Historically, egg-bound reptile embryos were thought to passively thermoconform to the nest environment. However, recent observations of thermal taxis by embryos of multiple reptile species have led to the widely discussed hypothesis that embryos behaviorally thermoregulate. Because temperature affects development, such thermoregulation could allow embryos to control their fate far more than historically assumed. We assessed the opportunity for embryos to behaviorally thermoregulate in nature by examining thermal gradients within natural nests and eggs of the common snapping turtle (Chelydra serpentina; which displays embryonic thermal taxis) and by simulating thermal gradients within nests across a range of nest depths, egg sizes, and soil types. We observed little spatial thermal variation within nests, and thermal gradients were poorly transferred to eggs. Furthermore, thermal gradients sufficiently large and constant for behavioral thermoregulation were not predicted to occur in our simulations. Gradients of biologically relevant magnitude have limited global occurrence and reverse direction twice daily when they do exist, which is substantially faster than embryos can shift position within the egg. Our results imply that reptile embryos will rarely, if ever, have the opportunity to behaviorally thermoregulate by moving within the egg. We suggest that embryonic thermal taxis instead represents a play behavior, which may be adaptive or selectively neutral, and results from the mechanisms for behavioral thermoregulation in free-living stages coming online prior to hatching.
Bibliographical noteFunding Information:
We thank the US Fish and Wildlife Service and the US Army Corps of Engineers for access to the Upper Mississippi River National Wildlife and Fish Refuge. For constructive comments and conversation, we thank C. Adams, M. Angilletta, L. Buckley, L. Hoekstra, and R. Huey, as well as American Naturalist editors J. Bronstein and M. Kearney. This research was conducted under an approved animal-care protocol (IACUC 4-14-7784-J) and an Illinois Department of Natural Resources permit (NH14.0073). The research was supported by National Science Foundation (NSF) grant DEB-1242510 to F.J.J. and an Environmental Protection Agency Science to Achieve Results (EPA STAR) Fellowship to R.S.T. Additionally, R.S.T. was partially supported by an NSF grant to L. Buckley (EF-1065638), E.J.G. was partially supported by a fellowship from the Office of Biotechnology at Iowa State University, and T.S.M. was partially supported by an NSF Postdoctoral Research Fellowship in Biology (DBI-1402202).
- Chelydra serpentina
- Snapping turtle