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König, H. E., Wissdorf, H., Probst, A., Macher, R., Voß, S., & Polsterer, E. (2005). Considerations about the function of the mimic muscles and the vomeronasal organ of horses during the Flehmen reaction. Pferdeheilkunde, 21(4), 297–300.
Abstract: Additional to the olfactory epithelium, the equine vomeronasal organ serves to the perception of odorous substances and specially for pheromones. In a middle-size horse this organ has an extension in length from an imaginary transverse plane about 10 cm caudally the nostrils to a transverse plane through the middle of the second premolar tooth. During the Flehmen reaction the levator labii superior, nasolabial, caninus and lateralis nasi muscles contract. The upper lip and the tip of the nose are lifted. The opening of the nostrils is narrowed, caused by the convergence of the plate and horn of the alar cartilage. In this manner in case of Flehmen reaction air is directly conducted towards the opening of the vomeronasal organ into the nasal cavity during inspiration. During the “Flehmen” horses assume a characteristic posture.
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Weeks, J. W., Crowell-Davis, S. L., & Heusner, G. (2002). Preliminary study of the development of the Flehmen response in Equus caballus. Appl. Anim. Behav. Sci., 78(2-4), 329–335.
Abstract: The flehmen response is commonly seen in most ungulates as well as in several other species (e.g. felids). The behavior is most often thought to be part of the sexual behavioral repertoire of males. One reigning hypothesis suggests that this behavior allows the male to determine the estrous state of a female through the chemosensory functions of the vomeronasal organ. However, females and young of both sexes also exhibit this behavior. Horse foals most frequently show the flehmen response during their first month of life with colts showing the behavior more often than fillies. This study tested the flehmen response on male and female foals throughout their pre-pubertal period. Foals were separately presented estrous and non-estrous urine weekly during the first month of life and then monthly until they were approximately 7 months of age. No significant differences were found between male and female foals for the following variables: latency to flehmen, duration of flehmen, frequency of flehmen and sniffs.
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