Naturalist Notebook

Snakes Eaten, Snakes Eating

Recently a friend asked me if there are birds that eat garter snakes. A few, I told him. Red-tailed hawks are known for making off with snakes. It is always an odd sight to see a snake writhing like a strand of kelp in the sky. Crows and ravens eat garter snakes. They eat everything. And, I told him, I have seen a great blue heron wrestle with a garter snake. Beyond that, I said, few birds would be brave enough to try.

Then reader Mira Thompson of Bay Lake told me a story that blew my mind. On a quiet road she came upon a robin flapping on the ground. It gathered itself and took flight with a snake dangling from its beak. From 10 feet up it dropped the snake. It swooped down, paused to watch the snake, then fluttered around to peck at it. The snake reared into a defensive posture and the robin attacked. It pecked, grabbed the snake and dropped it again and again. The fight moved inch by inch across the road. Finally, the snake was weak enough that the robin took it in its beak and flew off.

Garter snakes do some impressive eating of their own. Gardeners love them for their appetite for slugs and snails. They’ll eat earthworms, insects, frogs and small fish. They also eat our local celebrity, the rough-skinned newt. This is a feat worth investigating. Rough-skinned newts are packed with tetradotoxin, the same neurotoxin that makes Japanese pufferfish so deadly. Studies have shown that every potential predator of rough-skinned newts, from herons to raccoons, will die within minutes of ingesting them. In fact, newts will climb unharmed from the mouths of dead fish 20 minutes after being swallowed.

The common garter snake, Thamnophis sirtalis, is the lone exception. Research on populations across the Pacific Northwest has suggested that as the snakes acquire more and more resistance to the newt’s toxins, the newts become more and more toxic — an evolutionary arms race. But how can the race proceed if the newt dies when a snake tries to eat it, even if the snake dies too? More recent studies have found an answer: snakes will partially swallow newts before deciding whether or not they are too toxic and either spitting them back out or digesting them. The snakes, in other words, test their own resistance against the toxicity of the individual newt.

The cost is high. Within minutes of catching a newt, even if the snake will successfully digest it, the snake cannot hold its head steady. It opens its mouth as wide as it can and rubs it on the ground. In extreme cases, it writhes and bites itself. Its decision to digest or not digest may not be a decision at all; the newt might simply escape if the snake is sufficiently incapacitated. But if the snake manages to keep the newt down, the few hours it spends as a drunken wreck are rewarded with a meal that might last it over a week. And newts are a common prey and easy to catch.

How the Highest-ever Temperatures Cooled

On the final day of June’s record-smashing heat wave, I braved the oven outside with a laser thermometer. It’s well-known that cleared land can be 10 degrees hotter than adjacent forests on a hot day, and urban areas another 10 degrees hotter still. But the heat wave made it clear just how important night time temperatures are when a region desperately needs to cool. I measured different surfaces at 2 p.m., when the air temperature was around 100 degrees. Asphalt was 158 degrees in the sun; forest floor was 95 degrees in the shade; pasture was 115 degrees in the sun and 100 degrees in the shade; and, surprisingly, the coolest ground was the asphalt of a driveway under total shade, measuring 87 degrees. I began to wonder about not just maximum temperatures but the rates at which different parts of our landscape cool.

So, over the course of that evening, in its own right an extraordinary event with air temperatures dropping 50 degrees in less than five hours, I returned to my measuring points.

By 5 p.m. the air temperature had reached 105 degrees. Yet all the surfaces measured cooler than before. How could that be if the day had grown hotter? The answer, I eventually realized, is that after the sun leaves its apex, much of the day’s heat comes from surfaces rather than the sun. As they slowly release their heat, they keep the air temperature higher than it should be based on the angle of the sun. It’s the same reason that July and August are the hottest months when maximum sun exposure is in June.

By 8 p.m., with the air at 90 degrees and shade everywhere, the pastures matched the forest floor around 80 degrees. The road was still over 100 degrees. By 10 p.m., with the air at a merciful 72 degrees, the pastures and forest floor were in the mid-60s. The road was 86 degrees. The shaded driveway, which was coolest at midday, was now at 72 degrees, ten degrees warmer than the natural areas.

That shaded driveway tells an interesting story. Rock, whether natural or manmade, can hold a lot of heat energy. Because of that capacity, it takes time to bring its temperature up. The sun has plenty of raw power to quickly heat unshaded concrete to absurd temperatures, but when it is shaded it might actually function as a heat sink — for part of the day. As I saw, while natural surfaces cool quickly in the evening, even the shaded concrete with its load of heat energy would be radiating increased temperatures deep into the night.