Twinning

Twinning is the conception and subsequent birth of two offspring. Its frequency in human populations varies with geographical populations (high in Nigeria; low in Japan) and occurs in chimpanzees with a higher frequency (human rate ~0.43%; chimpanzee rate ~2.36%; monozygotic rate 0.43%)1. Two types of twins exist: ‘identical = monozygotic (MZ)’ and ‘fraternal = dizygotic (DZ)’. They can be distinguished at times by sex, by genes and, in a population, by the “Weinberg rule”, which states that the rate of dizygotic twinning is twice the rate of twin maternities in which the twins are of opposite sex. The cause of MZ twinning is unknown and it occurs randomly with the same frequency through all populations. DZ twins result from double ovulation which is the result of increased levels of pituitary FSH, a partially hereditary trait, even in chimpanzees 2. In 1,865 maternities 52 sets of chimpanzee twins have been recorded and two sets of triplets. With advancing maternal age FSH levels and, thus, twinning increase. Both Gorilla and Orang twinning rates are reported to not differ significantly from human fraternal tinning rate. For captive gorillas, 8 twin births in 1066 gorilla births in captivity registered in the International Gorilla Studbook in 2004 means that statistically there is one twin birth in 133 births (0.75%). For Orangutans, there are published twinning rates of 1.1% (9 out of 538 captive births, 1946 to 1978)3. Data on twinning from wild great ape populations are very limited.