Researchers have found a number of same-sex pairs of penguins at aquariums in Japan, with an imbalance between the numbers of male and female birds suspected to be the cause, a report said.

　　A research group led by Keisuke Ueda, professor of behavioral ecology at Rikkyo University in Tokyo, found about 20 same-sex pairs at 16 major aquariums and zoos, Kyodo news agency said.

　　Penguins in captivity "may be more likely to form same-sex pairs" due to the difficulty of finding partners of the opposite sex because breeding facilities in Japan only have an average of 20 birds, the agency quoted Ueda as saying.

　　It is not known if the frequency of homosexuality is higher than in the wild, where telling the sexes apart is tough, he said.

　　Many of the gay male pairs and two of the female pairs were seen performing mounting behavior, it said.

　　Ueda was not available for comment on the report.