Biology Reference
In-Depth Information
Hidden benefits
In long-tailed tits, the benefits of cooperation are clear, with the presence of helpers
leading to a significant increase in chick survival (Fig. 12.3a). However, in other
species it can be harder to determine the benefits of cooperation, because they can be
hidden and/or delayed. A nice example of this comes from a study by Andy Russell,
Becky Kilner and colleagues on the superb fairy-wren,
Malurus cyaneus
, a small
cooperative breeding passerine bird found in south-eastern Australia. Observational
data showed that the presence of helpers did not lead to an increase in chick mass
(Fig. 12.4a), so there was no obvious fitness benefit of helping to the chicks being
raised (Russell
et al
., 2007).
Russell
et al
. went on to test whether this could be explained by the mother of the
offspring reducing her reproductive effort when she had helpers. Consistent with this,
it was found that when females had helpers they laid 5.3% smaller eggs with lower
nutritional content (14% smaller yolk; Fig. 12.4b). This suggests that the benefit to
chicks from helpers is exactly compensated for by the reduced investment into eggs by
mothers. Russell
et al
. separated these effects of egg provisioning and rates of helping
with a combination of multivariate statistical analysis and a cross-fostering experiment
that moved eggs between nests with different numbers of helpers. This showed that in
nests where the eggs had come from nests with the same number of helpers, and hence
were the same size, the presence of helpers led to a significant increase in chick size
(Fig. 12.4c).
This raises the question of why do mothers with helpers reduce their investment into
eggs, such that this cancels the benefit of helpers to their offspring? One possibility is
that the presence of helpers leads to greater competition for food, so the mothers have
less resources to allocate to eggs. Another possibility is that females with helpers invest
less in reproduction, in order to save resources for future breeding opportunities. The
latter explanation was supported by the observation from a 16-year data set, showing
that the presence of helpers led to a 11% increase in the probability that mothers
survived to breed in the next year (Fig. 12.4d). Overall, this suggests that the benefits
from helping accrue in the long term, by increasing the chance that breeders survive to
breed again in future years. It would be extremely interesting to know if individuals are
more likely to help in groups where they are more closely related to the breeding female.
In the superb
fairy-wren, the
presence of
helpers doesn't
lead to an
increase in chick
size …
… because
mothers with
helpers lay
smaller eggs
Helping in superb
fairy-wrens
provides a
delayed benefit to
the breeders in
the group
By-product benefit
In some cases, cooperation can provide a benefit as a by-product or automatic
consequence of an otherwise 'self-interested' act. The idea here is that cooperation is
always the best option from an individual or selfish perspective, but that this also
provides a benefit to others.
This can be illustrated with a cooperative hunting game (Table 12.3), which Ken
Binmore (2007a) has referred to as the Prisoners' delight. Imagine that two players
have the opportunity to take part in a potentially cooperative endeavour, such as
hunting for prey, and that taking part costs one unit of energy. If the hunt is successful,
all food is shared between two players irrespective of whether they took part or not. If
only one player takes part, the hunt is a mild success, such that the food returned
