Evolution, 4th Edition

306 CHAPTER 12

parent that pays the greater cost in caring for the offspring (in terms of lost oppor-

tunities for further reproduction).

This theory may explain major patterns in parental care [15]. In birds and mam-

mals that must feed their young, parental care is more costly for males than for

females, because males could potentially obtain other matings in the time they

spend rearing a brood. That may explain why parental care in those animals is

generally provided by females or by both mates (FIGURE 12.7A). In contrast, many

fishes and frogs guard their eggs and young, but most do not feed their offspring.

Males can often mate with additional females while they guard the eggs already

in their nest (FIGURE 12.7B). In contrast, females can increase their reproductive

success only by replenishing the massive resources they have spent on producing

eggs. To do that, they must abandon the nest to forage. That may explain why in

fishes and frogs, males provide parental care more often than females.

Murder in the family

Sometimes an individual’s fitness is enhanced by killing the young of its own spe-

cies [15, 38]. In lions (see Figure 10.12), baboons, and many other social mammals,

males kill the offspring of other males, then father their own offspring with their

victim’s mothers (FIGURE 12.8). Selection can favor this behavior for two reasons:

it eliminates the genes of competing males, and females become fertile and sexu-

ally receptive sooner if they are not nursing young. This behavior occurs mostly

in species in which social groups contain more females than males and in which

sexual selection is likely to be strong [48]. In this situation, there are few possible

fathers of the new offspring from females whose offspring are killed, so the fit-

ness benefit is more likely to go to a murderous male than if there were an even

sex ratio.

While the murder of unrelated young might make evolutionary sense, how can

we explain the fact that in some species parents kill their own children? Infanticide

can be a way of adaptively regulating brood size [55]. A bird’s fitness is propor-

tional to the number of its surviving offspring, which equals the number of eggs

laid, multiplied by the probability that each egg survives. Survival may decrease

as number of eggs increases because of competition among the offspring for food,

and because parental care of a large brood can reduce the parent’s subsequent

reproduction (see Chapter 11). Female mice kill some of the young in their litter

Futuyma Kirkpatrick Evolution, 4e

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Evolution4e_12.07.ai Date 11-02-2016

(A) (B)

FIGURE 12.7 Parental care. (A) Great crested grebes (Podiceps cristatus) exemplify

the many bird species in which both parents care for the young. (B) A male three-

spined stickleback (Gasterosteus aculeatus) builds and cares for a nest containing egg

clutches fathered by him. This activity can attract additional females to mate with him.

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