Caudal duplication (dipygus) in a Rock Pigeon (Columba livia)

Congenital defects are those abnormalities present at birth that result from errors arising during development (Noden and de la Hunta, 1985). Congenital duplications are interesting among congenital defects because they are composed by two individuals. Multiple births most frequently result from fertilization of separately ovulated female gametes. However, complete or partial separation of cleavagestage blastomeres and blastocysts, or duplication during gastrulation can also result in the development of multiple organisms (Noden and de Lahunta, 1985). Thus, they form a graded series from slight duplication to near separation of two individuals (Hiraga and Dennis, 1993). There is a wide range of external variations according to the degree, site, and angle of fusion, and they are classified as free (unattached) or conjoined and symmetrical or asymmetrical (Hiraga and Dennis, 1993). But perhaps conjoined twin cases are described less frequently in the literature.