2003b-491 575s575 altruism altruisticbehaviorinanimals animal behavior biodiversity birds courtshipinanimals evolution evolution(biology) generativeorgans genetic humanbeings humanevolution naturalselection origin physiology ql761r42581988 ql761z94s2002 reproduction selection sexdifferences sexualattraction sexualbehavior sexualbehaviorinanimals sexualdimorphism(animals)