Global positioning system (GPS) units are now lightweight and compact. They have proven useful for analyzing the behavioral characteristics of horses in
pastures. Because the GPS records data in latitude and longitude, it may be feasible to calculate the distance between GPS units. The present study aimed to
confirm the applicability of GPS units in behavioral studies on horses. For this, we analyzed the accuracy of the distances calculated from GPS units using
Hubeny’s distance formula and of the monthly changes in interindividual distances obtained from GPS units worn by Thoroughbred dams and their foals in a pasture
until weaning. The calculated inter-GPS distances were highly accurate. The regression line was linear, and the squared correlation coefficient (r2)
was 0.9998. During the first month of age, the interindividual dam–dam and foal–foal distances were significantly greater than the dam–foal distance. During the
second month of age, the dam–foal distance increased once and gradually decreased up to the fifth month of age. During the sixth month of age, the dam–foal
distance was significantly greater than the foal–foal distance. The GPS distances calculated using Hubeny’s distance formula were useful for analyzing changes
in interindividual distances in a herd of Thoroughbred dams and their foals. Most likely, calculation of the distance between GPS units worn on equine head
collars is likely to become a very useful tool as an objective index for quantifying equine behavioral observations.
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