Women have not always been thought to influence fighting behaviour in men, apart from the standard green-eyed monster, the intricate plots of novels and in criminal cases. We may need now to rethink how the sexes interact when aggression between our near-relatives is researched in detail. Female vervets, Chlorocebus aethiops pygerythrus, could use grooming as a carrot, so they can manipulate the male monkeys into fighting other groups. They could also simply be aggressive with their own males to stimulate the fighting behaviour. In fact, the carrot was used after males had fought while the stick of aggression was used on defectors.
At times of abundant food availability, the recruitment of fighters, so that smaller groups could defeat larger groups in some circumstances, became regular behaviour. Males are 1.5X larger, with great big canine teeth, but the cost to them in a fight could be loss of fitness to breed or even feed. The authors, led by T. Jean Marie Arseneau-Robar of the University of Zurich, study there and at the Inkawu Vervet Project in Kwazulu Natal in South Africa and the University of Neuchatel. They consider that social grooming and tolerance of males may have a disproportionate effect on male behaviour. Because other females will obviously note any other female reaction, the males reputation and social interaction level will change radically. Male fitness is based on their mating success, so this low-cost incentive or disincentive is capable of creating a high-cost consequence.
The interest also lies in the aggression (punishment) towards males being exclusively observed during inter-group fighting. Obviously, other aggression could have been taking place, especially towards other females and juveniles, but this seems unlikely. Males often sat at the front line of the fight without participating and began fighting if their offspring were at risk. This made them the obvious targets for their females aggression, as they were available!
A typical vervet fight began with female vocalisation as they approached the neighbours. This was to encourage male support until the groups were 1m apart, when the scrummage began. The researchers are now investigating how the decision to attack or retreat is made. Pauses between these fights were useful for the research in that males could be observed participating in later events, whether or not they had received reward or punishment. This is the first time that both carrot and stick have been documented outside the human species. Social incentive seems to be the root of these behaviours, and very obviously creates similar fight situations in humans, too.