If you handle the egg of a delicate skink, you’ll quickly end up with a sticky mess on your hands (That’s the last double entendre I’m using today I swear). You see, delicate skinks (Lampropholis delicata) have a really cool adaptation: If they are disturbed in-ovum by a would-be predator, they hatch early and make a bolt for cover.
This adaptation was first documented by J. Sean Doody (now of the University of Tennessee) and his student Philip Paull of Monash University, Australia. Their study, published in the March 2013 issue of Copeia, describes the early hatching as “explosive”, with the hatchlings sprinting, on average, about 40cm. The embryonic skinks appear to interpret certain vibrations as a predatory threat, prompting them to forgo the remainder of their eggy downtime, thus avoiding their fate as a lizard omelet. The lizards can emerge up to 3 days earlier than they would normally do, to avoid the threat of predation.
This fascinating adaptation is an example of environmentally cued hatching. Environmentally cued hatching (ECH) allows egg-dwelling animals to respond to their external environment in one of three ways: delayed hatching, early hatching and synchronous hatching. Delayed hatching allows the animal to wait for an appropriate environmental trigger before hatching. For example, pig-nosed turtles (Carretochelys insculpta) hatch when the nest is flooded by rainwater during the rainy season. In so doing, they avoid hatching during the less hospitable dry season.
Synchronous hatching, as the name suggests, helps to ensure that all hatchlings in a clutch will emerge at roughly the same time. In this way, the young may receive the anti-predatory benefits of large numbers and there is an additional benefit in that the early risers won’t alert predators to the presence of the remaining clutch. Synchronous hatching can be triggered by factors such as the vibrations of emerging clutchmates (as seen in turtles) or the vocalisations of clutchmates (as seen in crocodilians). Early hatching, as displayed in this study, can be triggered by adverse conditions such as fungal infections or anoxia. This study however, appears to demonstrate early hatching as an anti-predatory strategy. Phillip Paull explains the significance of this finding: “So far, early hatching in response to predation is known in amphibians, fish, and invertebrates, but our study is the first to extend this ability to a terrestrial vertebrate, specifically a reptile. Anecdotal evidence of early hatching in lizard eggs in response to physical handling has existed for some time, but has never been investigated and quantified.”
Perhaps the best known example of an animal that hatches early to avoid predation is the red-eyed tree frog (Agalychnis callidryas). These frogs lay their eggs on the underside of leaves overhanging waterbodies. If the young are disturbed by the vibrations of an egg-predator they hatch early and drop to the waterbody below (In an especially cool adaptation, the young can differentiate between the vibrations of a predator and those caused by weather). In this case, dropping from a terrestrial environment to an aquatic habitat allows the tadpoles to escape a predator with relative ease, as a terrestrial egg-predator will not follow the tadpoles into the water.
However in the case of the skinks, hatching early is not enough as an egg-predator is likely to be just as happy eating a freshly hatched lizard. This may explain why the skinks hatch so explosively: By “hitting the ground running” they gain a head-start on any predator that decides to pursue them.
In the case of the red-eyed tree frog there is a cost associated with hatching early, as prematurely hatched tadpoles are generally less developed than their full-term counterparts. Doody and Paull had similar findings, with prematurely hatched skinks generally being smaller than those who enjoyed a full development, and the vacated eggs of early-hatchers contained more yolk than full term eggs. Weighing up the cost of early hatching against the anti-predatory benefits will require further research, as Paull highlights: “An embryo that foregoes complete incubation in order to evade an imminent threat from a predator is subject to profound costs and benefits, and we are only just beginning to understand how natural selection operates in this situation.”
While this is the first documented case of anti-predatory ECH in a terrestrial vertebrate, anecdotal evidence suggests it may actually be quite a common adaptation amongst reptiles. Since the publication of the study, Paull has “…seen many stories from people who’ve had lizard eggs hatch in their hands, so it’s entirely possible that the ability of embryos to hatch in response to environmental cues is widespread in reptiles.”
The study has further implications in the field of invasive species research, as Paull explains: “A secondary benefit of studying delicate skinks is that they are also an invasive species, having been accidentally introduced to New Zealand, Hawaii, and Lord Howe Island, where they compete with native species. Understanding their reproductive biology could lead to management strategies, and increase our understanding of the general strategies that make invasive species so successful.”
As our understanding of ECH grows, so to does our understanding of the subtleties of the interactions between animals and their environment. It is easy to dismiss the egg or the womb as isolated micro-habitats that are sheltered from the influence of the external environment. Studies such as this serve as a reminder that evolutionary pressures do not kick in postnatally. What better illustration could one ask for of the struggle for survival than a baby lizard running from a predator before it’s even finished hatching?
*This post highlights some of the other great work on reptile eggs coming out of Monash University.