Harem stallion changes are not associated with diminished reproductive performance of females in semi-feral Konik polski horses (Equus caballus).

This research investigates whether changes in the dominant male, or “harem stallion,” in a group of semi-feral horses affects the reproductive performance of the females. The findings suggest that the changing of a harem stallion doesn’t affect the successful reproduction of the female horses, contradicting previous theories of potential infanticide or foal abandonment.

Objective of the Study

• The aim of this study was to investigate and understand the effect of a change in the leading stallion on the mares’ subsequent reproduction performance, specifically focusing on semi-feral Konik polski horses.

Methods and Approach

• The researchers conducted a long-term study (31 years) in which they observed 8 horse bands in a semi-feral environment.
• During this period, the harem stallion changed 10 times, affecting 26 of the 48 mares and accounting for 60 of the 609 observed mare-seasons (a season in which a mare experiences a reproductive event).
• Various factors were studied including changes in the harem stallion, the age of the mares, and the year of each reproductive event.

Findings of the Study

• The researchers found that the mares’ reproductive performance remained consistent, regardless of changes in the harem stallion. This indicates that the mares’ ability to give birth and rear foals to at least one year of age was not affected by a stallion change.
• The likelihood of a mare staying barren increased as she got older (21% increase per year).
• Of the observed mares that were pregnant at the time of a stallion change, virtually all successfully gave birth (25 out of 26).
• In the 31 years of study, there were no observed cases of the new stallion showing aggression towards the foals.

Implications of the Findings

• These findings disapprove previous theories of infanticide and feticide related to a new stallion’s aggression and/or pheromonal influence causing loss of a fetus sired by another stallion and a female’s counteraction to infanticide.
• The fact that all foals born in a new harem reached adulthood without suffering any aggression refutes the “Bruce effect” hypothesis, which suggests the presence of a new harem stallion leads to the termination of pregnancies sired by another stallion as a biological strategy to reduce investment in pregnancy and avoid potential infanticide.