Ancient frog mystery solved

Scientists at Bournemouth University and Museum of London Archaeology have finally identified the culprits, ten years after 8000 ancient frog bones were discovered in a single ditch. 

The bones were discovered on the National Highways A14 Cambridge to Huntingdon Improvement Scheme between 2016 and 2018. The origins of this frog cemetery were a genuine mystery and left archaeologists exploring multiple avenues of investigation.

After seeing the discovery in the news, Dr Paul Clarkson at Bournemouth University contacted MOLA Senior Zooarchaeologist, Dr Vicki Ewens, with the aim of conducting further analysis.

After comparing the damage to the frog bones to previous studies, Dr Clarkson found they had been eaten by birds of prey, with short-eared owls and hen harriers being the most likely suspects.

But the story didn’t end there. Onsite archaeological evidence shows the ditch filled within a single year and therefore this still left the question of how so many bones appeared in such a short period of time. So, the Bournemouth team investigated further. 

By measuring the size of the frog humeri (arm bones) – a pioneering new method of analysis – they discovered most of the frogs died young, having been eaten in the winter of their first year of life. Radiocarbon dating by the University of Vienna also showed the remains dated back 1700 years to Roman period.

The Fen waters were retreating in this valley in the Roman period, creating ideal conditions for the frogs to breed in - leaving an abundance for the birds to eat.

As short-eared owls hunt over grassland and this valley had been wooded in the Iron Age, researchers also looked at snail, pollen, and charcoal evidence. This revealed that significant areas of woodland were gradually felled over time for grazing livestock. By the Roman period, large patches of open ground left the frogs with fewer trees to protect them from the grassland hunters. 

“We don’t often get an owl’s eye view of what it was like in the past from archaeological sites,” said Dr Paul Clarkson of Bournemouth University who led the study. 

“The mystery of these frogs has been solved - a feast of frogs that were eaten and their bones left behind by a group of owls. These bones not only tell us about life for people living near the Roman Fens, but they also give insights into the natural world - what wild animals were getting up to in the past, and what life was like for them,” he added.

Solving this puzzle also provides valuable clues as to the movement of people during the Roman period. The birds hunting activities were perhaps aided by a less crowded landscape, with communities visiting the area on a more seasonal basis in order to move livestock to new grazing pastures.

As Dr Ewens concludes: “I had so many theories as to why such as large collection of frog bones were recovered from this small feature. It’s been great to collaborate with Paul to finally answer this question. This research shows how even the smallest animal recovered on a site can help us to understand not only past environmental conditions but human activity within a wider landscape.”

The full research paper ‘An Owl’s Eye View: Human–Animal Entanglement at the Roman Fen Edge’ is free to access on The Journal of Human Paleoecology.