The occurrence of twins and accompanying behavioural changes in the banded langur,Presbytis melalophos

The occurrence of twins in arboreal Old World monkeys and apes is extremely rare. A wild adult femalePresbytis melalophos, however, gave birth to female twins. This stimulated many patterns of behaviour which were rare or absent when single infants were born. Allomothering increased greatly and maltreatment of twins by allomothers was common, even though this was never observed with single infants. Results indicate that the amount of allomothering inP. melalophos is normally controlled by the mother rather than by potential allomothers. The behaviour of the twins also differed from that of single infants. Except when they were taken by allomothers, the twins moved away from their mother much less than did single infants by their second month of life.