Foaling appears to cause the opposite of a stress response, researchers at the University of Veterinary Medicine Vienna found.
Foaling in horses is extremely fast. Labour and the active part of foaling, resulting in delivery of the foal, take 10 to 20 minutes and are considerably shorter than giving birth in humans or in cows.
Researchers set to find out if this brief period was stressful for the animals or were horses more relaxed than humans when giving birth.
They closely observed 17 foalings at the Brandenburg State Stud in Neustadt (Dosse), Germany, as well as recording electrocardiograms before, during and after foaling.
The researchers also took samples of saliva and blood and analysed the levels of stress hormones such as cortisol and epinephrine.
"Normal foaling appears to cause just the opposite of a stress response," said researcher Christina Nagel.
Researchers found that during labour the heart rate of mares does not increase. On the contrary, the mares even miss some individual heart beats due to delayed stimulus conduction in the heart.
In humans, such second-degree atriventricular (AV) blocks often require medical treatment but many healthy horses show AV blocks at rest.
On physical activity, eg when the horse is ridden, the heart beat becomes regular and the beat frequency increases.
The finding of AV blocks during foaling suggests that mares are strongly influenced by the parasympathetic nervous system, which usually causes a state of rest and relaxation.
Its antagonist, the sympathetic nervous system, would prepare the organism for a stress response but does not seem to be active while the animals are giving birth.
The level of stress hormones remains low in foaling mares and the researchers did not find an adrenaline rush at any point.
The need to care for the newly born foal was also not perceived as stressful: contact between the mare and the foal was associated with a further state of relief and relaxation.
Horses thus experience giving birth very differently from human mothers. They need a safe environment to give birth: all the foals in the study were born at night, when the stable was quiet, researchers said.