Tetrapod Brain Evolution Flashcards
what do brains do?
sensory information –> information processing –> bodily response
= allows organisms to process information from their surroundings allowing them to make better decisions regarding aspects such as food ect
= with out brains all behaviour would have to be random as no infomration can be acted upon
PROCESSING IS THE KEY THING MAKING BRAINS A SUCCESSFUL AND BENEFICIAL ORGAN
Give some examples of organisms which have not evloved brains
1) porifera = sponges
2) echinoderms = starfish
3) cnidaria = jellyfish
= many species havent evloved brains however are still able to process information, they lack a centralised nervous system, instead it is distributed throughout their body
outline why some species have evloved brains
it provides a centralised area in which information processing can take place
= electrical and chemical processes needed have a high energy expenditure, the further these signals need to travel the more energy is needed
= by having all processing units clustered in a central area (the brain) allows a lower energetic cost and higher efficiency
there is a lot of commonality between vertebrate and tetrapod brains, list some of the homologous structures in all tetrapod
also mention briefly what differs
1) cerebellum = motor control
2) optic tectum = vision in most lineages except mammals
3) medulla/brain stem = all autonomic functions
4) cerebral hemispheres = high cognitive functions such as speech, critical thinking and social behaviours
= the only thing which differs is relative size e.g. cerebral hemispheres are much larger in birds and mammals compared to other species such as reptiles
outline how the mammalian brain has such diversity
there are homologous structures present however there is great variation in the brains gross morphology e.g.
= highly folded in elephants vs smooth in manatees
= small in hedgehogs vs big in dolphins
outline the relationship between absolute brain size and absolute body size in species
body size accounts for over 90% of variation in brain size however despite a strong correlation between body and brain sizes the brain size doesn’t increase in direct proportion with body size with a scaling exponent of 0.7/0.8
= therefore small bodies have a larger relative brain size than larger bodies BUT bigger bodies have a larger absolute brain size
outline some of the costs of having a larger brain
brains are very metabolically expensive
- humans = brains make up 2% of body mass but use around 20% of resting energy
- chimpanzees = 13% resting energy
- mice = 8.5% resting energy
= per unit weight brains consume 10x more energy than other somatic tissues
however this cost must be paid for by some advantages otherwise the evolution of bigger brain sizes would not occur
outline some of the benefits of having evolved a larger brain
having a larger brain has been linked to having greater intelligence
being more intelligent allows for greater behavioural flexibility meaning individuals can respond better to changing environments where optimal solutions don’t remain the same all the time
- in humans enabled technological innovations and the production of societies
what is meant by the expensive brain frame work produced by Isler and Van Schaik, 2009,
explains the ways of mitigating the costs of big brains in two ways
1) changing energy allocations in the body
2) increasing overall energy intake
outline what Isler and Van Schaik meant by changing energy allocations to mitigate effects of big brain size in their expensive brain framework
large brains are associated with increased survival and reduced mortality but also reduced fecundity
= enlarged brains require increase maternal investment due to the developmental time needed for it to growth in mammals
= this maternal investment is a trade off against fecundity
= there is a trade off between the fitness components of growth and reproduction- more energy is allocated to growth and less to reproduction
outline what Isler and Van Schaik meant by increasing energy intake to mitigate effects of big brain size in their expensive brain framework
trend showing an overall increased basal metabolic rate in larger brained species = this is however not a tight relationship
= having a larger brain allows for adaptations such as improved foraging efficiency, identifying social cheats and behavioural innovations which all increase energy extractions
what are some challenges in measuring cognitive abilities in animals
defining what intelligence is and how it can be compared across species
how can we measure cognitive ability across different species
list the consequences of increased brain size
1) the number of neurones
2) connectivity
3) composition
outline how the number of neurones has changed as a consequence of increased brain size
larger brains have more neurones which means they also have greater processing
= found linear correlation between the number of neurones and overall brain mass suggesting larger brains have overall greater processing power
= Herculano-Houzel 2009
outline how connectivity has changed as a consequence of increased brain size
larger brain sizes have lead to changes in patterns of brain connectivity
- bigger brains = more neurones = more connections (synapses)
- at 100% connectivity, axon density increases exponentially but it is impossible for large brains to have 100% connectivity due to physical constraints on the number of synapses which can fit in the brain - there isnt enough for all neurones to be connected
= need a solution
