Biology Reference
In-Depth Information
whom it has a genetic stake. If these are full siblings, then the cost curve will be the
same as that for the parent because relatedness is 0.5 in both cases; a unit of extra
investment costs the parent investment in future offspring of relatedness 0.5, and costs
the offspring investment in future siblings, also of relatedness 0.5. Fig. 8.8 shows that
the optimal parental investment from an offspring's point of view is greater than that
from the parent's point of view. Between these optima there will be conflict, where it
pays the offspring to continue to demand care but it pays the parent to resist offspring
demands. Beyond the offspring optimum, both parent and offspring agree that
investment should cease.
The key point from Trivers's theory is that it is sibling rivalry which leads to parent-
offspring conflict. If a parent was designed to have just one offspring in its lifetime, there
would be no conflict.
Sibling rivalry
leads to parent-
offspring conflict
Sibling rivalry: evidence
We now consider the evidence, firstly for sibling rivalry and then for parent-offspring
conflict. There is abundant evidence that siblings compete for parental resources. This
often arises because food availability in the environment is unpredictable. It then pays
mothers to produce an optimistic brood size, in the hope that conditions will be good.
If food turns out to be scarce, sibling competition leads to brood reduction.
Facultative siblicide
The Galapagos fur seal
Arctocephalus galapagoensis
provides a good example of
interbrood
conflict
arising from unpredictable food. In the seas around the Galapagos Islands, the
availability of fish varies with seasonal and yearly changes in currents. Female fur seals
have one pup at a time. When fish are abundant, a mother produces plenty of milk and
can wean her pup when it is 18 months of age, but in poor conditions pups grow more
slowly and suckling can continue for two to three years. As a result of this variation, up
to 23% of pups per year are born while the older sibling is still being nursed. The two
pups then compete for the mother's milk and, in most cases, the younger pup dies within
a month, either from starvation or from direct attacks by the older sibling, who may
grab it and toss it in the air. Mothers sometimes intervene, leading to a fatal tug of war
as the older sibling pulls one end of the newborn pup while the mother attempts to
retrieve it (Trillmich & Wolf, 2008).
Unpredictable fishing can also lead to
intrabrood conflict
. The blue-footed booby
Sula
nebouxii
is a tropical seabird which lays two eggs. Incubation begins after the first egg is
laid, so the first chick has about four days' growth before its younger sibling hatches.
This size advantage means the older chick can reach up higher to intercept the
regurgitated fish from its parent's bill, and only after it is satiated does the younger chick
get fed. If food is abundant, then both chicks can take their turn. However, when food is
scarce the younger chick rarely gets fed and it starves to death within the first two to
three weeks. The key predictor of the younger chick's survival chances is the weight of
its elder sibling. When the elder sibling is 20-25% below its expected weight, it attacks
the younger sibling by pecking it. The younger sibling then cowers, becomes reluctant
Sibling rivalry in
fur seals and
boobies
