new guinea pig owner questions

Question
Hi I recently got 2 guinea pigs. One is a 2 month old male and the other is possibly a year old male. We put them together and the older one puts his head under the younger ones body and it humps his head. What does this mean? Its gross but Im not sure what it means. Also Everything Ive read about guinea pigs diets is confusing. I dont want to make my pigs unhealthy. Are the feeds with pellets and all the seeds and foods bits good for them or not? Are the birdseed looking treats that say they are for pigs actually good for them? Whats good and bad? Please help I really want to be a successful owner of guinea pigs but theres so many do's and dont's.

Answer
This mounting behavior is normal for males just being introduced. It's also a dominance thing. The older pig is trying to assert his authority and let the younger pig know he intends to be the boss hog.

Guinea pig diets don't have to be that confusing. You need to purchase plain guinea pig pellets, no seeds, no fruit loops or colored pieces, and absolutely no bird seed looking things. Just purchase a good quality plain green pellet with 18 - 20% protein. Stay away from the grocery store feed that has all the colored chips in them. They have no nutritional value and are just eye candy for the owners.