Maternity Leave

In ecology, animals can be divided into two broad categories: R strategists and K strategists. The R strategists are creatures like bacteria or insects - generally small and fast reproducing which provide their offspring with virtually no parental care beyond laying eggs in safe place. When they hatch the offspring are left to fend for themselves. K strategists, on the other hand, often include a developmental phase before reaching sexual maturity. They will produce fewer offspring, but typically offer a greater degree of parental care to ensure survival. This occurs across a wide range of species from reptiles to mammals and has some presence in the fossil record.

The fossil of Philydrosaurus surrounded by its young

Dinosaurs like Maiasaura are thought to have taken care of a newly hatched clutch of eggs in a similar fashion to that of a bird, by protecting the hatchlings and bringing them food. Now a fossil from China has pushed the record of post natal care back into the Jurassic.

An artist's impression of the 160 million year old diapsid
Philydrosaurus caring for its young

It is of a small species of aquatic diapsid reptile known as Philydrosauras which is part of a 160 million year old group known as the choristoderes. It is surrounded by no less than six additional members of the same species. Their small size suggests that they are juveniles and the clustering behaviour is thought to be a mechanism designed to reduce their chances of being snaffled by a predator.

'Their relatively small size would have meant that choristoderes were probably exposed to high predation pressure and strategies, such as live birth, and post-natal parental care may have improved survival of the offspring,' said Dr Charles Deeming, from the University of Lincoln's School of Life Sciences. 'This specimen represents the oldest record of post-natal parental care in diapsids to our knowledge and is the latest in an increasingly detailed collection of choristoderes exhibiting different levels of reproduction and parental care.'

Unequivocal evidence of post natal care in the fossil record is extremely limited, but its existence in a multitude of modern diapsid groups and in diapsids in the fossil record suggests that it may have existed in the common ancestor of all diapsids. This creature, whose remains have yet to be found, lived in a terrestrial environment full of predators both reptilian and amphibian. Post natal care would have been a useful behavioural adaptation to ensure a greater portion of the species survived. More evidence is needed, however, before this hypothesis can be confirmed.