Seasonal change in male reproductive investment of a fish

Many animals are sexually dimorphic, whereby males may display brighter body coloration and more distinctive ornamentation than females. Fishes in temperate regions markedly change their energy allocation toward reproduction in response to the seasonal environment. Seasonal change in reproductive investment affects the expression of sexually dimorphic traits in males through gonadal weight change. Here, we report seasonal changes in body size, testis weight and sexual dimorphism of the fins (anal fin length and dorsal fin length) in the northern medaka Oryzias sakaizumii. Fish were collected periodically from a wild population (Aomori). Gonad weight increased from May to July in both males and females, corresponding to the reproductive season. Moreover, during this period, the degree of sexual dimorphism in fin length increased. To investigate the relationship between testis weight and individual differences in male fin length, we analyzed relationships among morphological traits using structural equation modeling. In the reproductive season, increased testis weight was associated with longer fin length in males, but the relationship disappeared after the reproductive season. These observations suggest that the sexually dimorphic fin in this fish is a mating signal. Results from a mating experiment also support this view. Males with larger size and/or longer fin attracted more mates than those with smaller fins, suggesting that sexual selection operates through females choosing to mate with males having longer fins.

Methods
Seasonal change in Gonad-soma index(GSI)

 |------- aomoriGSI_data2004.csv(2020/04/01): Wild individual's morphological data, standard length, anal fin length, body weight, and gonad weight

 |------- Fig2_seasonalChange_BW_GSI.R(2020/04/01): Boxplot of the seasonal change in GSI

 |------- Fig4_TableS2_SEM_BW_SL_TW_AFL.R(2020/04/01): The association among morphological traits using structural equation modeling.

Seasonal change in sexual dimorphism in fins, anal-fin length(AFL), and dorsal-fin length(DFL)

 |------- aomoriGSI_data2004.csv(2020/04/01): Wild individual's morphological data, standard length, anal fin length, body weight, and gonad weight

 |------- SLvsFin_Data2013.csv(2020/04/01): Wild individual's morphological data, standard length, body depth, anal fin length, and dorsal fin length.

 |------- Fig3_seasonalChange_AFLandDFLratio.R(2020/08/07): The ratio in AFL or DFL per standard length 

Mating experiment to test the effect of male morphology on reproductive success

 |------- mateNumberAndfertilizationRate.csv(2020/08/14): Egg number, fertilization rate, female size, and male morphological traits in the mating experiment

 |------- Table1_FigS3_mateNumber.R (2020/08/14): Wald test for the mate number and male morphological traits

 |------- Table1_FigS4_fertilizationRate.R(2020/08/14):  Wald test for the fertilization rate and male morphological traits

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# Sharing and access information

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# Methodological information: (reference)

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Description of methods for data collection or generation: https://doi.org/10.1007/s10641-021-01059-x

The R scripts were written and performed using R version 3.6.1 for Windows (R Core Team, 2019, www.r-project.org/).