I agree that girls do respond well when they feel close to their team and that team chemistry is important. But I think boys feel the same way - they will race harder if they feel they are part of the team and that the effort of all is important. Making sure each kid feels that they contribute to the program is an excellent way to get them motivated to do their best, even if they aren't actually scoring for the team (yet).
I've watched two very successful programs recently and was surprised at the differences. The girls team has a ton of girls out, so their success is partly due to sheer numbers. But the coaches try to "raise" the game by isolating the top girls and giving them special privileges in order to get the others to run harder. (I've actually read recently on this board that that method was the best way to get girls to run faster -- make being on varsity special). But it doesn't work - it creates resentment among the girls, creates a spirit of divisiveness and most girls don't race hard. (Since they don't really work with the "almost in varsity, but not quite" girls, they don't really get kids with potential to improve (they just bring in fast new ones). It's pretty sad.
OTOH, a different successful program makes an effort to tell all its kids that each one of them is important, even if they aren't scoring. The kids (boys in this case) are told that each person pushes the guy in front of him to be better and therefore even the slowest kid on the team is helping the group be better. As a parent of a slower kid, I've noted that he feels a sense of ownership in the team and has exhibited the desire to do his best and race hard even if he's no where near varsity.
As an athlete, I've always run a lot better when I feel like I'm a real part of a team and I think that is true for most people and both genders. Be inclusive, create unity (bus rides and road trips are very helpful) and you will see them start to push harder (not just to be beat their teammate so that she gets more attention from the coaches) but because it helps the team goal.