by Andrew Farrer
Avoiding predation is important. From an evolutionary view being eaten is a poor strategy and I think we can all appreciate the more proximal aversion to being lunch for something else. The world is full of a myriad of strategies to avoid becoming a meal; herding (or flocking), poisonous or foul tasting chemicals, mimicry, camouflage, etc. However, the method that first comes to mind is simple: run. Nothing can beat distance for its defensive capabilities.
The tegu (of the genus Tupinambis) is another lizard who responds to mechanical stimulation, or in this case, tickling. Credit: Varnyard/YouTube
Eggs, though, are not good at running. If there is no parental care then all they may have at their disposal are camouflage and safety in numbers. For a predator, a clutch of eggs is a nutritious and easy meal. As such, the best strategy for an embryo is to hatch at the earliest possible developmental stage, leave the confines of the egg and become mobile. On the flipside, however, the world can be a harsh place and remaining in the egg as long as possible can protect the embryo. Also, once hatching competence has been reached development does not flat-line. More time developing in the egg could make a hatchling more robust. The solution would seem to be for the embryo to monitor the external environment and adjust its hatching date. Conveniently for them (and this post) that is exactly what they do; it’s called Environmentally Cued Hatching (ECH).
In 2011 J.S. Doody stated that ECH was the “..variation in time, age, or developmental stage at hatching, facilitated by an extrinsic cue..”. It can be divided into three areas: Early Hatching, Delayed Hatching, and Synchronous Hatching (hatching at the same time as other eggs in the clutch). Research is currently scattered and unconnected but in recent years researchers have been drawing together the few papers and the anecdotes that support the ECH hypothesis. The ability to react to environmental cues has been noted in a wide range of organisms. Parasitic flatworm eggs can survive in harsh environments that the larvae cannot, so hatching is delayed until conditions become more favourable. Some molluscs can delay due to food scarcity, and Spitting-spider mothers can trigger early hatching of the eggs clinging to her body so she can better defend herself. There are also examples from nematodes, crustaceans and insects. In the vertebrates, fish may need an extrinsic trigger as well as an intrinsic one to hatch at all and pig-nosed turtle eggs can detect vibrations from hatching siblings, leading to synchronous hatching.
A recent study by J.S. Doody and P. Paull studied how mechanical stimulation affected the eggs of the Delicate skink (Lampropholis delicate – a lizard native to Eastern Australia). Laboratory experiments showed that mechanical stimulation (in this case, being placed on an orbital shaker for one minute a day) of wild and lab lain eggs resulted in these eggs hatching 3.4 days earlier than a non-stimulated control group. The skink certainly reacted to mechanical stimulations but what about direct threat? Moving to the field the experiments and the results got a bit more interesting.
Delicate skink lay their eggs in narrow, rocky crevices. Doody and Paull located a nest with eggs at the minimum hatching age. To simulate a predator they poked the eggs with the blunt end of a bamboo cooking skewer. Within 10 seconds of being poked 16 of the 19 eggs hatched, the young lizard immediately launching itself from the crevice, falling 1.4 metres to the safety of the leaf litter below (a long way when you’re less than 40 mm long). The remaining 3 eggs also hatched but, being less dare-devilish than their siblings, opted for hiding deeper in the crevice. Simulating an egg being knocked from the nest Doody and Paull pushed 42 eggs out of nest crevices; 36 hatched as result with 12 of those emerging from the egg as it landed. The lizards’ piece de resistance, however, was hatching on open ground; they erupted from the egg and immediately sprinted away. These youngsters could sprint, on average, 41.6 cm (S.D. ±22.76 cm, Range; 5 – 89 cm, n=30) to cover. Not bad for a creature that has never seen the world before, or used its legs.
It’s the explosiveness that indicates an anti-predator response. However, wild skink that hatch early are significantly smaller than those who hatch spontaneously, and their eggs display yolk traces that spontaneous hatchers do not. The loss of energy (and thus body size at hatching) is a smaller cost than death and so worth paying. However the premature hatch could put youngsters at a disadvantage alongside spontaneous hatchers; presumably more so the earlier the hatch was triggered. Still, if you’re going to flee from danger, exploding from your egg beforehand is the way to do it.
Doody, J S. (2011). Environmentally cued hatching in reptiles. Integrative and Comparative Biology, 51(1), 49–61. doi:10.1093/icb/icr043
Doody, J. Sean, & Paull, P. (2013). Hitting the Ground Running: Environmentally Cued Hatching in a Lizard. Copeia, 2013(1), 160–165. doi:10.1643/CE-12-111
Warkentin, K. M. (2011). Environmentally cued hatching across taxa: embryos respond to risk and opportunity. Integrative and Comparative Biology, 51(1), 14–25. doi:10.1093/icb/icr017