Visible veins on the back of my bunny's ears?

[Marble]

Is this normal?

 

[Imbrium]

Rabbits have a more difficult time regulating body temperature (when it comes to heat, especially) than other animals. They can't sweat and they can't pant. Not all rabbits have visible veins on the back of the ears - it depends on the breed. For example, wild rabbits that live in desert climates will have larger ears and veins showing on both sides - some domestic breeds have similar characteristics. The veins in their ears allow for heat exchange, which is why they're so close to the surface. More surface area (ie back and front and/or larger ears) covered by surface veins means the rabbit can cool themselves faster.