Why are female reindeer the only kind of deer that except caribou to have?

It's not accurate to say that only female reindeer have antlers, excluding caribou. Both male and female reindeer (and caribou) can have antlers.

Here's why this is a common misconception:

* Antlers are seasonal: Antlers grow in the spring and are shed in the fall. Male reindeer typically grow larger antlers than females, and they shed their antlers earlier in the fall.

* Antlers are used for mating: Male reindeer use their antlers for dominance displays and fights during the mating season.

* Females need antlers for other reasons: Female reindeer also use their antlers to fight for resources, protect their young, and clear snow for foraging.

Therefore, while males often have larger antlers during the mating season, both male and female reindeer (and caribou) can and do grow antlers. The key difference is the size and timing of shedding, related to their specific roles in the species.