The long-tailed sun skink is Taiwan's largest lizard; its body is up to 12.5 cm in length, and its overall length including its tail can be up to 30 cm. It lays an average of two to 13 eggs each time. The skink often chooses to lay its eggs in cement drainage holes that are blocked on one end; whereas the open end of the drainage hole may allow predators to enter and eat the eggs or mother lizard.
After laying eggs, Female long-tailed sun skinks (Eutropis longicaudata) on the island of Lanyu will remain on their nests and guard their eggs. If the small Taiwan kukri snake (Oligodon formosanus) tries to steal any of the eggs, the mother lizard will immediately attack it, and drive it from the nest. This kind of egg-protecting behavior continues until the eggs have hatched.
While 98% of lizards and 95% of snakes do not stick around to protect their offspring after laying eggs or giving birth, the long-tailed sun skink of Lanyu is one of the few such reptiles to remain at the nest and protect its eggs. The female long-tailed sun skink's aggressive behavior only occurs during the breeding season, however, and the lizard only tries to attack egg-stealing Taiwan kukri snakes. Female lizards do not pay any attention to other species that pose no threat to its eggs, nor do they care about snakes entering their nest when it is not the reproductive season; these types of behavior contrast strongly with the mother lizard's instinctive protection of its offspring after it has laid eggs.
Huang Wen-san, director of the Biological Section at the National Museum of Natural Science, has performed tracking research on the long-tailed sun skink of Lanyu for more than a decade. According to Huang, in order to protect its offspring, the long-tailed sun skink will engage in egg-protecting behavior whenever its eggs are in danger of being stolen, but the same species of long-tailed sun skink living on the Taiwan proper, and in other parts of the world, does not exhibit this type of behavior. Nevertheless, compared with other species that exhibit similar types of parental care, such as birds and mammals, the Lanyu long-tailed sun skink does not display extensive care behavior. This may be because the behavior is currently evolving in the Lanyu environment, and the Taiwan kukri snake is playing an important role in its evolution.
The Taiwan kukri snake has a length of 35 to 70 cm, and chiefly eats the eggs of other reptiles. In the past, when the summer laying season arrived, many green sea turtles crawl onto the beaches of Lanyu to lay their eggs. At those times, the Taiwan kukri snake used its keen sense of smell to locate and gorge on turtle eggs (nowadays sea turtles only lay their eggs on the beach at Xiaobada Bay, which is less than 100 meters in length; the sea turtles currently lay 10-30 nests of eggs each year). With plentiful food in the form of abundant green turtle eggs, the kukri snake thrived, and was found in large numbers on the island.
The only problem for the kukri snake is that it must eat during the times when the green turtle is not laying its eggs. Because, among the eggs of various reptiles, the eggs of the long-tailed sun skink are relatively accessible for the Taiwan kukri snake, these eggs form a major food source for the snake.
With a stout, short body, the long-tailed sun skink is the largest lizard in Taiwan. This is why the mother skinks are almost always able to defeat kukri snakes in a fight, and the reason they are able to push the snakes out of their nests.
From an ecological perspective, the long-tailed sun skink and Taiwan kukri snake have an antagonistic relationship. Nevertheless the animals that prey on both are similar, and include the Taiwan stink snake, many-banded krait, and Chinese cobra.
Lanyu is a very small island, and natural enemies of the long-tailed sun skink and Taiwan kukri snake are not very numerous. As a result, to ensure that its offspring will not be eaten by kukri snakes, which are common and have few natural enemies, and since it is not too worried that its few natural enemies on the island will threaten it while it is in its nest, the Lanyu long-tailed sun skink remains behind to guard its nest after laying eggs.
In contrast, on the main island of Taiwan, both the Taiwan kukri snake and long-tailed sun skink have many natural enemies, and there are consequently few kukri snakes that might eat the eggs of the long-tailed sun skink. Furthermore, researchers working in the Pingtung area have discovered that snakes such as the Taiwan stink snake, many-banded krait, Chinese cobra, Taiwan pit viper, bamboo viper, and oriental ratsnake all prey on long-tailed sun skinks. If a mother lizard stayed in her nest to guard her eggs, she might run the risk of being eaten by a snake before her eggs hatched. This is likely to be an important reason why long-tailed sun skinks on Taiwan proper have not evolved egg-protecting behavior.
We can therefore conclude that the ecological environment on the island of Lanyu causes its long-tailed sun skinks to remain in their nests to protect their eggs.
Translated by Glen E. Lucas
Date:14 Jun 2016