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Review
. 2017 Sep 19;372(1729):20160313.
doi: 10.1098/rstb.2016.0313.

Estimating Adult Sex Ratios in Nature

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Free PMC article
Review

Estimating Adult Sex Ratios in Nature

Sergio Ancona et al. Philos Trans R Soc Lond B Biol Sci. .
Free PMC article

Abstract

Adult sex ratio (ASR, the proportion of males in the adult population) is a central concept in population and evolutionary biology, and is also emerging as a major factor influencing mate choice, pair bonding and parental cooperation in both human and non-human societies. However, estimating ASR is fraught with difficulties stemming from the effects of spatial and temporal variation in the numbers of males and females, and detection/capture probabilities that differ between the sexes. Here, we critically evaluate methods for estimating ASR in wild animal populations, reviewing how recent statistical advances can be applied to handle some of these challenges. We review methods that directly account for detection differences between the sexes using counts of unmarked individuals (observed, trapped or killed) and counts of marked individuals using mark-recapture models. We review a third class of methods that do not directly sample the number of males and females, but instead estimate the sex ratio indirectly using relationships that emerge from demographic measures, such as survival, age structure, reproduction and assumed dynamics. We recommend that detection-based methods be used for estimating ASR in most situations, and point out that studies are needed that compare different ASR estimation methods and control for sex differences in dispersal.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

Keywords: adult sex ratio; bias; mark–recapture; two-sex matrix models; unmarked populations.

Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
Proposed terminology for sex ratios. Modified from [4]. Note that ‘mature’ means reaching the minimum age of first breeding, and the category of ‘post-breeding’ adults includes those males and females who survive beyond reproductive cessation. SR, sex ratio.
Figure 2.
Figure 2.
Intra-class correlation in ASR in bird species that have (a) several ASR estimates (up to six estimates per species, rICC = 0.643, F = 6.40, p < 0.001, n = 57 species) or (b) obtained by different methods (rICC = 0.717, F = 6.469, p < 0.001, n = 18 species). Methods were classified into six categories: counting breeding adults in individually marked populations, counting non-breeding adults, capturing breeding or non-breeding birds, demographic modelling, counting dead birds and counting birds in museum collections. Horizontal dashed line denotes unbiased ASR (value = 0.50) (A. Liker & T.S. 2017, unpublished data).
Figure 3.
Figure 3.
Comparison of ASRs (males/(males + females)) of green-rumped parrotlets estimated by counts of the number of breeding and non-breeding adults of each sex resighted annually (line) and after correcting for detection probability by dividing each count by the annual sex- and stage-specific (non-breeder and breeder) probability of detection (bars) for the upland population (a) and lowland population (b). Adapted from Veran & Beissinger [2].
Figure 4.
Figure 4.
(a) Life cycle diagram composed of four stages (NBF, non-breeding females; BF, breeding females; NBM, non-breeding males; BM, breeding males) for green-rumped parrotlets based on pre-breeding censuses. (b) Projection matrix used to deduce the ASR from the model based on proportions of the stable-stage distribution of males and females in the appropriate nodes of interest [2]. Notation includes: ϕ, probability of local survival; ψ, probability of becoming or remaining a breeder; R, fecundity; ρ, primary sex-ratio; I, immigration rate. Subscripts include: JF, juvenile females; JM, juvenile males. Adapted from Veran & Beissinger [2].

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