Normally, animals give birth or lay eggs. An Australian skink, a chestnut with a snake-elongated body and small lizard limbs, has been shown to do both in the same pregnancy.

Researchers at the University of Sydney have documented the observation of a female of Saiphos equalis laying three eggs and, a few weeks later, giving birth to a small: it is the first time that something similar is observed in the same vertebrate, and in the same reproductive event.

These reptiles originating from the Australian east coast are among the few animals to be capable of bimodal reproduction. In some areas, such as in New South Wales, they give birth to babies after feeding them with their body (viviparous reproduction); in others, as in the area around Sydney, they lay eggs (oviparous reproduction). Until now it was thought that one or the other modality was genetically determined: but the observed skink seems to be in a "gray zone" between the two. The research was published in Biology Letters.

See also: how reptiles change sex

Gradual passage. "The oldest vertebrates were oviparous, but over thousands of years, developing embryos in some species began to be kept inside the body longer, until some animals arrived at birth, " explains Camilla Whittington, head of the study.

In the evolutionary history of vertebrates there are at least 150 evolutionary transitions from oviparous to viviparous reproduction - which, contrary to what one might think, is not unique to mammals. There is also a third type of reproduction, the ovoviviparous one, in which the embryos grow inside eggs which provide them with nourishment and which open inside the maternal organism. Both the small and the remains of the eggs are expelled with the birth: it is typical for example of the salamanders and some squaliformes.

Evolution in progress. The Australian skink described here would have been caught in a moment of transition between oviparous and viviparous reproduction: as if it were testing the field, before opting for a final choice. Microscopic analysis of his eggs showed that they were thinner than those normally laid by oviparous skinks.

This versatility could make the Australian sauro the ideal organism to study the evolution of pregnancy and birth. In evolutionary terms, having both reproductive possibilities could be used to choose the safest option for children, based on environmental conditions.