Two life cycles, one species: the Alpine newt in all its forms

Why the Alpine newt?

During its life, the Alpine newt, like many amphibians, goes through several stages (cf. triphasic): it begins life as an egg, then becomes a larva, a juvenile, and finally an adult. These different stages are associated with significant morphological changes, particularly during metamorphosis. Driven by hormones, this stage leads to a complete restructuring of the organism: certain organs and tissues transform to enable the newt to transition from an aquatic lifestyle to a primarily terrestrial one.

However, not all populations of the Alpine newt follow this pattern. In some populations, found mainly in Italy and the Balkans, metamorphosis can halt before it is fully complete. These individuals then become adults and can reproduce while retaining certain characteristics of their larval form and remaining in the water.

This strategy, called paedomorphosis, is optional and reversible: if environmental conditions become less favorable, these individuals can still resume their metamorphosis and transition to a terrestrial lifestyle. Facultative paedomorphosis can thus be considered a form of life cycle flexibility, allowing newts to take advantage of favorable conditions in the aquatic environment while avoiding the energy cost of a complete metamorphosis when it is not necessary. It is an example of variation in the life cycle within a single species, in other words, intraspecific variation.

We wanted to understand how this difference in life cycle might influence the morphology of adults, particularly that of the body and the masticatory system (the bones of the head), which also undergoes significant changes during metamorphosis.

To this end, we studied specimens preserved in European museum collections.

Body morphology was studied using measurements taken from various parts of the body. For the head bones, which is much more difficult to study directly, the specimens were X-ray scanned to obtain three-dimensional models of the bones. We then used a method called geometric morphometry, which allows for precise comparisons of bone shapes.

What did we find?

Our results show that paedomorphic individuals are smaller and exhibit morphological characteristics adapted to aquatic life. For example, their metacarpals (certain bones in the forelimbs) are relatively shorter, which may be related to a different use of their limbs when moving through water. The bones of the head also exhibit a morphology particularly well-suited to suction, a highly efficient feeding method in an aquatic environment.

We also sought to determine whether paedomorphosis alters the way different bones develop in relation to one another. Some bones can develop relatively independently; in such cases, they are said to be more modular. Conversely, when changes in the shape of one bone are strongly linked to those of others, the system is more integrated. Our results show that, despite differences in morphology, paedomorphosis does not alter this organization of bone development in Alpine newt.

In contrast, paedomorphic populations exhibit greater morphological diversity (disparity scores). This diversity may be related to the fact that the cessation of metamorphosis does not occur at exactly the same time in all individuals. Some have therefore undergone more remodeling than others, resulting in greater variation in the shape of their bones.

These findings are particularly interesting in the current context, as paedomorphic populations are dependent on permanent aquatic environments that are relatively free of predators. They are therefore highly sensitive to environmental disturbances, such as the introduction of fish, habitat degradation, hydrological changes, and climate change. Gaining a better understanding of the morphological consequences of different life cycle strategies thus provides insight not only into the constraints these populations face but also into their ability to cope with changes in their environment.