Cp. Windle et al., Unrearable litters and prenatal reduction of litter size in the common marmoset (Callithrix jacchus), J MED PRIM, 28(2), 1999, pp. 73-83
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.