Parting Company
If a ranger treats his followers well, they'll remain with him indefinitely. If the ranger has not neglected the well-being of his followers, or violated their trust, loyalty checks for them are rarely necessary.
When a Follower Abandons a Ranger
However, there is a limit as to how much abuse a follower will tolerate. The actions--or inaction--of the ranger may necessitate morale checks, as determined by the DM. Should a morale check succeed, the follower remains with the ranger. Should a check fail, the follower goes its own way, in most cases never to return. Followers who permanently abandon their ranger are considered lost and are not replaced.
Here are some situations that may result in a follower abandoning its ranger. It's up to the DM to decide when a given situation becomes stressful enough for the follower to require a morale check.
Inhospitable Terrain. An animal may hesitate to enter an environment radically different from its own. A hawk follower native to the mountains probably won't resist following a ranger into a forest or plain, but it may balk at entering an arctic region. Even if an animal follows a ranger into hostile terrain, it may not remain there for long; for example, a hawk may tolerate the freezing climate of the arctic for no more than a day or two.
Reckless Endangerment. Followers trained for combat willingly participate in encounters that may result in injury or even death. However, if the ranger forces an animal to participate in an unrelenting series of battles, resulting in serious damage or chronic fatigue, the follower may rebel and leave. Likewise, if a ranger regularly compels an animal to follow him into caverns, ruins, and similarly dangerous places, the animal may flee.
Starvation. Though followers usually find their own food, in certain situations they may depend on their rangers for nourishment. If a ranger neglects to keep a follower fed, the follower may decide to look for greener pastures.
Mistreatment. Most followers won't stand for beatings, whippings, or other physical mistreatment. (Being of noble character, most rangers won't administer such punishments, but exceptions do occur.) If the ranger persists in this behavior, abandonment is inevitable.
Inattention. Some followers, particularly dogs and other domestic animals, have emotional as well as physical needs. A ranger who consistently ignores or withholds praise and affection from his followers risks losing them.
Mating Season. An animal's urge to mate may overwhelm its loyalty to its ranger. An animal may disappear during mating season and never return, particularly if it has trouble finding a suitable partner. Morale checks usually aren't applicable in these situations; instead, the DM may take advantage of mating season to restore balance to the game by eliminating a powerful follower, or getting rid of a cumbersome follower that's proving to be too much of a burden to a ranger.
Impending Death. When certain followers reach the end of their days, due to illness or old age, they may abandon their ranger to die alone (an aged elephant journeying to an elephant's graveyard). Morale checks aren't usually necessary. Instead, the DM may engineer a poignant scene where the ranger realizes that death is near for his loyal companion, and must come to terms with the loss.
When a Ranger Abandons a Follower
Situations may arise where the ranger wants to rid himself of a particular follower. A follower with negligible trainability may prove to be more annoying than helpful. A filthy or frightening follower may make the party uncomfortable. A follower may eat too much, move too slowly, or scare off too many NPCs. Whatever the reason, the ranger has two options for abandoning a follower, either of which he can exercise any time he likes.
Release. This is a form of temporary abandonment. Using commanding gestures and a firm voice, the ranger lets the follower know that he wants it to remain behind. Because of the special bond between the follower and the ranger, the follower instinctively understands what the ranger is telling it, and responds by reluctantly wandering away into the wilderness. Should the ranger return to the general area where he released the follower, and call out to the follower or otherwise make his presence known, the released animal may show up again, ready to resume its role as a follower. (The DM decides if a released follower returns; generally, if the ranger attempts to locate the follower within a few months of its release, the animal will show up within a day or so. Otherwise, the ranger should assume that the released follower has died or relocated.)
Dismiss. This is a form of permanent abandonment. As with release, the ranger communicates his desire to dismiss a follower through a series of gestures and vocal commands. The dismissed follower moves away into the wilderness, never to be seen again. A dismissed follower will not return to the ranger. A ranger can't replace dismissed followers; they still count against his normal limit.
Other Options
A ranger who doesn't want to release or dismiss an unwanted animal follower has a host of other options, limited only by his imagination and the DM's approval. He can arrange for a farmer, a zoo keeper, or other NPC to care for the follower (making sure, of course, the caretaker is of good alignment). He can keep it in his stronghold, asking a human or demihuman follower to care for it. He can also give it to a friendly NPC as a gift (again, presuming the NPC is of good alignment; failure to do so may be taken as a sign of betrayal by the ranger's remaining followers).