Tuesday, July 29, 2014

Hard boiled selection?

On Thursday June 18th I walked from my office to a corn field on the Saint Michael's College campus. Along the way I nearly tripped over two painted turtles in the middle of the gravel road used to truck food waste to our compost facilities.  After perhaps pointlessly returning the first one to the nearest pond I recognized that the second one was digging a hole to lay eggs.

When I got as far as the corn field I found a third turtle backed up to a freshly covered hole where I presume she had deposited her eggs.  No eggs were visible and I was not about to dig to confirm my hunch.  In my days as a summer camp nature study instructor I learned that unlike bird eggs that actually require turning, turtle eggs should not be turned over during incubation.  Developing turtles attach to the inside surface of their eggs and turning can harm the growing embryo.

I did confirm that the cornfield was quite attractive turtle nesting habitat.  As I skirted the edge of the plowed and planted field I encountered no fewer than 17 turtle nests.  With the exception of the nest that was being completed, each and every nest that I found had been plundered and generally only egg shells remained.  I found intact eggs that had been dug up from just one nest but I suspect that digging up alone may have been sufficient to destroy the developing turtles.

With brief research I learned that it is not unusual for more than 90% of painted turtle nests to be plundered.  Racoons and skunks are frequent predators of turtle nests.  At first blush one might consider that all of this nest plundering would damage the turtle population.   However, turtles can reproduce each year for between ten and twenty years so there are multiple opportunities for each female to contribute to the next generation.

It is worth noting that I have no real way of knowing how many intact nests I walked past.  It seems that these turtles all lay their eggs in a relatively short time window.  This strategy is sometimes called "predator dilution".  If there is a large number of simultaneously available nests, then the chances of one particular nest being plundered is reduced.  There would therefore be selective pressure on turtles to nest at the same time as their neighbors.  Genes that favored solo nesting would be rapidly eliminated because each solo nest would be the only available target for predators.

Supporting the predator dilution hypothesis would require some field work and experimentation.  I'd first need to confirm clutch synchrony in the population; seems a worthy excuse for a regular walk to the cornfield.  An experiment to confirm that clutch synchrony was protective of the nests would be easy to design if not to implement.  I have no desire to in any way interfere with nesting turtles, but an experiment with an inexpensive substitute might do the trick.  My fake turtle nests might consist of shallow nests dug along the corn field and seeded with chicken eggs.  If predators are being satiated by availability of eggs, then I'd expect proportionally fewer eaten eggs where I placed 20 nests and proportionally more consumed eggs where I placed just 1 or 2 nests.

As with any good experiment, we should entertain alternative hypotheses.  In this case, it may be that the availability of multiple nests of eggs may attract multiple predators on a feeding binge.  One version of this hypothesis is called "local enhancement".  Hungry predators are drawn to rich food sources by the presence of other predators actively feeding.  It's far easier for a vulture to spot a flock of feeding vultures than to spot a carcass from the sky.  The same may well be true for fake turtle nests.

I'd need to replicate the experiment, collect, and analyze the data.  This how scientists know what we know: experimentation, replication, and data analysis.  This particular piece of science will remain a thought experiment for now.  But one never knows; there may just be a Saint Michael's College student in need of a viable project and this may be just the thing.  At least I'll gather the data on clutch synchrony next year and perhaps be a little fitter as a result.

All images for this post were taken on using a cell phone camera.

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