The Warm Forests of North America

The Americas have had a second great interchange, and this time, North America was the loser.

With no advantage as great as what the Andes gave South America, a great many North American organisms perished in the sudden freeze. Even those with similar skillsets, such as the North American llama-like genus Hemiauchenia, were largely doomed by their geological placement alone. With this considerable die off, the land was open to southern invaders.

That’s not to say that no North American organisms flourished, for indeed, a great many of them live on. And that’s also not to say that North America has no geological or geographical features that helped life return to normal, either. It is home to the unrivaled bastion of life on this continent: a stretch of warm, wet temperate forest roughly bordering the Gulf of Mexico. Despite its relatively northern latitude, its comfortable climate and considerable humidity make it peerless among the temperate forests. It is comparable only to those forests on the coast of eastern Asia, though even these are cooler and drier. This temperate American forest begins in Florida and reaches the newly-formed Gulf of Panama, not straying too far from the sea’s edge, for it is the sea that gives it life. See, when the amount of heat and light reaching Earth crashed those three million years ago, the energy available to fuel complex meteorological systems was reduced sharply. As a result, many oceanic and atmospheric phenomena collapsed, unable to keep going under the new energy budget. Among these was the Gulf Stream, a warm-water current that once flowed from the tropical Atlantic, through the Gulf of Mexico, then to northern Europe. The current kept Europe relatively warm, giving it temperate forests at a latitude where areas of Canada and northern Asia had chilly taigas. As the Gulf Stream weakened, Europe froze over, while the remaining current instead emptied in the Gulf of Mexico. The quickly falling sea levels made much of the Caribbean islands fuse with Florida, trapping the current there. Now it is the Gulf of Mexico that receives tropical heat transported from the sea, and though it is a weaker effect than the one that had heated Europe, it is enough to support a warm, wet forest this far north.

The organisms that inhabit this area have a wide range of origin stories. Most that aren’t southern invaders trace their ancestry from the Rocky Mountains, with lesser contributions from the Sierra Madre Occidental. These montane denizens supplied many of the trees and shrubs here: Oaks, sycamores, gigantic chokecherry descendants, and occasional conifers such as junipers. The dependable rain feeds a healthy understory of tall shrubs and a tangled, diverse groundcover of grasses, ferns, vines, and more, all of which become blanketed in snow each winter. As for animals, most of the larger ones here came from South America. Guanacos are common browsers, sometimes seen beside rhea descendants, while relatives of the sicklebird run about the ground, often targeting flightless tinamous. Even large predatory culpeos have climbed to the top of the food chain here just as they did in the South American grasslands. Despite this large influx, some native North American animals have held their ground, as shall be seen.

Many of these survivors live in the trees: A great variety of passerine birds whose ancestors made it here through their gift of flight sing a rich chorus in the warmer months. More surprisingly, these trees house squirrels, which are some of the few rodents to fare well in this new world. Without the urge to hibernate, this patch of familiar trees saved their lives as the cold pushed them out of their native lands. Upon these prey items feeds another native, a descendant of the red fox (Vulpes vulpes). Red foxes, with their widespread distribution and varied diets, and with a body size not too large yet not too small, survived on a global scale. In this locale, they have managed to displace the weasels, whose smaller body size, strong bias toward carnivory, and dependence on small endotherms did them no favors. As a result, one lineage of red foxes took to the trees, chasing the squirrels and small birds for food and producing the flying fox.

Flying foxes can’t fly, unlike the long-extinct animals they share a name with. Leaping and running through trees, even up very thin branches afforded to them due to their much reduced size, one could say that they “fly” in the old sense of the word. However, the loose skin between their limbs gives away their namesake, for they can glide. This they use to extend their jumps and to gain ground on fleeing prey. 

Their gliding is not well developed as of yet; more just controlled falling. Flying foxes use it exclusively as a leaping aid, extending their pounces significantly and making landings softer. Their puffy tails provide some measure of control, yet they seem uneasy in the air and are quick to aim for the nearest branch. Still, the evolution of the patagium and gliding behavior is an impressive feat in only two million years. Perhaps what sped it along were the common and varied birds of prey in this area, most of which would find a small arboreal fox an easy meal. The flying fox is thus wary and skittish, knowing that its muddy brown coat isn't enough to evade capture.

Flying foxes are much better climbers than gliders, making use of the flexible digits in their paws to grasp branches. These digits are unusual in that they don't grasp by opposing each other; instead, the fox folds its toes inward to grip surfaces between its digital and metacarpal foot pads which have hardened for the purpose. Feeding on eggs, small birds, squirrels, and the occasional snake, the fox has little reason to be on the ground. However, older pups do sometimes make it to the ground after a playful bout of scurrying, only for their worrisome mother to pick them up and carry them back to the familiar trees. 

Upon closer examination, there are plenty of North American species that managed to succeed despite southern invasion. Among large animals, the most notable are descendants of Harrington's mountain goat (Oreamnos harringtoni). Sharing the genus Oreamnos with the Rocky Mountain goat, Harrington’s mountain goat would have gone extinct with the arrival of the first humans, but by a stroke of sheer luck, they have lived on. Like their cousin, Harrington’s mountain goat was adapted to cold temperatures and climbing cliffs. Unlike them, however, they were considerably smaller and occurred as far south as the caves of the Grand Canyon, while Rocky Mountain goats peter out in Colorado. As such, Harrington’s mountain goat survived while their congener died out, an odd reversal of what would have happened otherwise. In this warm temperate forest they have evolved into cow-like herbivores, browsing the many low-lying plants in the undergrowth.

In this undergrowth, quite a large assortment of reptiles and amphibians eke out a living. Though amphibians did better globally, reptiles have crawled through the extinction as well. Their ectothermy allowed them to spend weeks or months without food while similarly-sized endotherms starved. North America now has an assortment of unique reptiles such as the cherrytail garter (Thamnophis foetidus). 

Cherrytail garters are seemingly typical garter snakes. They are 45 cm (1.5 ft) long, cryptically colored, and prey on virtually any animal smaller than themselves with their barely venomous saliva. When threatened, however, the snake will hold its tail up, displaying the red patch of ventral scales that gives it its name. This is a warning for a newly-refined weapon, for although most garter snakes can release a distasteful smell from their cloacas, the smell this one has is much more putrid. If pressed further, the snake will spray, filling the predator’s nostrils with an overwhelmingly musky and rotten stench. This works a treat on virtually all mammalian predators, whose sensitive noses work against them, and on some reptilian ones as well.

If the stench doesn’t work, however, the snake is quick to find a crevice or pile of leaves to dig into, for not all predators are susceptible to its defenses. Most non-ratite birds have only a weak sense of smell, so the snake often hides away instead of spraying if it perceives a threat from above. Similarly, a few species of large frogs also inhabit this forest which will cram a garter snake into their mouths regardless of what it smells like, so the cherrytail garter is quick to flee from these threats as well.

Interestingly, as if to make up for these drawbacks, cherrytail garters have an additional use for their red tails and smelly defenses. When young, these snakes can supplement their diets with carrion flies by flipping their tails upside-down, then spraying. Attracted by the red color and the rot-like smell, these flies will swarm around and land on the tail, providing food to the young snake without the need for venturing into the dangerous world outside. 

One occasional visitor to the warm forests draws its line of origin from the semi-arid lands of southernmost Mexico to northern Guatemala. The area was hot and semi-arid before the extinction, so the warm and semi-arid conditions proved quite tolerable for the creature. Here it found refuge in specially made structures made of dry plant material. With a taste for blood, these wingless creatures now fly across the continent, though their range has yet to reliably expand. These animals are perhaps odd candidates for this sort of lifestyle, for their kind is dubbed the feather solifuge (Acariphagus volans). 

Solifuges are perhaps unfamiliar animals, so a description is in order. Also known as wind scorpions, camel spiders, and sun spiders, these ancient arachnids of order Solifugae are most notable for their gigantic chelicerae which form powerful jaws. They also sport large chemosensory pedipalps — used for combat, touch, and sometimes locomotion — which often look like legs or perhaps arms. Their first pair of legs are also rarely used for locomotion, instead finding use as long feelers. This means that, despite their eight legs, these animals are largely hexapodal. The feather solifuge in particular belongs to the subfamily Eremobatinae, which are North American solifuges with a clearly defined, straight border between the head and chelicerae and which have lost a claw on each front leg, a result of their use primarily for feeling instead of walking.

The dry plant material the feather solifuge is so fond of living in is in fact a bird nest. These animals reside in the nest walls of medium to large flying birds, such as descendants of turkey vultures or various eagles. Here they find a plethora of food, mostly bird parasites such as ticks and lice. With a small size for a solifuge – just a smidge over 2 cm long – they can prey upon these parasites without attracting much attention. In fact, through some unspoken understanding, the resident birds will even allow the small solifuges to climb right onto their bodies. Here they move about on their shortened legs, combing through the feathers with their robust pedipalps, squeezing through tight spaces thanks to their slightly dorsoventrally flattened bodies in search of any parasites they can find. Precisely how the bird knows not to attack the solifuge is a bit of a mystery, though the solifuge’s coloration certainly plays a role. The birds it associates with don’t usually eat animals so small anyway, which likely exapted them for taking on the arachnid. However, on rare occasion, the host bird may attack its stowaways.

The solifuge has one behavior besides eating parasites that benefits the host. When the bird molts and is covered in growing blood feathers, some will inevitably break and stop growing, becoming oozing sites of pain. The feather solifuge has evolved a habit of targeting these damaged feathers, grabbing on with its large jaws and shearing the top off to slurp up the excess blood. It will continue to shorten the feather until it is a stump in the skin, in which case it will clamp down and slowly pull it out to lap up what it can. This clears the pore of the damaged feather, allowing the body to make a new one to replace it. Miraculously, despite the pain, the birds are still unlikely to attack the solifuge during the process. This is because the solifuge tracks damaged blood feathers by following the smell of infection, which means that the feather has been a major problem for some time.

Most solifuges reproduce about once a year, but feather solifuges do so twice. Females gorge themselves with food before mating time begins since they will not feed again until their young hatch. Males and females will climb off their hosts into the nest, and after the typical mating practice where the male flips the female on her back to give her his spermatophore, the female digs into the nest wall to lay her eggs. She guards the eggs alone, leaving only when the babies hatch. Young feather solifuges eat small prey such as mites and fleas, never crawling on a bird until adolescence. The feather solifuge's practice of going from bird to nest and back again throughout their lives allows these animals to spread to new areas. As such, although they have yet to make a permanent presence in the warm North American forests due to the humidity, they are somewhat common visitors.