Were animal ancestors bisexual?

Scientists suggest we might have been working on some incorrect assumptions.

Snow geese are among more than a thousand animals species to exhibit same-sex relations.

James Ronan / EyeEm, via Getty Images

Natalie Parletta

The paradox of same-sex relations in animals and humans has long confounded evolutionary scientists because it produces no offspring and, by their reasoning, should thus have been snuffed out through the laws of natural selection.

Yet, same-sex behaviour has been identified among more than a thousand animal species, from variegated sea urchins (Lytechinus variegatus) and molluscs to bonin flying foxes (Pteropus pselaphon), snow geese (Anser caerulescens) and domestic cows (Bos taurus).

Perhaps we’ve been looking at it all wrong, suggest Julia Monk, from Yale University, US, and colleagues, by assuming that heterosexuality is the default position.

First, they write in the journal Nature Ecology and Evolution, research assumes that same-sex relations come at a high cost and so there must be other adaptive benefits to it, or it will be soundly selected against where possible.

It also presumes that the ancestors of animals with same-sex partners were exclusively heterosexual.

They propose an alternative notion that, “through a subtle shift in perspective, moves away from the expectation that the origin and maintenance of SSB [same-sex sexual behaviour] is a problem in need of a solution”.

How about we shift the lens, they suggest, by considering that animal ancestors had indiscriminate sexual relations all along, and instead of asking “Why engage in SSB?”, ask “Why not?”.

Logically, when a trait is so prevalent, Monk and co-authors argue, it’s reasonable to presume it arose in a clade’s roots. Yet this likely explanation has been neglected in evolutionary studies of homosexual behaviour.

In challenging previous assumptions, they develop a complex model drawing on ecological, evolutionary and developmental factors that influence same sex relations in the same way as they do other traits.

This extends how costs and benefits can be perceived and how they “vary with factors such as levels of sexual selection, sexual polymorphism and sex recognition as well as encounter rate and social complexity, in addition to individual plasticity”.

As to why indiscriminate sex has not been widely accepted as something that ancestral species did from the start, the authors note that the dominating heterosexual, patriarchal culture has biased evolutionary biology’s underlying, culturally acceptable assumptions.

They are “excited” to see how research into the ecology and evolution of animals will progress as these conventional restraints are relaxed.

“The notion that SSB is a recently evolved and distinct phenomenon from ‘heterosexual’ sex... is symptomatic of the kinds of binary essentialism that hinder not only social liberation and equity, but also scientific discovery.”

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Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
  1. https://www.sciencedirect.com/science/article/pii/S0169534709001542
  2. https://phys.org/news/2019-05-scientists-explore-evolution-animal-homosexuality.html
  3. https://www.nature.com/articles/s41559-019-1019-7
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