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Maegan Yeung 2. "Bigger is better". Is this true in biology? "Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius – and a lot of courage – to move in the opposite direction." ~Albert Einstein In the essay “On Being the Right Size”, the author J. B. S. Haldane discussed the effects of changing size on an organism’s structure: “For every type of animal there is a most convenient size, and a large change in size inevitably carries with it a change of form”. 1 Given the implications of size to the state of anatomy, physiology, and behavior of an organism, it is rational to wonder whether larger sizes are fundamentally better for its survival. The impacts of body size on an organisms’ evolutionary advantage For animals, body size is a key determinant of many aspects of an organism’s biology, such as anatomy, physiology, 2 reproductive success, 3 longevity,4 and overall interactions with the physical and biological world. Concurrently, body size is exceptionally heterogeneous and wide-­‐ranging at almost every level of investigation.5 Within a given population, mean size generally contrasts amongst congeneric species, conspecific species and even between sexes within a particular species.67 By way of illustration, populations of ectothermic species may include independently functioning individuals over a massive range of body sizes: the smallest mammal, the white-­‐
toothed pygmy shrew, weighs just two grams, whereas the blue whale can reach lengths up to 31 meters and weigh up to 146 metric tons.8 This wide range of intrapopulational variation in body size invokes the question: what body size is optimal? In conventional wisdom, natural selection is suggested resemble the story of Goldilocks and the three bears: stabilizing selection should favor average individuals in a population to avoid selective pressures against extreme phenotypes, and the ones that survive and reproduce most effectively should be not too big, nor too small, but “just right” in size.9 However, as paleontologist Edward Cope proposed more than a century ago, there is a macroevolutionary tendency for species within a lineage to evolve towards an increase in size.10 This postulation, Cope’s rule, is bolstered by a variety of research evidence, such as the exponential growth in body mass of horses within the past 70 million years,11 as well as recent research conducted by the scientists from the Stanford School of Earth, Energy & Environmental Sciences indicating that the biovolume of five major phyla of marine animals has increased 150-­‐fold since the Cambrian explosion.12 Various explanations to this phenomenon have been proposed through indicating advantages of increased body size. Larger bodies ostensibly contribute to a marked increase in survival, fecundity and mating success13, as well as provide ecological advantages, such as ability to ingest larger prey and endure predator attacks or environmental extremities.14 Research to assess Cope's rule and miniaturization using a phylogenetic comparative method with extant Oryzomyini Rats as a study model indicated that the evolutionary trend of increased body size contributes to high diversity and specialization.15 Correspondingly, homoeothermic animals have a within-­‐species tendency to have increasing body size with increasing latitude or decreasing environmental temperatures to conserve heat through having a greater body mass (Bergmann’s Rule),16 as demonstrated by the proliferation of megafauna during the Pliocene Ice Age. Furthermore, gigantism may be favored on islands due to territorialism, deep seas due to reduced vertical temperature gradient and increased hydrostatic pressure, and overall due to reduced vulnerability to food shortages.17 Maegan Yeung Despite the advantages presented, there are several drawbacks to larger body sizes. Although evolution favors large sizes, conversely, extinction favors small.18 The occurrence of the mass extinction of megafauna, such as the widespread extinction of giant ice age animals during the glacial period at the end of the Holocene epoch, illustrates that large organisms are more susceptible to environmental crises, and the mass extinctions account for the cap on body size increase.19 A research conducted at Queen’s University on competitive interactions among 246 bird species pairs involving vultures at carcasses and hummingbirds at nectar sources suggested that as species deviated evolutionarily, the advantages of a large body size was decreased as smaller species were more likely to “dominate aggressive contests when interacting with more distantly-­‐
related species”.20 Moreover, the r/K selection theory suggests that large organisms have to trade off the quantity of offspring with a corresponding increased parental investment. In general, the gap of time from birth to reproductive age of a species is a function of allometry, so larger, slower-­‐
reproducing animal populations are generally unable to compensate for severe selection pressures. Animals with long generation times, such as African elephants which only fully mature 11 to 12 years after birth, are particularly vulnerable to hunting as the potential for population recovery in these animals over short time scales is low.21 As Galileo aptly suggests, “I am certain you both know that an oak two hundred cubits high would not be able to sustain its own branches if they were distributed as in a tree of ordinary size; and just as smaller animals are proportionately stronger and more robust than the larger, so also smaller plants are able to stand up better than the larger”.22 These competing paradigms suggest that although large body size may provide a high ability to compete and specialist niche in the short term, small body sizes may account for increased flexibility to thrive and exploit an ecological niche, even under increase selection pressures in the long term. Brain size, cognitive capacity and evolutionary advantages – Are bigger brains better? The extensive research on the relationship of body size to evolutionary advantages suggests the widespread interest to research the correlation between the two factors. Stemming from the same basis, an interesting question has arisen: does an increased brain size correlate to any significant evolutionary advantage? The answer appeared to be deceptively simple – increased brain size should increase brain capacity to facilitate information processing about its environment in an organism, and the more information a system, such as the brain, can process at a fast rate, the more effectively it can retort to environmental fluctuations and hence the chances of survival should correspondingly increase. Ever since the development of phrenology in the 1800s, it has long been widely assumed that brain volume determines cognitive capacity, with a surge of publications indicating a correlation between different indicators of brain size and derivatives of cognitive capacity. The increase in intracranial volume of the human brain throughout history from 435g in Australopithecus afarensis to the current average of 1,350g in Homo sapiens has been attributed to an increase in intelligence in humans.23 Furthermore, a genome-­‐wide study was reported to have revealed that the HMGA2 gene, in which certain variations have been associated with enhanced IQ, is correlated with increased brain mass.24 Maegan Yeung While increases in cranial size is proposed to affect cognitive capacity, current understandings suggest that the volume of the brain is not fully representative of intellectual ability. Correlational studies have only established a weak linear relationship between brain size an cognitive capacity; a more in-­‐depth look at the study of the HMGA2 gene indicate that the correlation established between variations of the HMGA2 gene and brain size could be further attributed to the fact that the gene also contributes to the overall physical development of humans (eg. human height).25 In his review “Are Bigger Brains Better?” in Current Biology, Professor Lars Chittka states that, "In bigger brains we often don't find more complexity, just an endless repetition of the same neural circuits over and over. This might add detail to remembered images or sounds, but not add any degree of complexity.”26 This is exemplified in the example of the sperm whale, which is the animal with the largest average brain volume of 9kg. The sperm whale has a thicker neocortex than most mammals at 2.63mm, which is approximately the same thickness of the neocortex in human brains. However, the organization of the sperm whale’s neocortex is revealed to be simpler than that of other mammals: the brains of sperm whales only have five neocortical layers whereas humans have six, due to the fact that their brains do not contain the cortical layer IV. The cellular census also indicated that sperm whales have only 12.8 billion neocortical neurons, which is 2/3 the total amount of neocortical neurons present in the human brain.27 This information, in a traditional viewpoint, indicates a compromise in mental processing abilities. This knowledge leads to the development that because larger animals require more neurons to represent their bodies and control specific muscles, the relative size of the brain, measured by brain-­‐to-­‐body mass ratio, would better denote the perceivable variation in behavioral complexity in animals.28 This theory has been generally accepted, but it does have its pitfalls, as this measure seems to favor animals of a smaller body mass. For example, tree shrews have the highest brain-­‐to-­‐
body mass ratio 1:10 (compared to 1:40 in humans),29 but seem to only have “laser beam" intelligence, in which a specific solution is used to solve a specific problem, and find it hard to apply to new situations or to solve different kinds of problem.30 Nevertheless, an increase in brain size also has its fair share of disadvantages. A study by Dr. Kamran Safi of the Max Planck Institute on demonstrates that there are ecological situations where a reduction in neuronal mass is more advantageous to reduce energetic requirements in fast-­‐flying bats.31 Similarly, a Swedish study that involved two different lines of guppies with different sized brains indicated that the evolution of larger brains compromises certain biological abilities. The guppies were genetically modified until one line of guppies had brains that were 9% larger in volume. The guppies with larger brains had to trade-­‐off certain traits, resulting in a reduced gut size by 20%, and 19% fewer offspring in females. This indicates that the larger brains result in an opportunity costs to sacrifice traits that may have been an evolutionary advantage in order to meet the higher energetic demands for the larger brain.32 Importance of brain modularity and interconnectivity If increased cranial volume and body-­‐to-­‐brain mass ratio are not always “better”, per se, for a given organism, then what exactly contributes to increased cognitive performance? Increasing amounts of research are pointing towards one key factor: the modularity of the brain.26 To refer to the computer analogy, a larger computer does not necessarily process data more efficiently, and more effective processing software would not necessarily require a larger hard disk drive to function. Further investigation of the composure of the brain under the microscope hints to the intricacy of Maegan Yeung cellular and molecular arrangements of synapses and neurons as the key determinant of the brain’s cognitive capacity. Yes – one may argue that an increased number of neurons in bigger animals due to biophysical limitations may contribute to larger brains, but the exhibition of the advantages of interconnectivity within the brain is also largely present in the brains of smaller animals. Research by Professor Lars Chittka explains how the effects of highly interconnected neural circuits are particularly evident in not only the rapid neural information processing, but also in the “highly differentiated motor repertoires, extensive social structures and cognition” of insects, especially social ones such as honey bees (Apis mellifera). Bees can learn to associate all the colors within their spectral range (from 300 nm to 650 nm), and can learn different kinds of patterns and shape based on position of “visual field, spatial orientation, motion contrast and bilateral symmetry”. Analysis of the neural network of insects and mushrooms also suggest that various complex intelligent traits, such as “numerosity, attention and categorization-­‐like processes”,26 and other forms of critical thinking may be replicated with only very limited neuron numbers. This supports the perspective that brain size may have less of an impact on cognitive capacity and behavioral repertoire than previously anticipated. The human brain contains greater than 100 thousand kilometers of synapse connections, an estimated storage volume of 1.25 × 1012 bytes, and up to 100 billion neurons.33 The electrical signals fired to from one neuron to another are considered a fundamental feature in generating human intelligence and cognizance. Given these remarkable figures, it may come as a surprise to some that the size of the human brain is shrinking. Research has suggested that that if our brain keeps dwindling at the current rate, the size of the human brain would be reduced by up to 20%, approaching the brain size of Homo erectus, a species of human that lived half a million years ago, at 1100g.34 As some may argue that human intelligence will decrease as a result, this may in fact indicate a conversion of humans to quicker, more agile thinkers. A noteworthy instance of this is the development of the “social brain” during adolescence, where cortical grey matter “decreases in volume by about 15% between age 10 and 20” and brain improves in cognitive performance as cranial wiring became increasingly effectual.35 Some may argue that a reduction in brain size would be detrimental towards social development of humans, as large brains are required in the “complex social systems that characterizes our species”. However, the hypothesis remains challenged due to the fact that the evidence presented in support are “highly anecdotal”. Robin I.M. Dunbar indicated that for example, the social brain hypothesis implies that “it was assumed that brains were only evolved to deal with essentially ecological problem-­‐solving tasks”, which is open to various interpretations. Conversely, smaller brains indicate increased domestication, and are a “signature of selection against aggression” and “increase in tolerance”, according to Duke University anthropologist Brian Hare. This is evident in various domesticated species, such as dogs and cows, which have “lighter and more slender skeleton, a flattened forehead” compared to undomesticated counterparts – indicators of decreased brain size. This is also demonstrated in comparisons of morphological differences between the brains of Homo neanderthalensis, which had larger brains with significantly larger visual cortices that were mainly devoted to vision and body control, and Homo sapiens, which have comparatively smaller brains that contain enlarged frontal lobes wired for social interactions and complex cognition.36 Future trends of body and brain size Maegan Yeung Freddie Mercury flamboyantly stated in a 1985 interview that “bigger is better: for everything.” Unfortunately, this popular generalization may not be applicable in circumstances mentioned above. Major changes are projected to occur on earth in the future caused by anthropologic and environmental reasons, causing increased volatility in the biophysical environment. This, along with the progressing biotic crisis resultant of rapid technological developments, is likely to expedite a major extinction of species. Given this presumption, larger animals may suffer as they are relatively predisposed to extinction risks due to biomechanical and energetic constraints.1 Additionally, smaller but more interconnected brains seem to be the way forward in terms of positive intellectual and social development. In light of the above analysis, bigger may not always be better in terms of body and brain size in the long term, but given that an effect may take millions, if not billions of years to demonstrate its full impact, who knows what changes will happen in the transient and unpredictable future. Bibliography: 1.
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"Haldane, On Being the Right Size." 2003. 28 Feb. 2015 <http://irl.cs.ucla.edu/papers/right-­‐size.html> Fox, CW. "Selection on body size and sexual size dimorphism differs ..." 2006. <http://www.ncbi.nlm.nih.gov/pubmed/16780517> Berg, S. "Body size and reproductive success of drones (Apis ...) 1997. http://www.apidologie.org/articles/apido/abs/1997/05/Apidologie_0044-­‐
8435_1997_28_6_ART0011/Apidologie_0044-­‐8435_1997_28_6_ART0011.html Speakman, JR. "Body size, energy metabolism and lifespan." 2005. <http://www.ncbi.nlm.nih.gov/pubmed/15855403> Altshuler, DL. "Allometry of hummingbird lifting performance." 2010. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2821213/> Vencl, FV. "Allometry and Proximate Mechanisms of Sexual Selection in ..." 2004. <http://icb.oxfordjournals.org/content/44/3/242.full> EMELIANOV, I. "Genetic differentiation in Zeiraphera diniana -­‐ University ..." 1995. <http://www.ucl.ac.uk/taxome/jim/pap/emelianov95.pdf> "From tiny to massive: Mammal size evolution explained ..." 2013. 28 Feb. 2015 <http://www.sciencedaily.com/releases/2013/06/130625092012.htm> "Stabilizing Selection -­‐ A Type of Natural Selection -­‐ Evolution." 2011. 28 Feb. 2015 <http://evolution.about.com/od/NaturalSelection/g/Types-­‐Of-­‐Natural-­‐Selection-­‐Stabilizing-­‐Selection.htm> Stanley, SM. "An Explanation for Cope's Rule -­‐ JStor." 1973. <http://www.jstor.org/stable/2407115> "Cope's Rule & Passive versus Driven Trends." 2014. 28 Feb. 2015 <http://faculty.chas.uni.edu/~groves/CopesRuleandTrends.pdf> "Researchers say larger size is a genuine pattern in ..." 2015. 28 Feb. 2015 <http://www.uncommondescent.com/evolution/researchers-­‐say-­‐larger-­‐size-­‐is-­‐a-­‐genuine-­‐pattern-­‐in-­‐
evolution-­‐not-­‐neutral-­‐drift/> Nenko, I. "Fertility, body size, and shape: An empirical test of the covert ..." 2009. <http://www.ncbi.nlm.nih.gov/pubmed/19402036> "Why did large animal evolve in nature? -­‐ UCSB Science ..." 2012. 28 Feb. 2015 <http://scienceline.ucsb.edu/getkey.php?key=427> Avaria-­‐Llautureo, J. "Body Size Evolution in Extant Oryzomyini Rodents: Cope's ..." 2012. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3318010/> "Mass Extinction of Large Ice Age Mammals Linked to Climate." 2014. 28 Feb. 2015 <http://www.natureworldnews.com/articles/5898/20140205/mass-­‐extinction-­‐large-­‐ice-­‐age-­‐mammals-­‐linked-­‐
climate-­‐induced-­‐vegetation.htm> McClain, CR. "The island rule and the evolution of body size in the deep sea." 2006. <http://craigmcclain.com/Papers/McClain_J%20BIOGEO_2006.pdf> Gibbons, R. "Examining the Extinction of the Pleistocene Megafauna." 2014. <http://web.stanford.edu/group/journal/cgi-­‐bin/wordpress/wp-­‐
content/uploads/2012/09/Gibbons_NatSci_2004.pdf> "The Pleistocene-­‐Holocene Event -­‐ Sixth Great Extinction." 2005. 28 Feb. 2015 <http://www.rewilding.org/thesixthgreatextinction.htm> Maegan Yeung 20. Martin, PR. "When David beats Goliath: the advantage of large size in ..." 2014. <http://www.ncbi.nlm.nih.gov/pubmed/25250781> 21. "r and K selection -­‐ YouTube." 2012. 28 Feb. 2015 <http://www.youtube.com/watch?v=Bu6ouKt9zhs> 22. "Scaling: why giants don't exist -­‐ Galileo and Einstein." 28 Feb. 2015 <http://galileoandeinstein.physics.virginia.edu/lectures/scaling.html> 23. "Brains | The Smithsonian Institution's Human Origins Program." 2010. 28 Feb. 2015 <http://humanorigins.si.edu/human-­‐characteristics/brains> 24. Nishino, Jinsuke et al. "HMGA2 promotes neural stem cell self-­‐renewal and increase brain mass p16 Ink4a and p19 Arf expression." Cell 135.2 (2008): 227-­‐239. 25. Claim: HMGA2 gene linked to brain size and intelligence." 2014. 28 Feb. 2015 <http://www.emersona4e.com/examples/claim-­‐hmga2-­‐gene-­‐linked-­‐to-­‐brain-­‐size-­‐and-­‐intelligence/> 26. Chittka, L. "Are bigger brains better?." 2009. <http://www.ncbi.nlm.nih.gov/pubmed/19922859> 27. "Are Whales Smarter Than We Are? -­‐ Scientific American." 2014. 28 Feb. 2015 <http://www.scientificamerican.com/blog/post/are-­‐whales-­‐smarter-­‐than-­‐we-­‐are/?id=are-­‐whales-­‐smarter-­‐
than-­‐we-­‐are> 28. "Cosmic Evolution -­‐ Epoch 7 -­‐ Cultural Evolution." 2011. 28 Feb. 2015 <https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/text/text_cult_3.html> 29. "Brain-­‐to-­‐body mass ratio -­‐ Wikipedia, the free encyclopedia." 2009. 28 Feb. 2015 <http://en.wikipedia.org/wiki/Brain-­‐to-­‐body_mass_ratio> 30. "Hauser presents theory of “humaniqueness” | Harvard Gazette." 2010. 28 Feb. 2015 <http://news.harvard.edu/gazette/story/2008/02/hauser-­‐presents-­‐theory-­‐of-­‐humaniqueness/> 31. Safi, K. "Bigger is not always better: when brains get smaller." 2005. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1617168/> 32. Kotrschal, A. "Artificial Selection on Relative Brain Size in the Guppy ..." 2013. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3566478/> 33. Hofman, MA. "Evolution of the human brain: when bigger is better." 2014. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3973910/> 34. "If Modern Humans Are So Smart, Why Are Our Brains ..." 2010. 28 Feb. 2015 <http://discovermagazine.com/2010/sep/25-­‐modern-­‐humans-­‐smart-­‐why-­‐brain-­‐shrinking> 35. "The Bigger An Animal's Brain, The Greater Its -­‐ Edge.org." 2014. 28 Feb. 2015 <https://edge.org/response-­‐
detail/25284> 36. The social brain hypothesis -­‐ ResearchGate." 2013. 28 Feb. 2015 <http://www.researchgate.net/publication/243779167_The_social_brain_hypothesis>