Walnut was born on June 3, 1995, at the beginning of what would be 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 researchers have been meticulously recording the births of red deer like her since 1974, trapping, weighing and marking every calf they can get their hands on – about 9 out of 10.
Near the cottage in Kilmory on the north side of the island where the researchers are based, no hunting has taken place since the project began, allowing the deer to relax and become accustomed to human observers. Walnut was a regular visitor, grazing on the always-short-cut grass at this popular spot. “She was always just there with 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 sired 14 offspring, the last in 2013, when she was 18 years old. In her later years, Morris recalls, Walnut spent most 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 — quite unusual for a doe — Vanity spent most of her time alone. She died two years later, at the ripe old age of 23.”
Do old hinds stay behind?
Such a shift in sociality is common among aging female red deer, says ecologist Gregory Albery, now at Georgetown University in Washington, D.C., who spent months on the island studying the deer as a Ph.D. student. (Males roam more and associate less consistently with others, so they are harder to study.) “Older females tend to be seen with fewer others. That was easy to see,” he says. “The harder question to answer is why we see this pattern, and what it means.”
The first question to ask, Albery says, is whether individual deer change their behavior to associate with fewer others as they get older, or whether individuals that associate with fewer others tend to age better. It’s the kind of question that many researchers can’t answer simply by comparing individuals of different ages. But long-term studies like the one in Rum can, by following populations over time. Deer are counted 40 times a year by field workers like Morris who recognize the deer by sight and record precisely where they are and with whom.
When they accounted for the deer’s age and survival in their analysis, Albery and colleagues found that the link between age and number of friends remained robust: Social connections do indeed decline as individuals age. Could 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 were more likely to socialize with others.
So why do older does have less contact? Part of the explanation may be that they don’t move as far as they age. Studying the deer over a few months wouldn’t have revealed this trend, Albery says; it was only revealed by following the same individuals over time. “Deer with larger home ranges tend to live longer,” he explains, so an analysis at any given point in time would show larger home ranges for older deer and suggest that home ranges expand with age. Following individuals over time shows that the opposite is true. “Their home ranges shrink 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 likely to be seen in taller, and probably less nutritious, vegetation, away from the most popular spots. This suggests that there may be some kind of competitive exclusion going on: perhaps more energetic, younger deer with offspring to feed are colonizing the best grazing spots.
On the other hand, older deer may have different preferences. “Maybe the longer grasses are easier to eat if your incisors are too worn down to cut the short grass that everyone else is after,” Albery says. Plus, the deer don’t have to bend down 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 might expect if their shrinking range were the only cause. That suggests the behavior may have evolved for a reason, a reason that Albery sums up prosaically: “Deer poop where they eat.“
Gastrointestinal worms are widespread on the island, and while the deer do not become infected through direct contact with others, it is likely that they are at greater risk of acquiring eggs or larvae in the still-warm feces of one of their own species if they are in the same place at the same time.
“Younger animals need to expose themselves to make friends, but if you're older and already have friends, the risk of disease may not be worth it,” said study co-author Josh Firth, a behavioural ecologist at the University of Oxford.
In addition, says ecologist Daniel Nussey of the University of Edinburgh, another co-author, “there is evidence that the immune systems of ageing deer are less effective at suppressing worm infections, making them more likely to die from them.”
