Turtles follow the sun even before they have muscles

Although turtles have a limited control over their internal body temperature, they do respond to temperature differences in complex ways. These include things like basking in the sun to raise their temperatures, responding to the cold by hibernating, and even determining the sex of their offspring via the ambient temperature at which the eggs develop. So, it's not a shock to find out that turtles start physically responding to temperature differences before they even hatch from an egg. What is surprising is that these responses occur before there's much in the way of a skeleton or nervous system to support it.

Normally, embryos tend to float to the top of the yolk, ending up closer to the top of the egg itself. This means that they tend to be warmer, since nests are warmed by the sun heating soil and debris above the eggs. Being a bit warmer is typically a good thing for the turtles, since it hastens development and should allow the embryos to hatch earlier.

It would be tempting to suspect that this positioning was just a fortuitous result of buoyancy, but some researchers tested that by placing a heat source to one side of some eggs. The embryos followed, ending up on the warmer side of the egg (although still towards the top of the yolk). Shift the heat source to the opposite side and, within a few days, the embryo followed it again. It's tempting to envision a tiny turtle embryo swimming its way through the yolk, but the effect was visible as early as three days, at which point the embryo is little more than a poorly differentiated mass of cells. Even once something recognizable as a turtle appears, it takes a considerable amount of time for its cartilage to solidify into bone and nerves to connect with the developing muscles.

All of which implies that we are probably still observing a process that's driven by fluid mechanics, and not some instinctual response to temperature differences. Given that, the authors' use of "bask" to describe the phenomenon is rather unfortunate. The authors also speculate that embryos might be repelled from heat sources if the temperatures reached a point where it becomes harmful, or to even out hatching days among eggs that are at different temperatures in a single nest. If either of those happen to be occur, then we might be looking at an active biological process, which would be quite interesting.

PNAS, 2011. DOI: 10.1073/pnas.1102965108  (About DOIs).

Listing image by NSF