I feel like this was published in the 1990s or 2000s. It featured a group of anthropologists who come to study these aliens. They seem to be lazy and quite docile, and so the anthropologists come into their town and happily integrate. The climax of the book occurs when it turns out that after the rains, all of the teenagers are gathered together and forced to stand in the elements for some long amount of time, causing most of them to die. Then they are sent out on a quest to find something "interesting" to bring back to the tribe (or possibly this happens later?). I do remembered there was a "quest"-like coming of age that happened for the aliens.
The humans are forced to take part in this winnowing, but escape, and the rest of the story is about them attempting to get back to their ship, which they'd parked on the larger continent quite a way from the alien village. This is made more difficult by being hunted by the aliens who see them as having violated the tribe's precepts.