for National Geographic News
Memories of drought remain with old female elephants and could help their clans survive during hard times, new research suggests.
Scientists made the discovery after reviewing data on elephant herds gathered in Tanzania's Tarangire National Park, which experienced a severe drought from 1958 to 1961.
When a second extreme drought hit the area again in 1993, elephant groups with mothers who lived through the drought 35 years earlier left the park to seek food and water, ensuring a better survival rate for their clan.
"The data show that the family groups that left the park fared much better than those that remained," said lead study author Charles Foley, a researcher with the Wildlife Conservation Society (WCS).
During the more recent drought, two elephant groups that left the park in search of better food and water lost fewer than 10 percent of their elephant calves, while a sole family group that stayed lost 40 percent of its calves.
During nondrought conditions, only 2 percent of calves die each year. Foley and colleagues from WCS and the Zoological Society of London detailed their findings earlier this month in the journal Biology Letters.
In the wild, elephants can live well into their late 60s. In East Africa, extreme droughts occur every 45 to 50 years, on average.
Experts believe there could be strong selective pressure for long-lived animals to have good memories, allowing elephants to retain key information such as where to migrate to survive severe droughts.'
In Tarangire National Park, the oldest elephant matriarchs of groups that left during the 1993 drought were five years or older when the 1958-61 drought occurred. These females likely guided the family groups in their clans to drought refuges outside the park, researchers say.
The group that stayed behind in Tarangire in 1993 had no individual old enough to have experienced the severe drought of the past. That probably explains why the group didn't leave, Foley said. "In other words, this would provide a selective reason for why 'elephants don't forget,'" Foley said. Iain Douglas-Hamilton, a zoologist and founder of Save the Elephants, says that researchers long suspected the evolutionary advantage of having matriarchs as a memory bank.
"The speculation was there and was written about," Douglas-Hamilton said, noting that the new study is the first "to really prove that there was a selective advantage in terms of survival that occurred with groups that had the older members."
In elephant society, females lead their family groups, which is why the ability to remember carries more weight in females than males.
Researchers behind the new study say it shows the importance of protecting veteran elephants as the frequency of droughts may begin to increase as a result of global warming. "If the few remaining older individuals are eliminated from a population, the impact can extend far beyond just their family group," Foley said.
"The effects of removing old individuals may not be seen for 10, 15, even 20 years after the event. But will eventually impact the population during the next severe drought," he added.
Foley said park managers should strive to protect older elephants, particularly in countries where culling is used as a management tool. "The shooting of older animals should be avoided at all costs," he said.
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