Biology Reference
In-Depth Information
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Weight ratio (
/
)
Fig. 13.10
Ratio of investment (measured by dry weight) in 21 species of ants. The
x-axis is the ratio of female:male weight and the
y
-axis is the ratio of numbers of
males:females in the colony. The lower line is the prediction if the investment ratio is 1:1
and the upper line is 3:1 in favour of females. The data are closer to the 3:1 line, as
predicted if workers control the sex ratio. However, some analyses have suggested that
things are not so simple, because: (a) dry weight overestimates investment in females,
so the average investment is actually closer to 2:1, and (b) queens mate with multiple
males in some species, which leads to workers favouring an investment less biased
towards females. (To understand how the lines are drawn take the example of a
♀:♂
weight ratio of 6:1. Equal investment would mean six
♂
per
♀
, and a 3:1 investment
ratio in favour of
♀
would mean a ratio of 2
♂
per
♀
.) From Trivers and Hare (1976).
Reprinted with permission from AAAS. Photograph shows a mating pair of the rover
ant,
Brachymyrmex patagonicus
, in which females are considerably larger than males.
Photo © Alex Wild.
Although Trivers and Hare's results were highly impressive, several workers pointed
out a number of potential problems with how the data were analysed and interpreted.
One of these was that female-biased sex ratios could also be caused potentially by other
reasons, such as local mate competition or local resource competition (Alexander &
Sherman, 1977), as discussed in Chapter 10. Although these alternative explanations
were later refuted (Nonacs, 1986; Boomsma, 1991), it also became clear that across
species data were unlikely to provide definitive evidence for worker control of sex
allocation. Much stronger evidence has since been provided by within species studies on
sex ratio variation between colonies.
Alternative
explanations
are possible
Split sex ratios
When examining the sex ratios produced by individual colonies, rather than at the
species level, it is often observed that some colonies tend to produce all or predominantly
males (drones), while other colonies tend to produce all or predominantly females (new
winged queens; often referred to as gynes). Koos Boomsma and Alan Grafen (1990,
1991; Boomsma, 1991) showed that such 'split sex ratios' could be predicted when
workers are in control of the sex ratio, and if the relatedness structure varies across
colonies.
Some colonies
specialize in
producing males,
while others
specialize in
producing females
