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Summary: A global study of vertebrates reveals that body temperature is the key factor behind the evolution of brain size. Warm-blooded species, such as mammals and birds, can sustain the energetic demands of larger brains, while cold-blooded species are limited by fluctuations in external temperatures.
The researchers also found that species that produce larger offspring tend to develop larger adult brains, as these young ones are better able to handle the initial energy costs. Together, constant warmth and large, well-fed offspring paved the evolutionary path for humans to develop larger brains relative to body size.
Key facts
Link to body temperature: Warm-blooded vertebrates can support larger brains because stable internal heat supports a constant flow of energy. Developmental restriction: Species with larger offspring can afford to develop and maintain larger brains into adulthood. Evolutionary Perspective: Endothermy (warm-blooded) first evolved for activity and stamina, but inadvertently made large brains possible.
Source: Max Planck Institute
Vertebrates have extremely different brain sizes: even with the same body size, brain size can vary hundreds of times.
As a general rule, mammals and birds have the largest brains relative to their body size, followed by sharks and reptiles. Amphibians and most fish, on the other hand, have the smallest brains of all vertebrates.
Why is this the case? In some groups of animals, species that live in groups have larger brains than solitary species. They have to deal with rapidly changing social situations and therefore need a more powerful brain.
Additionally, mammals and birds, which generate their own body heat and therefore have a higher, more stable body temperature, have larger brains than most other vertebrates, whose body temperature is determined by ambient temperature. But so far we don’t have a solid explanation for this difference. Furthermore, even within these groups important differences persist.
Brain tissue requires a constant amount of energy. Unlike other organs, the brain cannot simply shut down during sleep or during periods of hunger. So, when the brain grows, the body must find the energy to supply it.
According to the “expensive brain hypothesis,” the brain can only grow if it produces the additional energy itself or if it improves the organism’s chances of survival so much that it can afford to grow and reproduce more slowly.
This explains, for example, why monkey species that do not have to endure periods of hunger and therefore loss of energy throughout the year have larger brains, and why the brains of sedentary birds are larger than those of migratory birds.
Researchers at the Max Planck Institute for Animal Behavior in Konstanz have investigated whether these correlations apply to all vertebrates.
They found that in all vertebrate groups body temperature has a significant influence on brain size. Species that can keep their bodies constantly warm can usually afford larger brains, as they are more efficient in warm environments.
This also applies to so-called cold-blooded species that live in warm waters or specifically choose such places. Furthermore, according to the researchers, the size of the offspring also limits brain size in adulthood. Since the costs of a large brain relative to weight are particularly high for young animals, it is worth keeping the value low at first.
Those lineages that manage to keep their bodies warm and produce large offspring have the largest brains for a given body size.
“We humans were lucky to be warm-blooded. Furthermore, our babies are large and feed for years. This allowed the evolution of the largest brain of all vertebrates in relation to weight,” says Professor Carel von Schaik, head of a group at the Max Planck Institute for Animal Behavior.
Therefore, a consistently high body temperature was a prerequisite for evolution to produce larger brains. However, this ability was originally developed for other reasons: presumably, so that mammals could remain active at night and birds could fly longer distances.
Only then was the door opened to brain growth. Therefore, in evolution, innovations can have unexpected consequences and open up completely new possibilities.
Key questions answered:
A: Its ability to maintain a constant body temperature provides the constant supply of energy needed to fuel and sustain larger brains.
A: Social complexity, offspring size, and environmental stability all contribute: species that are warm-blooded and produce large offspring tend to develop larger brains.
A: The warm-blooded nature of humans and the long-term care of their large offspring created the energetic conditions that allowed our species to develop the largest brain relative to body size.
About this evolutionary neuroscience research news
Author: Carla Avolio
Source: Max Planck Institute
Contact: Carla Avolio – Max Planck Institute
Image: Image is credited to Neuroscience News.
Original research: Open access.
“Parental investment and body temperature explain encephalization in vertebrates” by Zitan Song et al. PNAS
Abstract
Parental investment and body temperature explain encephalization in vertebrates
Systematic variation in relative brain size between vertebrate classes remains poorly understood.
Here, based on the costly brain hypothesis, we propose that two general constraints explain much of the variation: 1) the ability to produce large offspring and thus provide them with the energy needed to build larger brains, and 2) the ability to maintain continuously high body temperatures, because cooler and more variable brain temperatures reduce brain performance and thus fitness.
Therefore, we predicted that encephalization (major evolutionary increases in brain size) only occurred when changes in physiology or natural history created these abilities.
First, comparative analyzes of all major vertebrate classes (n = 2600 species) revealed that protecting or provisioning eggs or embryos is associated with larger newborns.
Subsequent class-level analyzes confirmed that newborn size and adult brain size underwent correlated evolution in birds, mammals, and cartilaginous fishes, but not in other fishes, amphibians, and reptiles.
Second, we found a positive relationship between mean body temperature and brain size within each class (although sometimes insignificant).
Third, a pooled analysis of all vertebrates revealed a positive interaction between the effects of body temperature and newborn size.
In conclusion, encephalization became more pronounced in vertebrate lineages that can produce large offspring, reflecting internal fertilization with matrotrophy, and maintain a high body temperature, partly related to endothermy.
