Friday, November 26, 2010

Catching Some Rays

     Perhaps you’ve seen them lined up on a log sticking out from a lily pad covered beaver pond.  Maybe as you’ve floated down the river, waving your arms to maintain your balance and keep your beer can from dipping into the warm water, you’ve spooked some into plopping in from the overhanging branches with a splash.  A turtle’s domed shell enjoying the sun’s rays above the water is an iconic image.  As winter begins its snowy embrace, day dreaming about basking in the sunlight is not an unpleasant way to pass the time.

              Just about every species of turtle basks to soak up the sun.  Even highly aquatic turtles bask, like snapping turtles, but they do so without leaving the water.  These species may simply crawl into shallow water where they can benefit from the sunlight.  But, why do they go into all the trouble?

            One reptilian myth I’ve heard is that turtles must periodically crawl out of the water to dry off so they don’t become waterlogged and sink.  Well, this isn’t true.  Becoming waterlogged and sinking isn’t more of an issue for turtles than it is for fish.  As you might suspect, turtles bask to regulate their body temperature.

            Turtles are ectotherms, which means their body temperature is dependent on external forces.  The term, “cold blooded” has been largely abandoned because it gives the mistaken impression that these animals are relatively cool.  They’re not. In fact, some reptiles keep their body temperature warmer than we do.

            So, if the water’s cool, the turtles will climb out to warm up.  Warming their body helps them stay healthy and keeps their bodily functions running smoothly. For example, turtles tend to bask for longer periods of time after they’ve eaten a meal; they seek the added warmth to help their digestive system operate.

But, not just any basking spot will do.  Turtles often choose not to bask on structures too close to the shore; perhaps they perceive an increased risk of predation in these areas from animals such as raccoons or foxes.  They also prefer to bask in areas that allow them to have a clear view of their surroundings, to better see predators before it’s too late.  Another characteristic of basking spots include nearby deep water.  If a turtle is startled, it wants to be able to dive well below the surface, where it can wait until it’s safe.

            Who knew turtles could be so selective? With so many conditions about preferred basking areas, sometimes prime spots are in short supply.  And if there are a lot of turtles and only a few basking spots, it can get ugly.  Although many may picture a serene landscape of motionless turtles along logs when they imagine them basking, if you pay close attention you can watch turtles biting, crawling on top of each other, and even pushing their neighbors into the water!  Despite all the fierce competition among turtles, they actually benefit from sharing at least some of the space on their log.  More turtles means more eyes, and the more eyes there are, the more likely a turtle will spot any approaching predators.  If one turtle is startled, its neighbors will notice and they can all escape into the water together.

     This escape behavior has lead some to be concerned.  In many highly trafficked areas, where boats are a constant presence, turtles are often startled into the water while basking.  In some cases, disturbances are so frequent the turtle never feels comfortable basking at all.  This might affect the health of digesting turtles, or females with eggs inside.  Another basking-related concern is due to the questionable practice of “plinking”.  Plinking occurs when gunowners take target practice on basking turtles, maybe from bridges or boats.  For some rare species, plinking could deplete already small populations.  I'm not sure I understand the appeal of this activity and it's legal in several southern states.  Should it be?

Photos courtesy of Sean Sterrett.  For more information about river turtles and the conservation issues they face, the following book is a comprehensive review: The Ecology, Exploitation and Conservation of River Turtles (Enviromental Science)


MattK said...

I love the little musk turtle photo. I have rarely seen them basking but when I have they seem to choose more out of the way spots (sometimes not even in direct sunlight) than emydid turtles. I always figured this was in keeping with their general strategy of relying on crypsis over vigilance.

I have doubts about the group basking hypothesis that it allows predators to be spotted allowing all turtles to dive in. I did some field work with Northern Map Turtles and they would often bask singly in small groups in places where basking sites were abundant although other turtles basked nearby. They would also not synchronize their escapes regularly. I'm sure that they use cues from other individuals to some extent but I have doubts about how important this is.

I had no idea that there was a) a specific word for some bored jack-ass with a gun shooting turtles and b) that it was legal anywhere in North America. Thankfully it's not legal here in Canada at least although it still does occur.

David Steen said...

Thanks Matt...those musk turtles are great climbers. Sometimes I've seen something fall into the water from what seems like ten feet above. My first instinct is someone's throwing a rock at me, only later do I realize it's musk turtles dropping in.

Mark Bailey said...

I've always been puzzled by basking turtles' exceptionally keen long-distance vision, and their wariness. Why would a turtle need to drop off a log when the safety of deep water is a split second away and the potential threat coming into view is 100 meters or more away? Have we selected for this behavior in only a century or so of "plinking?" Seems unlikely to me. Have you ever seen/heard this addressed? In the absence of harassment turtles do become habituated to people, of course. You can take the "jungle boat" ride at Wakulla Spring and glide right past dozens of big Suwanee cooters.

David Steen said...

Hm...good question. Here's a stretch, maybe they're so wary to evade something like a diving osprey or other raptor...a predator that would make every second count. I can't find much to support this. But, a turtle is no match for something like an otter, which will pursue them through the water. Perhaps the quicker they can find refuge under a log or rock in the water, the better. For a turtle to become adapted to disturbance, I would imagine they would need to be frequently spooked enough to flee into the water, but without the positive reinforcement of a chase. The otter hypothesis is consistent.