What can cause rust coloration in a rabbit's fur?

Rust coloration in a rabbit's fur is often attributed to over-exposure to sunlight. Prolonged exposure to UV rays can lead to the fading or alteration of fur color, resulting in a rusty appearance. This occurs due to the breakdown of pigment in the hair follicles, which may be more pronounced in certain breeds or colors of rabbits.

While other factors like nutrition and grooming can impact overall fur health and appearance, they are less directly responsible for the specific phenomenon of rust coloration. A lack of proper nutrition generally affects general health, coat condition, and may lead to dullness rather than a specific rust color. High humidity levels can affect grooming and cleanliness, but they do not specifically cause rust coloration. Excessive grooming might lead to wear and damage on the fur, but it would not typically change the fur color to rust. Therefore, the connection between over-exposure to sunlight and its effect on fur pigmentation is the most direct and clear relationship among the provided options.