Answer:
It will be their antlers.
Step-by-step explanation:
Bigger/stronger antlers tend make male deer seems more attractive in the eyes of female deer since it can be used as weapons to defend themselves from attackers.
When female deers are ovulating, their urine will smelt differently by Male deers. This will invite a couple of male deers to the female deers so they can copulate.
On most cases, male deers will have to fight each other to won the female deer. They will ram their antlers toward another until one of them fled away or heavily injured.