On the strictly protected island of Golem Grad in North Macedonia, the tortoises are destroying their own population. During prolonged courtship, aggressive males are exhausting the females and frequently pushing them off the cliffs. Consequently, there are now one hundred males for every female capable of laying eggs. This is the only known example of demographic suicide in the wild to date.

Under favourable, stable and protected environments, large animal populations have no reason to die out. This should not happen unless a catastrophe, such as a devastating fire or the destruction of their habitat, or over-exploitation, wipes out all individuals or weakens the population, making it vulnerable to disease and other disturbances and hazards.

Well sheltered by the steep cliffs that line the island of Golem Grad on Lake Prespa in North Macedonia, Hermann’s tortoises (Testudo hermanni boettgeri) thrive on the wooded plateau.

After basking in the morning sun, they graze in the meadows, rest, and court, with the males emitting high-pitched sounds during mating. At first glance, nothing seems to threaten this population.

As is the case with other long-living species, maintaining populations requires high survival rates among adults. On Golem Grad, the adults have no predators, as wild boars, dogs, rats and humans are absent from this strictly protected island. The mild Mediterranean climate of this lake, situated at an altitude of 850 metres, is also favourable for reptiles.

All these factors explain the extraordinary population density, which stands at around 50 individuals per hectare – the highest ever recorded for tortoises. The ease with which these tortoises can be observed and studied led to the establishment of the field-monitoring programme in 2008. This was the result of a fruitful scientific collaboration between North Macedonia, Serbia and France, and this long-term monitoring programme was awarded the CNRS’s SEE-Life label in 2023.

But appearances can be deceiving: this population is in a critical state.

The extensive demographic, behavioural, physiological and experimental data collected over nearly 20 years show that, although highly active sexually and reproductively, this population is effectively committing suicide!

Demographic suicide is a strange and counter-intuitive theoretical process. The conditions under which it may arise are quite........

© The Conversation