Animals that mate with the same sex

Elephants - Same-sex relationships among elephants are long-lasting, unlike their heterosexual relationships.

Giraffes - Male giraffes are much more likely to perform mating rituals with other males than they are with females.

Marmots - Female marmots are more commonly known to mate with each other than with males.

Spotted hyenas - Hyenas live in a matriarchal society where females are very aggressive and routinely mate with each other. 

Whiptail lizards - Whiptail lizards have an asexual reproductive system. Females increase their hormones by having sex with other females.

Bed bugs -
Bed bugs are attracted to any newly-fed member of their species, including members of the same sex.

Mallards - Up to 19% of male mallard ducks will mate with other males after impregnating female ones.

Polecats - European polecats are believed to form romantic same-sex relationships for no evolutionary purpose.

Amazon river dolphins -
Dolphins have sex for pleasure and Amazon river dolphins have mating groups of three to five, often including one or two females.

Bonobos - Bonobos live in a matriarchal society where most are bisexual and most sexual activity involves two or more females.

Vultures - Male griffon vultures have been observed mating and raising adopted eggs together.

Ibises - Scientists observed that ibises exposed to methylmercury had a significantly higher rate of mating with the same sex than those that were not.

Pigeons - Both male and female pigeons are known to have same-sex relationships, with female pigeon pairs laying infertile eggs and attempting to nest them.

Bats - Up to 20 species of bats are known to form same-sex relationships.

Dragonflies - Between 20 and 80% of male dragonflies couple with other males. 

Magellanic penguins - Two male magellanic penguins raised an adopted baby penguin together in the San Francisco Zoo.

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