Biology Reference
In-Depth Information
Kin selection
In Chapter 11, we explained how altruistic cooperation can be favoured between relatives.
This idea is formalized by Hamilton's rule, which states that altruism will be favoured if
rB
0, where
B
is the benefit to the recipient,
C
is the cost to the actor, and
r
is the
genetic relatedness of the recipient to the actor. In the context of the Prisoner's dilemma,
cooperation can be favoured when individuals A and B are related, because the pay-off to
player B (weighted by
r
) will then matter to the inclusive fitness of the player A.
−
C
>
Kin selection
explains altruistic
cooperation
between relatives
Kin discrimination in long-tailed tits
One of the best examples of kin selection favouring cooperation comes from an extensive
study by Ben Hatchwell and colleagues on the helping behaviour of long-tailed tits,
Aegithalos
caudatus
, a small bird which breeds in most of Europe and Asia. In the non-breeding season,
long-tailed tits live in fluid winter flocks containing on average 16 individuals, including
overlapping generations of kin from one or more families, and also unrelated male and
female immigrants (Hatchwell & Sharp, 2006). Each flock occupies a large non-exclusive
range. In early spring, monogamous pairs form, with each pair occupying part of the range
of their winter flock. All birds start the season by attempting to breed independently and
there are no helpers associated with nests at this stage. However, many nests fail due to
predation. Whilst failed breeders may make a second attempt at nesting, some instead go
and help feed the chicks at other nests, leading to larger chicks with a substantially increased
likelihood of surviving to the following year (Fig. 12.3a; Hatchwell
et al
., 2004).
Using observational data from 52 helpers, Andy Russell and Ben Hatchwell (2001)
found that 79% of helpers were closely related to one or both of the breeders that they
were helping. They then went on to test a role of kin discrimination with a choice
experiment, where potential helpers were approximately equidistant between two breeding
nests, one belonging to close relatives and one to non-relatives. The results showed clear
kin discrimination - in 16 out of 17 cases (94%), individuals chose to preferentially go and
help at the nest of their relatives, rather than their non-relatives (Fig. 12.3b). Furthermore,
an analysis of a long-term data set on the same population suggests that indirect fitness
benefits are the major reason for the helping behaviour of long-tailed tits (MacColl &
Hatchwell, 2004). A role for direct fitness benefits can be ruled out because individuals
who help only rarely go on to breed in their later life, due to a high mortality rate
between seasons and a low rate of successful breeding. This means that individuals tend to
either gain direct fitness from breeding, or indirect fitness from helping, but rarely both.
As discussed in Chapter 11, kin discrimination can occur through direct genetic cues
of relatedness, or more indirect environmental cues, such as prior association or shared
environment. In long-tailed tits, a major cue for kin discrimination appears to be the
'churr', a contact call given frequently by both sexes for short-range communication
during behaviours such as nest building or aggressive interactions (Sharp
et al
., 2005).
The churr calls of relatives are more similar and individuals are more attracted to nests
where the recorded churr calls of relatives are played. Hatchwell and colleagues went
on to investigate the relative importance of genetic and environmental influences on
the development of the churr call with a cross-fostering experiment, where eggs where
switched between nests. They found that foster siblings (unrelated individuals reared in
the same nest) developed relatively similar calls that were just as similar as true siblings
Helping provides
clear benefits to
chick survival in
long-tailed tits
Long-tailed tits
preferentially go
and help at the
nests of close
relatives
Individuals gain
fitness through
breeding or
helping, but
rarely both
Long-tailed tits
distinguish kin
from non-kin by
an environmental
cue learned
as chicks - the
churr call
