Sexual Signals

In case you haven’t noticed, science has a real problem with language.  Sometimes you almost hear scientific jargon sucking the life out of things.  For instance, science, contemplating some of the most staggering beauty in nature, describes it simply as “sexual dimorphism.”

By itself, sexual dimorphism simply means that the sexes are physically different.  All of that rare beauty we see in the peacock, the majesty we admire in the maned male and unmaned female lions, and the fantastic size difference between the giant black widow female and its tiny mate fall under the flat rubric of “sexual dimorphism.”

Of course, whatever the size, plumage, or behavior differences in dimorphic species, it’s all about mating.  All the prancing mating dances, the roaring and butting heads to prove dominance, and the rampant display of whatever the female seems to like, however grotesque, is nothing more than an effort to bias the selection process in one’s favor.

But I want to talk about two species that, while dimorphic in a mild way, lack any dramatic plumage or seasonal changes to announce their status to the opposite sex.  Although vastly different, they have things in common: humans and elephants.

We humans live in a crowded world.  And Lord knows we are willing to do a lot to stand out.  We have no gay plumage, no annual antlers, no embarrassing parts of our anatomies that change color when we are in season.  But we do have elaborate rituals for getting the attention of the opposite sex.  A great deal of what we do is bent on convincing them that, if we are not really mate material, we are still good candidates for a little connubial experimentation.

Elephants have another problem.  The female not only has to pick the right candidate, she must first attract the candidates to be picked.

Like us, male and female elephants look pretty much alike, except for a slight size difference.  Even the obvious tusks aren’t sexually specific.  Female African elephants have them, while a large percentage of Asian males don’t.  And the only significant social difference between males and females actually gets in the way of breeding.  Females and the young herd together, while the males of breeding age live apart from them, sometimes alone and sometimes in loose male herds.

Worse yet, the females are not seasonal.  They spend so much time and energy raising their young that they only come into estrus and breed about once every five years.  How, then, do those separated males know when a female comes into season?

First of all, there’s smell.  The elephant has an acute sense of smell.  The bulls, as they roam around, are constantly (and optimistically) checking the females’ urine and droppings for hormonal cues.  Second, rather like a female cat in heat, the cows bellow loudly while in estrus and those big ears help the males hear the news.

But even those odors and that highly audible trumpeting covers only a small distance when compared to the upwards of 10 miles a day roaming range of elephants.  To get a decent selection of competing males, something more is needed.

Enter infrasound.

Turns out that while the higher frequencies take less power to generate, they are also absorbed more easily.  Low frequency sound, while harder to generate, travels much further.

Obviously elephants have lots of power to generate sounds, so they add a significant low frequency component to any really urgent messages (like – “Look out, there’s a lion pride hunting our calves,” or “Hey, big boy, am I ever ready.”).  Operating in the 10 – 20 Hz range, and inaudible to us, these are the elephantine equivalent of the famous jungle drums.

But wait, there’s more.

While most sound simply travels through the air bouncing off and being absorbed by things in the environment, infrasound is different.  Specifically, if the sounds can be coupled into the ground, they can travel for miles as seismic waves.  Which is pretty cool, but not terribly useful unless you happen to have some method of picking them up again.

Enter corpuscles.

There are a couple of skin corpuscles types, Pacinian and Meissner, that detect low frequency pressures.  (Interestingly enough, humans have them, too.  Think of feeling a really great bass riff that seems to rattle your chest, or listening to a piece of music that makes your skin tingle.) These corpuscles are shaped rather like onions, with lamellar layers surrounding a nerve cell.  The layers, with a slippery gel in between, serve to make the corpuscles extremely sensitive to vibrations from the 10 – 20 Hz range.  In humans many of them are concentrated in sensitive areas like the finger tips.  In elephants, they are concentrated in the trunk and (particularly Pacinian) in the feet.

Seen in cross-section, an elephants foot looks a lot like a human foot in a pair of shoes with those old wedge heels.  The elephant stand on tip toe, supported under the arch and heel by a big lipid wedge.  And that wedge, it turns out, has a high concentration of Pacinian cells.

So when an elephant sends out a blast of seismic messaging, others can be seen freezing in place, leaning forward to put their weight on their toes, and often resting their trunks on the ground as well.  But are they really listening with their feet?  Couldn’t that simply be their posture for listening to airborne infrasound?

No.  Field tests have proven that the elephants respond this way to seismic messages even when there is no airborne component.  And elephant infrasounds have been detected over 20 kilometers away.  For a low density population this is a pretty good way to get the word out and attract likely candidates.

And what, I hear you ask, does this have to do with humans and our own mating rituals?

Just this: The next time you see, hear and feel a boom box rolling by filled with testosterone, I would ask you not to think of all the money that was wasted pimping it out.  And please don’t think of the damage being done to the half-witted occupants’ hearing.  Don’t even think of the bare-chested discourtesy involved in blasting out their infernal thump, thump, thump as far as their amps will carry.

Instead, think corpuscles.

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