It is well-known that the oldest ancestors of modern animals laid eggs, but in time some species evolved and started to give birth to live progeny. Vertebrate reproduction tends to be clearly divided into two categories: laying eggs or live births. However, there are a few species of animals that straddle the line in the middle.
The tree-toed skin has attracted the attention of a team of researchers, and current data infer that the species is a transition phase from egg-laying to live births, a process that is fascinating and rarely observed.
Biology classifies animals which lay eggs as oviparous. This trait is present among many species of fish that rely on external fertilization to reproduce. Most oviparous species encountered on land rely on eggs that are fertilized inside the mother and then laid in a nest. The egg yolk will provide the nutrients required for the development while the eggshell provides protection.
The lizard that gives live births
Viviparous animals will carry and feed the embryos within their body, giving live births. The evolutionary process from laying eggs to live births was a major milestone for many animal species since a series of complex physiological changes have to take place. Among the changes, we can count the need to develop the space required to house the growing embryos, the umbilical cord required for transferring oxygen and nutrients, and the mechanism which facilitates the birth.
In rare cases, there are species with use both reproduction means. The phenomenon is known as bimodal reproduction, and it is quite rare. For example, only three lizard species out of 6500 feature the ability.
Researchers have discovered that egg-laying tree-toed skins are at the center between live births and egg-laying, being capable of performing both within the same pregnancy. Some data suggests that their uterus is evolving in an attempt to facilitate live births, but more research is needed.