Which of the following is a common sign of illness in animals?

A change in appetite is a well-recognized sign of illness in animals. When an animal is not feeling well, it may exhibit a reduced appetite or, in some cases, an increased appetite depending on the condition. Changes in eating patterns indicate that something might be wrong, prompting the need for further observation and possibly veterinary evaluation. Monitoring an animal's appetite can provide critical insight into its health status.

In contrast, increased energy levels typically suggest good health, as does normal weight gain under appropriate circumstances such as growth in young animals. Excessive grooming, while sometimes indicative of anxiety or skin issues, does not universally signify health problems in the same direct way changes in appetite do. Therefore, a change in appetite stands out as a classic indicator of potential illness in pets.