The scientific name for it
is mate-choice copying. It's quite
easily observed in guppies
and at least one other species.
As often is the case, the male
guppy is the colorful one,
always seeking to prove himself
to the female, who chooses from
among her suitors. Typically,
he'll put on a display, and she
will show some interest, or not.
Sometimes, if the female guppy
swims away, uninterested,
and the determined male seeks out
another partner, who this time
reveals a willingness to mate,
the previously unimpressed
female will then come rushing back,
as if to say, "I was here first."
Any guy can tell you, you are
more attractive to the other
gender if you have a woman
on your upper arm, or further
down, a ring. Even an old ex
is better than nothing. It goes
to prove you had some value once
to someone, somewhere. Why not now?
It isn't really jealousy
exactly, or even envy.
More like the very same reason
a comic gave when he explained
why certain people seemed to take
an almost instantaneous
dislike to him. "It just saves time."