Cost-benefit analyses
Hibernation (cost: risk death in winter cold > benefit: possible nourishment, aka. energy gain) -- so many animals opt to keep out of the cold and reduce activity
Migration (cost: long-range flight is energy-intensive, aka. energy expenditure < benefit: better climatic conditions for offspring) -- so many birds opt to migrate
Predator/prey -- i.e. foraging (cost: risk of being eaten/killed vs. cost: nourishment from food, aka. energy gain)
Carnivore/Omnivore/Herbivore (protein and other nutrients in meat, aka. energy gain vs. 10% inefficiency between trophic levels, aka. energy loss) -- no clear winner, so we see diversity of species
Parental Behavior -- i.e. care for young -- also applies to Type I, II, and III survivorship curves (cost: energy expenditure for finding food, keeping warm, etc. vs. benefit: offspring more likely to survive) -- can go either way, so we see diversity of species