Dictionary of Human Evolution and Biology

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1. either of two offspring produced at one birth (parity event); the product of a litter of size two.

2. one of two individuals closely related or resembling each other. Twins may be either (a) identical and always same-sexed, or (b) fraternal, in which case the pairs may be either same- or opposite sexed. Technically, the former are termed monozygotic twins and the latter dizygotic twins. Twins are normal in tree shrew, mouse lemur, dwarf lemur, slender loris, marmoset, and tamarin births, but are the exception among humans and most other higher primates. A female marmoset may have up to two litters per year, a total of four offspring annually.

See chimeric twin, fraternal twins, and identical twins.

See multiple births for incidences of higher-order births.

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