The effect of body size evolution and ecology on encephalization in cave bears and extant relatives

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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-017-0976-1) contains supplementary material, which is available to authorized users.

Hibernation constrains brain size evolution in mammals

“…In sum, using one of the largest mammalian brain size data set to date, we found that hibernators have significantly smaller brain sizes relative to body mass than nonhibernating species overall (species‐level), and within five of seven orders. This result adds to numerous previous studies supporting the idea that experienced seasonality ( in extremis where hibernation is necessary for survival) imposes an energetic challenge and thus acts as an evolutionary constraint on brain size (van Woerden et al ., ; Jiang et al ., ; Weisbecker et al ., ; Luo et al ., ; Veitschegger, ). This energetic challenge imposed by the environment also provides one explanation why ectothermic species such as reptiles, amphibians, fishes and insects do have smaller brain size relative to body mass compared to endothermic species such as mammals and birds, as previously pointed out by Gillooly & McCoy ().…”

“…Support for this hypothesis derives not only from the present study but also from an intraspecific study in Andrew's toads ( Bufo andrewsi ), which found that populations with longer periods of hibernation had smaller brains (Jiang et al ., ). Furthermore, a study in extant and extinct bear species reveals that brain size is smaller in species that exhibit dormancy and have a low calorie diet (Veitschegger, ).…”

Comparative skeletal anatomy of neonatal ursids and the extreme altriciality of the giant panda

“…The unusually small size of bear cubs relative to the mother may be a result between natural selection’s strong emphasis on large adult body mass and the legacy effect of the small body size in bear ancestors. Interestingly, in the other obligate herbivorous member of Ursidae, the cave bear ( Ursus spelaeus ; Veitschegger, ), body size seems to be evolving at a faster rate than brain size, the latter of which also directly correlates with gestation time, neonatal mass and litter size (Finarelli, ).…”

Palaeohistology and life history evolution in cave bears, Ursus spelaeus sensu lato

“…The investigated haplotypes of cave bears exhibit different growth patterns indicating variation in life history strategy during early phases of speciation. There is evidence that cave bears had a faster life history compared to close relatives because of their smaller relative brain size [ 71 ]. However, the overall pace of growth reconstructed for cave bears in this study is similar to those of their close relatives.…”