Return of the Howlers

Apologies for the long radio silence. Travelling and the obligatory pre-travelling frenzy shut down the blogging assembly line for a couple weeks. Having wrapped up my west-coast jaunt (thanks to the great crowd that came out for the CSPAN taping at Stanford), I can write a bit about some of the new science that has caught my eye.

Crouching on top on the pile are howler monkeys. Howlers have become frequent visitors to the Loom, much to my surprise. For some reason they've recently started to have a lot to say about evolution--particularly, as odd as it may seem, about the evolution of our own species. As I wrote in an earlier post, we humans have good eyesight compared to many other primates. We have three genes that make receptors for light in our eyes, each sensitive to its own band of the spectrum--red, green and blue. The combined sensitivity of these genes lets us tell the difference between yellow, organge, pink, and red. Other apes and monkeys in the Old World also have trichromatic vision, as it's called. On the other hand, almost all monkeys in the New World have only two color genes, as do lemurs, which are the most primitive of living primates. One gene is sensitive to blue, and the other is broadly sensitive to the red-to-green part of the rainbow. As a result, they can't discern colors as well as we can.

Scientists have proposed that the first primates had only the blue and red/green genes. When some monkeys colonized the New World, they took with them this poor color vision. Only later, in the ancestors of today's Old World monkeys and apes, was the red/green gene accidentally duplicated. The two copies gradually mutated until they became sensitive to different colors. What would drive the rise of better color vision? It seems that some 30 million years ago, the climate in Africa cooled and dried, altering the forests. Leaves became a much more abundant source of food than before. With eyes sensitive to the colors grading between red and green, Old World monkeys could make out tender young leaves lurking in the dappled foliage.

Enter the howlers. Unlike all other New World monkeys, howlers eat a lot of leaves. And it turns out that unlike all other New World monkeys, they also have trichromatic vision. They appear to have independantly evolved these genes some 10 million years ago.

Another striking thing about Old World monkeys and apes is their sense of smell. Many of the genes (half or more) that build receptors in their noses are broken. In other words, they have mutated to the point that they unable to be used by a nerve cell to build a receptor. Mice and dogs, which have intense senses of smell, have mostly intact olfactory receptor genes. So do lemurs, and so do almost all New World monkeys. One possible explanation has to do with food. To check fruit to see if it's ripe or rotten, it helps to have a keen sense of smell. But if you're eating leaves, smell becomes less important than vision. Howlers, as leaf eaters, offer an independant test. Not only do they have trichromatic vision, but they have lots of broken genes for smelling.

Now here's the twist: noses can do more than just smell. In many land vertebrates, there's a special clump of neurons in the nose called the vomeronasal organ. This mysterious organ is specialized for detecting only one particular kind of molecule: pheromones given off by other animals. Many animals can recognize relatives with pheromones, and males can tell whether females are recptive for mating by sniffing pheromones in their urine or released from special glands. But some land vertebrates have lost some or all of their ability to detect pheromones. Birds, for example, don't have a vomeronasal organ. Nor do Old World monkeys and apes. Regardless of some ad may promise about pheromone-laced cologne, we humans have little if any ability to detect pheromones. The genes that build pheromone receptors in other species are broken in our own genome.

One explanation for our missing vomeronasal organ is that our eyes destroyed it, much as they destroyed our sense of smell. With powerful eyes for searching for leaves, our ancestors became more sensitive to visual displays in the opposite sex. The females of many Old World monkeys and apes get red, swollen genitals when they're ovulating; males take that as a signal to try to mate. As these primates depended more on this visual language of love, their pheromones became less important. Birds support this hypothesis--they have four genes for color, giving them even better vision. And instead of pheromones, they depend on beautiful feathers and combs to attract mates. (Female humans, along with the females of a few other Old World primates now conceal their ovulation. That shift did not, however, bring back our vomeronasal organ.)

Recently, a group of researchers asked the next logical question: what about the howlers? In a paper in press at Molecular Biology and Evolution, they reported a surprising result: howlers have plenty of perfectly good pheromone genes. So three-gene color vision doesn't automatically wipe out pheromones. There are a couple potential explanations. One is that the link between vision and a loss of pheromones doesn't exist at all. The other--which the authors of the report favor--is that good color vision only raises the possibility of abandoning pheromones. They point out that Old World monkeys and apes tend to live more on the ground than their New World cousins, in open forests and savannas as opposed to dense jungles. It's a lot easier to see a distant potential mate in Tanzania, in other words, than it is in Brazil. For howlers, pheromones may still have an edge, even with color vision.

I have no idea what secrets howlers will reveal next. I'm assuming that they didn't invent the axe, the wheel, and the jet ski on their own. But beyond that, nothing will surprise me.