While infanticide committed by new males moving into a troop may be motivated by the need to ensure lactating females come into oestrus more quickly, I suspect this is merely one reason behind the instances of infanticide that occur.
The few baboon infanticide cases I have witnessed have occurred under different and varied circumstances. A number of fatally injured juvenile and infant monkeys have been brought to me with head wounds or severe spinal injuries, clearly caused by adult male monkeys; considering the extent of these injuries, I have assumed that the motivation was to kill. Biting the victim in the middle of the spine ensures paralysis of the lower body, immobilizing the victim. Adult male monkeys sometimes practice this same kind of attack on adult male opponents.
Taking all the cases I have seen into consideration, I have concluded that both chacma baboons and vervet monkeys commit infanticide or attack juveniles to:
1. Bring lactating females into oestrus: this hypothesis is only relevant in the infanticide cases as killing juveniles who are already weaned would be implausible.
2. To fatally wound a helpless young individual that is assumed to be genetically close to one’s adult opponent (re-directed aggression). This occurs during territorial disputes and when new adult males move into a troop.
3. The behaviour of chacma baboons and vervet monkeys in captivity is vastly altered and higher incidences of violence are more likely between adults (of both sexes); infanticide and juvenile attacks would therefore also be more possible as individuals are unable to escape.
Young primates are used as weapons during conflict situations between adult males. Young juveniles who have lost their mother (due to being shot, electrocuted, run over etc), and have too few adult allies, are likely to be prime targets for infanticidal males.
When rehabilitating a troop of vervet monkeys or chacma baboons, the risks of infanticide and fatal attacks on juveniles, need to be taken into consideration. All individuals need to be strongly bonded, with each member having enough adult allies to protect them under risky conflict situations. Sex and age ratios need to be as appropriate as possible. The troop needs to be large as small troops are at great risk of not surviving.