The Ecology Book

115

See also: Animal ecology 106–113 ■ Animal behavior 116–117 ■ The food chain

132–133 ■ The ecosystem 134–137 ■ Ecologica l resilience 150 –151

THE VARIETY OF LIFE

that the parents are likely to be able

to sustain. So, if a pair of birds can

only find enough food to feed six

chicks, but the female has laid 12

eggs, those young will be hungry

and may starve. If she has laid just

one egg, although the chick will

be raised successfully, most of

the available food will have been

unused. So neither the 12-egg nor

the one-egg scenarios are good

reproductive strategies; instead,

laying six eggs offers the best

chance of raising the most offspring.

This theory became known as

the food limitation hypothesis, or

Lack’s principle, and it was later

generalized by him and others to

cover litter size in mammals and

clutch size in fish and invertebrates.

The “latitude trend”

Lack’s hypothesis also suggested

an answer to another puzzle: why

most bird species have bigger

clutches at higher latitudes. On

average, birds near the equator lay

about half the number of eggs laid

by the same species in the far

north. This “latitude trend” could be

explained by a greater availability

of food during the long day-length

of summer compared with the

shorter day-length in the tropics.

However, other factors may

also apply. Higher mortality rates

in high latitudes—where winters

are harsh—may have led to the

evolution of large clutch sizes. This

is because the chances of survival

until the next breeding season

are low, and the reduced population

results in more food being available

for the survivors next season.

In 1982, Tore Slagsvold, a

Norwegian evolutionary ecologist,

advanced the nest predation

hypothesis, which proposes that

high rates of nest predation result

in smaller clutches. If a nest with

many chicks is found by a predator,

more work by the parent birds will

have been wasted than if the nest

contained fewer chicks. Also,

parents raising a large clutch are

more likely to be seen by predators,

because of the extra activity. Some

ecologists have argued that the

relative abundance of predators in

the tropics has been more important

than food supply in the evolution of

small clutch sizes at low latitudes. ■

Siblicide and the

Blue-footed Booby

Blue-footed Boobies are

seabirds native to the Pacific

Ocean. They get their food

from the ocean, but come

to rocky shores and cliffs to

breed. The female lays two

eggs, roughly five days

apart, so that by the time the

second chick hatches, the first

one has already grown

considerably. When food is

plentiful, the parents can find

enough to feed both offspring

until they fly the nest (fledge).

However, when food is scarce,

the larger chick will peck its

junior sibling to death. The

older chick can then get more

food, and is more likely to

fledge. If it does not murder

its sibling when food is scarce,

both chicks may starve.

This behavior, based

exclusively on the availability

of food, is called “facultative

siblicide.” In contrast, masked

boobies practice “obligate

siblicide”—the first-hatched

chick nearly always kills its

brother or sister, regardless

of how much food is available.

Blue-footed Boobies are driven

to siblicide by genetic factors.

The murder of a sibling can benefit

the perpetrator while also ensuring

the survival of the entire species.

Clutch size increases

with increasing latitude

and day length because ...

a longer day enables the

parents to find more food.

David Lack

Laying a clutch which

will result in a smaller

brood than ... could be fed

and reared successfully ...

confers advantages.

Tore Slagsvold

US_114-115_Clutch_control.indd 115 12/11/18 6:24 PM