The One-Eyed Monkey

What does a one-eyed monkey and broken jaw have to do with natural selection? While I’m not sure I know the answer, I will venture the following hypothesis. However, before I explain, I feel the need to begin with the back-story…

So I’m thoroughly focused on a butterfly larva that looks like a snake’s head. The larva was perched on a large leaf displaying a scale-like pattern and false eyes. More bark than bite, its appearance is intended to be a threat that invokes fear. Since I’m no bird, I recognized the bluff to be an example of mimicry and proceeded to photograph the “sheep in wolf’s clothes.” With about thirty images in the tank, I began to hear the approach of howler monkeys crossing the river valley. The Sarapiquí River divides a patch of rainforest and acts as a natural barrier between two competing troops of monkeys. Hoots and howls from one side of the river inspire a nearby troop to respond. This type of vocalization is a declaration of territory and is a howler monkey’s way of saying… “don’t cross the line.” 

I’ve wanted to photograph howler monkeys at eye-level without distracting highlights since my first visit to Costa Rica in ‘96. Up until now, most of my howler shots consist of indistinguishable black blobs in the crowns of tropical trees. Because I was working near a suspension bridge that crossed the Sarapiquí (think Raiders of the Lost Ark) I knew that I finally had a chance to get eye to eye. 
As the monkeys approached I grabbed my gear and ran to the bridge. The howlers effortlessly leapt from branch to branch. Big males negotiated large distances by wrapping their prehensile tail on one tree limb and, by bending the thickened branch, they would catapult themselves to the other side. Their apparent knowledge of basic physics seemed surpass that of my students.
A big male crossed overhead and I ducked in fear of a fecal bomb. As I swerved away from the incoming excrement, I began to shoot. A review of these first images revealed a large male with an eye to right and a hollow cavity to the left. The socket was empty! After a bit of bravado, the male grabbed a distant branch with its tail and ambled away. Exhilarated by all the activity, I continued along the bridge where every step sent a vibrating pulse to and fro. Another large howler peaked through the trees to my right and appeared to grin with a lopsided smile. At first I thought… “Hey this guy’s got Down Syndrome!” The droopy eyes and extended mandible reminded me of a chimpanzee that I had seen with trisomy. Later, during my image edits, I realized that howler must have broken its jaw in the past. It appears that the broken bones healed naturally through time.
This was a troop of derelicts and has-beens. They were retread monkeys with nowhere to go. How could a one-eyed primate that jumps from branch to branch survive in the treetops without the binocular vision that is the pride of our shared evolution? Furthermore, if nature were “red in tooth and claw,” as Lord Tennyson wrote in his epic poem In Memorium, what enabled the survival of a broken jawed leaf-eating giant?
The dominant male and his female were the last two monkeys to traverse the bridge. She was the mother of two, and these juveniles were the prize. The males in this troop encircled the youths, made body bridges between large gaps, and guarded the progeny from my eyes. While the rainforest appears to be a cornucopia of resources, suitable food is ephemeral and intraspecific competition is fierce. Two troops feeding on a common resource will fight for access to the limited food, territory and mates. While cries of howlers define a territory, physical interactions are inevitable. Competition results in winners and losers, and the losers often die. My troop of derelicts consist of survivors at the edge. Not threatened by the has-beens, the dominant male appears to have traded altruism for protection. The injured monkeys survive in the security of a troop, while the family unit has numbers to protect and ensure the survival of the progeny. I can't explain why a monkey with a broken jaw or one eye have survived; I can only assume that natural selection has somehow played a role.
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