In the early 1930s, researchers found a strong correlation between body size and other characteristics in a number of species; generally speaking, the bigger you are, the bigger body parts you have.

"This is called the rule of allometry", says Masahito Tsuboi at the Centre for Ecological and Evolutionary Synthesis (CEES) at University of Oslo (UiO).

However, Tsuboi and his colleagues have now found an exception to this apparent fundamental rule. An exception that can be of great importance to our understanding of the evolution of intelligence in birds and mammals: we have bigger brains than our body size would indicate. The findings are so groundbreaking that they were recently published in the renowned journal Nature Ecology & Evolution.

Law breakers

Tsuboi and an international team of researchers collected data on the body and brain size of various mammals, birds, bony and cartilaginous fish, reptiles and amphibians from the existing literature. On the list of species were blue whale, panda, whale shark and the extinct Tasmanian tiger. Then they found something sensational: mammals and birds appear to have broken the rule of allometry!

"Unlike other traits that can change in just a matter of generations, such as height and hair colour, allometric relationships are believed to change very slowly. The relationship between brain and body size in cichlid fishes, for instance, has remained largely unchanged for about ten million years. I was therefore very surprised and intrigued when I discovered that mammals and birds have managed to break this rule", says Tsuboi enthusiastically.

Prolonged development of the brain

It has been known for some time that the brain of humans and certain species of domestic animals, such as cows and chickens, develop through a two-step process. Once again, Tsuboi took a deep dive into the literature to investigate how the brain and body develop, from early embryonic stage to adulthood, in two species of fish.

"As in mammals and birds, the fish brain develops in two stages. The brain grows rapidly during the first phase, while this growth diminishes during the second phase, when body growth takes over. But the big discovery we made is that the first phase is much longer in mammals and birds than in fish", he exclaims.

This, Tsuboi believes, has enabled us to develop bigger brains. He reaches for his calculator and shows me that at 500 grams bodyweight, mammals and birds have 10-15 times the brain size as most other vertebrates (i.e. animals with spinal cord).

Energy-intensive parental care

The brain is one of the most energy-expensive organs in the body, which impose limitations on how big it may grow. So what evolutionary event has allowed us to develop large-sized brains? Tsuboi believes that the evolution of parental care could be the answer.

Three to five days old hatchlings of tree swallow (Tachycineta bicolor). No animals, except for some birds and mammals, are so vulnerable when they are born. This may been the key to why they have developed large brains. Photo: Andrew Iwaniuk, University of Lethbridge, Canada

"Mammals have a long pregnancy period with direct exchange of energy over the placenta and subsequent parental care, while many birds feed their chicks like crazy after hatching. This provides the brain with a large amount of energy over a longer time period, which is rare in other vertebrates, such as reptiles and amphibians", says Tsuboi.

"Are there any other theories that might explain this?"

"Mammals and birds also differ from other animals by being warm-blooded, i.e. the body temperature is independent of the temperature of the environment. However, I believe all of these aspects – temperature regulation, parental care and bigger brains – are connected", Tsuboi continues.

Nevertheless, despite the apparent benefits of such characteristics, they have occurred only rarely during evolutionary time.

"Why this is the case, is a very intriguing question", he smiles.

Something fishy

There are several more interesting and unanswered questions out there, some of which can be found in certain rivers of Africa and Latin America. Here, a very rare and strange type of fish lives, which also seems to have broken the rule of allometry: they have brains equivalent to mammals.

The elephant nose fish hs a big brain - considering its body size. Photo: GnathoMichel/Wikipedia/CC BY-SA 3.0

"There are a few hundred species of fish in the order Osteoglossiformes. These, including the funny-looking elephant nose fish, have big brains that are believed to produce electrical signals to communicate, search for food, and so on. There is lot of on-going research on how these species have managed to evolve such big brains", Tsuboi says, while showing a picture of an elongated fish with a trunk-like mouth.

"I think, therefore I am? But are big brains necessarily associated with higher intelligence? And what really is intelligence?" Tsuboi becomes pensive.

"It is difficult to define intelligence and how it relates to brain size. Generally, all organisms make the most intelligent choices in order to survive and reproduce in their respective environments. Still, studies have shown that animals with larger brains are more able to change their behaviour in response to novel information, he says.

Birds living in cities for example, where there is a continuous flow of new information, have larger brains than corresponding species living in their natural habitat. Large brains, says Masahito Tsuboi, may have allowed new behavioural pattern to emerge, such as the removal of caps from milk bottles.

Opportunities and challenges

The study published in Nature Ecology & Evolution involved almost one and a half years of reviewing the literature in search of original data on brain and body sizes.

"This study shows the potential of analysing large amounts of data from many different species to investigate whether the evolution of different groups of animals follows the same patterns or if certain groups stand out. Another critical aspect of the study was that our analyses included a combination of traditional and more advanced statistical models, says Kjetil Lysne Voje, also a researcher at CEES and a co-author of the article.

Future studies will to a greater extent rely on practical work in the laboratory, but this poses challenges.

"A bottleneck is the immense amount of physical work. It goes without saying that it is difficult to store hundreds of individuals of various species, especially since these have to be followed throughout all life stages. With reptiles it is also difficult to follow the embryonic development as they have closed eggs, as opposed to the transparent eggs of many fish species", says Tsuboi.

However, the hope is that such and similar studies will continue to shed light on the relationship between brain and body size in more species, its evolutionary significance, how it relates to intelligence, and not least the underlying genetics.

"There are a lot of exciting and unanswered questions out there that I would love to investigate, if only I had more time", Masahito Tsuboi smiles.

Contact:

Masahito Tsuboi, researcher at CEES, Department of Biosciences

Read the article:

Breakdown of brain-body allometry and the encephalization of birds and mammals

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