Biology Reference
In-Depth Information
phylogeny and found two important results. Firstly, monogamy appeared to be the
ancestral state in all the independent transitions to eusociality that they examined
(Fig. 13.6). This suggests that monogamy originated first, giving a high relatedness, and
then when ecological conditions led to a consistently favourable
B/C
, eusociality
evolved. The same pattern of monogamy being ancestral to the evolution of eusociality
has also been found in other eusocial taxa such as the termites and sponge-dwelling
shrimps (Boomsma, 2007; Hughes
et al
., 2008; Duffy & Macdonald, 2010).
The second result found by Hughes
et al
. (2008) was that all the instances of multiple
mating were in derived lineages, where castes had already evolved. In these cases, the
reduction in relatedness due to multiple mating has not led to the loss of eusociality,
because workers had already lost the ability to mate and realize full reproductive
potential. Furthermore, these species had also already evolved division of labour and
specialized helping behaviours, potentially giving them a substantial
B/C
that would
allow Hamilton's rule to be satisfied, even when relatedness (
r
) becomes lower.
Monogamy is the
ancestral state
when eusociality
evolved
Multiple mating
evolved after
the transition
to eusociality
The ecological benefits of cooperation
The monogamy hypothesis shows how a constantly high relatedness (
r
0.5) could
occur between potential helpers and the offspring they would help raise (full siblings).
However, for cooperation and eusociality to arise it is also required that the ecological
conditions lead to a high enough benefit/cost ratio to make cooperation worthwhile.
Specifically, that it is more efficient for a female to raise her siblings than her own offspring
(
B
/
C
> 1). In this section we will discuss three ecological factors which have been
suggested to be of key importance: life insurance, fortress defence and food distribution.
=
The benefits of life insurance
Dave Queller (1989, 1994) pointed out that in species where there is a period of extended
parental care, such as ants, bees and wasps, helping could be favoured as a kind of 'life
insurance'. If a solitary female dies during the period when she is raising a brood, the
offspring dependent upon her care will die as well. If, however, the same female was part
of a group helping to rear the brood, her death would not condemn the offspring to die,
because others would continue to carry out brood care. Thus, as part of a group, a
female has at least some 'assured fitness returns'. This advantage could, in theory, make
it better to help others breed than to breed as a solitary female. In an attempt to quantify
the importance of this 'insurance effect', Raghavendra Gadagkar (1991) studied the
social wasp
Ropalidia marginata
. On average, the developmental period from egg to adult
in this species lasts 62 days. An individual reproductive female has a probability of only
0.12 of surviving for 62 days. Thus, her expected reproductive success from solitary
nesting is not very high. In fact, Gadagkar estimated that a female
Ropalidia marginata
could increase her expected success by 3.6-fold as a result of nesting in a group! This is
clearly much greater than the value of just over 1.0 required when there is monogamy.
Jeremy Field and colleagues tested the importance of insurance effects experimentally.
With observational data such as that collected by Gadagkar, it cannot be ruled out
that some other confounding factor might vary between nests with and without
helpers. For example, lower quality females might be less able to attract helpers,
leading to a lower brood survival rate, independent of the effects of helpers. To avoid
Death of females
before the end
of brood care
can favour
cooperative
breeding
