Callitrichids have been proposed as communal breeders, with non-reprod
uctive animals acting as helpers. Reports in the literature attest to
the high variability in the behaviour of helpers. The present study as
sessed four proximate factors involved in helping in captive Callithri
x jacchus: age, gender and experience of helpers and composition of th
eir groups. Forty-eight helpers from 15 families from Natal (Brazil) a
nd Reading (UK) colonies were observed during the first 4 weeks of lif
e of new-born twins. Infant carrying, transfers and retrieval, and dur
ation of carrying bouts mere registered. Results showed that the prese
nce of helpers decreased the costs of infant carrying, with a negative
ly significant correlation between group size and mean percentage of c
arrying. Age, but not gender, was a significant factor for infant carr
ying transfers and retrieval, and mean duration of carrying bouts, wit
h older helpers contributing more than younger ones. Singletons and ol
dest non-reproductive offspring carried infants more, and longer, than
twins and offspring with older siblings present in the group. Experie
nce decreased motivation and actual carrying time, but not to a signif
icant level. Age, experience and group composition appeared important
factors in the display of infant care by helpers in captive Callithrix
jacchus.