When a ranger acquires an animal follower, he gains no special way of communicating with it. Unless the ranger trains the creature to respond to specific vocal sounds or physical signals, the follower passively accompanies the ranger on his travels, oblivious to his commands.
Animal followers feel loyal and friendly to their rangers. Most animal followers would no more harm their rangers than they would their own mothers. A follower would be unlikely to retaliate violently if the ranger mistreated it; instead, the follower would simply leave.
The ranger's presence has a calming influence on wild animal followers which tempers their reactions to the ranger's other companions. The animal followers will generally leave other player characters alone, so long as the PCs keep their distance and don't antagonize them. However, if a PC comes too close to a lion, tiger, or other wild animal follower, the follower may respond with a warning snarl or even a swipe of the paw. If the PC doesn't get the message, the follower may attack. Such an attack continues until the PC withdraws or the ranger intervenes. If the ranger has trained the follower to attack only when ordered, fellow PCs won't have to worry about assaults. Otherwise, the ranger's companions are advised to keep their distance. Even a ranger will not approach a predatory follower just after it has made a kill.
Naturally docile animals, such as sheep and mice, pose no threat to the party. Neither do domesticated creatures, such as farm animals and pets. Unless a trained animal is responding to its ranger's commands, the DM will decide how docile followers react, exactly as he does for followers that are wild animals.
Wild animal followers respond to non-player characters in much the same way as they do the ranger's companions; that is, they ignore NPCs who keep their distance and make no hostile actions, but may attack NPCs who get too close or threaten them. Docile animals respond timidly to unfamiliar NPCs, possibly cowering behind the ranger or seeking cover.
A ranger's calming influence also extends to followers who would normally consider each other predator and prey. If a deer and a lion are both among a ranger's followers, they co-exist harmoniously so long as they remain with the ranger. Though it's unlikely the pair would cuddle up together to go to sleep, neither would the lion eat the deer. At the same time, the lion follower would consider all other deer fair game, hunting them as necessary to satisfy its hunger. Should the ranger abandon or dismiss his lion and deer followers, the animals would shortly revert to their natural states, and the deer might stand a good chance of becoming the lion's next meal.
Animal followers provide many benefits to rangers, but there can be drawbacks as well. Here are a few typical complications, which the DM can use to add color to a campaign, serve as story springboards, or enliven an otherwise routine encounter.