Identifying individual animals in the wild has long meant tagging, branding or clipping. For amphibians, the standard approach has often been toe-clipping, a procedure that can affect survival, physiology and behaviour.
A new study suggests that, for at least one species of treefrog, nature has already provided a more humane solution: the eyes.
Published in the open-access journal Herpetozoa, the research was led by Dr Mélissa Peignier and colleagues at the Division of Behavioural Ecology at the University of Bern, with contributors from the Institute of Electronic Music and Acoustics at the University of Music and Performing Arts in Graz.
The team examined Osteocephalus oophagus, an Amazonian treefrog whose golden irises are crossed by fine black radial lines, forming a pattern reminiscent of a human fingerprint.
Fieldwork took place over three rainy seasons, from 2021 to 2023, on a river island of roughly five hectares in the Nouragues Nature Reserve in French Guiana. Across 392 captures, the researchers identified 103 individual frogs.
On later encounters, they photographed each animal’s eyes without touching it and compared the images with a reference library held on a rugged tablet. Frogs recognised in this way were simply left undisturbed. The identifications were independently verified using Wild-ID, a software package for photographic mark-recapture analysis.
The iris patterns proved highly distinctive. Within a single season, between 56% and 80% of frogs were recaptured and reliably re-identified, showing that the patterns are stable enough for short-term monitoring. Photographs could even be taken of frogs in amplexus, the mating embrace, without disrupting them.
The project began with simple admiration.
“At the time, I was doing my PhD, studying Allobates femoralis in the field in French Guiana. My first reaction was simply something along the lines of, ‘Wow, what beautiful eyes!’ That initial curiosity gradually turned into a genuine interest in the species, and I realised how little we actually know about them.”
Dr Peignier
The team’s motivation was also ethical.
“Other studies have used toe-clipping, which involves removing some of the digits. Personally, I was uncomfortable with this approach, especially given that these are arboreal frogs that rely on their limbs to climb trees.”
Dr Peignier
Dr Peignier hopes the method will spread.
“In a world where biodiversity is declining at an alarming rate, it is essential that we minimise our impact on the animals we study.
If we can reliably identify individuals using naturally occurring features, we may be able to reduce the need for more invasive marking techniques while still collecting the data needed to answer important scientific questions.”
The authors stress that suitability will vary between species, and that the approach should be tested case by case before wider adoption. Even so, they see considerable promise for amphibians with complex irises.
Fieldwork, Dr Peignier adds, had its own rewards.
“Honestly, the frogs themselves are incredibly beautiful and surprisingly chill, which makes working with them a real pleasure.”
Original source:
Bégué L, Petel F, Premel V, Raboisson G, Ringler E, Ringler M, Toszeghi M, Vacher V, Peignier M (2026) Iris pattern recognition as a non-invasive method for individual identification in Osteocephalus oophagus (Hylidae). Herpetozoa 39: 381-387. https://doi.org/10.3897/herpetozoa.39.e208000