Anyone who’s enjoyed the scenic beauty of a placid river, lake, or pond has undoubtedly also seen turtles sunning themselves on sunken logs, etc. This usually takes place in the morning and late afternoon - presumably because the turtles need the most warming at these times (after the chill of the night [insert music] and when day is done) - but they also sun themselves during the day.
Frequently the turtles will cluster around a particularly choice rock or fallen log. Such places often have many turtles crammed into the space available. So many, in fact, that they often climb on top of each other.
Now that we’ve set the scene, here’s my question: Why do the turtles tolerate their (I assume) competitors crawling on them and blocking the light? More particularly, why do large turtles tolerate smaller turtles crawling on top of them? I’ve seen big, bad mofo turtles with little pipsqueak turtles perched right in the middle of their carapaces - why don’t the big turtles just pitch the others off? If you’re competing for space in the sun, then why take guff from some shrimp turtle who wants your light?
Mainly because there is no reason to. The larger turtles would have to expend a great deal of energy to rid themselves of the smaller turtles, likely reentering the water, and it isn’t worth it. The smaller turtles also have a higher turnover rate, as they warm faster and leave more quickly. They also don’t significantly decrease the amount of heat that the larger turtles get, as they transmit some through their shells while increasing the effective exposed surface area (though not the actual surface area.)
I guess that the simple answer to your question is that they aren’t competing for the space.
The small turtle is simply not blocking enough light to make it worthwhile expending energy to dislodge him.
If the large turtle moves to dump the small turtle, it may fall in the water, thus losing some of the heat it has already gained, or else lose its spot to another turtle that may not be easy to dislodge.
One site I read says that when space is scarce they sometime pile up three deep. This leads me to think that maybe the most heat is taken in via warming the blood in the head, neck and legs. That would make shading the carapace less of a factor because not a lot of heat input is lost because of it.
Thanks everyone. I had considered the idea that it simply wasn’t worth the effort (i.e. the offending turtle is too small to make the result worth the expended energy), but turtles don’t seem to dislodge any other turtles, even when the size difference is less dramatic.
Since the behavior appears to be endemic to all turtles (anyway, all the turtles I’ve seen) I was thinking of a systematic answer along the lines of
Turtles are (all) stupid. [Colibri]
Turtles don’t absorb much radiation through their shells and so don’t care when their covered. [David Simmons]
Since being stupid hasn’t stopped lots of creatures (e.g. my sister) from acting in sophisticated ways, I think we can rule this out. That leaves David Simmons’ suggestion, which I’ll bet is the answer.
Now all we need is an experimentalist to shield the carapaces of some turtles and we’ll see if this is really what’s happening. Anyhow, thanks again.
Perhaps because there is a good chance the smaller turtle is the offspring of the larger one. Natural selection tends to favor groups whose elders are kinder towards the weaker members of the group.
I’m not sure if I believe this, but I think it’s a possibility…