The proportion of communally nesting
reptiles was underestimated partly because
reptile nests are so hard to find, and also
because many species do not nest.
Image: iStockphoto

Reptiles are not known to be the most social of creatures. But when it comes to laying eggs, female reptiles can be remarkably communal, often laying their eggs in the nests of other females. A new study from the Australian National University suggests that this out-of-character behaviour is more common in reptiles than was previously thought.

Scientists from the Research School of Biology at ANU performed an exhaustive review of literature on reptile egg-laying. Dr Sean Doody and his colleagues Dr Scott Keogh and Dr Steve Freedberg found that communal nesting has been reported in 255 lizard species as well as many species of snakes and alligators. The behaviour was also documented in 136 amphibian species. This suggested that communal egg-laying was much more common than generally recognised.

“Most of what we know about communal nesting in animals with backbones comes from birds, which typically have conspicuous nests,” Dr Doody said. “In contrast, nests of reptiles and amphibians are generally difficult to find. It was previously thought that about five per cent of reptiles and amphibians lay eggs communally. However, if we exclude species for which nests are not known that proportion rises dramatically to 85 per cent or more, at least for Australian lizards. This suggests that communal egg-laying is much more common than previously thought, and it is now necessary to explore why it occurs in these species.”

Other scientists have explored theories about shared parenting or nest-site scarcity being behind communal nest behaviour among reptiles, but the ANU researchers believe that there may be another evolutionary advantage for female reptiles in sharing nesting sites.

“Building a nest can be hard work for reptiles,” Dr Doody said. “Some female lizards may spend days digging a hole deep enough to deposit eggs. During that time she is not doing other important things such as finding food. She is also more vulnerable to predators. Females can avoid these costs by simply laying eggs in a nest that someone else has gone to the trouble to build. But sharing nests can also have a downside. When the eggs hatch, babies are immediately forced to compete with each other for resources. In addition, closely packed egg groups have an increased risk of disease transmission.”

Using a mathematical model, Dr Doody and his colleagues show that if the benefits to the mother outweigh the costs to the offspring, communal nesting makes evolutionary sense for reptiles. But when the costs of nesting together outweigh the benefits, we should expect to see solitary nests. This would explain why many reptile species occasionally use both shared and solitary nests.

More study needs to be done to confirm the model, Dr Doody says, but it is a starting point for explaining why communal nesting is so common in otherwise solitary reptiles. The research is published in the September issue of the Quarterly Review of Biology.