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By Grace Wade, Popular Science
Earlier this year two male penguins at the Berlin Zoo made headlines
for co-parenting an abandoned egg—but the pair aren’t an anomaly. To
date, scientists have recorded same-sex sexual behaviors in more than
1,500 animal species, from domestic cattle to nematode worms.
Scientists have proposed countless hypotheses to explain why same-sex
sexual behaviors (SSB) persist despite the supposed Darwinian paradox—why would animals spend time and energy on sexual activities that have zero chance of resulting in offspring?
A new theoretical paper published in Nature Ecology and Evolution
shifts away from the traditional question of “why” and instead asks…
well, why not? The authors of the paper propose that these behaviors
occurred in a common ancestor from which all animals evolved, and have
persisted because they have few, if any, costs. Instead SSB are
ecologically “neutral” and, therefore, there would be no reason for
natural selection to weed them out. In fact, the paper argues, some
degree of sexual flexibility may be an evolutionary plus.
The theory of natural selection—proposed by the one and only Charles Darwin—is
one of the main mechanisms fueling evolution. In the simplest terms,
the theory goes like this: Within an organism or species, there is a
diversity of traits. Some of these traits may be beneficial while others
may be detrimental. If certain traits are either really helpful
or really harmful, they will have an impact on an organism’s ability to
survive and pass on their genes—this is an animal’s “fitness.” Animals
with the really beneficial traits might have more offspring, which can mean the trait becomes more common in that species’ gene pool. However, this does not happen with every adaptation.
“Sometimes
when populations change over time, it is just because of chance, not
because any particular variant is better or worse in terms of fitness,”
says Erin Giglio, one of the co-authors of the paper and a PhD
candidate at the University of Texas Austin studying ecology, evolution,
and animal behavior. She is describing something called genetic
drift—another mechanism of evolution and a possible explanation for
SSB’s persistence.
Past research on the topic of same-sex sexual
behaviors relied on the assumption that animals’ common ancestor
exclusively engaged in different-sex sexual behaviors (DSB, or what
humans call heterosexuality). Therefore, past hypotheses presume SSB
evolved independently across different animal lineages. The scientists
behind the new paper believe the opposite—that SSB is a trait found in
one common ancestor—a species that practiced indiscriminate mating as a
reproductive strategy.
“In any trait so widely seen across
different animal species, you would usually at least consider the
hypothesis that the trait was there from the origin,” says Julia Monk,
the lead author of the paper and a PhD candidate in ecology at the Yale
School of Forestry and the Environment.
Monk and the other
co-authors suggest the common ancestor was a multi-cellular, sexually
reproducing, immobile creature, that most likely could not determine one
sex of its species from another. Organisms that exclusively practice
different-sex sexual behaviors rely on characteristics such as differing body sizes, colors, or chemical signals
to distinguish between sexes. The theoretical ancestor didn’t have
those dimorphic traits or the ability to distinguish them yet, so it
engaged in both SSB and DSB to maximize mating opportunities.
“A
lot of studies take it for granted that the cost of same-sex sexual
behavior is obviously high. Thus, there was either some huge unknown
benefit for this behavior that would explain why it evolved, or that it
was just kind of an accident of maladaptation,” says Monk.
But
Monk and her co-authors argue there are low costs, if any, associated
with SSB. Compared to other hindrances to an animal’s reproductive
fitness—such as infertility or mate competition—SSB’s consequences are
far lower. Therefore, selection for or against the trait would be
relatively weak.
“The idea that everything about the world and
about a given species is a product of selection, and that there is some
perfect animal that evolution is striving towards, is a big problem in
evolutionary biology,” says Giglio. “Sometimes diversity and different
ways of approaching a problem are equally valuable.”
Historically,
evolutionary biologists have viewed the natural world through a
human-centric perspective—one that for many years viewed SSB as
unseemly, or at least irrelevant. That’s precisely why the authors chose
terms like SSB and DSB instead of “homosexuality” or “heterosexuality”:
To further prevent conflation between human cultural biases and
non-human animal sexual behaviors. While the authors of the paper
acknowledge their new hypothesis may be incorrect, they hope to
encourage other scientists to break free from old norms so as to better
understand the world.
“Thinking critically about the way we
approach given phenomena and trying to understand them is really
important in order to get clear answers about where we came from or who
we are,” says Giglio. “If we don’t ask the right questions, we are never
going to get the right answers.”
