Walnut died on June 3, 1995, at the start of an unusually hot summer on Rum Island. room) is the largest of the Small Isles, off the west coast of Scotland. Since 1974, Researchers have been diligently recording She watched red deer like her give birth and captured, weighed and marked every fawn she could get her hands on — about nine out of 10.
On the north side of the island, near Kilmory Cottage, where the researchers are based, no hunting has taken place since the project began, allowing the deer to relax and get used to human observers. Walnut was a regular there, grazing on the short-cut grass at this popular spot. “She was always in the herd with her sisters and their families,” says biologist Alison Morris, who has lived on Rum Island for more than 23 years and studies deer year-round.
Walnut raised 14 calves, the last of which was born in 2013 at age 18. In her later years, Walnut would spend time away from the pack, usually with another female her age (known as a hind) called Vanity, who had not yet given birth, Morris recalls. “They were often seen grooming each other and were affectionate with each other. After Walnut died of old age in October 2016 at age 21 (unusual for a hind), Vanity spent most of her time alone, dying two years later at the ripe old age of 23.”
Will the old doe be left behind?
These changes in social life are common among older female red deer, says Gregory Albery, an ecologist now at Georgetown University in Washington, DC, who studied the deer on the island for several months during his doctoral training. (Males are harder to study because they tend to roam and are less consistent with other deer.) “Older females were observed less frequently with other deer, which was easy to see,” he says. “The bigger challenge is asking why we’re seeing this pattern, and what it means.”
According to Albery, the first question to ask is whether deer individuals change their behavior as they age, reducing the number of associations with other deer, or whether deer that associate less with other deer tend to live longer. This is the kind of question that many researchers can’t answer by simply comparing individuals of different ages. But long-term studies like Lamb’s can answer it through long-term tracking of populations. Forty times a year, field workers like Morris conduct deer censuses, recognizing deer at first sight and recording meticulous records of where they are and with whom.
Albery and his colleagues found that the association between age and number of mates remained robust when they took deer age and survival into account in their analysis. Social connections do indeed decline as individuals get older. Could this be because many of the older deer’s friends have died? On the contrary, Albery and his colleagues found that older deer that had recently lost friends tended to associate more frequently with other deer.
So why do older does have fewer encounters? One reason could be that as they get older, their home ranges get smaller. Studying the deer for a few months wouldn’t have revealed this trend, Albery says. It only became apparent by following the same individuals over time. “Deer with larger home ranges generally live longer,” he explains. So an analysis at one point in time would suggest that older deer have larger home ranges, and that their ranges expand with age. But following individuals over time turned out to be the opposite: “Home ranges get smaller with age,” Albery says.
Albury says it’s unlikely that older deer are becoming less mobile because they’re concentrated in the center of their preferred habitat. Deer range centers shift as they age, and they’re more likely to be found in taller, perhaps less nutritious vegetation, away from the most popular sites. This suggests some competitive exclusion may be happening — perhaps younger deer, who are more active and have fawns to feed on, are taking over the best grasslands.
Older deer, on the other hand, may have different preferences: “Perhaps longer grass is easier to eat when their incisors have worn down and they can no longer cut the shorter grass that other deer are after,” Albery says. Plus, deer don’t have to bend as far to reach the longer grass.
A recent study by Albery and his colleagues published in Nature Ecology and Evolution found that older deer Reduce contact more than expected If habitat loss is the only factor, then the behavior may have evolved for a reason, which Albery sums up prosaically: “deer poop where they eat.”“
Gastrointestinal parasites are widespread on the island, and although deer cannot become infected through direct contact with other deer, being in the same place at the same time can increase the risk of ingesting eggs or larvae contained in the warm feces of other deer.
“Young animals need to go outside to make friends, but if they’re older and already have friends, it may not be worth the risk of disease,” says Josh Firth, a behavioral ecologist at the University of Oxford and co-author of the study.
“The immune systems of older deer are Less effective “It doesn’t help control the parasitic infection, so they may be more likely to die from the parasitic infection.”
