1200px-Concentrated_animal_feeding_operation,_Missouri_(2)If you live next to a concentrated animal feeding operation – facilities that houses thousands of farmed animals in fetid conditions – there’s no point in buying perfume.  The smell of animal excrement overwhelms any scent you could wear.  If you’re interested in a romantic dalliance, you’ll have to woo people with your looks.  Or, sure, conversation.  But a charming scent won’t do it.

In other environs, scent contributes to your allure.  We humans choose our mates based on a huge number of considerations, including the way people smell.  Back in 1995, zoologist Claus Wedekind proposed that human females are most attracted to the scents of men whose immune genes differ from their own.

oldspiceDuring college, a friend tried to convince me that the best route to romantic success was Old Spice aftershave.  “It reminds women of their fathers,” he said.  This is, of course, the opposite theory from Wedekind’s – that females would seek out partners whose scents mirror their own genetic lineage.

But this much is uncontested – by overwhelming our sense of smell, air pollution makes humans less sexy.

Stick_insect_WGWe’re not the only animals who use aroma to identify attractive mates.  Stick insects can have a wide range of physical appearances, and multiple species sometimes live in overlapping areas.  Each subpopulation of stick insects secretes a different mix of oily aromatic chemicals from their skin.  These oils protect them from scrapes and dehydration – and help them find mates of their own kind.

If stick insects couldn’t smell, they might mate wantonly.

That’s what happens with fish.

When we pollute water, fish lose the ability to recognize each other.  In the same way that humans near a CAFO won’t notice each other’s scents because they can only smell ammonia and sulfurous shit, fish living near human dumping grounds – whether it be farm run-off, factory effluents, or untreated sewage – find their sense of smell overwhelmed.

fishMany types of fish behave the way my Old-Spice-sporting friend hoped humans would – they seek mates who smell like their forebears.  Which they can’t necessarily do in polluted waters.  And so fish mate across species.  Their chimeric children dissolve the old boundary lines.

Perhaps you thought this couldn’t happen – the traditional definition of a “species” is a population of organisms that can produce fertile offspring only by mating with each other.  But the traditional definition is wrong; scientists don’t actually know what a species is.  Whatever boundaries exist seem porous.  The Neanderthal genes carried by modern Homo sapiens show that humans also mated with other species, at least until we drove our relatives into extinction.  Chimpanzees are the closest we have left, sharing 98% of our DNA, but now they’re endangered too.

Although – maybe that’s fine.  Not murdering our relations, or endangering the chimps; maybe it’s fine for multiple lineages to merge back into one.  I hate to find any virtue in pollution, but dissolving species boundaries doesn’t sound so bad.

Contemporary biology textbooks claim that species boundaries arise whenever subpopulations cease interbreeding.  For the “Advanced Placement” biology test, students are expected to know that speciation can be triggered by migration, or a geographic impediment like a new highway, or even cultural barriers.

A strong preference for certain types of scent might qualify as a cultural barrier.  Or tropical birds that want their mates to look or dance a certain way.  And so would anti-miscegenation laws in the United States.  Except for the gene flow provided by pale-skinned rapists, those biology textbooks imply that epidermal melanin concentrations marked a species boundary until the 1960s in the United States.

In the contemporary U.S., parental wealth creates a similar mating barrier.  In many parts of the country, children born to rich, well-educated parents rarely even chat with children born to poor people, let alone marry them.  This phenomenon has persisted for only a generation or two, which is certainly too brief to create a species division, but shows no sign of abating.

Marrying somebody who shares your interests seems fine.  My spouse and I seem to be fairly similar people.  And yet – should I be alarmed that my own choice inches us closer toward the world of Metropolis?


Feature image: “Character study, strong smell” by Franz Xaver Messerschmidt.