The Triple Helix @ UChicago

Fall 2018

"On Animals and the Master Chef’s Brain" By Stephanie Zhang

 

Koalas are to eucalyptus as pandas are to ____.

In answering this analogy, we default to filling in the blank with “bamboo.” Why? Because koalas eat eucalyptus, and pandas eat bamboo. Granted, this statement is entirely true, but the deeper rationale would be that both species consume vegetation that is inefficient and unconducive to their survival. Eucalyptus is not ideal for koalas just as bamboo is not ideal for pandas, yet both depend on their respective food sources for survival.

Eucalyptus leaves are tough, fibrous, and low in nutrition.[1] While koalas are able to handle eucalyptus’ poison via their large caecum—part of the intestine containing the bacteria needed to break down the eucalyptus fibers—chewing the tough leaves wears doing their teeth over time.[2] While other animals have developed evolutionary mechanisms to counter the detriments of consuming vegetation (e.g. the wombat’s rootless teeth, which never stop growing), the koala has not been able to develop a similar mechanism. As a result, the koala derives little energy from this combination of a high-energy task and low-energy food, continuing to eat in this cycle until it dies—either by predation, natural hazards, or starvation from having ground its teeth down to an unusable level.

Pandas, too, consume food unfit for their physiology. Evolved from omnivorous bears, the giant panda’s digestive system remains oriented for that carnivorous lifestyle; the panda entirely lacks the enzymes required to digest cellulose. Without the gut microbiota of its herbivore counterparts, the panda is able to derive little metabolic benefit from bamboo—and consequently spends up to 14 hours a day eating nearly 12.5 kg of bamboo.[3]

Since both species are not able to acquire a substantial proportion of energy in each unit of consumption, they have to spend much more time eating than humans do. What an organism is able to do in a day is largely attributed to energy uptake, as well as brain quality. More than 25% of humans’ energy budgets are funneled into powering a mass of soft nervous tissue that accounts for 2% of body weight. Scientists have seen, though, that this statistic is not unique to the human; other mammals like tarsiers and certain lemurs, treeshrews, etc. possess similarly expensive brains.[4] The difference in energy consumption of animals’ brains most notably lies in brain size and number of neurons. 

Further, the human brain is peculiarly large compared to the human’s body size. In fact, assuming the average human is 65 kg, the human brain, at 1.3 kg, is 1.04 kg larger than expected for a mammal of that body mass.[5] 

The question then becomes, “Why us? Why did humans get the brains without the brawns?”

There exists a tradeoff between body size and neuron number, for both measures are incredibly energy-intensive to maintain.[6] Thus, to be large in both size and brain power would be disadvantageous for most, as fueling both assets would require allocating more hours to feeding over other activities like rearing young. The whale, both large and intelligent, is an exception because its feeding entails much less work than, say, a primate’s; while the whale simply opens its mouth to engulf volumes of water containing prey, the primate has to scour for its food. Another example is the giant panda; the bear cannot fulfill the caloric needs of both a big brain and body when it already has to spend at least half of the day eating. In fact, zookeepers take advantage of pandas’ “tradeoff” when they swap out twin panda cubs. Under natural circumstances, panda mothers will leave one twin out to dry and focus their efforts on only one; to counter this, zookeepers swap out the two cubs throughout the day. The panda does not notice the cub is any different, so she then ends up taking care of both!

Thus, an organism that is both brainy and brawny would likely be at an evolutionary disadvantage. So why do humans not spend more time in their day feeding? 

If you are burning to know the answer to this question, you are on the right track! The answer lies with fire. Some 1,000,000 million years ago, Homo erectus is theorized to have first leveraged the controlled use of fire in man’s history.[7] With fire, man began cooking his meats and setting the precedent for how we eat today. Relying on a purely raw diet, we would not be able to sustain the magnitude at which we form synapses and transmit neural impulses. Homo erectus’ predicted 62 million neurons would have required more than eight hours a day of feeding on raw food.[6] Not only can a greater amount of calories be derived from cooked food than raw, but cooked food is also readily chewable and therefore more energetically efficient.  As such, a cooked food diet can afford a scale of neurons unachievable with a raw food diet.

With the advent of cooking came more time for humans to invest in activities other than eating. While other animals scavenged and competed for food, humans were able to invent, philosophize, experiment, ideate, and employ their brains in ways other animals lacked the time and energy to. Humans were not able to escape the hunt-and-be-hunted cycle of nature by developing civilization through the sheer size of their brain but rather by how they learned to power it—and how we continue to power it.

Of course, if it were more socially acceptable to spend more time in a day eating, life might be easier. 

With all this in mind, maybe the question to ask now concerns whether Master Chef is the most evolutionarily advanced show. Either way, now that you have finished reading this article, you ought to reward your tired brain with some cooked food!

References

[1] Attiwell, Peter M., and Mark A. Adams. 1996. Nutrition of Eucalypts. Collingwood, Vic.: CSIRO.

[2] Lanyon, Janet M., and G. D. Sanson. 1986. “Koala (Phascolarctos Cinereus) Dentition and Nutrition. II. Implications of Tooth Wear in Nutrition.” Journal of Zoology 209 (2): 169–181. doi:10.1111/j.1469-7998.1986.tb03573.x.

[3] Xue, Zhengsheng, Wenping Zhang, Linghua Wang, Rong Hou, Menghui Zhang, Lisong Fei, Xiaojun Zhang, et al. 2015. “The Bamboo-Eating Giant Panda Harbors a Carnivore-Like Gut Microbiota, with Excessive Seasonal Variations.” MBio 6 (3). doi:10.1128/mbio.00022-15.

[4] Boyer, Doug M., and Arianna R. Harrington. 2018. “Scaling of Bony Canals for Encephalic Vessels in Euarchontans: Implications for the Role of the Vertebral Artery and Brain Metabolism.” Journal of Human Evolution 114: 85–101. doi:10.1016/j.jhevol.2017.09.003.

[5] Aiello, Leslie C., and Peter Wheeler. 1995. “The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution.” Current Anthropology 36 (2): 199–221. doi:10.1086/204350.

[6] Fonseca-Azevedo, K., and S. Herculano-Houzel. 2012. “Metabolic Constraint Imposes Tradeoff between Body Size and Number of Brain Neurons in Human Evolution.” Proceedings of the National Academy of Sciences 109 (45): 18571–18576. doi:10.1073/pnas.1206390109.

[7] Gowlett, J. A. J. 2016. “The Discovery of Fire by Humans: a Long and Convoluted Process.” Philosophical Transactions of the Royal Society B: Biological Sciences 371 (1696): 20150164. doi:10.1098/rstb.2015.0164.

 
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