Estivation- A Midsummer’s Nap

by Sarah Witcher

“Ah, summer, what power you have to make us suffer and like it.” ~Russell Baker

An Eastern box turtle cools off in the water, by Rebecca Bailey, 2023 DNR Photo Contest

Imagine that you are a box turtle. You slowly meander through woods and backyards, in search of water and snacks. The dappled browns and oranges on the top of your shell (your carapace) blend seamlessly with the dried leaves and speckles of unrelenting sunshine streaming through the trees, making you almost invisible to onlookers. 

It’s been oppressively hot for weeks, and the blood in your veins feels like dried up nail polish, sticky and sluggish. Speaking of slugs, you eat one off the bottom of a leaf, but it’s no use – you still feel thirsty. By midafternoon the heat is triggering an instinct you didn’t know you had; you are getting very sleepy. Slowly and methodically, you dig a shallow depression under the cover of jewelweed and a tenacious blackberry bramble. You settle in against the cooling soil, close your eyes, and wait.

A Maryland summer can almost always be relied upon to bring relentless heat and humidity. For those of us who don’t love the heat or who have to work in it, those days can bring to mind fantasies of escape, like wishing you could take a nap and wake up when the heat is gone! Unfortunately for humans, we are stuck in circadian rhythms and bodies that need near-constant feeding and watering to survive even a few days. Our fragile physiology can usually maintain the constant sub-100-degree temperatures we need to thrive, but we are notoriously susceptible to damage from exposure and extreme temperatures. This forces us to flee summer heat in the shade or the air conditioning rather than lean into the swelter.

American Toad. Photo by Jordan Kron

Most of us have heard of hibernation, a prolonged period of dormancy to escape cold temperatures, but did you know there’s a version of that for the heat? Fortunately, quite by design, several Maryland animals have figured out a lower-tech solution than swimming pools and forced air systems: it’s called estivation ( from aestas, the Latin root word for summer). Like hibernation, it’s not simply a behavior modification like choosing to sleep. Metabolic rates actually dramatically decrease, slowing or even halting some chemical processes that consume energy. This isn’t just to beat the heat for increased comfort! The decrease in metabolism can be between 70-100% of the normal rates in some animals, and it proves to be an essential survival strategy in times of food shortage and drought.

It’s easy to understand how animal residents of deserts and other arid climates might need to estivate as a survival strategy. While Maryland’s heat index may be less harsh, we still have a few species that utilize estivation – a tactic that may serve them very well as our global climate changes. Reptiles and amphibians are our best-known summer snoozers. Here are a few examples:

• American toad (Anaxyrus americanus, common): Like many toads, this one is mostly nocturnal in the hotter season. It’s been shown to estivate in burrows to avoid unfavorable conditions, at times even transitioning straight to hibernation as summer temperatures cool off.
• Eastern box turtle (Terrapene carolina, common but populations are declining): Tthis familiar species has been well-documented as a frequent summer napper.
• Tiger salamander (Ambystoma tigrinum, endangered in Maryland): Young of this rare species have been shown to estivate in older studies, burying themselves under dry, sandy soils to reach a moist layer beneath until conditions improve.

Tiger Salamander. Photo by Kevin Stohlgren.

That said, herps aren’t the only ones with tricks up their sleeves. Invertebrates get in on the summer shut-down, with numerous species of terrestrial snail sealing up the opening of their shells and secreting mucus barriers to keep from drying up like a raisin, depressing their metabolisms once they’ve secured themselves. Insects join in too, like ladybugs, some weevils, and even the hemlock wooly adelgid (Adelges tsugae). This invasive species of insect, native to Japan, was first observed in the eastern United States in the 1950s. Its life cycle is complex and involves multiple generations of offspring, but in this region, it notably goes dormant as a nymph around the month of July. Research to help scientists understand the timing of this process has been essential in targeting effective treatments to save native hemlock trees from the destruction caused by this tiny insect.

Maryland Conservation Corps members inoculate trees against damage from hemlock woolly adelgids. Staff photo.

This summer, as you meet animal neighbors on a toasty warm day, take time to respect and admire this superpower called estivation. Even tardigrades can’t take the heat like some animals who can estivate! These microscopic organisms (affectionately nicknamed “water bears”) are famous for their ability to survive in the vacuum of outer space or insanely cold temperatures. Tardigrades routinely enter a “tun” state where they shrivel up in suspended animation – but even that ability doesn’t function for more than a day in temperatures above 145 degrees. 

That said, estivation isn’t entirely magical in its protections. Organisms that use this strategy, even for very short periods of time, often wake up with issues like oxidative stress, buildup of toxins, muscle atrophy due to lack of use, and immune system deficiencies. Creating great wildlife habitat in our backyards is still the best thing we can do to support our wild comrades, and ensure the sleepiest Maryland animals recover from the hottest part of the year. This summer, remember to provide water, lots of shade, and an array of native plants to help us all beat the heat!