Fig 13. 2 shows how a female opossum got her energy at different stages of her life, and the functions to which she allocated that finite energy supply. Again there is a trade-off: allocating less energy to reproduction means having smaller litters (babies) If everything else remains equal, aging reduces an individual"s fitness: two theories on why aging persists, therefore aging should be opposed by natural selection, rate-of-living theory. Posits that populations lack the genetic variation to respond any further to selection against aging: evolutionary theory. Invokes a trade-off between the allocation of energy to reproduction versus repair. And yet, longer life spans have not evolved. Females given extra eggs had progressively smaller clutch sizes in subsequent years, whereas control females did not begin to show reproductive senescence until age 4. Researchers concluded that there is a trade-off in collared flycatchers between early-life and late-life reproduction.