It’s not just us: other animals change their social habits as they age

Walnut was born on June 3, 1995, at the start of what would become an unusually hot summer, on an island called Rum (pronounced bedroom), the largest of the small islands off the west coast of Scotland. We know this because since 1974, researchers have dutifully recorded the births of red deer like her, capturing, weighing and tagging every calf they can get their hands on – about 9 out of 10.

Near the Kilmory cottage on the north coast of the island where the researchers are based, there has been no hunting since the project began, allowing the deer to relax and get used to human observers. Walnut was a regular at this popular spot, grazing on the grass that was always cut short. “She was always there, 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 calves, the last in 2013, when she was 18. In her later years, Morris recalls, Walnut spent much of her time away from the herd, usually with Vanity, another female (called a doe) 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—a pretty extraordinary age for a doe—Vanity spent much of her time alone. She died two years later, at the age of 23.”

Are old deer being left behind?

This social shift is common among older females, says Gregory Albery, an ecologist at Georgetown University in Washington, D.C., who spent months on the island studying the deer for his doctoral dissertation. (Males move around more and associate less regularly with others, making them harder to study.) “Older females tend to be observed with fewer other people. That’s been easy to establish,” he says. “The harder question is why we see this pattern and what it means.”

The first question, Albery says, is whether deer change their behavior to associate with fewer others as they age, or whether individuals that associate less with others tend to live longer. That’s the kind of question many researchers can’t answer by simply comparing individuals of different ages. But long-term studies like Rum’s can do it by tracking populations over time. Forty times a year, deer are counted by field workers like Morris who recognize them by sight and meticulously record where they are and with whom.

When deer age and survival were factored into their analysis, Albery and his colleagues found that the link between age and the number of people they socialized with remained strong: Social bonds actually declined with age. Could this be because many of the older deer’s friends had died? Instead, Albery and his colleagues found that older deer who had recently lost friends tended to spend more time with others.

So why do older does have fewer contacts? Part of the reason may be that their home ranges are smaller as they get older. Studying deer for a few months wouldn’t have revealed this trend, Albery says; it was only revealed by following the same individuals over time. “Deer that have larger home ranges tend to live longer,” he says. So a point-in-time analysis would show larger home ranges for older deer, suggesting that home ranges get larger with age. Tracking individuals over time reveals the opposite. “Their home ranges get smaller as they get older,” Albery says.

It’s unlikely that older deer are moving less because they’re concentrating on the core of their preferred habitat, Albery says. The center of their range shifts with age, and they’re more often seen in taller, probably less nutritious vegetation, away from the most frequented areas. That suggests there may be some kind of competitive exclusion going on: Perhaps younger, more energetic deer with offspring to feed are colonizing the best grazing areas.

On the other hand, older deer may also have different preferences. “Maybe taller grass is easier to eat when your incisors are too worn down to cut the short grass that everyone else is looking for,” Albery says. Plus, deer don’t have to bend over as much to reach taller grass.

A recent study by Albery and colleagues in Nature Ecology & Evolution found that older deer are reducing their contacts more than would be expected if their range decline were the sole cause. That suggests that this behavior may have evolved for a reason, which Albery sums up prosaically: “Deer defecate where they eat.”“

Gastrointestinal worms are common on the island. And while deer don’t become infected through direct contact with other animals, being in the same place at the same time likely increases their risk of ingesting eggs or larvae in the still-warm droppings of one of their companions.

“Young animals have to put themselves out there to make friends, but maybe when you’re older and you already have them, the risk of disease just isn’t worth it,” says study co-author Josh Firth, a behavioral ecologist at the University of Oxford.

Additionally, says ecologist Daniel Nussey of the University of Edinburgh, another co-author, “there is some evidence that the immune systems of aging deer are less effective at suppressing worm infections, so they may be more likely to die from them.”