: Both individuals benefit from the interaction. In social groups, this often takes the form of "reciprocal altruism," such as vampire bats sharing food with those who failed to hunt.
When it had a pouch full of sounds, Zooseks climbed the willow and began to sing. It did not mimic anyone exactly. Instead, it braided the thumps, chatters, croaks, and rumbles into a single, strange lullaby. The first notes drifted low and slow, then rose in playful staccato, then settled into a rhythm as steady as the river. The tune wound down the valley like warm honey.
