| Mobile ectothermic animals can control their body temperatures by selecting specific thermal conditions in the environment, but embryos trapped within an immobile egg and lacking locomotor structures have been assumed to lack that ability. I studied thermoregulatory behavior in reptile embryos to answer (1) does thermoregulatory behavior occur to embryos?(2) Is the embryonic movement in response to heat an active or passive process?(3) Is thermoregulatory behavior widespread in reptile embryos? and (4) what is the cost of this behavior?We first carried out experiments in the Chinese soft-shelled turtle (Pelodiscus sinensis) to see if thermoregulatory behavior occur to embryos. Falsifying the assumption of inability of behavioral thermoregulation, our experimental studies show that even early stage turtle embryos move within the egg to exploit small-scale spatial thermal heterogeneity. Behavioral thermoregulation is not restricted to post hatching life and instead may be an important tactic in every life-history stage.The embryos of soft-shelled turtles can reposition themselves within their eggs to exploit locally warm conditions. In the second experiment in an emydid turtle (Chinemys reevesii), we further ask whether turtle embryos actively seek out optimal thermal environments for their development, as do post-hatching individuals. Specifically,(i) do reptile embryos move away from dangerously high temperatures as well as towards warm temperatures? and is such embryonic movement due to active thermoregulation, or (more simply) to passive embryonic repositioning caused by local heat-induced changes in viscosity of fluids within the egg? Our experiments show that embryos avoid dangerously high temperatures by moving to cooler regions of the egg. The repositioning of embryos is an active rather than passive process:live embryos move towards a heat source, whereas dead ones do not. Overall, our results suggest that behavioural thermoregulation by turtle embryos is genuinely analogous to the thermoregulatory behaviour exhibited by post-hatching ectotherms.Does thermoregulatory behavior also occur in the embryos of other reptile and bird species? Our experiments show that such behavior is widespread but not universal in reptile and bird embryos. We recorded repositioning within the egg, in response to thermal gradients, in the embryos of three species of snakes(Elaphe bimaculata, Xenochrophis piscator, and Zaocys dhumnades), two turtles (Chelydra serpentine and Ocadia sinensis), one crocodile (Alligator sinensis) However, we detected no significant thermoregulation by the embryos of two lizard species (Takydromus septentrionalis and Phrynocephalus frontalis). Overall, embryonic thermoregulatory behavior is widespread in reptile species but may be unimportant in the small eggs laid by most lizards.The cost of behavioural thermoregulation is well recognized in the post-hatching stage of ectothermic vertebrates. However, the cost of thermoregulatory behaviour remains unknown. We manipulated the intensity of thermoregulatory behaviour in turtle embryos (P. sinensis), and determined the hatching success, body mass and righting response to assess the energetic cost associated with this behaviour. Hatchlings from embryos that had experienced intensive behavioural thermoregulation were smaller and contained less energy than those from the control group, which indicated that behavioural thermoregulation by turtle embryos incurred energetic costs. Nonetheless, the smaller hatchings did not exhibit a lower hatching success or a slower righting response, suggesting that the cost incurred by behavioural thermoregulation is relatively low in turtle embryos. |
