One of the craziest things I ever did was to climb a pochote tree.
One of the craziest things I ever did was to climb a pochote tree so I could take a picture of a bird. If you've ever seen a pochote you'll know why I say that. It has long, thick, sturdy spikes about the size of your big toe. At Hacienda Barú National Wildlife Refuge, many of our guided tours pass through a pochote plantation, and curiosity about the spikes has prompted many of our visitors to ask why the pochote trees have such long spikes. The truth is that nobody really knows for sure, but there is one theory that seems to fill in all the blanks and tie up all the loose ends. It was told to me by a young biologist, and I didn't think to ask him the source of his information. But right or wrong I think you will find the theory interesting.
Around 10 million years ago, North and South America were separate continents each with its own peculiar flora and fauna. At that time the Bombacaceae family of trees, to which the present day pochote tree (Bombacopsis quinatum) belongs, was found only in North America. They didn't have spikes then, like they do today. That was about the time when volcanic activity and tectonic plate movements were combining forces to create the land mass which now connects these two large continents. That land bridge, of course, is called Central America. It is impossible to place an exact time when the connection was completed, because sea levels were not constant.
According to Anthony G. Coates, editor of the book Central America, A Natural and Cultural History, beginning about three million years ago, the earth began to experience a regular cycle of ice ages approximately every 100,000 years. During each ice age much of the earth's water froze, tying it up in glaciers and the polar caps. This caused sea levels to drop. After each ice age the polar caps melted and sea levels rose again. The difference between the highest and lowest sea levels was 130 meters or more. Due to this variation, the land bridge, in its early stages wasn't always complete. During glacial periods when sea levels were low all sections of the bridge were above water and the land connection between the continents was complete. Then when the ice melted, sea levels rose, and water again covered the lowest sections of land bridge. Accordingly the migration of species between the two continents happened sporadically over a period of several million years.
According to S. David Webb, a contributor to the book Central America, A Natural and Cultural History, the first South American land mammals migrated to what is now Central America nearly 8 million years ago, when the land bridge was probably no more than an archipelago of widely spaced islands. The first to swim the wide gaps were two kinds of large ground sloths, described by Webb as "bear-sized." One of these eventually extended its range as far north as Alaska and spread through all the regions of what is now the continental United States. There is no evidence of further mammal crossings until about 5 million years later. The rest of the ground sloths and tree sloths didn't begin arriving until about 3 million years ago, by which time the land bridge was looking more like a bridge and less like a string of islands. Today Central America still harbors two species of tree sloths, both of which are seen regularly on Hacienda Barú National Wildlife Refuge and Manuel Antonio National Park.
The sloths of today are raccoon-sized mammals that hang in trees, often beneath branches, and sleep a lot. They are categorized as folivores, meaning that they eat mostly foliage, or leaves. I have seen them eat the fruit of the strangler fig and the cecropia trees, but most of the time they eat only leaves. That's probably why they sleep so much. Leaves are high fiber and low energy and are quite difficult to digest. Sloths eat many different kinds of leaves, but they seem to have their favorites and will sometimes eat all the leaves off of certain trees leaving the branches denuded.
The pochote plantation at Hacienda Barú borders primary forest. We rarely see sloths in the pochote trees, presumably because of the spikes, but the boundary between the pochote and the natural forest is an excellent area to observe them. Both the three-toed sloth (Bradypus variegatus) and the two-toed sloth (Choloepus hoffmanii,) hang out in the unspiked branches of the rain forest trees, at the edge of the pochote plantation. From their comfortable perches the sloths reach over, pull the pochote branches to their mouths and eat the leaves. Very seldom will one actually venture over onto the spiked branches. Nevertheless, the branches of a pochote aren't nearly as scary looking as the trunk, and it is possible for a very careful sloth to maneuver through them. It's tricky and they do it very gingerly, but there are occasions when we find sloths hanging, eating and sleeping in the spiked branches.
Some of the giant ground sloths that came from South America were 5 to 6 meters tall or about as high as the peak of the roof on a small single story house in Costa Rica. Presumably the leaves of the trees from the Bombacacae family were as appetizing to sloths then as they are now. The theory goes that the ground sloths devastated the vast majority of the Bombacacae family eliminating the young trees before they had time to mature and produce seed. The only individuals that managed to survive the onslaught were a few individuals that had an rare gene that caused them to grow spikes on their bark. Those were the only trees that the sloths left alone and the only ones to flower, seed and pass on the genes for spiked bark.
The individuals with the most and longest spikes were the most likely to survive and pass on their genes to future generations. In a relatively short time, perhaps 1000 years the entire Bombacaceae family acquired the spiked characteristic in the young growing trees. Once the trees reach a diameter too thick for a sloth to climb, most of the spikes disappear. You can believe this theory or not. To me it seems reasonable.
The relationship between the sloth and the pochote didn't end with the appearance of spikes, but before I get into that, let me tell you a little more about the three-toed sloth. According to G. G. Montgomery in Costa Rican Natural History, baby sloths are weaned off of milk at about six weeks of age and, from that time forward, eat only leaves. The infant's first taste of foliage comes at about two weeks of age when it begins to lick the green saliva that foams around edges of its mother's mouth while she is chewing leaves. I had the good fortune to see this once, at very close range, while climbing in the crown of a giant rain forest tree, at Hacienda Barú. Even though the baby sloth quits nursing milk at six weeks it stays with its mother for another four and a half months, after which time it is weaned socially.
In other words it goes one way and mom goes the other. In practice, however, it isn't quite that simple. Weaning time is very traumatic for the juvenile, and once the mother decides it is time to make the break, she needs to figure out how she is going to ditch junior. Out running him is hard work.
Every year the spiked pochote trees lose all their leaves around the beginning of December. They remain bare branched until April when the rains resume. In the interim they flower and produce a seed pod which opens and sends the pochote seeds floating on the wind in puffs of cotton. Social weaning time for three-toed sloths comes along about the time the trees are putting on tender new leaves. At the same time we often find weaning aged sloths in the pochote branches as far at 30 meters into the plantation. For the first two years it was only one, then a second one appeared. In 2001, the fifth year, there were three.
One day while guiding a group of visitors, we saw a mother sloth with her large male offspring about 20 meters into the pochote plantation. A couple of hours later when we returned on the same trail, the youngster was alone at the spot where we had seen both earlier, and the mother was almost back to the rain forest. The weanling, being much less adept at moving through the tree tops took a week to work his way out on the spikey branches. In the meantime he had plenty of tender young pochote leaves for nourishment. When he finally made it to the rain forest, mom was long gone. For about two weeks junior hung in the tree where he had last been before his mother took him into the pochote and abandoned him. Finally the youngster began to venture out and explore his surroundings. It appears that one female figured out this weaning strategy and later at least two more started using the same strategy. Whether each mother figured it out for herself, or the first one taught the other two, is a mystery.
Sloths are learning other ways to exploit the pochote, but first a little more background. Back in the early 1970's, when I first came to Hacienda Barú, it was a cattle ranch and rice farm. There were only 3 forested areas in the coastal lowlands. All three were surrounded by deforested land. Over the years we created natural corridors to allow wildlife to move freely between these forest fragments. By the mid 1990's all of the forested areas on Hacienda Barú were connected to one another. None remained isolated and the increased movement of wildlife was amazing. I first saw a sloth in the lowlands in 1996. Today they are a common sight along the corridors and in all the forested areas.
Near our house, in the lowlands, my wife Diane has some horses and enough pasture for them to graze. The fences are lined with pochote interspersed with other tree species, and lines of pochotes cross the pastures in many places. One day while walking over to the office I spotted an adult, female, three-toed sloth hanging in the spikey, leaf less branches of a pochote tree, the third pochote in that line. It was such an unusual sight that I called the Hacienda Barú naturalist guides to come and see it. Most seemed to think that it had come to the edge of the secondary forest and continued on, looking for more food and mistakenly followed the line of bare-branched pochotes in hopes of finding better foliage farther down.
But that sloth hung around for more than a week. We saw it almost every day and always in a pochote. There were other species of trees nearby, ones without spikes. Obviously this sloth was either a masochist, or there was more to the situation than we were observing. Then one afternoon I saw the familiar sloth in a pochote tree about two hundred meters from its original appearance. This time I had my spotting scope and quickly focused it on the sloth. She was eating the pochote flowers and newly formed seed pods. We named her flower girl. She had learned what the squirrels and birds figured out years ago. Pochote flowers and pods are very tasty, so much so, that it is worth learning to do an obstacle course in order to get them. Who says you can't teach an old sloth new tricks
When you take into consideration that prior to 1985 there were no pochote trees anywhere in this area, these two new aspects of sloth behavior become even more interesting. Pochote grows naturally in Guanacaste Province, but all the pochote in our area was brought here in the last 20 years. By planting it at Hacienda Barú National Wildlife Refuge, in places where sloths could get to it, we unwittingly set the stage for the sloths to demonstrate their innovative talents. In other words they learned a couple of new tricks. I guess you could say the sloths have a love-hate relationship with the pochote; love those leaves, flowers and pods, but hate those spikes. And, from the pochote's point of view; hate those sloths.
For further reading I recommend the two books cited above: Central America, A Natural and Cultural History, edited by Anthony Coates, and Costa Rican Natural History, edited by Daniel H. Jansen.
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