SOCIAL living is a complicated business. The more buddies you have around, the harder it becomes to keep track of friends and enemies. Two studies of primate societies suggest how they might cope.
Reciprocal grooming is one way that apes and monkeys forge and maintain social bonds. But as groups get larger, the practice of 鈥淵ou scratch my back and I鈥檒l scratch yours鈥� becomes too time-consuming. Some have suggested that early human language evolved as a way to effectively groom several people at the same time.
Karen McComb at the University of Sussex and Stuart Semple at Roehampton University, both in the UK, tested this idea. Using published data from 42 primate species, and factoring out similarities between species arising from shared ancestry, they showed that both call-repertoire and the time spent grooming increases with group size (Biology Letters, DOI: 10.1098/rsbl.2005.0366). The number of calls used by the primate species varied from just two in the Calabar potto to 38 in bonobos.
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鈥淭his tells us that communication is right there at the base of social behaviour,鈥� says McComb. 鈥淗aving a larger vocal repertoire allows you to have a more complex social set-up.鈥� Robin Dunbar of the University of Liverpool, UK, who put forward the idea that language arose as a form of group grooming, agrees. This analysis fits nicely with the theory that as group size gets too big for bonding by grooming alone, vocal exchanges can step in to maintain relationships, he says.
It鈥檚 a long way from primate vocalisations to human language, but Dunbar maintains that increased vocal complexity could be a step on the way. 鈥淟anguage probably did not evolve suddenly out of nowhere but rather developed piecemeal to fill the bonding gap.鈥�
The second cross-species comparison shows that it is female society that matters in primate brain evolution. Patrik Lindenfors of the University of Virginia in Charlottesville looked at how the number of males and females in a society varies with the size of the neocortex, the executive part of the brain. He found that the size of the neocortex increases with the number of females, but the same was not true for males. This is consistent with the idea that primates that have to cope with more social interactions need larger brains, but suggests that female social interactions matter most.
This is no surprise, says Lindenfors, because males and females get together for different reasons. 鈥淲hile females mostly group according to conditions set by the environment, males instead simply go where the females are,鈥� he writes in Biology Letters (DOI: 10.1098/rsbl.2005.0362). Dunbar agrees. 鈥淢ales have less need for exceptionally good social skills.鈥�
