Biology Reference
In-Depth Information
(b)
0.8
(a)
0.6
0.4
0.2
0
Orange
controls
Black
controls
Orange
chicks
Black
chicks
Experimental broods
Fig. 8.10
(a) American coot chicks have orange-tipped ornamental plumes. Photo © Bruce Lyon. (b) Parental
provisioning to broods where all the chicks have had their orange plumes trimmed (black controls) is no
different than to normal broods (orange controls). However, in experimental broods where half the chicks have
been trimmed, the black chicks are fed less than their orange sibs. (Lyon
et al
. 1994).
preferences in other contexts. Nevertheless, it is clear that in all these examples increased
sibling rivalry from lower relatedness leads to increased signalling for parental resources.
Parent-offspring conflict: evidence
Behavioural squabbles
While the evidence for sibling rivalry is compelling, it has proved more challenging to
provide convincing evidence for parent-offspring conflict (Kilner & Hinde, 2008).
Simply observing behavioural squabbles does not necessarily demonstrate an
evolutionary conflict. Robert Trivers (1974) suggested that weaning tantrums might
reflect an offspring's attempts to manipulate parents into continued investment beyond
the parental optimum, but equally they may be part of behavioural development with
no underlying conflict (Bateson, 1994). A parent might gain information about
offspring needs when observing an older offspring attack a younger sibling, and even
brief interventions on behalf of the younger offspring, as observed in fur seals, may be
the way the mother assesses their relative fitness. In fact, all the examples of sibling
rivalry we have discussed so far might reflect the outcome of a parental strategy to
optimize investment in relation to variable resources.
David Lack, who pioneered the idea of optimal clutch size (Chapter 1), proposed that
brood reduction was a parent adaptation (Lack, 1947). He suggested that parents
