Brain Size Evolution In Lizards And Mechanisms Of Ecological Adaptation

As the central system of learning,memory,cognition and information processing,the ecological adaptation of the organism's brain is one of the core contents of evolutionary biology.It is well known that brain is a kind of organ with high energy consumption of vertebrate,which is tightly related to another high energy consumption organs,such as the gut tract are also critical driving factors for the evolution of the brain.The Cognitive buffer hypothesis states that relative larger brains can enhance the cognitive abilities to select high quality food and hence improve the digestion and absorption to gain more abilities.And it is noteworthy that better food can decrease the gut tract length via enhancing the rate of digestion.At the same time,diet type and habitat type can take selective pressure to the morphology of the brain function,and thus they can affect the evolution of morphology of brain size.Therefore,to analyze the evolutionary trade-off between brain size and gut tract length,as well as explore the effects of food type and habitat type on brain size,it's conducive to a comprehensive understanding of the ecological adaptation mechanism of brain size evolution.At present,several researches about the evolution of vertebrate brain size mainly focus on the taxa of fish,anuran,birds and mammals.Meanwhile,several mainstream scientific hypotheses have been proposed to explain the ecological adaptive evolution of vertebrate brain size,including the Cognitive buffer hypothesis,the Constraint brain hypothesis,the Expensive tissue hypothesis as well as the Developmental cost hypothesis.However,the similar and systematic studies on evolutionary brain size of lizard species are very scarce.In this study,32 lizard species in China were selected as model systems by dissecting and measuring the brain size and gut tract length of all individuals,we explored the relationship of evolutionary tradeoff between brain size and gut tract length.At the same time,we elucidated the influence of habitat type on evolution of brain size.The main results and conclusions are as follows:1.By dissecting and measuring the brains of 32 lizard species,one-way analysis of variance(ANOVA)shows that there are significant differences in the brain morphology and size of different brain structures(telencephalon,mesencephalon,cerebellum,rhombencephalon and olfactory bulbs)among species.The linear regression analysis of brain size and body size shows that there is a significant positive correlation between brain size and body size,which supports the assumption of which “Larger species have larger brains”.2.There is a phenomenon of allometric growth between the total brain and five different brain structures,but the whole brain shows a trend of coevolution,which supports the “Constrained brain evolution hypothesis”.The growth rate of olfactory bulb was much higher than that of other brain structures,which supports the “Mosaic brain evolution hypothesis”.3.The phylogenetic generalized least squares regression with controlling phylogenetic relationships and body size shows that there was no correlation between gut tract length and brain size,but in other brain structures,gut tract length is positively correlated with telencephalon size,which is speculated that the enlargement of the telencephalon may be related to the improvement of functional requirements,such as the availability of food.4.Habitat type significantly influenced the evolution of brain size.The results show that the terrestrial type species have relatively small brains,while the saxicolous type species have relatively large brains,which provide new evidence for supporting the Cognitive buffer hypothesis.In conclusion,these results above reveal the relationship between brain size and gut tract length in 32 lizard species in China as well as illustrating the importance of species' habitat type for the ecological adaptation of brain size.In addition,the study on the evolution of brain size in ectotherms provides new experimental evidences for the verification of the Cognitive buffer hypothesis and the Constrained brain evolution hypothesis.Meanwhile,it also offers a new perspective to dig out the deeper relationship between different brain regions and their specific functions in adaptive evolution of vertebrates.