Speak to professionals from various disciplines and you will notice something funny: Even when they are off duty, they tend to view the world through the lens of their professional background. For example, a psychiatrist at a dinner party might pause to think a bit about the possible neuroses of the guests. A police officer might scrutinize an ordinary scene — and the people present — for any sign of illegal activity. It's not that they will do anything about it or even speak their mind, but I bet that's what goes on in their heads — it's the same with evolutionary biologists.
A dinner party is a rich source of observations of animal behavior and a biologist automatically categorizes human activity according to what we've learned from animals. Try it next time you're at a party or in a bar. One of the most obvious behaviors shown by men is what biologists call "mate-guarding."
This occurs when a man (or a male of any species) has reason to believe their mate may attract the attention of a rival. Females have many reasons why they might actually want to interact with other males but, for now, let's consider the male point of view.