Unrearable litters and prenatal reduction of litter size in the common marmoset (Callithrix jacchus)

It is widely believed that common marmosets (Callithrix jachus) typically g ive birth to tu ins under natural conditions. In captivity, however, births of triplets or even larger litters are common, although parents rarely suc ceed in rearing more than two offspring. The traditional interpretation is that captive conditions, notably the ready availability of food, have led t o increased reproductive output, perhaps involving a higher ovulation rate. The present paper provides evidence, combined from ultrasound examinations between ovulation and birth and hysterotomies conducted during the late em bryonic and early fetal phase, that the litter size can be progressively re duced during pregnancy without spontaneous abortion. There is an unusually long lag phase prior to the onset of embryonic growth in common marmosets; the fetal stage does not begin until day 80 of the 144-day pregnancy. Reduc tion in litter size occurs during embryonic stages (up to day 80), and cont inues into the fetal stages. These results indicate that the common marmose t is adapted for flexible modification of litter size between ovulation and birth. The high incidence of triplet births in captive colonies may theref ore be an expression of an adapted natural developmental process under arti ficial circumstances.