Summary: A global study of vertebrates reveals that body temperature is the key driver behind brain size evolution. Warm-blooded species like mammals and birds can maintain the energy demands of larger brains, while cold-blooded species are limited by fluctuating external temperatures.
Researchers also found that species producing larger offspring tend to evolve bigger adult brains, as these young can better handle early energy costs. Together, constant warmth and large, well-fed young opened the evolutionary pathway for humans to develop the biggest brains relative to body size.
Key Facts
- Body Temperature Link: Warm-blooded vertebrates can sustain larger brains because stable internal heat supports constant energy flow.
- Developmental Constraint: Species with larger offspring can afford to develop and maintain bigger brains into adulthood.
- Evolutionary Insight: Endothermy (warm-bloodedness) first evolved for activity and endurance, but it unintentionally made large brains possible.
Source: Max Planck Institute
Vertebrates have extremely different brain sizes: even with the same body size, brain size can vary a hundredfold.
As a rule, mammals and birds have the largest brains in relation 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 animal groups, species that live in groups have larger brains than solitary species. They have to cope with rapidly changing social situations and therefore need a more powerful brain.
In addition, mammals and birds, which generate their own body heat and therefore have a higher and more stable body temperature, have larger brains than most other vertebrates, whose body temperature is determined by the ambient temperature. But so far we do not have a solid explanation for this difference. Moreover, even within these groups, there are still major differences.
Brain tissue requires a constant amount of energy. Unlike other organs, the brain cannot simply shut down during sleep or periods of hunger. So when the brain grows larger, the organism 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 thus energy loss 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 these are more efficient in warm environments.
This also holds for so-called cold-blooded species that live in warm waters or specifically select such places. In addition, according to the researchers, the size of the offspring also limits brain size in adulthood. Since the costs of a large brain in relation to weight are particularly high for young animals, it pays to keep the value low at first.
Those lineages that manage to both keep their bodies warm and produce large young have the biggest brains for a given body size.
“We humans were lucky to be warm-blooded. In addition, our babies are large and fed for years. This allowed the evolution of largest brain of all vertebrates in relation to weight,” says Professor Carel von Schaik, head of a fellow group at the Max Planck Institute of Animal Behavior.
A constantly high body temperature was therefore a prerequisite for evolution to produce larger brains. However, this ability 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 open for brain growth. In evolution, innovations can therefore have unexpected consequences and open up completely new possibilities.
Key Questions Answered:
A: Their ability to maintain a constant body temperature provides the steady energy supply needed to fuel and sustain bigger brains.
A: Social complexity, offspring size, and environmental stability all contribute—species that are warm-blooded and produce large young tend to evolve larger brains.
A: Humans’ warm-blooded nature and prolonged care for 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: The 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
The systematic variation in relative brain size among vertebrate classes remains poorly understood.
Here, based on the expensive brain hypothesis, we propose that two broad constraints explain much of the variation: 1) the ability to produce large offspring, and so provide them with the energy required for constructing larger brains, and 2) the ability to sustain continuously high body temperatures, because cooler and varying brain temperatures reduce brain performance and thus fitness.
We therefore predicted that encephalization (major evolutionary increases in brain size) only happened where changes in physiology or natural history created these abilities.
First, comparative analyses across all major vertebrate classes (n = 2600 species) revealed that protecting or provisioning eggs or embryos is associated with larger newborns.
Subsequent analyses at the class level 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 (albeit sometimes insignificant).
Third, a combined analysis across all vertebrates revealed a positive interaction between the effects of body temperature and newborn size.
In conclusion, encephalization became most pronounced in vertebrate lineages that can both produce large offspring, reflecting internal fertilization with matrotrophy, and sustain high body temperature, partly linked to endothermy.
