How Animals Use Physics To Survive
How does a gecko manage to walk on the ceiling? Do cats drink like we do? And what happens when a dog shakes water off its coat? A new book explores how animals use physics in their daily lives.
Book Excerpt: ‘Furry Logic’
By Martin Durrani and Liz Kalaugher
It’s getting hot in here
In Indiana Jones and the Raiders of the Lost Ark, dashing archaeologist Henry ‘Indiana’ Jones, played by Harrison Ford, faces his worst nightmare. Terrified of snakes, he must brave a secret Egyptian chamber teeming with the reptiles to stop the Ark of the Covenant falling into enemy hands. As in many movies, the scene draws on this animal’s classic image as a creature of both evil and power.
Steven Spielberg, however, had more than symbolism on his mind. After scouring every pet shop in London for snakes, the movie director’s staff had to cut up rubber hoses to make up the numbers. Even some of the ‘snakes’ weren’t snakes but legless lizards, a difference that’s crucial to a biologist, if not to a desperate film crew. Like the slow worm in your compost heap, legless lizards are – as their name suggests – lizards whose legs have shrunk or disappeared.
The actors’ motto ‘never work with children or animals’ could have been coined with snakes in mind. These reptiles bite. They slither. They’re scary. But it’s not just filmmakers who have problems. Biologists studying snakes in the wild have a tricky time too. Snakes are hard to track down, and once a snake has spotted you it’ll slide away or, worse, inject or spray you with venom that could kill if it gets under your skin or into your eyes.
Luckily for our story’s non-phobic hero, Rick Shine from the University of Sydney in Australia, one snake is an exception to this ‘difficult-to-work-with’ rule. Catch it at the right moment and it barely cares if you pick it up. Shine could, if he wanted to, put these reptiles in a car and take them for a ride. Up to a point, as we’ll find out later, he did. In autumn, winter and spring the red-sided garter snake (Thamnophis sirtalis parietalis) hangs out, like Indy’s nemeses in Raiders of the Lost Ark, in huge groups, sometimes tens of thousands strong (there’s a number to make a film director jealous). They won’t be in a secret Egyptian chamber, but in limestone cracks under the frozen soil of the Canadian prairies in the province of Manitoba. For this snake is a record-breaker: it’s the most northerly-living reptile in the western hemisphere.
Living where temperatures plummet to -40˚C and snow coats the ground for eight or nine months a year seems crazy. Reptiles are ectotherms (from the Greek for ‘heat from the outside’) and can’t generate their own body heat by burning food. Instead they rely on outside sources like the Sun, basking in its rays until they’re warm enough to move fast and reproduce. Faced with freezing conditions, red-sided garter snakes huddle together for warmth in their winter hidey-holes and brumate, the snake equivalent of hibernation.
But being in Manitoba brings benefits for the red-sided reptiles, and for those studying them too. For a start, once it arrives, summer is warm, with temperatures touching 30˚C. In April or May the snakes emerge and writhe around on the barren soil in groups hundreds or thousands strong. This sight, which looks like a giant tangle of squirming spaghetti, has intrigued people for years. What are the snakes up to?
With a plot that even Spielberg would be proud of, the mystery of the red-sided garter snake involves cool physics, lots of sex and a soupcon of gender-swapping. Not among Shine and his colleagues, we must stress, but the snakes themselves.
Where are our manners? We should get to know this snake before we pry into its sex life. First let’s meet the wider family. Garter snakes live throughout North America, although only those species that dwell where winters are extra-cold brumate. You’ll find these reptiles in woods, forests or grasslands as long as there’s water nearby. About half a metre long, they’re venomous enough to kill small prey but not humans. Favourite snacks include frogs and fish, though the snakes will also feed on earthworms, rodents and small birds.
As for red-sided garter snakes, these reptiles don’t, at first glance, live up to their name; they’re black with cream stripes running the length of their body. Their red sides lie beneath overlapping scales and you can only see them if the snake puffs up its body in annoyance. During Manitoba’s three or four months of summer, the reptiles make the most of the warmth and can stray more than 15km (9 miles) from their dens in search of food.
When the first chill hits the air – in August, no less – the snakes head back to their bunkers. At first they stay down there only at night or when it’s cloudy. Once the daytime temperature drops below freezing, however, the reptiles put themselves under house arrest and snuggle together ready for nine months of cold. Their winter homes lie 6m (20ft) underground, below the frostline. At 10˚C, the ‘indoor’ temperature is no summer’s day but balmier than the -40˚C outside. While they brumate, the snakes barely expend any energy, existing almost in suspended animation. They eat nothing and hardly breathe, getting up only now and then for a drink of water.
All of a slither
Stand by one of the snake pits in or around the village of Narcisse in late spring, the long-awaited sunshine warming your face, and you’ll enjoy one of the most unusual sights in nature. Facing you will be a writhing carpet of mud-caked reptiles that have just emerged from their burrow and are huddling together once more. Stare closely and you’ll notice something even more odd: almost all the snakes are males. At about 45cm (18in) long, they’re some 15cm (6in) shorter than the females.
Undaunted by their smaller size, the male snakes venture outside several weeks before the females. By lying in wait, each hopes to be first to mate. As they slither past one another, the early-risers flick their tongues in search of chemicals called pheromones that the females release through their skin. After nine long months of brumation, sex seems to be the males’ number one aim.
But there’s a hitch. As soon as the females emerge from their lair, most leg it (as far as that’s possible for a snake). Any who are slow off the mark become the centre of attention in a frenzied mating ball of tens or even hundreds of amorous males, each trying to loop his body around her so he’s in the right position to mate. The female finds this stressful and does what she can to escape. With males outnumbering females by 10 or more to one, a male’s chances of reproducing are slim.
The giant tangle of male snakes and these smaller mating balls are freaky enough, but something even weirder’s going on. Look carefully and from time to time you’ll see males give their full attention not to a female, but to another male. We’re not being sexist but some males’ behaviour is most ungentlemanly. Literally. He-snakes pretend to be she-snakes, or ‘she-males’ in the scientific lingo, giving off pheromones to impersonate females. She-males are easy to spot: they’re the same length as other males but, having slithered from underground later, are still coated in mud. Rarely courting ‘other’ females, these transgender snakes crawl around sluggishly instead. Soon the ‘real’ males jump on them.
Harder than identifying the she-males is understanding what they’re up to. If he wants to mate with a female, why does a male pretend to be the same sex as her? This puzzle set biologists scratching their heads. Perhaps becoming a she-male gives a male a reproductive edge so he can steal sperm from other males or avoid attack from larger rivals. But Rick Shine wondered if hanging out in a giant heap isn’t only about reproduction. Could it also be a matter of heat?
Reptiles in the bag
Fortunately biology was on the researchers’ side. You’d think desperate-to-mate garter snakes wouldn’t take kindly to interference. But in late spring, Shine and his colleagues can do what they like with the reptiles – male, she-male or female. Pick them up, measure them, put them in a bag; the snakes don’t have the energy to care, making them almost ludicrously perfect for study. That’s why Shine made a pilgrimage from Australia to snake dens near Narcisse seven years out of eight from 1997 to 2004. ‘Having 10,000 amorous snakes in an area the size of a living room is a snake biologist’s idea of heaven,’ he says.
To find out the she-males’ secrets, Shine and his colleagues simply sat in the grass alongside red-sided garter snakes that were fresh from their winter quarters. Grabbing individual she-males by the tail, the researchers presented them to ‘real’ males to see how they’d react. The males almost always found the she-male a turn-on, pressing their chins on him/her and lining up their bodies. So males definitely fall for the she-males’ pheromone charms. But what’s in it for the she-males?
Time for a more cunning plan. Shine kept one group of she-males at 10˚C, the temperature of their bunker. He warmed another batch of she-males to 28˚C by putting them in cloth bags and placing them on the electrically heated front seats of the team’s four-wheel-drive Yukon hire car. Next the team brought the two groups to a common temperature of 25˚C, heating the cool snakes up on the car seats, while letting the warm group chill off naturally.
Holding each 25˚C she-male by its tail, Shine presented him/her to five different males. As expected, the males flicked their tongues faster and tried to sidle up to the she-male. But their interest didn’t last forever. The guys stopped sniffing around a snake from the ‘warm’ group within about three hours. ‘Cool’ snakes won attention for five hours. The males’ loss of interest revealed that the she-males had stopped gender-swapping and gone back to being simple males, with the ‘warm’ she-males reverting to type faster than the ‘cool’ ones. The conclusion was clear: male red-sided garter snakes become she-males to warm up as fast as they can. By pretending to be a female, a she-male entices other males to press themselves against what they see as a potential mate. Rubbing against his/her warmer rivals, the cold she-male draws heat generated by their muscles into his/her own body. Heat, as we’ll hear later, only ever flows from hot to cold.
Excerpted from FURRY LOGIC by Martin Durrani and Liz Kalaugher. Copyright © 2017 by Martin Durrani and Liz Kalaugher. Excerpted by permission of Bloomsbury Sigma.