Notes

"An estimated one-quarter of all black swan pairings are of males. They steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs. The males spent time in each other's society, guarded the common territory, performed greeting ceremonies before each other, and (in the reproductive period) pre-marital rituals, and if one of the birds tried to sit on the other, an intense fight began.[1][41] More of their cygnets survive to adulthood than those of different-sex pairs, possibly due to their superior ability to defend large portions of land. The same reasoning has been applied to male flamingo pairs raising chicks."

"Female Laysan albatrosses, on the north-western tip of the island of Oahu, Hawaii, form pairs for co-growing offspring. On the observed island, the number of females considerably exceeds the number of males (59% N=102/172), so 31% of females, after mating with males, create partnerships for hatching and feeding chicks. Compared to male-female couples, female partnerships have a lower hatching rate (41% vs 87%) and lower overall reproductive success (31% vs. 67%).[43]

Warming waters from climate change have led to increased foraging times and thus increased mortality among female black-browed albatrosses on the Antipodes Islands in New Zealand. The skewed gender imbalance has led to many male albatrosses forming homosexual relationships. Male-male pairs now comprise between 2–5% of the albatross population on the islands."

"Mallards form male-female pairs only until the female lays eggs, at which time the male leaves the female. Mallards have rates of male-male sexual activity that are unusually high for birds, in some cases, as high as 19% of all pairs in a population.[7][page needed] Kees Moeliker of the Natural History Museum Rotterdam has observed one male mallard engage in homosexual necrophilia."

"Researchers have found that as many as 8% of rams (male sheep) demonstrate sexual attraction to other rams. A larger percentage demonstrates attraction to both males and females. While researchers continue to examine why these differences in sexual behavior occur, they have made one significant discovery related to animal brains.

The difference occurs in a region of the brain called the anterior hypothalamus, where researchers identified the existence of what they called an “ovine Sexually Dimorphic Nucleus”, or oSDN. A 2004 study found that the oSDN of male-oriented rams is, on average, smaller than those of female-oriented rams. The oSDN of the heterosexual rams also produced more aromatase, an enzyme that converts the hormone testosterone into an estrogen called estradiol. These findings present a potential pathway towards understanding the biological basis of sexual behavior in sheep."


"A total of 31% of laysan albatross belong to same-sex pairings (primarily female-female). Researchers suggest that female-female pairings increase fitness in colonies with fewer males than females, as the female birds can ensure their eggs are fertilized by the fittest males even if that male already has a partner and thus will not participate in raising the chick. "

"Researchers found that disabling the fucose mutarotase (FucM) gene in laboratory mice – which influences the levels of estrogen to which the brain is exposed – caused the female mice to behave as if they were male as they grew up. "The mutant female mouse underwent a slightly altered developmental programme in the brain to resemble the male brain in terms of sexual preference" said professor Chankyu Park of the Korea Advanced Institute of Science and Technology in Daejon, South Korea, who led the research. His findings were published in the BMC Genetics journal on July 7, 2010.[32][33] Another study found that by manipulating a gene in fruit flies (Drosophila), homosexual behavior appeared to have been induced. However, in addition to homosexual behavior, several abnormal behaviors were also exhibited apparently due to this mutation."

"The influence of hormones on the developing fetus has been the most influential causal hypothesis of the development of sexual orientation.[4][5] In simple terms, the developing fetal brain begins in a "female" state. Both the INAH3 (third interstitial nucleus of the anterior hypothalamus) area on the left side of the Hypothalamus, which stores gender preference, and the center area of the Bed Stria Terminalus (BSTc) area on the right side of the Hypothalamus, which stores gender identity, are small and function as female. The action of the SRY gene in the Y-chromosome in the fetus prompts the development of testes, which release testosterone, the primary androgen receptor-activating hormone, to allow testosterone to enter the cells and masculinize the fetus and fetal brain. If the proper amount of testosterone is received by the INAH3 while it is constructed at 12 weeks after conception, the testosterone overrides the estrogen that is also present, and enlarges the INAH3 which is known to be involved in directing typical male sex behavior, such as attraction to females. If the INAH3 does not receive an overriding amount of testosterone, in may not grow normal size for males, and may function as female or partially female, causein same-sex attraction to males.

It has been shown that the INAH3 in gay men has likely been exposed to low levels of testosterone in the of the brain compared to straight men, or had different levels of receptivity to its masculinizing effects, or experienced hormone fluctuations at critical times during fetal development. In women, if the INAH3 receives more testosterone than is normal for females, the INAH3 may enlarge somewhat or even to the size that is normal for males, increasing the likelihood of same sex attraction.[4] Supporting this are studies of the finger digit ratio of the right hand, which is a robust marker of prenatal testosterone exposure[disputed – discuss]. Lesbians on average, have significantly more masculine digit ratios, a finding which has been replicated numerous times in studies cross-culturally.[6] While direct effects are hard to measure for ethical reasons, animal experiments where scientists manipulate exposure to sex hormones during gestation can also induce lifelong male-typical behavior and mounting in female animals, and female-typical behavior in male animals."

"Maternal immune responses during fetal development are strongly demonstrated as causing male homosexuality and bisexuality.[8] Research since the 1990s has demonstrated that the more sons a woman has, there is a higher chance of later born sons being gay. During pregnancy, male cells enter a mother's bloodstream, which are foreign to her immune system. In response, she develops antibodies to neutralize them. These antibodies are then released on future male foetuses and may neutralize Y-linked antigens, which play a role in brain masculinization, leaving areas of the brain responsible for sexual attraction in the female-typical position, or attracted to men. The more sons a mother has will increase the levels of these antibodies, thus creating the observed fraternal birth order effect. Biochemical evidence to support this effect was confirmed in a lab study in 2017, finding that mothers with a gay son, particularly those with older brothers, had heightened levels of antibodies to the NLGN4Y Y-protein than mothers with heterosexual sons.[8][9] J. Michael Bailey has described maternal immune responses as "causal" of male homosexuality.[10] This effect is estimated to account for between 15 and 29% of gay men, while other gay and bisexual men are thought to owe sexual orientation to genetic and hormonal interactions."

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2912782/
https://en.wikipedia.org/wiki/Homosexual_behavior_in_animals
https://academic.oup.com/endo/article/145/2/478/2499800?login=false
https://www.thoughtco.com/homosexuality-in-animals-4164365
https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2826.2009.01828.x