Hamilton (1967) can occur in recently founded populations
in which male offspring of founding females compete among
themselves for mates. The mechanism of sex determination
in wood lemmings (Myopus schistocolor), collared lem-
mings (Dicrostonyx torquatus), and the South American
field mouse (Akodon azarae) has been proposed as a way
to mitigate local mate competition. In these taxa, the sex-
determining effects of the Y chromosome are suppressed by
a modified X chromosome (X) such that XY individuals
are female. As a result, population sex ratios of these spe-
cies are usually heavily female biased (Fredga 1988; Espi-
nosa and Vitullo 1996). In collared lemmings, XY females
produce both X- and Y-bearing ova, but compensate for
the loss of nonviable YY individuals by a higher ovulation
rate. Wood lemmings, on the other hand, show a double
disjunction during mitotic anaphase such that they produce
only X* eggs (Gileva et al. 1982).
Although the mechanism of sex ratio distortion is well
understood in these species, its underlying explanations are
not. The stability of most mammalian populations would
minimize the potential for the type of local mate competi-
tion that Hamilton envisioned in 1967, but it could occur
in patchy habitats where isolated populations are prone to
extinction followed by recolonization (Maynard Smith and
Stenseth 1978; Werren and Hatcher 2000). However, in
none of the species showing X* sex determination is there
evidence of the spatial clustering and inbreeding that current
models predict are necessary for such a system to evolveFacultative Sex Ratio Adjustment 135Table 11.2 Evaluation of selected literature relative to competing hypotheses of sex-ratio adjustment
Consis- Consis- Consis- Consis-
tent with tent with tent with tent with
Reference Taxa Evidence of bias? T-W?a FCA?b LRC?c EM?d
Rivers and Crawford (1974) Mus musculus Ye s Ye s — — Ye s
McClure (1981) Neotoma floridana Ye s Ye s — — Ye s
Gosling (1986) Myocastor coypus Yes Yes No No No
Goundie and Vessey (1986) Peromyscus leucopus Yes No Yes — Yes
Labov et al. (1986) Mesocricetus auratus Ye s Ye s — — Ye s
Meikle and Drickamer (1986) Mus musculus Yes (but significant in Partially — — Yes
opposite direction
for some treatments
McShea and Maddison (1986) Microtus pennsylvanicus Ye s Ye s Ye s Ye s Ye s
Armitage (1987) Marmota flaviventris No (except as a function No No No Yes
of maternal age)
Wright et al. (1988) Mus musculus Yes Yes — No Yes
König (1989b) Mus domesticus No No — — No
Krackow and Hoeck (1989) Mus musculus Yes (male biased for Yes Yes Yes Yes
2nd litters only)
Clark et al. (1990) Meriones unguiculatus No Yes — — Yes
Clark et al. (1991) Meriones unguiculatus No Yes — — Yes
Trombulak (1991) Spermophilus beldingi No No No No No
Hornig and McClintock (1994) Rattus norvegicus Ye s N o N o N o Ye s
Lambin (1994b,c) Microtus townsendii Yes No Yes Yes Yes
Meikle and Thornton (1995) Mus musculus Ye s Ye s — — Ye s
Moses at el. (1995) Neotoma cinerea Yes (but with maternal No — No Yes
mass, not treatment)
Sikes (1995) Neotoma floridana No No No No No
Hornig and McClintock (1994) Rattus norvegicus Ye s N o N o N o Ye s
Sikes (1996a) Onychomys leucogaster No No Yes No No
Krackow (1997) Mus musculus No No No No No
Duquette and Millar (1998) Peromyscus mexicanus No No No No No
McAdam and Millar (1998) Peromyscus maniculatus Yes No Yes — Yes
Moses et al. (1998) Neotoma cinerea Yes No — No Yes
Millesi et al. (1999) Spermophilus citellus Yes (varied with date) Yes No No Yes
Lamb and Aarde (2001) Mastomys coucha Ye s N o N o N o Ye s
Bond et al. 2002 Microtus canicaudus Yes No Yes Partially Yes
aConsistent with predictions of Trivers and Willard’s (T-W) 1973 hypothesis based on direction of sex-ratio bias or other evidence of differential investment?
bConsistent with predictions of the first cohort advantage (FCA) hypothesis?
cConsistent with local resource competition (LRC) hypothesis?
dConsistent with the extrinsic modification hypothesis (EM) for nonfacultative adjustment of parental investment?