A) because of the relative benefits animals can derive from current reproduction versus from living longer
B) because things must eventually wear down, like the transmission on a car
C) because everything must grow old and die as a part of the cycle of life
D) because animals with senescence, such as zoo animals or humans with medical care, live long artificially

A) Natural selection does this; most mammals will eventually evolve the ability to have multiple offspring per litter.
B) The number of offspring per litter has no effect on reproductive success because in most cases only one survives anyway.
C) Reproducing involves a number of trade-offs, including quantity (number) versus quality of offspring. Larger litters mean lower-quality individual offspring.
D) It is not possible to change the number of offspring per litter in mammals; all mammals have about the same number of offspring.

A) home-range size.
B) nocturnal activity patterns.
C) forms of tool use.
D) the amount of fruit in the diet.