Why did Portugal’s giant bears gradually shrink to almost half their original size?

A new study has shown that the body mass of Iberian brown bears has declined by more than 40 per cent. Scientists suggest that this may have been driven by environmental changes and centuries of human pressure, although there is as yet no definitive answer. The study has been published in the journal *Palaeogeography, Palaeoclimatology, Palaeoecology*.

Giants of the Ice Age

The researchers examined hundreds of brown bear bones found in caves in Portugal.

One of the most impressive finds was the skeleton of a male bear dating back around 22,000 years, which scientists estimate weighed approximately 385 kilograms. This is one of the largest brown bears ever discovered on the Iberian Peninsula.

Even more ancient bears from the Furninha Cave, which lived around 80,000 years ago, were also notable for their enormous size. Many of them weighed over 300 kilograms, and their massive bones resembled the remains of the famous cave bears.

Where did the giants go?

After the end of the last ice age, the animals’ size began to gradually decrease.

In the early Holocene, around 12–8 thousand years ago, male brown bears in north-western Spain weighed approximately 260 kilograms, whilst females weighed around 140 kilograms.

Today, the average weight of male Iberian brown bears is around 144 kilograms, and that of females is 102 kilograms.

Thus, the weight of males has fallen by more than 40 per cent, and that of females by more than 25 per cent.

Why have the bears become smaller?

The authors believe that climate change alone cannot explain this trend.

In their view, the long-standing coexistence of humans and bears may have played a significant role.

Initially, people began to use the same caves where the bears had set up their winter dens. Later, the development of agriculture led to a reduction in forest cover, whilst the advent of firearms made hunting considerably easier.

The researchers suggest that the largest and boldest animals were the first to perish, whilst smaller bears were less likely to engage in dangerous encounters with humans and were more likely to leave offspring.

However, the authors emphasise that this is, as yet, merely a working hypothesis requiring further confirmation.

Not just humans

There are other possible explanations.

For example, the ancient giant bears may have belonged to a different population from modern Iberian brown bears.

Furthermore, the animals’ size may have depended on their diet. A more carnivorous diet or a high intake of fish could have contributed to an increase in body mass.

Research into ancient DNA and chemical analysis of the bones, which scientists plan to carry out in the future, will help to shed light on this.

Why there were no cave bears in Portugal

Another unexpected finding was the absence of cave bear (Ursus spelaeus) remains in Portugal.

These animals were widespread across Europe during the Ice Age, yet no traces of them have yet been found in what is now Portugal.

The authors suggest that brown bears may have occupied part of the ecological niche of their larger relatives, which may also have contributed to their unusually large size.

Why this is important

Brown bears play a key role in ecosystems.

They regulate the populations of certain animals, compete with wolves for prey and disperse vast quantities of plant seeds. According to the study’s authors, a single bear droppings can contain more than 70,000 seeds, which serve as food for dozens of species of birds and small mammals.

Understanding how bears have changed over time will help to better protect modern populations, whose numbers remain low.

Source

Journal: Palaeogeography, Palaeoclimatology, Palaeoecology.

Study title: Pleistocene brown bears from Portugal: Morphometric trends, palaeoenvironmental drivers and palaeobiological inferences.