The first-cohort advantage hypothesis (FCAH) was formulated to explain patterns of biased sex ratios in litters of Virginia opossums (Didelphis virginiana). Females of this species typically produce 2 litters each season, and litters born early in a season are male-biased and those born later in the season are female-biased. Males born earlier are older and heavier in the next breeding season and likely to secure more mates. The hypothesis predicts fitness benefits to mothers that have more males in their 1st litter (the 1st cohort) each season. The feathertail glider (Acrobates pygmaeus), in Victoria, Australia, is another species that produces 2 litters per year and not all males mature before the start of the breeding season after their birth. Hence, feathertail gliders also might be expected to show a pattern of sex bias in litters consistent with the FCAH. They do not. This paper uses data from 2 field studies of feathertail gliders in Victoria over the past 18 years. Sex ratios were recorded for 53 first- and 42 second-cohort litters. There was a trend toward male-biased litters in the 1st cohort, but also in the 2nd, and neither group differed significantly from a binomial distribution of sex ratios. This provides no support for the FCAH in this species. This may be due to the much less sharply defined breeding seasons in feathertail gliders compared to those of Virginia opossums, and to their greater longevity.
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