What Can ‘Dead’ Anole Eggs Tell Us About Reptilian Development?

Incubating A. sagrei eggs.

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.

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3 Responses to What Can ‘Dead’ Anole Eggs Tell Us About Reptilian Development?

  1. Yoel Stuart says:

    Cool stuff Dan. My A. carolinensis breeding experiment reached a point around 4-5 weeks of incubation where all the eggs looked very healthy and plump and yet none were hatching, as expected given the incubation duration.

    Thinking that water balance may have been the problem, we added some water to the vermiculite in their petrie dishes and voila, the next night, our first lizard hatched, followed in rapid succession by many more over the next few days. It’s tempting to conclude that the watering did the trick.

    However, eggs in the next cohort, for which we moistened the soil more often, seem to have about the same incubation period as that first batch. Hard to say if the water did the trick for the first batch. Do you have any thoughts? Thanks!

  2. dan warner says:

    Hi Yoel,
    Tough to say for sure. Most studies show little to no effect of mositure on incubation duration. When there is an effect, it’s often by just 2-3 days on average – nothing like the effect of temperature. Disturbing the eggs (e.g., by adding water) towards the end of development may have stimulated the already full-term embryos to hatch out. This has been shown in turtles (pig-nosed turtle embryos come to mind) and I’ve heard of anecdotal observations like this on Lampropholis skinks.

  3. Rich Glor says:

    Very cool work. We’ve seen similar a similar phenomenon in eggs from Anolis distichus. Some eggs make it rather far in the process before failing. Some of these eggs have fairly well-developed embryos in them, but others appar to just have a mass of rotten yolk. By the time we pull them from the experiment, these eggs are usually pretty far gone.

    This is a somewhat unrelated question, but has anybody done work with yellow, uncalcified eggs? I assume that these are simply infertile, but I’m wondering if anybody has actually test whether this is the case or if something else might be going wrong during the process of egg construction.

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