Dogs and cats have a keen sense of smell, and they use it to gather information about their surroundings and other animals. When dogs and cats sniff each other, they are using their sense of smell to learn more about the other animal.

In particular, dogs and cats have scent glands located in their anal area, and these glands produce pheromones, which are chemical signals that animals use to communicate with each other. When dogs and cats sniff each other's rear ends, they are gathering information about the other animal's pheromones, which can provide important information about things like their age, gender, and reproductive status.

Additionally, sniffing is a natural way for dogs and cats to greet each other and establish familiarity and social bonds. Sniffing allows them to recognize other animals in their environment and identify members of their own social group. Overall, sniffing is an important way for dogs and cats to communicate and gather information about each other.