Biology Reference
In-Depth Information
produced an average of 0.9 offspring each, the subordinates were never observed to
father any offspring. Therefore, by helping the dominant to reproduce, the subordinate
'sacrificed' 0.9 of its own offspring in order to increase the dominant's reproductive
success by 6.1 offspring.
Plugging these values into Hamilton's rule gives (0.42
0,
and so Hamilton's rule is met. Consequently, the cost of helping by the subordinate is
outweighed by the indirect benefits they obtain by helping their kin (brothers) obtain
more mates. Indeed, the minimum relatedness required to meet Hamilton's rule is
r
×
6.1)
−
0.9
=
1.7, which is
>
Cooperative
courtship is
explained by the
benefit of helping
a relative obtain
more mates …
=
0.15 (i.e. the value of
r
at which
r
×
6.1
−
0.9
=
0), and so even half-brothers
(
r
0.25) would be expected to cooperate. It should be noted that the data are
observational, so these calculations do not allow for complications such as individuals
of different quality pursuing different strategies. For example, are subordinates low-
quality individuals that were incapable of breeding independently? Ideally, removal
experiments are needed to justify the assumption that, in the absence of its partner, both
the dominant's and the subordinate's success becomes equal to that of a solitary male.
Nevertheless, a number of alternative explanations can be ruled out, based upon
direct benefits to the subordinates, which may be important in other species (Krakauer,
2005). Firstly, there is the possibility that subordinates help to gain a 'share' of the
reproductive success, as appears to occur in the ruff (
Philomachus pugnax
; Lank
et al
.,
2002). This can be ruled out in the wild turkey because subordinates produced no
offspring. Secondly, subordinates may help to gain an increased likelihood of future
inheritance of a display perch, as appears to occur in
Chiroxiphia
manakins (McDonald
& Potts, 1994). In the wild turkey, coalitions form before adulthood and only change
through death. Consequently, if the dominant dies, the subordinate just becomes a
solitary male.
=
… and not by
immediate or
delayed direct
fitness benefits to
the subordinate
How do individuals recognize kin?
Hamilton's explanation for altruism requires a sufficiently high relatedness between
interacting individuals. One way to obtain this is kin discrimination, whereby an
individual assesses its relatedness to other individuals and adjusts behaviour accordingly.
There is now a rapidly growing body of evidence that individuals can indeed do this,
and even distinguish close kin from distant kin. How do they achieve this?
Greenbeards
An entertaining but theoretically unlikely possibility was proposed by Bill Hamilton
(1964) and coined the 'green beard effect' by Richard Dawkins (1976). The idea is that
there may be 'recognition alleles' which express their effects phenotypically, so enabling
bearers to recognize these alleles in others, and also causing bearers to behave
altruistically towards others with the recognizable phenotypic effect (Fig. 11.3). Such a
gene would, therefore, need to do three things: signal itself, recognize the signal in
others and direct cooperation towards those where it detects the signal. Such a gene
would, for example, confer on its owner both a green beard and a tendency to be nice to
others with a green beard.
Greenbeard genes
cause altruistic
behaviours to be
directed at other
individuals who
carry that gene
