All children are essentially monkeys. Ever since I was tall enough to reach the lowermost branches of a tree and strong enough to swing myself up, arboreal life was the life for me. I remember climbing the mastheads of swaying pines, reading books in the crooked elbows of maples, and challenging myself to steal the most unreachable apples. I felt safer in trees; they seemed less like a high wire and more like a net to me. They held me safely out of sight of real siblings and imaginary enemies. They put me closer to the birds I loved. Throughout my childhood, I felt myself pulled up into the treetops by some plant magnetism, and pushed up there by some rambunctious animal urge.
When I went to college, I decided to study canopy ecology. And one of the things that sparked my interest was a story I heard from a professor about fish living in the trees. It went like this: when the Amazon River floods, the water level can rise enough to temporarily cover the lower branches of trees. Small fish, my professor told me, will sometimes lay their eggs in the submerged bromeliads on the tree branches, and when the floodwaters subside, the eggs hatch in a leafy fishbowl of water, replenished by rain and oxygenated by the plants themselves. Voila, fish in the trees.
Of course, I can find ZERO evidence to support this claim. And if fish ever do wind up imprisoned in tree branches, it would be by accident; after all, guppy fry in an epiphyte would soon make a nice snack for a hungry coati. But it turns out that there are a few fish in the world who head for the trees on purpose. Their existence challenges the public preconception about the definition of “fish,” and serves to help us understand the soul of a tree.
“Everybody is a genius,” said certified genius Albert Einstein. “But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.” Well, guess what, Einstein. It turns out that such a genius fish exists.
The Mangrove Killifish is an extremely aggressive and territorial fish that lives in the coastal mangrove forests from Florida to Brazil. It fights ceaselessly to protect its sparse puddles in the muddy tidal thickets. But the problem with puddles is that they dry up, and when the water disappears, it’s time to climb. A remarkable transformation happens in the killifish during the dry season: They lose their aggressive habits, climb the nearest tree or rotting log, and stuff themselves into the cracks and insect-bored holes. Their physiology changes: instead of breathing through their gills, they breathe (and excrete waste) through their skin. They can stay that way, a literal fish out of water, for over two months, just waiting in the trees for rain to come.
Mangrove Killifish aren’t the only fish that climb. The climbing perch of India and the pernicious snakehead fish of Africa and Asia — and lately, seen taking over the waterways of North America — can not only breathe air, but get off the ground. Both have been found on low logs or tree trunks, taking huge gulps of air while they wait for danger to pass. Another species, the climbing catfish has even evolved modified fins that can grasp rocks and tree trunks. While these fish don’t possess lungs, a wrinkly substitute called a labyrinth organ allows them to absorb atmospheric oxygen, allowing them to flop between bodies of water and survive in stagnant pools where the levels of dissolved oxygen would otherwise be stifling.
Okay, hugging tree trunks and rotting logs isn’t exactly like climbing into branches. But it does raise a few interesting points about our evolutionary transition from water to land. You know that cartoon of fish walking out of the water and evolving via lizard and dog and ape into a human?
They’re leaving trees out of the picture. It is said that humanity only began once we descended from the trees, and terrestrial life really started with the evolution of climbing — first onto dry land, and then onto logs. If that picture were more accurate, they would be walking into the equivalent of a mangrove forest, not a beach. And maybe climbing some roots while they were there.
Arborescence is the term for a plant’s tendency to become tree-like. A tree is not part of a family of plants, but rather a form of plant: a tall, woody perennial with a central stalk. By this definition, majestic redwoods get the same label as grasses like bamboo, banana trees, and palms. Even bryophytes — ferns and mosses — can become trees; in fact, when the first fish came gasping onto the alluvial muck about 375 million years ago, ferns were the only trees. The thing that really defines a tree is arborescence, the desire to grow up. And unlike the opportunistic vines, it does so on its own initiative. Some plants are content to lie low in the shade, but the fecklessly competitive trees muscle each other out for sunlight, elevating their bounty of leaves, seeds, and fruit — and their promise of relative safety — out of the reach of earthbound animals. Everything shifted up.
So the animals followed. And they are still following. Though the fish-like amphibians that first stormed the beaches of Pangaea would have been comfortable for a while crawling on their bellies, the pressure to follow the perching insects and raise themselves above the jaws of their predators would soon have sent them scuttling up the nearest marshland stalks. Whether scooting on pelvic fins, or belly-flopping, or climbing with finger-like spines, they must have been compelled to climb not long after they were compelled to breathe air. We tetrapods have an arborescence of our own. We crave elevation.
Currently, the first known tree-climber is Suminia, an iguana-like reptile that preceded the dinosaurs, having browsed the treetops in the lower Permian Period, about 260 mya and around the time of Dimetrodon. But I wouldn’t be surprised if they found evidence that “fishapods” or early reptiles had been humping themselves awkwardly up trees at least 50 million years earlier in the Pennsylvanian Epoch, as the fern forests were growing taller and atmospheric oxygen was rising. Even if it was just a foot or two, as the Mangrove Killifish climbs, the need to escape upwards would have been so strong in that world of giant centipedes and toothy proto-crocodiles that our ancestors could not have resisted the urge to seek higher ground, above the ground.
There are animals that dive where submarines cannot follow, and have never heard of sunshine. There are animals that dig, or live in caves, or whose cervical vertebrae are fused and simply cannot look up. So it is probably the bias of a fallen species when I talk about the natural arborescence of life, the heliotropism that makes us seek the solace of heights. But knowing that there are fish that climb trees makes me glad, because then I know we’re not the only lowly creatures who are inclined to rise. It seems to be in our nature and our interest to reach upwards, to keep extending our existence further above the fray. The forests may dwindle or burn, and we may need to come back down to Earth many times in the course of our evolution. But the monkeys in us remember, and the children remember, and we will always find ourselves climbing again, resisting gravity, stealing apples from the tops of trees.