A scientist observes that the fur on the paws, noses, and ears of Himalayan rabbits tends to be dark brown while the fur on the rest of the animals' bodies tends to be white. The scientist biopsies a section of skin from the ear of a Himalayan rabbit and cultures the follicle cells.
When the cells are exposed to cold temperatures, they synthesize a certain pigment molecule. When exposed to warmer temperatures, the same cells stop producing the pigment molecule.
Which of the following BEST explains the results of the scientist's experiments?