Low incidence of twinning in the loggerhead sea turtle

Twinning has been reported to be much rarer in sea turtles than in freshwater turtles. However, data from sea turtles were inferred from unhatched eggs only. We hypothesized that the difference in twinning events among turtle taxa resulted primarily from the methods used for recording data. In this study, data have been recorded from 727 clutches of eggs of loggerhead sea turtles Caretta caretta laid between 2002 and 2009 at the Pelagie Islands, Italy (n = 36 clutches) and at Dalaman Beach, Turkey (n = 691 clutches). Twin embryos were found in seven out of 12160 unhatched eggs (twinning rate per unhatched egg: 0.058), but not a single case of hatched or emerged twins was recorded in 3571 eggs. The twinning rate recorded, based on post-hoc procedures, did not underestimate the occurrence of the phenomenon: the odds of obtaining a pair of twins in unhatched eggs were as large (0.92 times) as the odds of obtaining a pair of twins in the total eggs. Compared with other loggerhead nesting sites, and other sea turtle species, no statistical differences were found. However, twinning in C. caretta was found to be 6.9 times rarer than in freshwater turtle species.