As I wrote in the previous post, howlers use their voices to mark and hold territory--yelling takes a lot less energy than, say, chasing and fighting off other monkey troops. Which is good if you eat nothing but leaves and a little fruit. Pardon the blur. I got excited when he opened up that big ol' chamber at close range.
Male howlers are charged with the job of holding feeding territory for their troop; females busy themselves raising young. So males have some special equipment, other than the swag you can see. The hyoid bone is enlarged into a bony bulla, or sac, which makes a resonating chamber for his voice. It's as if he has a megaphone right in his throat. He's got one of the loudest voices in the animal kingdom. And as far as we know, this is the only mammal with a hyoid bulla made of bone, rather than esophageal tissue. Lots of monkeys and apes have large fleshy elastic laryngeal pouches (think gibbons and orangs) but only howlers have bony bullae.
1.These are the skulls of red howler monkeys (Alouatta seniculus). Female is on the left, and the male on the right.
2. is the rear view of the skulls. A female howler has a nice bulla, too, but the male's is almost three times its size. So it's the males who can really roar.
Check out the incredibly deep and massive jaw bone on both sexes (3). That tells me I do not want to get bitten by a howler monkey. With masseter muscle inserts like that, they must have some tremendous crushing power! Let's face it. Nobody wants to get bitten by a monkey. So I try to be polite when talking with them, and not escalate things with empty rhetoric.
I've borrowed these photos from the University of Edinburgh Natural History Collections. You can read more at their page. There's also a highly detailed discussion (condensed to a couple of sentences here) by Darren Naish at the excellent science blog, Tetrapod Zoology.
I thank both sources for publishing such cool stuff online, where pikers like me can share it. They're welcome to my photos of the live creatures!
How is it to trip so lightly over a palm frond? To have a tail that reaches out like a fifth hand to grab whatever's nearest? To know you're not going to fall?
It feels good to be a howler, hanging with my troop in the dry forest, waiting for sunset. We're going to sleep down the slope tonight, where the bougainvilleas are blooming. I'll wrap my tail around a limb and drift off, blinking at the moon.