The Economist - USA (2019-11-30)

68 TheEconomistNovember 30th 2019

1

W

hen it comesto sexual behaviour,

the animal kingdom is a broad

church. Its members indulge in a wide vari-

ety of activities, including with creatures

of the same sex. Flying foxes gather in all-

male clusters to lick each other’s erect pe-

nises. Male Humboldt squid have been

found with sperm-containing sacs im-

planted in and around their sexual organs

in similar quantities to female squid. Fe-

male snow macaques often pair off to form

temporary sexual relationships that in-

cludes mounting and pelvic thrusting.

Same-sex sexual behaviour has been re-

corded in some 1,500 animal species.

The mainstream explanations in evolu-

tionary biology for these behaviours are

many and varied. Yet they all contain a

common assumption: that sexual behav-

iours involving members of the same sex

are a paradox that does indeed need ex-

plaining. Reproduction requires mating

with a creature of the opposite sex, so why

does same-sex mating happen at all?

A paper just published in Nature Ecology

and Evolutionoffers a different approach.

Instead of regarding same-sex behaviour as

an evolutionary oddity emerging from a

normal baseline of different-sex behav-

iour, the authors suggest that it has been a

norm since the first animals came into be-

ing. The common ancestor of all animals

alive today, humans included, did not, they

posit, have the biological equipment need-

ed to discern the sex of others of its species.

Rather, it would have exhibited indiscrimi-

nate sexual behaviour—and this would

have been good enough to transmit its

genes to the next generation.

The group of young researchers from

institutions across America who wrote the

paper, led by Julia Monk, a graduate stu-

dent at Yale, argue that conventional mod-

els of sexual behaviour’s evolution take

two things for granted that they should not.

The first is that the cost of same-sex behav-

iour is high because energy and time spent

engaged in it do not contribute to repro-

ductive success. If that were true it would

indeed mean that maintenance of same-

sex behaviour over the generations re-

quires some exotic explanation whereby

such activity confers benefits that out-

weigh the disadvantage. The second as-

sumption is that same-sex activity evolved

separately in every species that exhibits it,

from an ancestral population that engaged

exclusively in different-sex behaviour.

Do you come here often?

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 re-

produce—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 neuro-

logical mechanisms needed to identify an-

other’s sex, and thus permit sex-discrimi-

nating mating behaviour, is high.

Sometimes, that will be a price worth pay-

ing, especially if a long-term relationship

is involved in reproduction, as it is in most

birds and some mammals. But it is the evo-

lution of sex-discrimination for which spe-

cial-case exemptions must be sought, not

the evolution of same-sex behaviour.

The second assumption is even easier to

Sexual orientation of animals

Basic instincts

A new hypothesis argues that same-sex sexual behaviour is an evolutionary

norm, not an aberration

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