I have a Druid and a Ranger in the game I'm DM'ing. They constantly change their animal companions. I know that a Druid can release an animal companion from service and get another one (or replace a dead one) for 24 hours of interrupted ritual. Is it the same for Ranger?
How does summoning happen exactly? Do the animals just pop into existence like a Summon Monster spell or does the Druid or the Ranger try to call an animal into their service with their rituals? Can they call any kind of animal regardless of the biome they are in? For example, can they get a camel in the woods, or can they get a wolf in a metropolis? Do they have to be in a biome where the animal could be found? It doesn't make any sense for them to be able to get a camel in the woods if they are not summoning the animal like a Summon Monster spell really.
When an animal companion dies, do they disappear like how a creature summoned by a Summon Monster would die, or do they die like a normal animal?
It would be great if I can get page numbers describing these rules.
Best Answer
The Ranger's entry for Animal Companions (page 48 of the Player's Handbook) specifically states
So Rangers also require 24 hours of uninterrupted time to replace a released or dead animal companion.
While there is nothing within the Player's Handbook regarding how the an animal companion is acquired, there is a section within Masters of the Wild that gives a more expansive set of rules on animal companions (page 34 onward). Please note though that Masters of the Wild is a 3e book, not 3.5, and as such certain elements of it have been superseded and other elements don't make a lot of sense (eg. the animal companion spell talked about in Masters of the Wild does not exist in 3.5), but it still does add a number of much more flavourful rules to a character's interaction with their companion.
As you surmise, the Animal Companion ability is not a magical effect - the animal does not simply "pop" into being like a summoned monster, the character must search within the typical terrain and climate that is home to the animal they are desiring. As such a dead animal companion would also leave a corpse like any other dead character within the party.
Masters of the Wild also requires a potentially greater amount of time to find an animal, saying that with a successful Wilderness Lore check to determine the animal's native terrain and climate and with the use of the detect animals or plants spell, that the character has a 30% chance per day of finding an animal of the desired type. While this is a much more flavourful rule for finding an animal companion, I can also understand why 3.5 has made this a flat 24 hour period without the need for spells or skills.
Masters of the Wild also expands on the bond between the PC and the animal, stating that it is a friend, not a slave, and it may grow tired of ill-treatment or constant damage-taking in fights. While the animal starts with an attitude of helpful, the GM should adjust the animal's attitude on the NPC Attitude table on page 149 of the Dungeon Master's Guide due to neglect or abuse, and that an animal that becomes hostile or unfriendly will do its best to leave the PC.
One further thing that your Ranger and Druid should consider is that Masters of the Wild classes abandoning a companion animal in a foreign land/terrain/climate, or the depths of a dungeon where most animals would not be comfortable, as an evil act, and that even neutral or evil Druids/Rangers would generally be against this practice.