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Finding Nemo Explained by Clownfish Hormones

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Finding Nemo Explained by Clownfish Hormones

Anemonefish spend their lives in close proximity to their anemones. Females are larger and usually defend the nest; males spend more time tending to the eggs. Credit: Photo by L. Brian Stauffer
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Two brain-signaling
molecules control how clownfish dads care for their young and respond to nest
intruders, researchers report in a new study. Because there are many
similarities in brain structure between fish and humans, the findings offer
insight into the fundamental nature of parental care, the scientists say.

"One of the benefits of studying fish is that their
behaviors are simpler," said Ross DeAngelis, a former graduate student who
conducted the work in the laboratory of Justin Rhodes, a professor of
psychology at the University of Illinois at Urbana-Champaign. "By
exploring these systems, we can understand the broader implications on
vertebrate behavior."

Clownfish live in pairs on sea anemones in the wild. Female clownfish
are more aggressive than their mates and help protect the nest from intruders.
Fathers have a more nurturing role. See video.

"Male clownfish are spectacular fathers," said
Rhodes, who led the research. "They nourish the eggs by fanning them to
provide oxygen and clear debris, and they nip at the eggs to clean them. When a
predator is around, they switch their behavior to become aggressive - they try
to bite and fight the predators away."

Previous research focused on only one facet of parental care:
either nourishment or defense. The new study aims to understand both aspects
together in the presence of intruders.

Two hormones play a role in guiding the trade-off between
caring for one's offspring and defending them. Arginine vasotocin increases
aggression, while isotocin boosts egg care. Their effects on parental behavior
in the presence of intruders was previously unknown.

The researchers used inhibitory compounds, known as
antagonists, to block the binding of arginine vasotocin or isotocin to their
receptors in the brain. They injected these compounds into the abdomens of the
fish. From there, the antagonists were carried to the brain through the blood.

"Blocking arginine vasotocin reduced aggression and
increased parental care in male clownfish," DeAngelis said. "This is
an unusual result because they are such good dads - we didn't think it was
possible for them to be even better."

Blocking isotocin had the opposite effect. It increased
aggression, and the fish spent less time nipping and fanning their eggs, DeAngelis

"The results are similar to what we see in humans,"
Rhodes said. "Oxytocin, which is the human version of isotocin, is known
to be important for nurturing. Arginine vasopressin, which is the human version
of arginine vasotocin, plays a role in social and affiliative behavior in the
slightly different context of mating."

The two hormones have very similar structures and bind to
similar receptors in the brain, so the researchers cannot be certain that the
antagonist for one isn't also affecting the other. They also have not yet
determined how the hormones specifically modify brain-signaling.

"Individuals across the animal kingdom have to make
decisions on how to maximize their fitness, and most of those decisions are
based on environmental context," DeAngelis said. "It is interesting
to see that the neurochemical pathways can be modulated by the current social

Reference: DeAngelis, R., Dodd, L., & Rhodes, J. (2020).
Nonapeptides mediate trade-offs in parental care strategy. Hormones and Behavior,
121, 104717. https://doi.org/10.1016/j.yhbeh.2020.104717

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