Turtles are one of the most endangered marine animals. One of the most vulnerable aspects of their life cycle is the egg stage – where the little baby sea turtle eggs are buried in nests and left behind on the beach during their incubation period. On average, sea turtles lay clutches of between 50-130 eggs, depending on the species, in nests on tropical or subtropical beaches. As they lay their eggs on the beach and then return to the ocean providing no parenting, the eggs are vulnerable to predation by scavengers and can experience predation rates of more than 50% on some beaches. Asian mongooses commonly scavenge and feed on turtle eggs, which they are able to sniff out soon after the eggs have been laid.
New research carried out by Burns et al. (2021) from the Institute of Biodiversity at the University of Glasgow and published in the Royal Society Open Science journal provides evidence that turtles make ‘decoy’ nests to throw predators off the location of the nest that contains their eggs. Two species of turtles, leatherbacks and hawksbills, were chosen for this observational study and their behavior was observed at night during the nesting period from June to August in hawksbills and June and July in leatherbacks over a few years. Although there is variation in egg laying between different species, most turtles follow the following process with slight variations.
Sea Turtle Egg Laying Process:
- Emergence from the sea and movement up the beach
- Selection of nesting site
- Preparation of nest site (body pitting)
- Excavation of egg chamber
- Oviposition (egg laying)
- Refilling of the egg chamber
- Scattering or throwing sand around the nest site with flippers (often referred to as the ‘camouflaging’ or ‘disguising’ stage).
- Return to the sea
The researchers measured the duration of four phases of egg laying (evacuation of nest, oviposition, refilling and sand scattering) as well as the respiration rate of each individual hawksbill turtle and compared the count of and distance between sand-scattering stations, the cumulative distance traveled and the angle of each turn made by the leatherback and hawksbill turtles. On average, the leatherback turtles had 11 sand-scattering stations and traveled a distance of around 100 cm between each station whereas the Hawksbill turtles had 7 sand-scattering stations and traveled about 30 cm between each station. Overall, the turtles devoted significant metabolic energy, inferred from respiratory rates, and time to conduct these sand-scattering activities – extending the duration of their stay on land and putting themselves at risk.
The sand-scattering behavior of the turtles is thought to disperse and de-localize any signs of the nest’s presence and any indicators of recent egg laying activity which would give away the position of the nest such as disturbed sand, texture or scent. The sand dispersing behavior of the turtles is chaotic and unpredictable, and varies substantially between individuals, which researchers think helps prevent predators from learning and recognizing the path to the nest.
Although leatherbacks and hawksbills are evolutionarily distinct, being separated by around 100 million years of evolution, they both perform sand-scattering flipper actions after laying eggs, immediately above the nest, after refilling it and then at a number of stations progressively displaced from it.
Leatherbacks (Dermochelys coriacea)
Leatherback turtles are unique among turtles and differ from other species, firstly, because of their enormous size (they are the heaviest non-crocodilian reptile) and also because of a number of structural, anatomical, physiological, behavioral, migratory and dietary adaptations. For example, leatherbacks do not have rigid scutes (similar to scales) which are thought to be an adaptation which allows them to dive to depths greater than 1200 m. To put this into perspective, recreational human scuba divers usually only dive up to 30 m and have to do a special course to be allowed to dive up to 40 m. Leatherbacks also spend prolonged periods in colder water, unlike other species. They typically munch on jellyfish and other soft invertebrates and have many sharp papillae lining their throat to allow them to eat these slippery creatures.
Hawksbills (Eretmochelys imbricata)
Hawksbill turtles are smaller than the leatherbacks and have the characteristic harder shells shared by other turtle species. They are distinct from other turtles because of their sharp curving beak and the saw-like appearance of their shell edges. The shells are reported to change color slightly depending on the water temperature.
Although researchers have been aware of the ‘sand-scattering’ behavior of turtles for a while, it was originally thought to play a role in camouflaging the nest or as an effort to re-establish the beach environment and optimize the temperature and moisture levels in the surrounding sand for the incubation of eggs. Burns et al. (2021) suggest that it is unlikely that the turtles perform this sand-scattering action as a means of disguising the presence of the nest as the area disturbed by this behavior is much greater and would actually draw attention to the vicinity of the nest. The authors also note that other studies have found that some predators, such as mongooses, are still able to identify the position of the nesting area – so it is not a very effective means of hiding the nest but rather creates confusion and wastes the predator’s time because not all ‘nests’ have eggs. Decoy nests are more likely to work against predators like mongooses which tend to dig up the nests shortly after they have been laid and likely rely on sensory cues such as scent to find the nests, compared to predators like ghost crabs which feed on the hatchlings when they emerge.
Regarding the second hypothesis, the researchers’ observations of the prolonged and extensive sand-scattering actions of the turtles suggest that the sand-scattering is not to re-establish optimal conditions at the nest as it would be ineffective at the distance at which the turtles were observed to carry out this behavior, although it may be that at the nest this behavior does re-establish the beach conditions.
Although it was originally believed that sand-scattering behavior be sea turtles was to disguise the location of the nest or re-establish optimal incubation conditions, Burns et al. (2021) provide a convincing case that this behavior is rather an attempt by the turtles to create decoy nests to confuse terrestrial predators. Given the risk of prolonged periods on land, it is likely that this behavior is important to the reproductive success of sea turtles and improves the chance of offspring survival.