Many things happen when people are ageing. Apart from the greying hair and wrinkled skin, there is a(n) 1 change which comes with older age. When humans reach their later years, they favour more long-term 2 and their social cirele is reduced.
Now, for what appears to be the first time, scientists have seen the same 3 in another species. Twenty years of observations of chimpanzees (猩猩) reveal that older males choose to keep contact with their 4 friends at the expense of other relationships.
The researchers studied 78,000 hours of observations made between 1996 and 2016 that followed the social 5 of 21 male chimpanzees between the ages of 15 and 58 years old. They classified the chimps' 6 depending on the amount of time they sat with others and groomed (梳毛) them. They then rated (分类) the various pairings as mutual (相互的) friendships, where both chimps seemed to enjoy the relationship; 7 friendships, where one chimp was more keen to be friends than the other; and non-friendships, where neither chimp showed 8 the other.
When the scientists looked at the 9 of friendships, they found that the older chimps had more mutual friendships and fewer one-sided friendships than younger chimps. Another 10 seen in older humans was also spotted in the chimps. As the males got older, their levels of 11 gradually become less, meaning they started fewer fights and tended to threaten others in their group less often.
The observations have left the researchers 12. According to an idea in psychology known as socio-emotional selectivity theory, older humans prefer more 13 relationships because they are aware that time is running out. However many animal experts argue that chimpanzees 14 the human sense of mortality (死亡) , suggesting something else is driving the behaviour.
Robin Dunbar, a professor of evolutionary psychology at the University of Oxford, said in humans, the 15 social circles with age is due to declining social motivation to get out and meet people combined by lack of opportunity. In chimpanzees, as older males compete less for mates, they may focus on close, reciprocal (互惠的) relationships with trusted partners, he said.