Abstract
BackgroundThe evolution of larger brain volumes relative to body size in Mammalia is the subject of an extensive amount of research. Early on palaeontologists were interested in the brain of cave bears, Ursus spelaeus, and described its morphology and size. However, until now, it was not possible to compare the absolute or relative brain size in a phylogenetic context due to the lack of an established phylogeny, comparative material, and phylogenetic comparative methods. In recent years, many tools for comparing traits within phylogenies were developed and the phylogenetic position of cave bears was resolved based on nuclear as well as mtDNA.ResultsCave bears exhibit significantly lower encephalization compared to their contemporary relatives and intraspecific brain mass variation remained rather small. Encephalization was correlated with the combined dormancy-diet score. Body size evolution was a main driver in the degree of encephalization in cave bears as it increased in a much higher pace than brain size. In Ursus spelaeus, brain and body size increase over time albeit differently paced. This rate pattern is different in the highest encephalized bear species within the dataset, Ursus malayanus. The brain size in this species increased while body size heavily decreased compared to its ancestral stage.ConclusionsEarly on in the evolution of cave bears encephalization decreased making it one of the least encephalized bear species compared to extant and extinct members of Ursidae. The results give reason to suspect that as herbivorous animals, cave bears might have exhibited a physiological buffer strategy to survive the strong seasonality of their environment. Thus, brain size was probably affected by the negative trade-off with adipose tissue as well as diet. The decrease of relative brain size in the herbivorous Ursus spelaeus is the result of a considerable increase in body size possibly in combination with environmental conditions forcing them to rest during winters.
Highlights
The evolution of larger brain volumes relative to body size in Mammalia is the subject of an extensive amount of research
Many of these studies focus on the external morphology of artificial, fossil, or virtual endocasts [34, 35, 39,40,41]
Some authors suggested a small brain size compared to body size and speculated that the increase of skull size in the evolution of U. spelaeus outpaced brain size [35, 36]
Summary
The evolution of larger brain volumes relative to body size in Mammalia is the subject of an extensive amount of research. Cave bear brains are among the earliest ones of an extinct species to be investigated and several studies discuss different aspects of its evolution [34,35,36,37,38,39,40,41,42]. Many of these studies focus on the external morphology of artificial, fossil, or virtual endocasts [34, 35, 39,40,41]. Absolute as well as relative brain size can be influenced by social structure [46,47,48], environment [48,49,50,51,52], sensory systems [53], evolutionary history [54,55,56,57], body size evolution [42], and different physiological as well as life history trade-offs [43, 52, 57,58,59,60,61,62,63,64,65,66]
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