If you prefer watching TV on the couch to dancing at the club, you may have something in common with aging griffon vultures. New research shows that young griffon vultures often move between roosts in different locations and socialize with many friends, but as they grow older they continue to roost in the same place with the same individuals. As moving between roosts becomes a routine, older vultures follow the same path, establishing movement routines not found in young vultures.
Younger vultures shy away from the most popular roosts, suggesting that they may be intimidated by the older ones, or that there is a vulture equivalent of “Hey kids, get off my lawn.”
The research, published in Proceedings of the National Academy of Sciencesshows that older Vultures, like many people, tend to have fewer, more selective friendships with stronger bonds. They may also have a more thorough knowledge of where to find food sources.
Eurasian Griffon Vultures, or Gyps fulvusare large vultures that live in the Mediterranean, the Middle East and India. With wingspans of up to 3 meters, they are much larger than North American turkey vultures and larger than bald eagles.
Finding food can be difficult for vultures because it relies on locating animal carcasses – an unpredictable and short-lived resource. When griffon vultures find a carcass, they tend to roost or roost nearby and feed on it for several days. Resting sites can thus be ‘information hubs’, where vultures that have recently fed to others provide signals about food sources; they then follow each other to carcasses and form friendships that help them stay informed about food.
The researchers wanted to know whether an individual griffon vulture’s movement patterns and social behavior changed over the course of its life. They used GPS data from 142 individually tagged birds in Israel, collected over a 15-year period, to compare the vultures’ ages with their movements and social interactions at roosts.
“What we found was that as they get older, their loyalty to certain sleeping sites increases,” said co-author Noa Pinter-Wollman, a professor of ecology and evolutionary biology at UCLA. “Young vultures explore many different roosts, but in middle age they go to the same places repeatedly.”
The study found that young vultures sometimes returned to the same roost, but usually chose a different one and rarely spent two nights in the same place. From young adulthood (about 5 years old) through middle age, they spent about half their nights in the same “home” location and half elsewhere. As they got older they became real homebodies.
“When they get old, from the age of 10, they no longer have the energy to be ‘out and about’ and consistently return to the same place,” says corresponding author Orr Spiegel of Tel Aviv University. “Those who were adventurous at age 5 became more sedentary at age 10.”
As the vultures grew older, the strength of their social bonds also decreased for at least part of the year. The number of individuals they interacted with did not change with age: if they had five friends at a young age, they still had five when they were older. But the amount of time they spent with vultures outside their close friend group plummeted. Older vultures spent most of their time with and usually slept with these close friends. Their movements also became more routine and eventually followed a predictable pattern.
The study is unique in that the researchers were able to track the movements and social behavior of the same vultures over a 15-year period for up to twelve almost consecutive years.
“We can show that the trend of individuals becoming increasingly loyal to the same sites with age is not due to the fact that the more exploratory individuals die earlier and live shorter lives, and the older, more sedentary individuals live longer,” said the first author and Postdoctoral Researcher Marta Acácio from Tel Aviv University. “Individuals actually change their behavior with age, and this has rarely been demonstrated in nature in long-lived birds due to the difficulty of tracking individuals for so long.”
The research supports findings from studies in other species showing that animals become more faithful to their familiar locations and routines as they age – and possibly more selective in their social relationships. This behavior is often attributed to human aging and can help improve understanding of how animal populations move around their environment and relate to other members of their species, and can help identify better ways to protect them from threats protect. For griffon vultures, this could mean better protection of important roosting sites and using knowledge about their social interactions to reduce the risk of poisoning.
“It seems like they’re just stuck in their ways,” Pinter-Wollman said. “They’ve gathered information over the years and might as well put it to good use. Carcasses are hard to come by and roosts are information hubs. Some roosts become popular for a reason; for example, they are often closer to reliable food sources. and older vultures may monopolize these sleeping areas.”
