People have an optimal walking speed that minimizes energy expenditure. This optimal speed varies with physical features like mass and lower limb length, and therefore males in any given population tend to have faster optimal walking speeds than females. Given this difference, it is not clear what happens in walking groups of mixed-sex. In order to walk together, someone in the pair will need to pay the energetic cost of deviating from his or her optimal speed.
The authors here examined individuals' speed choices when they walked around a track alone, with a significant other (with and without holding hands), and with friends of the same and opposite sex. They found that males walk at a significantly slower pace to match the females' paces, only when the female is their romantic partner. The paces of friends of either same or mixed sex walking together did not significantly change, suggesting that significant pace adjustments occur only for romantic partners.
These findings could have implications for both mobility and reproductive strategies of groups, and could help interpret fossil footprint trails and hunter gatherer strategies.