Biology Reference
In-Depth Information
The queen is equally related to her sons and daughters (
r
0.5 in each case) and so
Fisher's theory of equal investment says she should produce equal numbers of male
and female reproductive offspring (Chapter 10). To be more precise, the queen's ESS is
to
invest equally
in the two sexes. So, for example, if the production of queens requires
twice as many resources as the production as drones, then the Queen should produce
twice as many drones as queens. It is important to emphasize that we are referring to
equal investment in
reproductive
offspring, not sterile workers. Recall that in Chapter
10 the argument was that a 50:50 sex ratio was stable because the expected
reproductive success
of a male and a female is the same. Hence, the discussion of
sex ratios is only pertinent to reproductives.
Now for the twist: because workers are more related to their sisters (
r
=
Queens prefer a
1:1 investment
ratio, investing
equally in male
and female
reproductives
=
0.75) than to
their brothers (
r
0.25), they would rather rear a higher proportion of sisters. But how
much bias in favour of reproductive sisters should they show? Once again we search for
the ESS sex ratio, this time from the workers' point of view. If the workers rear too many
sisters then the sex ratio in the population will become so female biased that a drone will
have very much greater reproductive success than a queen. It turns out that the stable
sex ratio for the workers is a 3:1 investment in favour of reproductive females (Trivers &
Hare, 1976). When female reproductives are exactly three times as common as males,
drones have three times the expected success of queens because on average each drone
has three times the chance of finding a mate. From the workers' point of view this
would exactly compensate for the fact that brothers are only one third as closely related
as are sisters: a worker expects to get three nieces or nephews from her brothers for
every one she gets from her sisters. Nieces and nephews on her sister's side are three
times as closely related to her, so the total gain per unit investment via brothers and
sisters is the same.
To summarize, the queen prefers an equal investment in male and female reproductive
offspring, but the workers prefer a ratio biased 3:1 in favour of females. There is a
direct conflict of interest over the sex ratio between workers and the queen. Who wins?
And how?
=
Workers prefer a
female-biased sex
ratio, investing
three times as
much in females
as males
Queens and
workers disagree
about the optimal
ratio of
investment in
reproductives
Tests of worker-queen conflict
Robert Trivers and Hope Hare (1976) attempted to test whether the queen or workers
win by analysing the ratio of dry-weight investment (more accurate than simply looking
at numbers) in male and female offspring in 21 species of ants. The ant species were
chosen because they were ones in which the conditions for the hypothesis were likely to
hold (one queen, monogamous). Despite a considerable amount of scatter in their data,
Trivers and Hare found that, on average, the ratio of investment was much closer to 3:1
than to 1:1 (Fig. 13.10). They concluded that the workers win the conflict and
successfully manipulate the sex ratio towards their own optimum and away from that
of the queen. To put it bluntly, the workers are successfully farming the queen as a
producer of sisters and brothers: a far cry from the idea of workers as subordinate
females making the best of a bad job! Trivers and Hare suggest that the workers win
simply because they have both practical and numerical power; they are usually many
and do all the offspring provisioning, and are thus in a position to selectively prioritize
new queens rather than males.
Measuring
the ratio of
investment in ant
nests shows a
female bias …
… workers appear
to win the battle
over the
investment ratio
