The Times Australia
The Times World News

.

How – and why – did homosexual behaviour evolve in humans and other animals?

  • Written by Jenny Graves, Distinguished Professor of Genetics and Vice Chancellor's Fellow, La Trobe University
How – and why – did homosexual behaviour evolve in humans and other animals?

Since gay couples have fewer children, the high frequency of same-sex relationships in humans is puzzling from an evolutionary point of view. Perhaps there are social advantages such relationships confer on a group, or perhaps “gay genes” are selected for other reasons.

A group of Spanish researchers have studied same-sex sexual behaviour and social relationships in more than 250 species of mammals – and in a recent paper[1] in Nature Communications, they conclude it arose independently many times, and is related to other kinds of social behaviour.

Darwin’s paradox

Research[2] has shown the basis of male homosexuality in humans is at least partially genetic. I know of no work on a genetic basis for female–female sexual behaviour.

Why then is male–male sexual behaviour so common? You’d think, because gay couples have fewer children, these gene variants would be passed on rarely, and their frequency would decline over time.

Geneticists, sociologists and psychologists have advanced many possible explanations for this conundrum.

One is that gay genes are really “male-loving genes[3]”. In this case, though gay males have fewer children, their female relatives who share these gene variants may be more inclined to mate earlier and have more children, making up the deficit.

Read more: Born this way? An evolutionary view of 'gay genes'[4]

Other hypotheses referenced[5] in the new paper propose that same-sex behaviour has beneficial effects for human groups. One idea is that same-sex relationships are important for forming and maintaining bonds and alliances within the group. This predicts same-sex behaviour should be more frequent in social species than in non-social species.

Alternatively, same-sex behaviour may help to diminish conflict between members of the same sex, and contribute to establishing social hierarchies. If this is so, we would expect same-sex behaviour to be more common in species where aggression and killing among members is also common.

The big picture of same-sex relationships

Human aren’t the only mammals to show a high frequency of same-sex relationships. There are reports of same-sex behaviour (courtship, mounting, genital contact and copulation, pair bonding) in 261 (out of 5,747) mammal species.

Mostly this behaviour is frequent and overt, occurs in the wild, and in half the species is displayed by both sexes. It is very widespread. These species represent about half of all mammal families.

Primates are strongly represented. Fifty-one species, from lemurs to great apes, show same-sex sexual behaviour.

Photo of a group of lemurs
Same-sex sexual behaviour has been observed in 51 primate species, including lemurs. Shutterstock[6]

The even bigger picture is given by studies on many other animals, which reveal same-sex behaviour in birds, reptiles, frogs and fish, as well as many invertebrates.

Most studies of same-sex relationships focus on a particular species, which makes it hard to test these competing hypotheses.

The new research[7] explores same-sex relationships across a wide range of mammals. It asks whether this behaviour was ancestral to all mammals, or whether it evolved independently in response to the establishment of different social systems.

Same-sex sexual behaviour evolved many times and quite recently

It has been proposed[8] that the common ancestor of mammals indulged in indiscriminate sexual behaviour, which manifested as a mix of same-sex and heterosexual relationships. The new study contradicts this.

Using a tree of relationships of mammals to each other – confirmed with DNA sequence comparisons – the patterns of same-sex sexual behaviour were mapped onto the relationships between species. The distribution of same-sex behaviour over all mammals didn’t fit the pattern we would expect if it were present in the common ancestor of all mammals, and was retained in some lineages but not others.

Read more: Homosexuality may have evolved for social, not sexual reasons[9]

A better explanation for the evidence is that same-sex sexual behaviour was rare in mammalian ancestors overall, but evolved independently many times in many different families. Species exhibiting same-sex sexual behaviour had shared ancestors much more recently than species not showing the behaviour. This suggests same-sex sexual behaviour has been gained and lost many times, and quite recently, during mammalian evolution.

Different lineages showed different times at which same-sex sexual behaviour evolved. It became more frequent in Old World monkeys (those found in Africa and Asia today) and increased again during the evolution of the great apes.

Same-sex sexual behaviour and social organisation

Next, the researchers examined the correlation of same-sex sexual behaviour to different measures of social organisation in different mammal species. They compiled information about sociality (how the animals live together) and aggression between members of the same species, and tested for correlations with male or female same-sex sexual behaviour.

The study found same-sex sexual behaviour, both male and female, was more common in more social species. This suggests same-sex sexual behaviour was selected for in social species.

The frequency of male, but not female, same-sex sexual behaviour was also correlated with the frequency with which animals of the same sex attacked and killed each other. This supports the hypothesis that homosexuality evolved to mitigate male–male aggression in mammals.

We conclude from this study that same-sex sexual behaviour in both males and females evolved as species shifted from solitary living to sociality. It helps to establish and maintain social relationships and alliances, resolve conflicts and avoid aggression.

The high frequency of same-sex sexual behaviour in ape and monkey species suggests it was present in a social great ape ancestor, and maintained in present day social species, including humans.

Everybody might be right

Establishing that homosexuality confers selective advantages in social species such as humans and other great apes does not rule out other explanations.

There may still be fertility advantages accruing to the other sex who inherit “male-loving” or “female-loving” gene variants, for example. These benefits are not necessarily the same in different mammal lineages, and may include others that have not yet been investigated.

In any case, the ubiquity and frequency of same-sex sexual behaviour in mammals means homosexuality cannot be considered aberrant or maladaptive in humans, or any other species. It was selected because it confers different and overlapping social and fertility benefits.

References

  1. ^ a recent paper (www.nature.com)
  2. ^ Research (link.springer.com)
  3. ^ male-loving genes (theconversation.com)
  4. ^ Born this way? An evolutionary view of 'gay genes' (theconversation.com)
  5. ^ referenced (www.nature.com)
  6. ^ Shutterstock (www.shutterstock.com)
  7. ^ new research (www.nature.com)
  8. ^ proposed (www.nature.com)
  9. ^ Homosexuality may have evolved for social, not sexual reasons (theconversation.com)

Read more https://theconversation.com/how-and-why-did-homosexual-behaviour-evolve-in-humans-and-other-animals-215331

Times Magazine

Data Management Isn't Just About Tech—Here’s Why It’s a Human Problem Too

Photo by Kevin Kuby Manuel O. Diaz Jr.We live in a world drowning in data. Every click, swipe, medical scan, and financial transaction generates information, so much that managing it all has become one of the biggest challenges of our digital age. Bu...

Headless CMS in Digital Twins and 3D Product Experiences

Image by freepik As the metaverse becomes more advanced and accessible, it's clear that multiple sectors will use digital twins and 3D product experiences to visualize, connect, and streamline efforts better. A digital twin is a virtual replica of ...

The Decline of Hyper-Casual: How Mid-Core Mobile Games Took Over in 2025

In recent years, the mobile gaming landscape has undergone a significant transformation, with mid-core mobile games emerging as the dominant force in app stores by 2025. This shift is underpinned by changing user habits and evolving monetization tr...

Understanding ITIL 4 and PRINCE2 Project Management Synergy

Key Highlights ITIL 4 focuses on IT service management, emphasising continual improvement and value creation through modern digital transformation approaches. PRINCE2 project management supports systematic planning and execution of projects wit...

What AI Adoption Means for the Future of Workplace Risk Management

Image by freepik As industrial operations become more complex and fast-paced, the risks faced by workers and employers alike continue to grow. Traditional safety models—reliant on manual oversight, reactive investigations, and standardised checklist...

From Beach Bops to Alpine Anthems: Your Sonos Survival Guide for a Long Weekend Escape

Alright, fellow adventurers and relaxation enthusiasts! So, you've packed your bags, charged your devices, and mentally prepared for that glorious King's Birthday long weekend. But hold on, are you really ready? Because a true long weekend warrior kn...

The Times Features

What Endo Took and What It Gave Me

From pain to purpose: how one woman turned endometriosis into a movement After years of misdiagnosis, hormone chaos, and major surgery, Jo Barry was done being dismissed. What beg...

Why Parents Must Break the Silence on Money and Start Teaching Financial Skills at Home

Australia’s financial literacy rates are in decline, and our kids are paying the price. Certified Money Coach and Financial Educator Sandra McGuire, who has over 20 years’ exp...

Australia’s Grill’d Transforms Operations with Qlik

Boosting Burgers and Business Clean, connected data powers real-time insights, smarter staffing, and standout customer experiences Sydney, Australia, 14 July 2025 – Qlik®, a g...

Tricia Paoluccio designer to the stars

The Case for Nuturing Creativity in the Classroom, and in our Lives I am an actress and an artist who has had the privilege of sharing my work across many countries, touring my ...

Duke of Dural to Get Rooftop Bar as New Owners Invest in Venue Upgrade

The Duke of Dural, in Sydney’s north-west, is set for a major uplift under new ownership, following its acquisition by hospitality group Good Beer Company this week. Led by resp...

Prefab’s Second Life: Why Australia’s Backyard Boom Needs a Circular Makeover

The humble granny flat is being reimagined not just as a fix for housing shortages, but as a cornerstone of circular, factory-built architecture. But are our systems ready to s...