Competition for females has become very fierce: it is not known how it is that there are up to 19 males for every female on this island in North Macedonia. To escape aggressive males, females throw themselves off cliffs.
A new study suggests that sexual aggression from males may be the reason females of Mediterranean turtle (Hermann’s tortoise) throw themselves off cliffs on the island of Golem Grad, in Great Lake Prespa, North Macedonia.
Scientists describe the phenomenon as a “demographic suicide”.
The research, in Ecology Letters, documents a case of extreme imbalance between the sexes of the species. According to data, in some areas there are even 19 males for each female. In total, the population is around 1,000 turtles on the island.
Researchers note that this imbalance leads to violent mating attempts. Some females, exhausted after being chased by several suitors, climb to the top of the cliffs and launch themselves into the void as a way to escape. Many end up dying as a result of the fall, which reduces the turtle population on the island.
The projection indicates that, in the current scenario, the last large female Golem Grad is expected to die in 2083. Under normal conditions, Mediterranean tortoises, or Hermann’s tortoises, can live up to around 100 years.
Violent mating attempts
Upon noticing that a high number of females died young, ecologist Dragan Arsovski, from the Macedonian Ecological Society, decided to examine the species’ reproductive behavior and found that several males simultaneously chase a single female.
“She is literally buried by the males,” Arsovski told . Furthermore, mating attempts are marked by violence: males attack females, they bite them until they bleed and pierce them with the sharp tip of their tail. As a result, up to three-quarters of Golem Grad females have lesions in the genital region.
Although there are also records of males falling from cliffs, Arsovski said there is a “significantly higher proportion of females that die this way”.
“Extinction vortex”
To deepen the results, other experiments reinforced field observations: when placing females from the island in front of simulated cliffs, the researchers observed that they voluntarily launched themselves in the presence of males, while females from a neighboring continental population did not do so.
The study also points out that harassed females reproduce less and have lower annual survival rates than females on the mainland.
Jeanine Refsnider, an ecologist at the University of Toledo, said that male sexual aggression “appears to be causing a extinction vortex” of the females. He added that he had never seen anything like it in a natural environment without direct human interference.
Until now, it is not known how it came about the extreme imbalance between males and females on the island. One theory suggests that the fluctuation may have initially been random, as there are slightly more females than males on the continent.
Another hypothesis points to a introduction of animals promoted by humansgenerating imbalance. This thesis is supported by numbers engraved on the hooves of some of the older males.
