Men only slowed their walking speed when they were ambling with a female romantic partner -- not when they were with other women.
This being a scientific study, the researchers added that the phenomenon might have evolutionary roots in an attempt by mobile couples to help preserve the female's fertility.
The study, led by Cara Wall-Scheffler of Seattle Pacific University, found that "males walk at a significantly slower pace to match the females' paces when the female is their romantic partner."
In a study involving 22 people, researchers assessed men's walking speeds as they strolled around a track alone, with a woman who was a romantic partner, or with friends of the same and opposite sex.
The result: men walked at a 7 percent slower pace with a woman who they were romantically involved with.
There was no significant change in the men's pace when walking with male or female friends, according to the study published Oct. 23 in the journal PLoS ONE.
The researchers noted that this phenomenon has been seen in other species, where the energy needed to move is conserved -- in part to help boost fertility. Humans have an "optimal" walking speed that minimizes their energy output, and men tend to have a faster walking speed than women.
So, when a man and woman walk together, one of them will have to switch from their optimal speed and pay the price in energy output, the researchers said.
In the human scenario, it seems that the man most often makes a sacrifice, the study found. He will slow down to match her optimum speed, so that the "female is spared the potentially increased caloric cost of walking together." The energy the woman retains might then "be allocated to reproduction," the researchers concluded.
Harvard Medical School explains the health benefits of walking.