Instant Sex Swap: How a Female Fish Becomes a Male Boss in Minutes

When the top spotty fish in a tank disappears, the runner-up turns aggressive within minutes, rushing and nipping rivals while its body quietly begins a weeks-long switch from female to male.

Otago scientists linked this lightning-fast coup to a dominance hierarchy based on size and to a neural decision-making network that fires up the moment power shifts. The findings reveal just how quickly behavior and brain chemistry can adapt, offering fresh clues about social flexibility across species—and practical insights for fisheries that rely on sex-changing fish.

Rapid Role Reversal Discovered

Scientists at Ōtākou Whakaihu Waka have made a fascinating discovery about a remarkable species of sex-changing fish. In a new study, researchers found that it takes just minutes for these fish to shift their behavior and seize power when their social structure is disrupted.

The research, led by the University of Otago’s Department of Anatomy and published in Proceedings of the Royal Society B, focused on the New Zealand spotty, or paketi, a coastal fish known for its ability to change from female to male during adulthood in response to social shifts.

When the dominant male in a group is removed, the sex change process in the next fish in line begins almost immediately.

Minutes to Dominate

PhD candidate Haylee Quertermous, lead author of the study, explains that while the full biological transition takes weeks, behavioral changes happen incredibly fast. Within minutes, a second-ranked fish starts displaying dominant behaviors to claim its new role.

“The aggressive behaviors (called ‘rushes’) involved the dominant fish swimming rapidly towards subordinate individuals,” she says.

“Sometimes the dominant fish will make physical contact with the subordinates, including taking bites at them, usually around their tail and fins. These aggressive behaviors are usually accompanied by the subordinate quickly swimming away (‘escaping’) from the dominant fish.”

Aggression Unleashed

While she expected to be able to see behavior changes within an hour of removing the dominant fish, she was surprised by just how rapid the change could be.

“In many of the tanks, second-ranked fish increased their aggression within just a few minutes after removal of the dominant fish.”

She cautions the dominant behavior that accompanies a female to male sex change in spotties does not indicate a change from typically ‘female’ to ‘male’ behavior, as other sex-changing fish species such as clownfish for example, change from male to more dominant female fish.

Size & Hierarchy Rules

The researchers observed that spotties form linear dominance hierarchies based on size, with larger individuals dominating smaller ones.

They sought to determine which fish in the hierarchy were more likely to change sex when the opportunity arose.

Results show dominant, larger fish are more likely to change sex, and when social hierarchies are disrupted, less dominant fish can quickly change their behavior to seize new opportunities.

Brain Wiring Behind Power

The study also delved into the neural mechanisms underlying spotties’ social interactions, finding that the social decision-making network in the fish brain is highly involved in establishing dominance.

Fish that attained dominant positions showed significant differences in this network compared to fish of all other ranks.

Dr. Kaj Kamstra, who led the neurobiological aspects of the research, says the findings provide valuable insights into the complex interplay between social behavior and neural processes in these fish.

Broader Impact & Applications

“They also highlight the importance of social context in shaping individual behavior, shedding light on the evolution of social behavior and the flexibility of brain mechanisms in adapting to changing social environments.

“The research has broader implications for understanding social dynamics in other species, even humans.”

The findings can be applied to other species of sex-changing fish where social dominance appears to be the most common trigger for sex change, and could prove beneficial for aquaculture and open water fisheries, with many commercial valuable fisheries dependent on fishes that change sex, for example, New Zealand’s blue cod.

Reference: “Behavioural and neural correlates of social hierarchy formation in a sex-changing fish” by Haylee M. Quertermous, Kaj Kamstra, Chloe A. van der Burg, Simon Muncaster, Erica Todd, Christine L. Jasoni, Culum Brown and Neil Gemmell, 30 April 2025, Proceedings B.
DOI: 10.1098/rspb.2024.2097

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