Ms Monk and her co-authors question the first assumption by pointing out that
many animals seem to mate at a frequency far higher than looks necessary merely to reproduce—
meaning that the proportional costs of any instance of sexual activity which does not produce offspring must be low.
If this is true, it reverses the burden of proof. The cost of the sensory and neurological mechanisms needed to identify another's sex,
and thus permit sex-discriminating mating behaviour, is high.
Sometimes, that will be a price worth paying, especially if a long-term relationship is involved in reproduction, as it is in most birds and some mammals.
But it is the evolution of sex-discrimination for which special-case exemptions must be sought, not the evolution of same-sex behaviour.
The second assumption is even easier to challenge.
Typically, evolutionary biologists assume that traits shared widely across a related group
are likely to have evolved in an ancestral population, not repeatedly and separately in each lineage.
Ms Monk and her colleagues argue that cognitive biases in the subject's practitioners
have pushed them to look for fantastic explanations for the evolution of same-sex behaviours in a range of animals,
rather than considering the perhaps more reasonable explanation for its persistence,
that it is a low-cost ancestral trait that has little evolutionary reason to disappear.
Although the idea that same-sex behaviour has always been a norm is scientifically intriguing,
the paper's authors are also making a broader point about human beings' pursuit of knowledge.
Ms Monk says that the paper's authors met through a Twitter account which promotes the work of LGBT scientists.
This was a serendipitous encounter which gave them space to explore an idea
that might have been dismissed at first sight in a more conventional setting. The group includes people with a range of sexual orientations,
so naturally they had an incentive to ask whether mainstream evolutionary biology's view of sexual orientation is correct.
Their hypothesis still needs testing.
That will mean zoologists gathering more observational data on sexual behaviour of animals in the wild—and doing so with an open mind.
The authors themselves are also mulling approaches involving computer modelling,
which might show that a group of organisms behaving according to their theory
is capable of reaching the distribution of sexual behaviours seen in the wild today.
If their hypothesis is confirmed, it raises the question of which other facets of scientific knowledge might be being obscured
because the backgrounds of practitioners in those fields do not lead them to ask unconventional questions.
Ms Monk's and her colleagues' theory may yet turn notions of the evolution of animal sexual behaviour on their head.
With a broader array of minds focused on other problems, other fields might follow, too.