Strength of sexual selection and sex roles vary between social groups in a coral reef cardinalfish

The strength and direction of sexual selection can vary among populations. However, spatial variability is rarely explored at the level of the social group. Here we investigate sexual selection and sex roles in the paternally mouthbrooding, socially monogamous, and site-attached pajama cardinalfish, Sphaeramia nematoptera. Females were larger, more aggressive, and had a longer dorsal fin filament, indicating reversed sex roles. At the scale of social groups, we show the Bateman gradient and reproductive variance depending on the sex ratio and size of the groups. In small and medium-sized groups with balanced or male-biased sex ratios, Bateman gradients were steeper for females, whereas gradients were equally steep for both sexes in large groups or when the sex ratio was female-biased. For both sexes, reproductive variance increased with group size and with a higher male-to-female sex ratio. In S. nematoptera, mating opportunities outside the socially monogamous pair appear to impact sex..., Study system and tagging
This study was conducted over five periods, spanning two years; October - November 2012 (observational period: 32 days), February - March 2013 (21 days), July - August 2013 (16 days), March - April 2014 (23 days) and September 2014 (9 days), in Kimbe Bay, Papua New Guinea (5°30’S, 150°05’E). To reflect approximately the same observational timespan and thus enable comparisons of reproductive output, data were grouped by year. Social groups of the pajama cardinalfish, Sphaeramia nematoptera, were defined as occupying the same distinct patch of the stony coral Porites cylindrica at a minimum distance of 2m from another group (Fig. 1a). A total of 379 individuals from 18 groups at 5-17 m depth on five reefs were included in the study (see map in Rueger et al. 2019). All fish were caught using handnets and diluted clove oil solution (Munday and Wilson, 1997), and individually marked using Visible Implant Elastomer (VIE) tags (Northwest Marine Technology). For each i..., , # Strength of sexual selection and sex roles vary between social groups in a coral reef cardinalfish

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Rueger et al. (Forthcoming 2024). Strength of sexual selection and sex roles vary between social groups in a coral reef cardinalfish. *The American Naturalist*.

The dataset contains the data underlying the Bateman gradient, reproductive output, and reproductive skew analyses (Bateman gradient file), as well as the data underlying the behaviour and trait analyses (aggression and Trait data files). We have also included the code for the Bateman gradient analysis and figures.

Data were collected over five periods, spanning two years; October - November 2012

(observational period: 32 days), February - March 2013 (21 days), July - August 2013 (16 days), March - April 2014 (23 days), and September 2014 (9 days), in Kimbe Bay, Papua New Guinea (5°30’S, 150°05’E).

## Description of the data and file structure

Each dat...