LMC and mating assurance
Under extreme inbreeding, when there is a single foundress, LMC
would predict a very female-biased sex ratio, with just enough males
to fertilize their sisters. However, stochastic male mortality may select
for foundresses to produce slightly more males than necessary to act
as insurance (Hartl, 1971). This has been found to occur in bethylid
parasitoids (Insecta: Hymenoptera) and probably malaria parasites. In
26 species of Bethylidae, the sex ratio was negatively correlated with
offspring number (Griffiths and Godfray, 1988); i.e. as offspring number
increases, because males can fertilize many females, the foundress
produces just enough males to fertilize the females and so the optimal
proportion of males (the sex ratio) decreases with increasing total number
of offspring. However, the fit was approximate and they observed that,
especially in large clutch sizes, more males than predicted (i.e. needed to
fertilize all the females) were found; these were interpreted as insurance
males.Habitat Quality
In the 1970s, Trivers and Willard (1973), while working on mammal
systems, developed the idea that the fitness of sons and daughters could
vary according to the maternal condition or circumstance: that is, the
Fisherian assumption that the relationship between fitness and resource
allocation is identical for sons and daughters does not always hold – there
are circumstances where one sex gains more from additional investment
than the other. Charnov (1979) and Bull (1981) extended this idea to all
situations where mothers vary in their resources available for repro-
duction and one sex gains more from additional investment. In solitary
parasitoids, the foundress tends to lay females in large host larvae and
males in small host larvae, because the female gains more from being large
whereas the male suffers less from being small: that is, larval host size
dictates the adult size of the developing parasitoid (Charnov, 1982; King,
1987). Size-dependent sex ratios have indeed been commonly found in
parasitoid species whose hosts stop growing upon parasitism (idiobionts),
but less so where the parasitized host continues to grow (koinobionts),
where initial host size would not be expected to be a good indicator of
resource potential (King, 1989). As well as the ability to assess the quality
of the larvae, foundresses may be able to identify previously parasitized
hosts. This is analogous to altering sex ratio with the number of
coinfecting foundresses where individuals are expected to increase their
male investment, as is the case for parasitoids and intestinal trematodes.
Thus, sequential parasitism will similarly be expected to lead to the
second foundress investing more in males, not only because of the
predictions of LMC but also because host quality may decrease because of
overexploitation. In species where sex is determined at conception, such206 R.E.L. Paul