We take our brains for granted, despite being so proud of our evolutionary advantage, but what disadvantages come with these cognitive benefits?
Do brains really steal energy from other organs or consume more energy than others? New research suggests that cutting down on energy requirements in other places in the body, are not enough to fuel the demands to grow a much larger brain.
Kimberley Sukhum and colleagues from the Department of Biology, Washington University, did some studies with the brains of a larger fish called mormyrid electric fishes, or elephantfish. These fish use electrocommunication, a form of electrical signaling invisible to other fishes, making them pretty smart fish.
They postulated that although a larger brain offers cognitive advantages, brain tissue has a high metabolic rate, therefore requiring either a decrease in other energy requirements, or an increase in overall energy consumption to evolve a larger brain.
The problem is that previous studies on frogs, toads, birds, fish and primates have found conflicting evidence for either of these hypotheses, leaving the costs and constraints unclear. This study hopes to shed more accurate light as to where the energy comes from. Their finding were published yesterday, Dec. 21 in the Proceedings of the Royal Society B.
The mormyrid electric fishes have extreme encephalization as do mammals. Encephalization is the amount of brain mass related to an animal's body mass. One such fish, the Gnathonemus petersii, has a brain that constitutes 3% of it's body mass, comparable with human brains at 2-2.5%.
Neuroanatomy of the knollenorgan electrosensory (red), electromotor (blue) and corollary discharge (purple) pathways in mormyrid electric fishes
Other species of elephantfish also have large brain sizes, but how they differ within the species is unclear. This variety of brain sizes within a particular type/specie of fish "presented a great opportunity to study the metabolic costs of braininess," Sukhum said.
Osteoglossomorph fishes display wide variation in relative brain size among lineages
The conclusion of the study is that extreme encephalization can be accommodated through an increase in overall energy consumption. The energetic trade-off within an animal's body alone, can only explain evolution of moderate increases in brain size. The energy requirements to produce extreme conceptualization seems to necessitate an overall increase in energy consumption to fuel such growth.
Relative brain size correlates positively with relative oxygen consumption
This was determined by measuring the oxygen consumption and the toleration of hypoxia which were used as proxies to determine energy use and demand. The largest brained species have the highest demand for oxygen, while the smallest brained had the lowest.
The EOD rate of large-brained lineages is more sensitive to hypoxia than for small-brained lineages.
In previous studies, frogs and toads were found to have smaller guts if they had bigger brains. Due to the energy expenses of both organs, one had to get smaller as the other got bigger. In earlier human studies, it was theorized that our metabolic rate was adjusted by having smaller human guts to accommodate a bigger human brain. The argument is that the gut shrinking possibly coincided with a more nutritious diet of cooked food and tubers to stimulate brain size development. However recent studies of our closer evolutionary relatives, the great apes, found that the metabolic rate and total energy expenditures run in tandem with brain size development.
The difference comes from how much of a brain size increase is being undertaken. To accommodate moderate increases in brain size, a shift in the energy use of other organs or modifying behavior can be enough to sustain this growth. But in extreme encephalization, the demand for a bigger brain requires an increase in total energy intake.
There is a downside to being smarter, and that is the requirement to consume more energy simply to exist.
Looking at the mormyrids and their electrolocation, this ability allows them to sense their environment and forage more efficiently. One fish, the Gnathonemus petersii, have a Schnauzenorgan which is a tube snout that allows them to extract calories from crevices.
Schnauzenorgan snout
The fish that have more extravagant energy requirements may need to stay in oxygen-rich environment's such as fast-moving rivers, while the smaller-brained cousins are able to survive in more areas where there are lower oxygen requirements such as swamps.
Even between the genders of a species, the electrical signals matter. Mormyrid electric fish rely on the waveform of their electric organ discharges (EODs) for communicating species, sex, and social status. A mismatch of wavelengths can greatly affect the survival of the individual and species.
mixed signals
For humans, we overcome the energy requirement shortfalls, by not only cooking our food and sharing our food so that we can all survived more efficiently as through cooperation, but also by storing our energy through fat. Our fat storage provides us with an important buffer against food shortages that would deplete our energy needs and affect our survival capacity.
Mammals certainly have this fat storage capacity to store energy for survival when food shortages occur, which support this theory for bigger brains in evolutionary development.
I also recently posted Bigger Brains - Bigger Burden or Bigger Benefit? New Study Has Answers related to this topic.
https://www.sciencedaily.com/releases/2016/12/161221132803.htm
http://rspb.royalsocietypublishing.org/content/283/1845/20162157
http://jeb.biologists.org/content/216/13/2365
http://jeb.biologists.org/content/210/13/2244.full
