What behavior may indicate an animal is feeling unwell?

Aggressive behavior and panting can indeed indicate that an animal is feeling unwell. Aggression may arise in animals when they are in pain or discomfort as a defensive mechanism. Similarly, excessive panting is often a physiological response to various conditions, including stress, fever, or pain. Both behaviors can signal that an animal is experiencing distress and may require further assessment by a veterinary professional.

In contrast, the other options generally represent behaviors associated with a happy and healthy animal. Wagging of the tail typically suggests that a dog is excited or happy, while sleeping a lot could indicate normal rest but may also hint at lethargy due to illness, yet by itself is not a definitive sign of being unwell. Interacting playfully with other animals is clearly a positive sign, suggesting the animal is feeling good and social. Therefore, the combination of aggressive behavior and panting is a more reliable indicator of discomfort or illness.