Scientists at Bournemouth University and Museum of London Archaeology have discovered the reason 8,000 frog and toad bones were found buried in a 45-foot-long trench at an Iron Age settlement near Cambridge in the UK.

The bones were unearthed during highway improvement works between 2016 and 2018. The trench was located near a Late Iron Age (400 B.C. – 43 A.D.) roundhouse, so the frogs were thought to have been buried between that range. It was a perplexing find. Frog bones had been found before in archaeological contexts, but only as an individuals. Mass burials of animal remains had been found before in archaeological contexts, usually from ceremonial feasting, but they were the usual food animals – livestock like cattle and sheep or chickens – not hundreds of tiny amphibians.

Archaeologists eliminated the possibility that they had been cooked in a feast because there were no cut marks or evidence of heating. Grain found nearby indicates the settlement grew crops that would attract amphibians, but it would take a Moses-level pestilence to explain the numbers. Another possibility was migrating frogs becoming trapped in the ditch on their way to breeding waters, or freezing to death in a sudden temperature drop while seeking shelter in the mud.

Well, it turns out all of those ideas were wrong. The frogs were eaten, but not by humans, and hundreds of years after the roundhouse was built.

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.

Humans were the owls’ accomplices in the mass slaughter. The area had previously been wooded, but in the Late Iron Age, people gradually felled the trees to make more land available for grazing their livestock. Short-eared owls hunt on grasslands, so the frogs were much more exposed with fewer trees to shelter them.

“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.