Biology Reference
In-Depth Information
demand an honest signal from offspring in investment choice. In both cases, the cost
of the signal ensures its honesty (Grafen, 1990a).
Experiments by Rebecca Kilner with canaries
Serinus canaria
support this view, that
costly begging might resolve parent-offspring conflict. Nestlings begged more vigorously
when they were more hungry, and parents provided more food as begging signals
increased (Kilner, 1995). Furthermore, unrewarded begging was costly because it
retarded chick growth. In an experiment, pairs of siblings were hand-fed with the same
amount of food, but one member of the pair had to beg for just ten seconds before it was
fed, while the other had to beg for 60 s (both times were within the natural range for
begging bouts). The sibling that begged for longer had lower mass gain (which reduces
survival to independence), demonstrating that increased begging is costly to chick
fitness and thus restrains chick selfishness (Kilner, 2001).
According to resolution models, therefore, offspring demand and parental provisioning
become coadapted, so that the underlying conflict is now obscured. Mathias Kölliker
and colleagues (2000) were the first to reveal this coadaptation by clever cross-fostering
experiments with great tits
Parus major
. This enabled them to measure offspring demand
and parental generosity independently. They swapped newly-hatched young between
nests, so that parents raised a mixed brood of foster-young, half of which came from
one foreign nest and half from another foreign nest. When the chicks were ten days old,
each had its begging measured in the laboratory at two levels of hunger; after 60 min
and 150 min of food deprivation. This gave a measure of a nestling's begging intensity,
or demand for food, in response to increased hunger. Parental responses to chick begging
signals were recorded in the field by measuring their increase in provisioning in response
to playbacks of high versus low intensity begging calls.
The results showed that a nestling's demand varied with the nest of origin. In other
words, nestlings from the same original brood, but reared in different foster nests,
tended to have similar demands. Furthermore - and this was the most exciting
discovery - a nestling's demand was related to its genetic mother's generosity; nestlings
with more generous mothers demanded more, while those with less generous mothers
demanded less (Fig. 8.11a).
Cross-fostering experiments with burying beetles
Nicrophorus vespilloides
have
shown the same positive correlation between offspring demand and parental
provisioning (Lock
et al
., 2004). These beetles lay their eggs in the soil near a vertebrate
carcass. When the larvae hatch, they crawl to the carcass and beg (rather like nestling
birds). Their parents feed them by regurgitating a digested soup of carrion into the
larva's mouth. Laboratory experiments revealed that a mother's level of care, when
provisioning a foster brood, was positively correlated with the begging levels of her
offspring, when they were reared by a foster mother (Fig. 8.11b).
In theory, such covariation between offspring demand and parental supply could
result from genetic variation, with mothers genetically pre-disposed to be more generous
having offspring genetically pre-disposed to demand more resources (Kölliker
et al
.,
2005). However, it could also result from a so-called 'maternal effect', where individual
mothers have the capacity to match their offspring's demands to the local resources
available for care. This may sound far-fetched but there are many cases where offspring
phenotype varies as the result of maternal control. For example, towards the onset of
winter vole mothers give birth to offspring with longer fur, and in ponds with more
predators small crustaceans give birth to offspring who have more spines for protection
Costly begging by
offspring may
resolve parent-
offspring conflict
Coadaptation of
offspring demand
and parental
provisioning
Experiments with
great tits and
burying beetles:
generous mothers
have more
demanding
offspring
