In many species, males exhibit phenotypic plasticity in sexually selected traits when exposed to social cues about the intensity of sexual competition. To date, however, few studies have tested how this plasticity affects male reproductive success. We initially tested whether male mosquitofish, Gambusia holbrooki (Poeciliidae), change their investment in traits under pre- and post- copulatory sexual selection depending on the social environment. Focal males were exposed, for a full spermatogenesis cycle, to visual and chemical cues of rivals that were either present (competitive treatment) or absent (control). Males from the competitive treatment had significantly slower swimming sperm, but did not differ in sperm count from control males. When two males competed for a female, competitive treatment males also made significantly fewer copulation attempts and courtship displays than control males. Further, paternity analysis of 708 offspring from 148 potential sires, testing whether these changes in reproductive traits affected male reproductive success, showed that males previously exposed to cues about the presence of rivals sired significantly fewer offspring when competing with a control male. We discuss several possible explanations for these unusual findings.

We used the eastern mosquitofish, Gambusia holbrooki, to test whether exposure to social cues presumed to indicate the level of sexual competition affects male investment in different sexually selected traits, and ultimately reproductive success. We experimentally manipulated the perceived level of sexual competition by placing males in either a competitive or control environments for a full spermatogenesis cycle. More specifically, we provided test males with visual and olfactory access to a female, who was either alone or housed with rival males. We then investigated whether this affected male behavior and/or sperm production. Crucially, we then tested for an effect of the social treatment on male reproductive success when males from each treatment competed one on one with each other to sire offspring. 

Males were exposed to the social treatments for five weeks and were then stripped of sperm to quantify sperm number and velocity. To restore their sperm reserves, males were returned to their treatment aquarium for another week. Next, we placed a pair of size-matched males from the competitive and control treatment with a virgin female and recorded their mating behavior on the first and third day. After one week, females were transferred to separate aquaria to give birth and we recorded the total amount of offspring produced. For a proportion of the offspring we performed paternity analysis to calculate male reproductive success. Finally, we also photographed and measured males and females. 

For more information see publication Spagopoulou et al. AmNat 2020.

Spagopoulou-etal_AmNat2020.csv contains the dataset used in this publication 

Spagopoulou-etal_AmNat2020-ReadMe.txt contains the explanation of the columns of the csv file

Spagopoulou-etal_AmNat2020.R contains the R script with the analysis and model selection