I have 3 boxers. My 2 males are 4 and 3 yrs old. My female is 1. They are all fixed. They have been very loving and close to each other. Lately however my 4 yr old male challenges and bucks up to my 3 yr old boxer. This has resulted in two heated fights between them. The 4 yr old is the aggressor. I don't know why this is happening. What can I do? And most importantly why is it happening?

Help!
Marilyn Rodriguez
Lutz, Florida