Anyone who has incubated reptile eggs knows that moisture is important. Without sufficient moisture, eggs quickly desiccate and shrivel beyond any chance of returning to a healthy, turgid state. Because of this, eggs must experience positive water balance during most of the incubation period for successful embryonic development, and the relative moisture content of the incubation medium can greatly impact how much water is taken up by eggs. In turn, this will affect embryonic metabolism and will have important effects on hatchling size.
Those of us who have incubated thousands of reptile eggs have probably come across the occasional ‘odd ball’ that swells up at a healthy rate, but never hatches, and upon dissection nothing but water and yolk oozes out with no sign of an embryo. These ‘odd balls’ have lead me (and others) to wonder whether embryos are really necessary to regulate water uptake by eggs, or if water uptake is simply a passive hydraulic process? This question could be simply addressed by comparing rates of water uptake by eggs with and without embryos present. However, the problem is that reptile eggs are often discarded when they die and researchers often don’t repeatedly weigh eggs throughout the incubation period. Moreover, infertile eggs frequently shrivel up and develop fungus soon after oviposition (with some occasional exceptions described above). Lastly, infertile eggs that do take up water for a considerable duration are rare enough that appropriate sample sizes are difficult to come by.
We recently conducted an incubation experiment using Anolis sagrei eggs to evaluate how hydric and thermal conditions affect rates of egg water uptake and phenotypic development. For unknown reasons, many of the eggs in our study looked healthy well into incubation (often 30+ days) and then suddenly showed signs of negative water balance (e.g., a dimpled or shriveled appearance). Upon opening eggs to inspect embryos, we noticed that most eggs did not have embryos or had embryos that had died at extremely early stages of development. Although this was unfortunate for our initial goals, we were given a unique opportunity to address the question posed above – are embryos actively regulating water uptake or is water uptake a passive hydraulic response to developmental environments?
In our recently published paper, we compared mass gain (due to water uptake) of ‘live’ versus ‘embryo-less’ eggs, and show that water uptake does not differ between these two groups during the first half of incubation, but the rate of water uptake by ‘embryo-less’ eggs dropped considerably during the latter half of incubation. These results suggest that water uptake by eggs is a passive process during early development, but may require active regulation by embryos (or extraembryonic membranes) during late development. In fact, the decline in water uptake by ‘embryo-less’ eggs coincided with the time when organogenesis would have been about complete in the ‘live’ eggs, which suggest that embryos might be capable of regulating water uptake at this time. These results provide some new insights into the physiological processes involved in reptilian development and incubation. But perhaps a more important message from this work (and one that isn’t addressed in the original paper) is that researchers shouldn’t always give up on failing projects (in this case, due to egg mortality) because you never know what interesting patterns might emerge.