麻豆传媒

In a forest clearing in the far south-east of Guinea, a mother chimpanzee gently puts down her baby so that she can crack open a nut. There鈥檚 something wrong with the apparently mundane scene, though: her baby is dead, and has been for weeks.

The 1-year-old chimp, known as Jimato, died of flu when an epidemic struck a community of 19 chimps living in the district of Bossou in 2003. The virus also killed 2-and-a-half-year-old Veve. Both corpses were carried by their mothers for weeks.

Such behaviour has been seen in primates before, but rarely. What鈥檚 unusual in these cases is how long the mothers clung onto their young. In other accounts of corpse-carrying primates, the body has been snatched out of the mother鈥檚 hands by rowdy males, or in wet conditions it has disintegrated within days. But the dead Jimato and Veve were carried for 68 and 19 days respectively before they were abandoned.

Their bodies were effectively mummified by dry-season conditions. 鈥淭hey dried out and lost all of their hair, leaving most of the body parts, even the genitals, preserved in a layer of dry, leathery skin,鈥� says at the University of Oxford, who has co-authored a paper on the behaviour.

Another new study looks at how a group of captive chimpanzees dealt with the death of an elderly female. Together, these episodes have given insights into how chimpanzees deal with death, and what finally causes the mothers to let go of their dead young.

Pansy

, a psychologist at the University of Stirling, UK, has written about the death of Pansy, the captive, elderly female chimp. After Pansy鈥檚 death, her daughter stayed beside her throughout the night. Then members of the chimp group cleaned the corpse and avoided the place where she died. Anderson says these may be 鈥渞ituals鈥� to cope with death.

On the other hand, he says, 鈥淏iro鈥檚 results suggest that a period of continued contact after the death of an infant may be important for a mother chimpanzee to adjust psychologically to her loss鈥�.

鈥淥ur combined research makes a strong case that chimps not only understand the concept of death but also have ways with which they cope with it,鈥� says Anderson.

In many ways, the mothers in Biro鈥檚 study treated the corpses as though they were still alive: they groomed them, swatted flies away, emitted high-pitched screams of distress when they accidentally dropped the bodies and placed them on the ground 鈥渨ith great gentleness and care, as though they were alive鈥�, says Biro.

But some of their behaviour suggests they knew that the corpses were dead. For instance, 鈥渨hen an infant called Fokaye playfully dragged a corpse around a tree, his mother flinched and moved away鈥�, says Biro. 鈥淏ut that鈥檚 the only time we saw a chimp show aversion.鈥�

Letting go

Eventually, both Bossou mothers did abandon their dead young, which Biro thinks is linked to their hormonal cycle. Chimps produce hormones that prevent them from getting pregnant while they鈥檙e breastfeeding: as soon as they stop, .

鈥淭he hormonal changes the chimps experience as their bodies gear up to reproduce may push them into 鈥榣etting go鈥� of the corpses,鈥� says Biro. 鈥淲e don鈥檛 know that for sure, but our observations suggest that may be true.鈥�

One question that remains is why the chimpanzee mothers carry their dead infants, given that the corpses make it hard for the mothers to climb trees and feed themselves. , a primatologist based at Emory University in Atlanta, Georgia, says it may be an extension of looking after sick infants. 鈥淭he infant may recover,鈥� he says.

Alternatively, corpse-carrying may have become something of a Bossou 鈥渢radition鈥�, says Biro. She believes that Veve鈥檚 mother may have learned to carry her dead infant from Jimato鈥檚 mother, (PDF) before Veve died.

Journal references: Biro: , vol聽20, p聽R351; Anderson: Current Biology, vol聽20, p聽R349