Saturday: 22 October 2011
I still have a couple of posts to make about box turtles, but with morning temperatures in the 30s for a couple days running, and daytime highs in the 60s over the last week, it seems likely that I’ve seen the last of the turtles until next year.
So how did we do on the box turtle census this year?
To preface, since 2005 I’ve had 39 turtle encounters involving 29 unique turtles in the study area. (Since I’ve been walking through the new property, which is not a part of the study area, I’ve actually seen 9 additional turtles). Six turtles have been observed more than once, and three of these have been observed four times.
This year I had twelve encounters. Nine of these involved new turtles in the study area, and three were old turtles previously observed.
The last turtle I saw was Ivan, once again this year, playing in the creek last week after a good rain:
What did we learn this year?
I took the turtle observations and used them as numbers for a mark and recapture analysis, and estimated somewhere around 40 turtles, which in the 20 acre study area is around 2 turtles per acre.
I did a re-estimate after encountering four new turtles in a row, here, and increased the estimate to 50 turtles, or 2.5 turtles per acre.
If I use 2011 as the recapture year, and all previous years as the marking years, then I’d estimate about 70 turtles, or 3.5 turtles per acre.
It was the last two re-estimates that cause me to suspect that turtles from the outside may be migrating into the study area in advance of hibernation.
I’ve already mentioned the interpretation of the last month’s observations suggesting an influx of end of season turtles, perhaps returning to their hibernacula.
Last year our neighbors discovered a lot of nest building turtles, and several turned out to be some of the ones I’d observed here. One of these was Ernest (oddly, a male). This year I found Ernest back in his old place, indicating a full round trip.
The nesting sites our neighbors have observed, and the one we observed in 2009, were all located well upland from any natural source of water - not much less than a thousand feet. This is a lot of distance to expect a tiny baby to cover, and baby box turtles have a semi-aquatic habit for at least a couple of years. Very strange, and I don’t know to make of it:
There could be some kind of selection strategy going on, similar to the weird and seemingly pointless nesting treks of Antarctic penguins (on a much smaller and more modest scale, of course).
Or these nesting turtles we’ve observed could have gotten themselves into a programmed nesting instinct entirely by accident, by some foolish ancestor whose progeny just happened to have survived despite all odds.
I still have to write up one more post, at least, but I did some scute ring counting and comparisons and showed very little change in the rings of older turtles over several years. We also took a good look at external carapace morphology - the bone structure beneath the keratin layer, and how it appears to grow.
All in all, a pretty good year for box turtle observations, with lots of suggestions for next year’s goals.