If you’re anywhere north of Florida right now, you have probably had it up to here with this winter $#!*. The human body changes in the colder months, as even in heated homes we simply spend more energy thermoregulating our bodies minute-to-minute. Besides gaining “winter weight,” we do something almost unique to mammals and birds: we shiver. When the body’s core temperature drops below a critical threshold, our muscles involuntarily twitch to generate heat. While exercising in the cold does heat the body somewhat, which is why your dad always told you to suck it up when you were out shoveling the driveway, most heat generated by exercise goes to waste as it is flung into the atmosphere. Shivering produces a nice, constant, and most importantly, internal heat that keeps the hypothermia at bay. Heat generation is unique to us “endotherms,” or what used be known as the “warm-blooded” animals. But there are always exceptions to the rule. If you can start a fire by rubbing two sticks together fast enough, it’s possible for even a cold-blooded snake to keep a fire inside.
The reticulated python of Southeastern Asia is the world’s longest snake. Experts disagree on precisely how long they can grow, but it starts approaching the kind of lengths by which you measure yachts; some say 28 feet, some say 34. Most reticulated pythons will only grow a mere 10-20 feet, but I’ll bet if you were accosted by a 15-foot python in the swampy jungles of Malaysia, you wouldn’t say “boo.” You probably wouldn’t say anything; pythons are constrictors that asphyxiate their prey, and very capable of putting a stranglehold on an adult human. Indeed, there have been several cases of humans being both killed and eaten by “retics,” usually swallowed whole and head-first. As a rule of thumb, most pythons are capable of swallowing and digesting animals their own weight, which can be as much as six grown men.
But such deadly coils can also be put to good use. Reptiles get a bad rap as uncaring parents; truly, if someone is called “cold-blooded,” it means they lack compassion. But while most reptiles prefer to lay eggs and skedaddle, a few are excellent mothers. And pythons are one of the few snakes that will coil around their clutch of eggs until they hatch, starving themselves in the process. And of the pythons, only a few species can actually help their young develop by rapidly vibrating their muscles to generate heat, up to 7 degrees above the ambient temperature. The snake has effectively taught itself to shiver. This technique costs them dearly; pythons can lose up to half their body weight tending a single clutch, and it may exhaust them so thoroughly that they may not be able to breed again for 2 or 3 years. But the pay-off is a clutch of hatchlings that are safe from predators and, at almost two feet long, are healthy from being incubated, both of which advantages cannot be taken for granted in the jungle.
Call it facultative endothermy — creating heat by exercise — or simply thermogenesis. It breaks the general rule that reptiles cannot regulate their body temperature except by moving their bodies to a warmer or cooler spot. When it comes to giving their offspring the best chance at life, some pythons can choose to make a fantastic leap forward in evolutionary adaptations, one which changed the world eons ago. You may hate winter, oh you endotherms, but do please try to imagine how much harder life would be if you had to move constantly to one spot or another simply to keep your body from shutting down. Being able to convert food into heating fuel gave us the power of choice; we were no longer confined by our immediate environment. We endotherms can go where we please, when we please. We can make snow angels without the fear of death, and live in Phoenix without spontaneously combusting. Shivering may not be a pleasant sensation, but it allows us a certain freedom. We can’t always trust the weather to stay sunny and mild, but we can always trust in ourselves.
Not every mammal is so lucky. Echidnas, the most primitive mammals still alive today, don’t maintain a constant 98.6 degrees F, but can let their bodies drop to 77 F before thermoregulation kicks in. To get a sense of what an internal temperature of 77 F feels like, consider that humans reach blue-faced hypothermia at 95 F. And naked mole rats have entirely lost their ability to thermoregulate. They are subterraneans, so the ambient temperature of their environment doesn’t change much, but their ectothermic physiology means that they couldn’t breach the surface of the soil and bask in the African sun if they wanted to. They are imprisoned in their tunnels forever.
And yet, pythons aren’t the only ectotherms to be able to raise their body temperatures manually. Great white sharks, for one, have the rare ability to heat their own blood by running it alongside their working muscles, thereby keeping the most important organs (the stomach and liver, mainly) up to 25 degrees F warmer than the surrounding water. Keeping your belly warm and functional in cold water is pretty important when you follow deep-diving and blubbery animals such as elephant seals. And many insects can warm themselves by vibrating their wing muscles. My favorite example of this is the case of the Japanese honeybee, which uses heat as a weapon to literally roast its enemies alive. That enemy is usually the Giant asian hornet, also called the tiger hornet or the sparrow hornet. Armed with a venomous sting that can dissolve human flesh, one hornet is capable of taking down dozens of bees by itself, but a scout hornet can even summon others to help finish the job. So when a tiger hornet invades a Japanese honeybee hive, it has to be neutralized immediately. When the hornet is detected, the colony will mob it in a defensive ball and start “shivering” their wing muscles rapidly. The immobilized wasp in the ball’s center starts feeling the concentrated heat of five hundred vibrating bees, which can reach a temperature of 117 degrees F. Essentially, its brain boils in its own blood. A few bees in the center of the ball may be lost, but most individuals can survive higher temperatures than the hornets, and the hive is saved.
So the ability to shiver has pretty profound effects. But while a glowing fireball of red-hot bees is a pretty awesome example of the phenomenon of facultative endothermy, the python is beginning to represent its most troublesome case. Consider the burmese and rock pythons of Florida, pets that have escaped into the Everglades and parts north and are breeding. Not only do they have no natural predators — Burmese pythons can easily eat an alligator — but their ability to protect and incubate their eggs gives them a clear advantage over other snakes in that subtropical jungle. More disturbing, the rock python, which can grow to 16 feet, has been known to eat children. A ten-year old in South Africa was swallowed by a rock python in 2002, and in 2009, a reticulated python that was being babysat for a friend was caught trying to asphyxiate and swallow the snake-sitter’s three-year old son. (Attention mothers: According to Dr. Spock, toddlers and 18-foot long pythons do not mix.) Though caring and attentive mothers, pythons are nonetheless deadly killers capable of choking the life from a full-grown guerrilla soldier and digesting him over the course of a month. Setting a python free in the backyards of America? It’s enough to give you the shivers.