Pregnant male pipefish defy evolutionary norms
January 2, 2025 Velentina Boulter
Velentina Boulter is science journalist based in Melbourne.
Greater pipefish, Syngnathus acus 54, and Sargassum pipefish, Syngnathus pelagicus 55,56. Handcolored copperplate engraving from Gottlieb Tobias Wilhelm’s Encyclopedia of Natural History: Fish, Augsburg, 1804. Wilhelm (1758-1811) was a Bavarian clergyman and naturalist known as the German Buffon. (Photo by: Florilegius/Universal Images Group via Getty Images)
A new study out of the University of Canterbury in Christchurch, New Zealand has called into question traditional perceptions of mating.
“In most species, males compete to attract females. But with pipefish, the males carry and protect the embryos,” says PhD student Nicole Tosto, who led the research.
“Pipefish are unique because they don’t follow the usual ‘rules’ of evolution.”
The research highlights how biological differences in male and female pipefish influence their survival and mating habits.
While females have genes to support egg production, males activate genes to strengthen their immune system.
This is a key adaptation that allows the males to nurture and care for embryos in their bodies.
The study, published in Molecular Ecology, also uncovered how this switch in activated genes impacts mating selection.
In most species, females prefer larger, dominant males as mates because it often increases their chance of having healthy offspring, as the strong male can provide security and defence from predators.
Instead, the study found that female pipefish swim against this trend and tend to choose smaller males with high fitness levels.
Tosto suggests that this selection is based on efficiency as smaller males may need fewer resources.
She also believes smaller males could be better suited for the synchronised water movements that are a part of the species’ courtship rituals.
In many animals, males and females of the same species can have physical features that are different between the sexes and are often used to attract mates. These visible traits are known as sex-specific ornaments.
However, the pipefish species involved in the study were monomorphic, meaning that male and female pipefish looked almost identical and had no visible differences.
“Nicole’s research has brought up important questions for evolutionary biologists when it comes to current vs past selection,” says her doctoral supervisor, Dr Sarah Flanagan, a senior lecturer in Biological Science at the University of Canterbury.
Natural selection is a process where individuals with traits that help them survive become more likely to reproduce and therefore pass on those traits to their offspring. Overtime these advantageous traits become more commonly inherited among the species.
“For example, whether the existence of sex-specific ornamentation is evidence that selection is currently acting strongly on those sex-specific traits or whether ornaments are evidence of selection having happened in the past.”
Pipefish don’t have sex chromosomes meaning both sexes share the same genetic blueprint, they just use the genes in different ways.
For example, females focus on producing egg-enhancing proteins, whereas males produce immune-boosting proteins for pregnancy.
“Knowing how these pressures shape mating systems helps us better understand how species survive and adapt to their environments,” says Tosto.
While there is no current extinction concern for dusky pipefish (Syngnathus floridae), the species of pipefish investigated in the study, other pipefish species such as the estuarine pipefish are critically endangered.
Scottie's Playtime 
