DOI: 10.5061/dryad.wm37pvmz6
title: Sexual dimorphism in fin size and shape in North American Killifish

description: Sexual dimorphism is intriguing because it suggests that males and females
 differ in phenotypic optima for traits and that sex-specific trait values
 can evolve despite a shared genome. Differences in sexual dimorphism
 across populations or species suggest that the nature of sexual selection
 and/or genetic constraints differs among species. Here, we measured sexual
 dimorphism in 20 species of North American killifish (Fundulidae) in size
 and shape of dorsal, anal, and caudal fins. We observed profound sexual
 dimorphism in anal and dorsal fin size and shape across all species,
 suggesting a common direction of selection. Sexual dimorphism was also
 present in caudal fin size and shape but was much lower in magnitude, with
 several species not differing from a null expectation of zero. There was
 little evidence for a phylogenetic signal in the levels of sexual
 dimorphism in dorsal and anal fin traits. We also found a strong
 phylogenetic correlation between sexual dimorphism in anal and dorsal fin
 shape but no phylogenetic correlation between fin area, base length, or
 ray length across different fins. Our results indicate that there is
 pronounced sexual dimorphism in anal and dorsal fin size and shape across
 fundulids. Similar patterns of sexual dimorphism in anal and dorsal fin
 properties have been documented in other groups, including gars, bichirs,
 graylings, minnows, and many species in the Atherinomorpha, suggesting
 that this pattern may be common across Actinopterygii.
description: Here, we measured sexual dimorphism in 20 species of North
 American killifish (<em>Fundulidae</em>) in size and shape of
 dorsal, anal, and caudal fins. We took digital photos of 363 individuals
 across 20 species. We measured standard length and fin base length for the
 dorsal, anal, and caudal fins. For the dorsal and anal fins, we also
 measured the lengths of the first and last fin rays as well as the lengths
 of median, first quartile, and third quartile fin rays. For the caudal
 fin, we measured the median fin ray length and the length of the upper
 dorsal and lower ventral fin ray. From these data, we estimated fin area.
 We also used published genbank data to estimate a phylogeny for the group.
 We analyzed the data in R. Because there is pronounced variation in
 standard length (i.e., size) across these species, we first performed a
 linear regression of the log of the trait value on the log of standard
 length and used the residuals as a measure of size-corrected fin
 traits. 
description: # Sexual dimorphism in fin size and shape in North American Killifish
 [https://doi.org/10.5061/dryad.wm37pvmz6](https://doi.org/10.5061/dryad.wm37pvmz6) ## Description of the data and file structure ### Files and variables #### File: Davis\_etal\_killi.csv **Description:**  ##### Variables * Fish_ID: ID number for code in original photo * Genus:self-explanatory * Species:self-explanatory * Sex: male or female * SL: standard length; length in mm from the tip of the snout to the base of the caudal peduncle * Dor_Base: dorsal fin base length (mm) * Dor_RF: dorsal first fin ray length (mm) * Dor_RQ1: dorsal first quartile fin ray length (mm) * Dor_RQ2: dorsal median fin ray length (mm) * Dor_RQ3: dorsal third quartile fin ray length (mm) * Dor_RL: dorsal last fin ray length (mm) * Dor_Area: dorsal fin area calculated as the product of dorsal base length times the average dorsal fin ray length (mm^2) * Anal_Base: anal fin base length (mm) * Anal_RF: anal first fin ray length (mm) * Anal_RQ1: anal first quartile fin ray length (mm) * Anal_RQ2: anal median fin ray length (mm) * Anal_RQ3: anal third quartile fin ray length (mm) * Anal_RL: anal last fin ray length (mm) * Anal_Area: anal fin area calculated as the product of anal base length times the average anal fin ray length (mm^2) * Caud_Base: caudal fin base length (mm); same as the caudal peduncle width (mm) * Caud_DM: caudal fin dorsal ray length (mm) * Caud_Mid: caudal median fin ray length (mm) * Caud_VM: caudal fin ventral ray length (mm) * Caudal_Area: caudal fin area (measured from photo) (mm^2) **Tree File** The tree file is the median fundulid tree pruned to include only the taxa included in the morphological data. **R Files** There is one large file that contains all of the analyses presented in the paper. It has the code to create the 4 main data tables in the paper (Tables 2-5) and to create 6 of the figures (figures 3-8). Table 2 is the correlations between trait values and standard length. Table 3 presents the analyses for each major trait, including the F-values from an Analysis of Variance that considers the effects of Species, Sex, and their interaction. It also lists the value for sexual dimorphism as measured as the effect size due to sex. It presents statistics on the extent to which there is a phylogenentic signal on standard length and other fin traits after being corrected for standard length. It also examines whether sexual dimorphism has a strong phylognetic signals. Finally, it presents the across-species correlation between male and female values after having corrected for phylogeny. Table 4 presents the same analyses above, except that it is performed on individual fin ray measurements. Table 5 presents data on the extent to which sexual dimorphism is correlated across different fin traits (i.e., is sexual dimorphism in dorsal fin area correlated with sexual dimorphism in anal fin area) across species after having accounted for the phylogenetic history. Figure 3 presents boxplots for standard length across all 20 species. Figure 4 presents the overall effect size of sex on different fin traits across the entire family. Figure 5 gives a visualization of dorsal fin size and shape after being corrected for standard length. For each species, the emmeans for the two sexes was determined, while holding constant the effect of standard length. The position of the fin rays was accounted for by using the mean fin base length for males and females and accounting for the position of the first fin ray, first quantile fin ray, median fin ray, third quantile fin ray, and last fin ray. This visualization allows one to see the size and shape of dorsal fins of males and females in different species. Figure 6 is the same as figure 5, except that it shows anal fin size and shape. Figure 7 shows the effect sizes due to sex for the area of dorsal, anal, and caudal fins across all 20 species of killifish. Figure 8 shows the effect sizes due to sex for the shape of dorsal, anal, and caudal fins across all 20 species of killifish. ## Code/software These data were processed using R version 4.4.2 (2024-10-31) -- "Pile of Leaves". The following packages were used: ape, diversitree, rfishbase, phytools, geiger, emmeans, broom, ggplot2, ggpubr, dplyr, car, and broom.