A 'slug' next to a healthy, freshly laid egg. The scale bars in millimeters.
In December, Rich posted on infertile eggs occasionally produced by anoles, also known as slugs. These eggs are small, yellow, and uncalcified. We have found that females typically lay slugs in different locations than fertile eggs. In our breeding colony, nearly every viable egg is deposited in egg laying substrate (moistened vermiculite in a plastic yogurt container), whereas slugs are found on nearly any surface but these cups, as described in a previous comment by William Baugher.
As our hybridization experiment in distichoids proceeds, it has become clear that these inviable eggs may be an important measure of the success of hybrid matings. There are some really great studies on viable anole eggs in the reproductive biology literature (1, 2, and 3, to name a few), but I have had no luck in finding papers that discuss, or even mention the production of, these inviable eggs. Since the last post on this subject AA readership has gone up and I am hoping that someone out there has some additional information on the phenomenon.
Specifically, we are most interested in learning:
Are slugs always the result of a lack of fertilization or do failed early stage embryos also appear as slugs?
Why are these eggs produced? Stamps (1975) reported the ability of Anolis aeneus to reabsorb eggs in the absence of a mating event. Are slugs eggs that can no longer be fertilized and are thus discarded? Why wouldn’t they too be reabsorbed?
Is slug production stimulated by interaction with males? Copulation?
Are slugs produced by other species? We’ve found them from a variety of distichoid species. And Martha commented that she has seen them produced by captive A. cybotes.
So, Anole Annals community, does anyone know where we can learn more about slugs?
Hi! I’m a grad student from a different branch of the lizard tree (Sceloporus), but I’ve been fascinated by your website! My advisor and some of my labmates have done breeding studies on the side-blotched lizard (Uta stansburiana), and I’ve definitely seen females of that species lay “slugs” as well. They look very similar to the one in your photo and can be produced in the same clutches as healthy eggs. I’m afraid I don’t know of any detailed studies on why or when U. stansburiana females lay slugs, but I’ll be excited to see what you find out from Anolis! Good luck!
Thanks Beth! I will be sure to provide an update here on whatever information I gather.
Hello, I have seen slugs laid by many lizards and snakes in a captive situation. Typically you see them when a female is just starting to lay, either when young or starting a new reproductive season or at the end of the season. It is not unusual to find a slug egg right before you find the first good egg laid by a young female Anole. Slugs can also mean that something is amiss with the female, possibly nutritionally and it is important to know if that is the case. You can also get slugs laid as a result of stress. Often transferring a female to another enclosure or introducing new enclosure occupants will result in a slug being found in the next couple days. I don’t have any answers to your questions but I wanted to make the point that slug eggs are not rare to find and hope someone does have some answers. I think trying to figure out why you are getting slug eggs in a situation where you are pairing different species will be pretty impossible until someone figures out why females lay them at all! Let me know if I can help with anything else.
Thanks David. I agree that understanding why they are being produced is key, and that slugs are certainly not rare. I get them in all of my crosses (pure species and hybrid) throughout the breeding season.
I’ve bred a lot of anoles, and we see these all the time. We almost always see them produced when a female is breeding condition but before she is placed with a male. After she mates, she tends to produce only viable eggs for 2-3 months, at which point she may start laying slugs again. (I did not know they were called this–thanks!) We have been taking the slugs as a sign that a female has no stored sperm, but this may not actually be true.
Joel says it , I also bred a lot of anoles.
But my old anole females laid also only slugs, they are more than 6 or 10 yeares old
hope thats help a litte bit
Back in the early stages of my embryological work I dissected 20 or 30 of these eggs and never found signs of a developing embryo – no embryo, no blood spot, etc. I also observed that the yolk of these eggs is much more fluid-like than any fertilized egg with a noticeable embryo.
Thank you Thom. This is great information! I think I will try dissecting some of the eggs we have collected. Would you suggest transferring yellow eggs to PBS following the embryological protocol you published in Herp Review, or just open them up?
While it’s more an anecdote than data, I just had something similar happen in breeding a pair of F1 hybrids between a Black rat snake and California king snake – 16 eggs, but only 3 seem fertile. Both have been bred before (including to each other). It could just be a spot of bad luck, or something more interesting, but I thought I’d add my input in case anyone finds it useful. I’m hoping to breed them again this season, but don’t know if it’ll work.