Fight or flight
When your ancestors encountered animals that were predators or prey, their hearts would beat faster, their blood pressure rose, and their pupils changed in size. These automatic responses allowed their bodies to move faster and to see better when escaping or pursuing animals.
The eyes show it
Nowadays, even when not escaping predators or pursuing prey, our pupils dilate automatically when we look at certain animals. How much our eyes respond can vary depending on the animal, and can be different for each person. How do you feel about rabbits? How do you feel about snakes? Your eyes will reveal the truth.