Size does matter

Wednesday 07 Jul 21

Much smaller male fish among the mosquitofish have been thought to have an advantage in reproduction due to their agility – but female mosquitofish prefer the larger males. This is the conclusion of a new surprising review study indicating female power over their offspring.

In Denmark, we have a saying that humorously says: It is not the size, but the doing, when we talk about what women find attractive in men. Turning to the world of fish, females of the fish genus Gambusia (commonly known as mosquitofish) have now proven to disagree. 

A new review study reveals that even though the gambusia males are smaller than the females – sometimes only half the size – and that biologists so far have assumed that smaller males have some advantages in terms of reproduction due to their agility and ability to sneak up on females, the larger male mosquitofish are more successful at reproducing. For example, they can better assert themselves against rivals, produce more sperm – and are preferred by females.

Read the scientific paper Male size and reproductive performance in three species of livebearing fishes (Gambusia spp.): a systematic review and meta-analysis

“The females often show a really strong preference for associating with larger males, which I think is because the large males can protect the females from harassment by other smaller males. So it may not be that the females specifically want to mate with the large males, but they prefer associating with them, and that gives larger males an advantage,” says Nicholas Patrick Moran, postdoc at the National Institute of Aquatic Resources, Centre for Ocean Life, DTU.

Nicholas Patrick Moran has co-supervised the new overview study, led by scientists from Bielefeld University in Germany in collaboration with Alfredo Sanchez-Tojar and Master’s student Bora Kim. The scientists will present their results today, 7 July 2021, in the Journal of Animal Ecology 

Sneak tactics versus female power 

Mosquitofish are small, shiny fish comprising around 45 species. While the females are up to seven centimeters in length, the males are often only four, but their size varies. Mosquitofish are therefore often used to research selection according to body size. Many studies have already investigated whether the size of the male has advantages in terms of reproduction, but the results so far have been contradicting.

“I think one of the more interesting insights from this study is how female choice seems to work in this system. Because the gambusia males use sneak tactics, females weren’t thought to be able to exert much control or choice over who they mate with. But now we see that the larger the males are, the more likely it is that females will associate and potentially reproduce with them.,” says Nicholas Moran and concludes:

“It indicates a more subtle and possibly overlooked form of female fish exerting influence over their reproduction.” 

Larger males doing better across the board

The scientists evaluated 36 individual studies in which the relationship between the size of male mosquitofish and their reproductive performance was examined. According to Nicholas Moran the discovery of the new study is surprising because gambusia species have often been considered an example of a system where smaller males have an advantage because they tend to be quite a lot smaller than the females, and they employ this coercive/sneaking type of copulation: 

“So it is really surprising that larger males seem to do better in all the categories of reproductive success we looked at. That they seem to do better across the board was the most surprising part for me”, says Nicholas Moran.

A pest and a significant component of freshwater ecosystems

The western and eastern gambusia, also called mosquitofish, are now some of the most widely distributed freshwater fish in the world. They are originally from North America, but they were introduced as a biological control for mosquitos. They are now an extremely invasive species globally. 

“This species, even though it is introduced and often a pest, is now a significant component of freshwater ecosystems in every continent except Antarctica. So understanding how they reproduce is fundamental knowledge that contributes to their management”, says Nicholas Moran.

When asked if these new results showing females’ preferences for larger mosquitofish males indicates that evolution is gradually wiping out the smaller males, Nicholas Patrick Moran replies:

“This definitely suggests that there should be some selective pressure towards larger males, but smaller males may have advantages in other ways. For example, larger males could be more likely to be eaten by a predator, while smaller males may need to eat less food to survive. So we can’t say here whether males are evolving to be smaller or larger, but I suspect that this large-male advantage in reproductive success might be somewhat balanced out by other advantages that the small males have.”

So even if this new study shows that size does matter, there is still hope for the smaller ones!

Photo courtesy of Andrew Kahn.


Nicholas Patrick Moran, Postdoc at the Centre for Ocean Life and the Section for Oceans and Arctic, National Institute of Aquatic Resources, DTU, contributed to this study as part of his Marie Curie Individual Fellowship.

While the work was co-supervised by Nicholas Patrick Moran and Alfredo Sanchez-Tojar (Bielefeld University, Germany), most of the work was done by student Bora Kim who did her Master’s at Bielefeld. She has now gone on to do a PhD at the Konrad Lorenz Institute of Ethology, in Vienna, Austria.
8 AUGUST 2022