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with males that had similar hydrocarbon profiles, the sex ratio was unbiased or f emale
biased. Put simply, if a queen mates with two males who smell the same by chance, then
her offspring will vary little in smell, leading to the workers thinking their queen only
mated once, and hence produce the wrong sex ratio. This provides a very elegant
demonstration of the general problem that information processing can constrain how
perfectly animals would be expected to behave.
Sundström and Boomsma's results also raise the problem of how could the workers
manipulate the sex ratio to their advantage? The queen controls the initial sex ratio of
the brood produced, so the workers must be doing something else. To determine what
was going on, Sundström and colleagues (1996) turned to another wood ant, Formica
exsecta , which also has split sex ratios in response to queen mating frequency. They
compared the sex ratio of eggs with that of pupae and found that whilst these ratios
were not different in colonies with multiply mated females, the proportion of males
decreased significantly between the egg and the pupal stages in colonies with singly mated
queens. This suggests that the workers are selectively neglecting or destroying males.
Eliminating males is just one way to manipulate sex ratio; others have since been found.
For example, in Leptothorax acervorum workers bias the sex ratio of reproductives by
adjusting the proportion of females that develop as queens or workers (Hammond et al .,
2002). This non-destructive option may not be open to F. excsecta workers, because
workers and reproductives are reared at different times in that species.
To summarize so far, workers are selected to manipulate the sex ratio of reproductives
in their colonies in response to queen mating frequency. In at least some well studied
species workers appear to do this, leading to split sex ratios, by either destroying males
or adjusting the proportion of females that develop as queens or workers. This provides
strong support for Trivers and Hare's original hypothesis that the workers win the
conflict, albeit via a more complicated route than they imagined. So, is that the end of
the story? No, because it seems that workers do not win all the time - sometimes the
queens win!
Luc Passera and colleagues studied the red fire ant Solenopsis invicta . This species is so
named because ants will swarm up a person and only start inflicting their painful sting
in response to a pheromone released by the first ant to attack. This leads to the ants
stinging in concert, causing a level of pain analogous to being burnt, which can cause
death in small animals by overloading their immune system. Passera et al . (2001)
experimentally switched queens between male- and female-producing colonies, and
found that the sex ratios produced in a colony post-switching was predicted by the
colony from which the queen came. Colonies produced predominantly females if
the queen came from a female-producing colony, and vice versa, irrespective of whether
the host colony (workers) was previously producing predominantly females or males
(Fig. 13.13). This suggests that, in this species, the queens have a significant amount of
control over the sex ratio! The queens appear to do this by varying the proportion of
eggs that are male. In some colonies, they produce almost no males, forcing workers to
rear only females, whereas in other colonies they produce only a small proportion of
females, forcing the workers to rear them as workers and the males as reproductives.
Overall, these results suggest that sex ratio conflict is a constant tug of war in the
hymenopteran social insects. The workers appear to often gain the upper hand, but not
always. This is a very active area of research and a major outstanding problem is
explaining the variation in who wins, and how different players try to gain the upper hand.
Workers make
mistakes when
queens mate
males who smell
the same
Workers
manipulate the
sex ratio by
destroying males
… or adjusting
the proportion
of females that
develop as
queens or
workers
But sometimes
queens win
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