Aggressive Mating Drives European Tortoise Population Toward Extinction on Macedonian Island

A severe decline in the European tortoise (Testudo hermanni boettgeri) population has been documented on Golem Grad, a small island in Lake Prespa, North Macedonia. This island, about 200 square meters in area and surrounded by steep cliffs, hosts approximately 900 tortoises, a density higher than nearby mainland populations due to the absence of predators. However, researchers have observed a critically skewed sex ratio and deteriorating health among females, threatening the population's survival.

Over 16 years, scientists monitored two tortoise groups on the island, one on the central plateau and another near the cliffs, and compared them to a mainland control group near the village of Konjsko. They found an extreme male bias: 737 males to 46 females on the plateau, and 47 males to 17 females near the shore. Females on the island were smaller, less fertile, and had lower survival rates than mainland females. The researchers attribute this to the males' aggressive mating behavior, which includes chasing, colliding with, and biting females.

This relentless male pursuit forces females to constantly evade, disrupting essential activities like feeding and basking, leading to poor physical condition and reduced reproductive success. Males also harass immature females, further endangering their survival. Behavioral tests showed island females were more willing to flee over dangerous rocky terrain, sometimes resulting in fatal falls. Evidence of crushed shells under the cliffs supports this.

The combination of skewed sex ratios, male aggression, and female decline creates a feedback loop accelerating the population's collapse, described as an "extinction vortex." Models predict the last female tortoise on the island will die by 2083, potentially leading to the species' local extinction. Researchers caution against anthropomorphizing animal behavior but emphasize the urgent conservation implications.

This case highlights how even in relatively stable environments, interacting biological and behavioral factors can drive species toward extinction, underscoring the complexity of conservation challenges amid the ongoing global biodiversity crisis.