Around the year 2006 my friends were receiving Molly fish as gifts from me for almost any occasion. My sister and I had an uncontrollable Amazon Molly in our fish tank: it was giving birth to more fish than we could handle and almost every other week! But we only had one Amazon Molly in the fish tank. How was she constantly pregnant?!
Among many other species (including other molly species), we initially only had one “Amazon Molly”, a freshwater fish native to Texas and Northern Mexico. The name has nothing to do with the Amazon River, but it’s in reference to the all-female societies of warriors and hunters (the Amazons) in Greek mythology. As it turns out, this fish species is all-female and has been reproducing asexually (through parthenogenesis) for at least 100,000 years.
In nature, there are different forms of parthenogenesis.
Parthenogenesis is a type of asexual reproduction where egg cells can develop into a full-grown adult without the need of sperm cells (i.e., “virgin birth”). In some cases, two egg cells fuse together, in others the chromosomes duplicate after meiosis. Many invertebrates are known to reproduce this way, like aphids, some snails, and many aquatic planktonic animals. To my knowledge, no mammals are known to reproduce this way – although some of today’s most practiced religions claim a certain story about a virgin birth in humans. Anyway…
It’s not just invertebrates!
In 2006, two captive Komodo dragons reproduced parthenogenetically in two different Zoos in the UK. Similarly, one year later, captive female hammerhead sharks gave birth after having lived a long time without males nearby. After exploring the possibility of long-term sperm storage, genetic evidence showed that these sharks had just reproduced parthenogenetically. Both cases exemplify how one species is not tied to only one type of reproduction but can reproduce both sexually and asexually.
Although asexual reproduction predates sexual reproduction in evolutionary history, many modern asexual species have sexual ancestors, and some still need some sexual stimulation to reproduce. These can reproduce through kleptogenesis or gynogenesis: types of parthenogenesis with a hint of sperm.
Underneath the logs of the swampy forests near the US Great Lakes live individuals of kleptogenetic salamanders. In kleptogenesis, an individual from a unisexual species (e.g. all-female salamanders) can steal sperm from a male of a closely related species and use that to reproduce. Most of these populations are polyploid (quick reminder: you are diploid). Kleptogenesis has been observed in several amphibian species.
Gynogenesis is the case of the Amazon Molly fish, where the females require a male to pretend to mate, using this male as a “hormonal trigger”. But none of the male’s genetic material is passed to the next generation. Instead, the female molly’s just trick a male of a closely related species to stimulate their ovulation, but the sperm cells degenerate and never fuse with the egg’s nucleus – it does not become part of the Amazon Molly genetic material.
Similarly, the whiptail lizard (Aspidoscelis uniparens) is an all-female species, but their stimulating mating behavior is between themselves. A lesbian dance, some have said.
Homosexuality in nature
Reproduction is overrated (sure, necessary, but it’s not all that it’s involved in a species survival). Let’s move past reproduction and into behavior and sexuality (which is not the same as sex).
The different ways of reproduction I’ve mentioned have been documented in only less than 100 vertebrate species so far, however, homosexuality has been documented in over 1500 animal species already: fish, birds, mammals (including us!).
Homosexual behavior is mostly common in social species. Evolutionary scientists tend to stubbornly look for the reason behind why traits exist. Several hypotheses have been proposed for homosexuality in the animal kingdom, some suggest that individuals attracted to the same sex will not reproduce but will contribute to take care of the young in the group; same sex attraction might have evolved to facilitate social integration; and even the idea that maybe same-sex behavior is the baseline and different-sex sexual behavior is the novelty.
Same-sex penguin couples have been observed several times already taking care of eggs and raising the chick penguins together. The same has been documented in vultures. As a matter of fact, a lot of bird species form same-sex couples.
One of our closest relatives, the Bonobos, enjoy having sexual relationships with anyone around them and they use sex as a peace-making tool (as opposed to our other close relatives, the Chimps, who are more aggressive and prefer a good fight over sex). Some researchers say Bonobos are an entirely bisexual species, and they don’t use sex just to reproduce – they seal deals, make peace, and just enjoy it. The number of animals that have been documented having same-sex couples and/or same-sex intercourse is really not hard to find online, from species closely related to us to others further away in the “tree of life”: lions, dolphins, bisons, mallards, sheep, bats, elephants, marmots, macaques, lizards – the list goes on.
Calling non-human animals homosexuals has been controversial, and we can only infer some things from their behavior. But the bottom line here is: homosexuality is natural and has always been there.
Sex-changes in animals
Animals can also change sex. If the movie Finding Nemo wanted to be more accurate, Nemo’s dad would have had to become female (protandry) after the mother was gone. Wrasses (other fish) do a similar thing but from female to male (protogyny). Some animals can go back and forth throughout their lifetime.
Other than changing sex, there are animals that are male and female at the same time (gynandromorphism), and some that have both types of reproductive organs (hermaphrodites). Most plants and a myriad of invertebrates are hermaphrodites. But it’s not exclusive to these very distant relatives, there’s different cases of gynandromorphs already documented in reptiles and birds.
“It’s XX and XY”: the lie of chromosomal sex.
The presence or absence of a “Y” chromosome in humans makes no big difference. The biological-sex-determination gene (SRY), which is mainly in the Y chromosome, has also been found in the X chromosome (see XX males, for example). This is probably an exchange that occurs during recombination (in meiosis).
Like my genetics professor said: “in genetics, anything is possible”. There are many other chromosomal differences and expressions, even people with XY chromosomes that physically look like girls (Androgen Insensitivity Syndrome) or people with XX chromosomes that have something apparently closer to a penis (Congenital Adrenal Hyperplasia). Discovering these genetic differences after the XX/XY idea has made us think the XX/XY is “the norm” and anything other than that is “abnormal”. Normal is a heavy word. But these different chromosome combinations or expressions are as common as twins, and no one is telling twins they’re not real. There is absolutely nothing wrong with anyone with these chromosomal differences.
There is absolutely nothing wrong with anyone that with or without any underlying genetics feels attracted to anyone and identifies however they identify.
Biology is diverse. Diversity is what’s natural.
“Biology” and “natural” are diverse. There is no absolute biological distinction between males and females, and we’re probably wrong in just having a “male or female” option anyway. To reduce one human life to genitals and the ability (and will) to reproduce is doing more harm than some people realize.
It’s unfortunately often (and frustrating) when people violently try to use subjective ideas of “nature” and outdated biological claims to defend their homophobia, transphobia, and other dangerous attitudes, and mockingly accuse us biologists of “not knowing biology” when we say that gender and sexuality are fluid, diverse, and definitely not binary. “Biology is biology” seems to be a common – and useless – argument, and I hope I’ve provided a good picture on just how diverse and non-binary nature actually is. But this is still not a satisfying answer for me.
Nature is amazing, yes, but it is not a moral guide for the human societies we live in. Yes, we are animals, our own species in biology, but we are also a social species and individuals capable of making conscious choices. Morale and ethics are a very human concept and are subjective in space and time. Nature should not be our moral compass: there are ducks that perform “forced copulation”, bugs that stick their genitals in any part of “their partner’s” body, and geese that takedown and drown anything that flies near their territory – that’s nature, and we should not do any of that. Stop trying to use nature to justify your misbehaviors: you should not need a scientific justification to be a decent and respectful human being.
Did you learn something new here? Consider supporting me on Ko-fi and get a personalized doodle!
[kofi]
Recommended:
The Invention of Heterosexuality, by Brandon Ambrosino (2017).
References:
- Barron & Hare. 2020. Prosociality and a Sociosexual Hypothesis for the Evolution of Same-Sex Attraction in Humans. Frontiers in Psychology.
- Monk et al. 2019. An alternative hypothesis for the evolution of same-sex sexual behaviour in animals. Nat. Ecol. Evol. 3(12): 1622-1631.
- Warren, W.C. et al. 2018. Clonal polymorphism and high heterozygosity in the celibate genome of the Amazon molly. Nature Ecology & Evolution. 2, 669:679
- Engelstädter, J. 2017. Asexual but Not Clonal: Evolutionary Processes in Automictic Populations. Genetics 206(2): 993-1009.
- Booth, W. et al. 2012. Facultative parthenogenesis discovered in wild vertebrates. Evolutionary Biology.
- Booth, W. et al. 2011. Evidence for viable, non-clonal but fatherless Boa constrictors. Biology Letters. 7(2):253-256.
- Stelzer, C. et al. 2010. Loss of Sexual Reproduction and Dwarfing in a Small Metazoan. PLoS One 5(9): e12854
- Bogart, J.P. 2009. Sex in unisexual salamanders: discovery of a new sperm donor with ancient affinities. Heredity (Edinb). 103(6): 483-493.
- Loewe, L. & Lamatsch, D.K. 2008. Quantifying the threat of extinction from Muller’s ratchet in the diploid Amazon molly (Poecilia formosa). BMC Ecology and Evolution 8(88).
- Chapman, D. et al. 2007. Virgin Birth in a Hammerhead Shark. Evolutionary Biology.
- Neiman, M. 2006. Embryo production in a parthenogenetic snail (Potamopyrgus antipodarum) is negatively affected by the presence of other parthenogenetic females. Invertebrate Biology 1(125): 45-50.
- Watts, P.C. et al. 2006. Parthenogenesis in Komodo dragons. Nature Brief Communications. 444
- Crews, D. et al. 1986. Behavioral facilitation of reproduction in sexual and unisexual whiptail lizards. Proc. Natl. Acad. Sci. 83: 9547-9550
Love this post Maria! Theres so much involved with sexuality in humans it really shouldn’t be surprising how immensely complex and varied it is in nature! Great post (and awesome website design too!)
Thanks Dillon! I’ve also really enjoyed your Instagram posts on #PrideMonth this last month.
I’ll link my fave here in case anyone’s reading this and is intrigued: “Are chemicals in the water turning frogs gay?!”