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Amazon mollies (Poecilia formosa) can be genetic clones and still behave differently. In controlled studies, individual fish showed repeatable differences in activity and feeding, even when researchers separated them at birth and raised them under standardized conditions. “Clone” describes their genetics; it does not guarantee identical behavior.
What scientists mean by behavioral individuality
In this research, individuality does not mean a fish has a human-like personality or behaves differently in every situation. It means that fish show repeatable, predictable differences from one another: a relatively active fish, for example, tends to remain relatively active across repeated observations.
That distinction matters because a single unusual observation could be noise or a temporary response. Repeated differences are evidence that individuals vary consistently, even within a clonal species.
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Different activity levels despite different amounts of early social contact
A 2017 Nature Communications study split clonal Amazon molly broods among three early-life conditions: immediate isolation after birth, seven days of group rearing, or 28 days of group rearing. The fish were then isolated and repeatedly assessed at seven weeks.
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Researchers found repeatable behavioral differences in all three groups. In the isolated baseline group, among-individual differences accounted for roughly 30% of total behavioral variation; estimated repeatability was 0.35 (95% confidence interval 0.13–0.52). The study did not find evidence that the amount of early social contact changed the magnitude of behavioral individuality in this setup.
This is a result about those treatments and observations, not proof that social experience never affects fish behavior. It shows that the measured differences did not depend on the tested durations of early group rearing.
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Differences were detectable from the first day
A 2022 high-resolution tracking study followed genetically identical fish separated at birth through their first ten weeks. It reported strong behavioral individuality on day one: early differences predicted behavior through at least ten weeks, and individuality strengthened as the fish developed.
Because differences were already detectable so early, the result points attention beyond post-birth social experience. It does not, by itself, identify what produced the day-one differences.
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Activity and feeding differences persisted in a longer study
In a 2023 study, activity during the first four weeks had a reported repeatability of R = 0.371 (95% CI 0.329–0.413), while feeding behavior had a repeatability of R = 0.183 (95% CI 0.145–0.224). These estimates indicate consistent differences among the fish in the study, with activity more repeatable than feeding by these measures.
Why can clones differ?
Clonal fish share a genetic lineage, but that does not make every influence on development identical. The studies establish that behavioral differences occur; they do not prove one specific cause. Researchers have discussed several plausible contributors:
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- Maternal provisioning: offspring may receive slightly different resources or developmental inputs before birth.
- Epigenetic variation: differences in how genes are regulated could arise without a change in DNA sequence.
- Developmental stochasticity: small, partly random variations during development may accumulate into measurable differences.
- Microenvironmental variation: subtle differences in temperature, chemical cues, or food experience may matter even when overall rearing conditions are standardized.
These are candidate explanations, not settled findings. The 2017 experiment standardized post-birth conditions but did not make each fish’s entire prenatal and later environment literally identical.
Do behavioral differences matter later in life?
A 2023 life-history experiment followed 34 fish in standardized conditions for 280 days. Across that period, the researchers recorded 152 broods and measured 2,522 offspring. They found repeatable differences among fish in both brood size and offspring size.
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Early feeding patterns were associated with growth, and growth in turn predicted offspring size. The study did not find a direct relationship between early behavior and later reproductive traits. The results therefore suggest a possible indirect pathway from feeding behavior through growth to offspring size, rather than showing that early behavior directly determines reproduction.
What the 2026 correction changed—and did not change
In a 2026 author correction to the 2017 paper, the researchers reported that body-size values had been assigned to the wrong individuals in the original analysis. They reran the analyses using the documented original measurements and said the main findings and conclusions about behavioral individuality and early social experience were unchanged. The corrected analysis also found a slight body-size effect: smaller fish were more active.
The correction updated body-size analyses, not the central conclusion that repeatable behavioral differences appeared among the clonal fish.
Individual fish and groups are different questions
A 2026 study abstract indexed by PubMed describes an experiment tracking replicate groups of clonal Amazon mollies for 55 days. Group-level behavioral rankings shifted around weeks four to five. The authors interpret the early group differences as reflecting consistent individual variation, followed by less predictable trajectories as social feedback increased.
This group-level pattern does not overturn findings about repeatable differences among individual fish. A group’s behavior can change through interactions among its members even when individuals show their own recurring tendencies. The result is reported here at the level available in the PubMed abstract.
Quick Recap
What the evidence supports
- Clonal Amazon mollies can show repeatable differences in activity and feeding.
- In the tested 2017 conditions, changing early group-rearing duration did not produce a supported change in the amount of behavioral individuality.
- Differences were detectable on day one and tracked to at least ten weeks in a later study.
- Several biological and environmental mechanisms remain plausible, but the studies do not establish a single cause.
- Behavioral differences may relate indirectly to later offspring size through growth, while group behavior follows a related but distinct pattern.
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