How frequent and important is behavioral thermoregulation by embryonic reptiles?

The debate about behavioral thermoregulation inside reptile eggs centers on the frequency (and hence, biological significance) of the phenomenon, not about its validity. Both sides of the debate agree that large eggs in shallow nests laid in sun-exposed soil will experience clines in mean temperature and (especially) diel thermal variance; that embryos in the middle phase of development have the ability to reposition themselves, and room to do so; and that small changes in developmental temperatures can influence offspring fitness. Equally, all protagonists agree that thermal clines will be too low in some other kinds of nests, and that embryonic repositioning is impossible very early and very late in development. Based on an array of other fitness-enhancing behaviors exhibited by tetrapod embryos, and general principles for recognizing adaptation, we conclude that behavioral thermoregulation inside the egg likely is adaptive in some but not all reptile species. We identify productive directions for empirical research to resolve points of contention.