Understanding the brain size of cave bears

The cave bear became extinct around 24,000 years ago, but its fossils are so abundant that we still have the capacity to study them. In this guest blog, Kristof Veitschegger discusses his research, published this week in BMC Evolutionary Biology, that questions the common perception of cave bears as small brained and 'stupid'

The Pleistocene was a time when giant mammals were common in Europe. One of them was the cave bear. This species is one of the most abundant fossil taxa from Europe due to its strategy of surviving the winter by resting, and consequently sometimes not surviving, in caves [1]. Thus, many fossil remains are perfectly preserved.

Most researchers agree that cave bears were predominantly herbivorous and mostly ate soft leaved plants such as herbs [2]. When I started my research on the brain size of cave bears, I was confronted with a short anecdote of the “stupid,” plant eating bear in the cave. The basis for this was the idea of an evolutionary brain size arms-race between predator and prey, where predators needed to always be smarter [3]. Here, it is important to note that overall or relative brain size does not necessarily reflect intelligence [4].

I wanted to look further into this anecdote and see if there is any validity to it. Thus, I started to collect data on dietary preferences of all studied bear species [2] and scored them on a scale where leaf consumption would result in small brains, and animal protein consumption in big brains. Additionally, I also wanted to see if the annual cycle of fattening up for the winter and starving until spring affected the costly brain matter [5].

Difference in brain volume between polar bear (left) and cave bear (right) (skulls are stored in PIMUZ, Switzerland). Glasses are filled with 6 mm glass beads as representation of the different brain volumes.
Kristof Veitschegger

My research showed that neither diet nor dormancy alone had a significant effect on the relative brain size of cave bears. Thus, the results do not give validity to the anecdote of the “stupid,” plant eating bear. However, there was a weak environmental influence that affected the brain size of cave bears as is shown in the combined score of diet and dormancy.

I also wanted to understand the effect that body size had on the brain of cave bears. Cave bears were one of the biggest bear species ever to roam the earth and body mass estimations are between 225 and 1,500 kg [1, 6]. My research showed that the body mass of cave bears increased at a much higher pace than brain mass. Thus, cave bears ended up with small brains compared to their body size.

 

References:

  1. Rabeder G, Nagel D, Pacher M. Der Höhlenbär. Stuttgart: Thorbecke Verlag; 2000.
  2. van Heteren AH, MacLarnon A, Soligo C, Rae TC. Functional morphology of the cave bear (Ursus spelaeus) mandible: a 3D geometric morphometric analysis. Org Divers Evol. 2016;16(1):299-314.
  3. Jerison HJ. Evolution of the brain and intelligence. New York: Academic Press; 1973.
  4. Roth G, Dicke U. Evolution of the brain and intelligence. Trends Cogn Sci. 2005;9(5):250-7.
  5. Navarrete A, van Schaik CP, Isler K. Energetics and the evolution of human brain size. Nature. 2011;480(7375):91-3.
  6. Christiansen P. What size were Arctodus simus and Ursus spelaeus (Carnivora: Ursidae)? Ann Zool Fenn. 1999;36(2):93-102.

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