Chapter 10

MONKEYS AND MAGGOTS...OR PARASITES FOR PEACE

Howler monkeys live in the jungles of Central America. They have a special resonating bone that amplifies their cries - hence the name "howler monkey." Even though they're only two feet tall, you can hear a howler monkey scream from three miles away.

Especially if they've got screwworms.

Meet the screwfly, which lives in the same jungles as howler monkeys. Screwflies look pretty much like normal houseflies, except bigger. They aren't parasites themselves, but their babies are.

When it comes time to have baby flies, screwflies look for a wounded mammal to lay their eggs in. They're not picky about what kind of mammal, and they don't need a very large wound. Even a scratch the size of a flea bite is plenty big.

When the eggs hatch, the larvae - also known by such charming names as "maggots" or "screwworms" - are hungry. As they grow, they begin to devour the flesh around them.

Most maggots are very fussy and only eat dead flesh, so they're not a problem for their host. They can actually help to clean the wounds that they hatch into. In a pinch, doctors still use maggots to sterilize the wounds of soldiers.

But screwworms - screwfly maggots - are another matter. They are born ravenous, and they consume everything they can get their teeth into. As they devour the animal's healthy flesh, the wound gets bigger, luring more screwflies to come and lay their eggs. Those eggs hatch, and the wound gets even bigger...

Eww. Yuck. Repeat.

At the end of this cycle is a painful death for many a howler monkey.

But screwflies also bring a message of peace.

Like all primates, howler monkeys want mates, food, and territory - all the stuff that makes being a howler monkey fun. So they compete with one another for these resources - in other words, they get into fights.

But no matter how angry they get, howler monkeys never use their teeth or claws. Even if one of the monkeys is much bigger, all it ever does is slap the other one around and (of course) howl a lot.

You see, it's just not worth it to get into a real fight. Because even if the smaller monkey gets its monkey ass totally kicked, all it has to do is get in one tiny scratch, and the fight becomes a lose-lose proposition. One little scratch, after all, is all a screwfly needs to lay its eggs inside you.

Many scientists believe that the howler monkeys developed their awesome howling ability because of screwworms. Any monkeys who resolved their conflicts by scratching and biting (and getting bitten and scratched in return) were eaten from within by screwworms. Game over for all the scratchy and bitey howler monkey genes.

Eventually, all that was left in the jungle were non-scratchy monkeys. Survival of the fittest, which in this case were the non-scratchiest.

But there were still mates and bananas to be fought over, so the non-scratchy monkeys evolved a non-scratchy way to compete: howling. Survival of the loudest. And that's how we got howler monkeys.

See? Parasites aren't all bad. They take primates who otherwise might be killing one another and leave them merely yelling.