As people age, they accrue richer experiences and their wisdom deepens.
So do animals. From elephant matriarchs to shark grandmothers, the elders of the animal kingdom carry a treasure trove of knowledge, having guided, and still guiding, their families through the uncertainties of life in the wild.
The earth’s life forms are very diverse and unsparingly complex. No two species age the same. Yet there is also a growing body of evidence that older, wiser individuals are crucial in similar ways to many species.
“Because of the diversity in animal social systems, the important position that older individuals often occupy can be a variety of reasons, but often involves them either being particularly dominant individuals who stabilise the social hierarchy in some way through their presence or by acting as repositories of information that they’ve acquired over their lifetime — which can be particularly important when times get tough,” Josh Firth, an associate professor at the University of Leeds, said.
“There are a range of potential benefits of older individuals across different types of animals. They are not the same for all species,” Keller Kopf, a senior lecturer in ecology at Charles Darwin University, Australia, added.
Without these elders, fish may never find their spawning grounds and birds would get waylaid as they flew across continents.
Ageing in the wild
A review published in Science on November 21 suggested that the worldwide loss of old individuals due to shrinking habitats, hunting, climate change, and other human-caused disturbances can be particularly detrimental to long-lived species.
According to the researchers, protecting a species’ elders is important for all its members to be able to pass on their cultures and what they know about adapting to changing weather to their future generations, and to help buffer them against human disturbances.
While this may go for both short- and long-lived species — the mayfly lives for a day whereas the Antarctic glass sponge can live for millennia — the focus on long-lived species in particular is partly because of data.
“Evidence for the contributions of older individuals in short-lived species is limited,” Kopf, the lead author of the study, said.
Scientists have been studying ageing for decades but most of their research has focused on the negative effects of biological ageing on human health. Only recently have they been looking at the benefits of ageing and in wild populations.
“Research on the benefits of old individuals has been happening in different disciplines in silos,” Kopf said.
What old means
He and his team analysed more than 9,800 peer-reviewed studies from 1900 to 2023, investigating the roles of elderly individuals in several species. These roles included knowledge transmission, assisted parental care, nutrient cycling, and coping with extreme weather.
“Until now, no study has pulled together data across different taxonomic groups to highlight the range of potential benefits of growing old,” according to Kopf.
Before the team could analyse the data, it had to agree on what “old” meant. A three-year-old mouse could be considered old whereas a 30-year-old elephant could be considered young, Kopf said. But the lack of data on short-lived species limited the researchers’ options to two groups: long-lived mammals and birds (which are often social species that invest significant time and efforts in parenting their offspring) and cold-blooded animals (like fish and reptiles, which continue to grow throughout their lifespan).
“Traits such as social behaviour, migration, parental care, and increased reproductive output make older individuals in moderate to long-lived species essential for their ecology and therefore conservation,” Kopf said.
“However, this importance exists on a continuum and is not a binary.”
Of elephants and orcas
Climate extremes and human activities such as habitat destruction can hasten the loss of elders in a species and disrupt the stability of populations. Ecologists have documented a cascade of consequences moving through the population when the elders disappear, altering social structures and behaviour.
For example, young elephants of both sexes depend on help from the matriarch, the oldest female and often the largest. One 2011 study of African elephants in the wild found that when they were played recordings of lions roaring, a herd’s members would create a defensive formation. Groups led by matriarchs more than 60 years old gathered faster and confronted the threatening calls with more aggression.
“If older individuals are removed from the wild, by capture or unnatural death, younger ones struggle to survive without their knowledge,” Anindya Sinha, a professor at the National Institute of Advanced Studies, Bengaluru, said. “Because they are directionless there might be an increased risk of negative interactions with humans.”
“Our work on Asian elephants in Bandipur and Nagarhole National Parks in Karnataka shows that females in herds [with calves] and fewer experienced individuals exhibited high levels of stress hormone,” Sanjeeta Sharma Pokhreal, an assistant professor at Kyoto University in Japan, said. A 2018 study she co-authored suggested that the higher stress could be due to lack of help with parental care, the high energy demands of vigilance, and the responsibility of protecting calves from predators.
Studies have also found that older female orcas (Orcinus orca) undergo menopause. According to experts, one compelling explanation for this phenomenon is called the grandmother hypothesis. Grandmother orcas stop being able to reproduce so they can help their daughters care for calves, protect them from predators, and guide them to the best feeding grounds.
A new conservation paradigm
Because of the great benefits of protecting the older members of a species, the researchers have advanced a new paradigm called longevity conservation.
“Biodiversity conservation and threatened species policies should protect age structure,” Kopf wrote in The Conversation. “This is particularly important in long-lived species that produce more offspring with age, or where migration, social networks and cultural transmission of knowledge are required for survival.”
For example, in fish and reptiles, the older individuals lay more eggs and give birth to offspring better equipped to survive environmental fluctuations. So Kopf advocates age- and size-based fisheries management strategies that better protect older individuals.
But significant challenges arise when the longevity conservation paradigm comes in contact with ground realities, such as in India.
More than 700 marine fish species are caught every year along both coasts of the country, with fishing vessels targeting multiple species simultaneously. How experts can implement a management strategy that protects individuals while allowing uninterrupted harvesting is unclear.
“Protecting larger, older fish is important for conservation, but in a multi-species fishery like India, it’s extremely challenging,” Mayuresh Gangal, a research affiliate at the Nature Conservation Foundation, said.
“Different species vary in size as they age and their life histories differ significantly. On top of that, the fishing gears cannot be very target specific in multi-species fisheries. It is difficult to tailor gear to target specific species or sizes effectively.”
Kopf also wrote in The Conversation that the “loss of old individuals is not yet recognised by the International Union for Conservation of Nature as a means of listing threatened species”.
Rupsy Khurana is Science Communication and Outreach Lead at the National Centre for Biological Sciences, Bengaluru.
Published – December 31, 2024 05:30 am IST
