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(there may be practical reasons, for example if one is smaller, but we ignore these for the
moment). However, from either offspring's point of view, it should be more interested in
its own welfare (it is genetically related to itself by a fraction, one) than its sibling (if its
sibling is a full sibling, then relatedness is 0.5; Chapter 11). Therefore, each offspring
should try to grab more than its fair share of parental investment.
There will also be
interbrood conflict
(Fig. 8.7b). Imagine a parent which has just one
offspring at a time (e.g. a seal). A time will come where it will pay a parent to terminate
care of this offspring and save further investment for the next one (the B-C maximum
in Fig. 8.2). However, the current offspring will benefit by continuing to demand care
because, once again, it is genetically more interested in its own welfare than that of its
future sibling.
We can illustrate parent-offspring conflict graphically (Fig. 8.8). The benefit and cost
curves from a parent's point of view remain the same as in Fig. 8.2; increased investment
in any one offspring brings diminishing benefits, yet costs continue to increase because
every unit of investment decreases resources available for other (current and future)
offspring. We now add benefit and cost curves from an offspring's point of view. For an
offspring, the benefit curve will be twice the parental benefit curve (an offspring is twice
as related to itself compared to the parent's relatedness to it). However, the offspring will
also experience costs from increased investment because this deprives its siblings, in
Interbrood
conflict: current
broods should
demand more, at
the expense of
future broods
Benefit to offspring
Benefit to parent
Cost to parent/offspring
Optimal PI
for parent
Optimal PI
for offspring
Parental investment per offspring
Fig. 8.8
Trivers's (1974) theory of parent-offspring conflict. The benefits and costs from
the parent's point of view are the same as for Fig. 8.2. However, an offspring will value
its own life (r = 1) twice as much as it is valued by its parent (r = 0.5), so the benefit
curve for the offspring is twice that for the parent. If siblings are full siblings
(r = 0.5) then the cost curve for the offspring is the same as that for the parent (see
text). The optimal parental investment from an offspring's point of view is greater than
the parental optimum. From Lazarus and Inglis (1986). With Permission from Elsevier.
