Which of the following is a case of altruism?
1. a shrimp digs a burrow that it shares with a goby that keeps watch for predators.
2. a prairie dog gives up a foraging opportunity to keep watch for predators.
3. a baby bird aggressively begs, resulting in less food available to its siblings.
4. a plant releases allelopathic chemicals, harming its competitors and itself.
5. individuals in a flock of birds are less susceptible to predators than individuals alone.