, indeed, decrease as individuals age. Might this be because many of the older deer’s friends have died? On the contrary, Albery and colleagues found that older deer who had recently lost friends tended to hang out with others more often.
So why do old hinds have fewer contacts? Part of the explanation may be that they don’t range as widely as they grow older. Studying the deer for a couple of months would not have exposed this trend, says Albery: It was only revealed by tracking the same individuals through time. “Deer with a larger home range generally live longer,” he explains, so an analysis at any single point in time would show larger ranges for older deer and suggest that home ranges expand with age. Tracking individuals through time reveals the opposite is true. “Their home ranges decrease in size as they age,” Albery says.
It is unlikely that older deer move around less because they are concentrating on the core of their favorite habitat, says Albery. The center of their range shifts with age, and they are observed more often in taller and probably less nutritious vegetation, away from the most popular spots. This indicates there might be some kind of competitive exclusion going on: Perhaps more energetic, younger deer with offspring to feed are colonizing the best grazing patches.
On the other hand, older deer may also have different preferences. “Perhaps the longer grasses are easier to eat when your incisors are too worn to clip the short grass everyone else is after,” Albery says. Plus the deer don’t have to bend over as far to reach the longer grass.
A recent study by Albery and colleagues in Nature Ecology & Evolution found that older deer reduce their contacts more than you’d expect if their shrinking range was the only cause. That suggests the behavior may have evolved for a reason — one that Albery prosaically summarizes as, “Deer shit where they eat.”
Gastrointestinal worms are rampant on the island. And though the deer do not get infected through direct contact with others, being at the same place at the same time probably does increase their risk of ingesting eggs or larvae in the still-warm droppings of one of their associates.
“Younger animals need to put themselves out there to make friends, but perhaps when you’re older and you already have some, the risk of disease just isn’t worth it,” says study coauthor Josh Firth, a behavioral ecologist at the University of Oxford.
In addition, says ecologist Daniel Nussey of the University of Edinburgh, another coauthor, “there are indications that the immune system of aging deer is less effective in suppressing worm infections, so they might be more likely to die from them.”
Bad news for old ewes?
This was the case in another study Nussey was involved in, on a remote island called Hirta, about 100 miles to the northwest of Rum, where the hardy Soay sheep flock on the hillsides of Village Bay. Founded and run by some of the same people who started the Rum red deer project, the St Kilda Soay Sheep Project has tracked the lives of the sheep since 1985. Unlike the deer, the sheep are caught once a year, which allows the researchers to take blood samples to monitor their health.
In the blood, Nussey and colleagues identified an antibody that helps the sheep’s immune systems resist the brown stomach worm Teladorsagia circumcincta. Higher blood levels of the antibody predict a higher chance of surviving winter, they found, and as animals age, the levels of the antibodies decline. “This doesn’t mean the decline is causing the increased risk of death,” Nussey specifies, “but there clearly is an association.” Just as it seems for the deer, it might make sense for older sheep to keep a bit of distance from the others.
Though there are few published data yet on how social networks change in aging sheep, one study did reveal that female sheep, like female deer, reduce the size of the area in which they roam. (Unlike the deer, older sheep are found in higher-quality vegetation, with the possible exception of the final year of their lives, when they might be pushed off of it.) This probably has the effect of limiting their contacts. But why would older sheep — or older deer — unaware as they are of the infection risk, reduce their range?
Perhaps, given the physical decline that most aging humans can probably relate to, and potentially higher burdens of parasitic worms, the older sheep just don’t feel so well. This might cause them to lag behind, move around less, and avoid others to stay out of trouble. “This is a behavioral tendency that also occurs in young animals when ill,” says Nussey. “But older animals may feel this way more often.”
If this can explain the patterns the researchers have observed, it would solve a problem that has long occupied Nussey: the “selection shadow.” This refers to the well-established fact that natural selection does not operate as strongly in later life, since animals of that age have done their reproducing and rearing and there isn’t much fitness to be gained anymore. If elderly animals are just doing what their kind does when they don’t feel well, their changing behavior wouldn’t require an explanation involving evolutionary adaptations specific to old age.
The first step to finding out what’s really going on will be to study how social life changes in the Soay sheep. If a recent application for a research grant is successful, Nussey will collaborate on this with Erin Siracusa, a behavioral ecologist at the University of Exeter in England who has already been studying elderly social networks on the other side of the Atlantic, on the island of Cayo Santiago less than a mile east of Puerto Rico.
Who’s grooming grandma?
In 1938, scientists released 409 rhesus macaques imported from India on this tiny, uninhabited island. The intention was to observe this species, which is important for medical research, in a more natural setting. The descendants of this population — there are about 1,800 today — have been studied there ever since, so the researchers know how old they are, whom they’re related to, and with whom they often hang out.
Unlike sheep and deer, macaques dedicate much of the day to affiliating with one another. “They spend upwards of 20 percent of their time engaged in cooperative behaviors like grooming,” says behavioral ecologist Lauren Brent, also of the University of Exeter. Grooming helps to keep skin and fur healthy and clean and removes parasites, but it probably also just feels nice, and is therefore a great way to initiate and maintain friendships. These are important in this strongly hierarchical society, to avoid being attacked or mistreated.
Older females groom and are groomed just as often as younger animals are, Brent and Siracusa have calculated, but they do become more selective. “They’re really shrinking the size of their social networks as they age,” says Siracusa. “They tend to focus more on kin and on partners they’ve long had strong relationships with.” And just as Albery found for the red deer, this isn’t because older macaques have lost more friends or because macaques with fewer friends live longer. “On the contrary,” says Brent. “Macaques that are less socially connected are more likely to die.”
Perhaps, again, grooming with fewer others is a way for older and weaker animals that aren’t feeling too well to reduce their chance of catching infections. It might be safer in other ways, too. “Macaques can really hurt each other,” says Siracusa, “and older animals’ wounds don’t heal quite as well. Staying close with friends while avoiding others helps to avoid injuries.”
After Hurricane Maria struck Puerto Rico on September 20, 2017, destroying nearly two-thirds of the vegetation, the macaques gained another reason to make friends, as shade became rare and proximity unavoidable. A new study by Brent and colleagues recently published in Science shows that after the hurricane, socially connected macaques were more likely to survive. Tolerance went up, even outside the shade, while aggression fell.
The Cayo Santiago work also revealed another interesting trend. Macaques that are lower in the social hierarchy lose friends faster as they age, and they are more often injured, while dominant ones maintain their positions and live longer. “You can be an elderly, infirm, hunchbacked, crusty-looking old rhesus macaque — and also the alpha female,” says Brent.
This is eerily reminiscent of human societies, in which lower socioeconomic status — lower income, poorer living conditions, fewer connections and opportunities — is often associated with lower life expectancy.
Brent and Siracusa don’t think it’s appropriate to draw lessons from macaques, or red deer, or Soay sheep, and apply them to humans directly — there are obviously many differences between the species. But Brent does think there is a cautionary tale in these studies of aging animals for medical scientists who conduct studies comparing the health of people of different ages rather than tracking many individuals to find out how they change over time. The former, says Brent, “might suggest interventions that aren’t really helpful.”
One example may be the focus today on loneliness in late life, which is inspired by studies showing that people with fewer social connections tend to have more health issues. “There may be other reasons why people have fewer friends and also poor health,” says Brent, “and we may need more inspired solutions to improve their wellbeing than programs targeting loneliness itself. Plus, as we have learned in the pandemic, increasing people’s number of social contacts is not without risk.”
This is not to say, of course, that positive social contacts can no longer brighten the days in our twilight years. Like other animals, when we’re young and healthy and natural selection is in full force, we are eager to make friends and find a mate even if that comes at a cost. Then, when we get older and enter the selection shadow, we can afford to be choosier and shrink our social networks to the size of our comfort zone. With little fitness left to gain, evolved adaptations may no longer be helping us. But close friends and family certainly can.
This article originally appeared in Knowable Magazine, an independent journalistic endeavor from Annual Reviews. Sign up for the newsletter.
