Why some animals got big brains and others didn’t

Warm bodies and big babies set the stage for large brains. A new study of 2,600 vertebrates shows that brain size grows when warmth and newborn size align.

The work centers on how living things pay the brain’s energy bill every day. The study was conducted by scientists in Konstanz, Germany, who examined animals from fishes to birds.

Why brain size varies

The research was led by Carel P. van Schaik, a professor of evolutionary anthropology at the Max Planck Institute of Animal Behavior (MPIAB). His research focuses on how energy budgets and parental care shape life histories.

Across species, encephalization, the evolutionary increase in brain size relative to body, has never been free. The brain draws power nonstop and the bill arrives daily without breaks.

The idea behind the expensive brain view is simple, bigger brains need extra fuel or cuts elsewhere. Studies of how organisms convert food into cellular energy point in the same direction.

“We humans were lucky to be warm-blooded. In addition, our babies are large and fed for years,” said Schaik.

Key constraints for brain size

The first constraint is temperature. Animals with endothermy, internally generated stable body heat regardless of the environment, can keep neural machinery running at peak efficiency.

The second is newborn size at birth. Large young arrive with more stored energy and parental support, letting early brain growth proceed without shortchanging the rest of the body.

When these two conditions align, brain size expands far more readily across clades. The analysis shows that birds and mammals cluster at the high end for brain size, while most amphibians and fishes do not.

How the team tested the idea

The team compared brain, body, and newborn sizes across 2,580 species using models that account for family ties among species.

A second data set linked brain size to typical body temperature for 1,059 species. When analyzed together, warmth and newborn size showed a positive interaction, each amplifying the other’s effect on brain size.

The researchers also tested basal metabolic rate, energy burned at complete rest, and found little added power once body temperature was included.

Energy math of a growing brain

A large brain is costly at any age. It is especially expensive in early life when growth competes with learning and motor development.

Because the neural budget is constant, shortfalls are hard to cover in practice. The brain cannot simply pause activity during sleep or hunger, and that limits how big it can get.

One pathway is to raise the energy intake available to juveniles. Another is to extend childhood so that brain construction continues while parents provide food and protection.

Larger newborns help on both counts. They bring reserves and usually receive longer care, which together reduce the risk of energy debt for a developing brain.

Sharks as a test case

Some sharks keep parts of their bodies warmer than surrounding water. That trait, called regional endothermy, helps them stay active in cooler seas.

Tag data back up the benefit, and a comparative analysis found regionally endothermic fishes cruise faster than cold blooded cousins. Warmer bodies make brain tissue more efficient per unit of energy during activity.

In the dataset, several shark lineages sit closer to warm-blooded birds and mammals than to other fishes on brain size for body size. Those patterns line up with another broad result from physiology.

Across vertebrates, relative brain size increases with higher temperature after accounting for body mass. The temperature link helps explain why many ectotherms remain small brained even when other traits look favorable.

Why newborn size matters

Brains are heavy users of glucose and oxygen every minute. Even in humans, this organ can claim one fifth of resting energy across the day.

The more a species invests in each baby, the easier it is to bridge the early energy gap. That is why matrotrophy, mothers supplying nutrients directly to embryos during gestation, is linked with larger young.

This link echoes earlier work that tied brain expansion to energy budgets. A unified framework argues that species grow bigger brains by finding extra energy or shifting it from other costs.

Humans fit this template neatly as a case. Our infants are large for our body size and they are provisioned for years.

Limitations and future research

Not every lineage that produces large newborns has large adult brains. Many amphibians and reptiles stay small brained because their body temperatures remain low or fluctuate widely.

Temperature and newborn size work together rather than alone. The combination explains why mammals and birds shot ahead, and why most ectotherms did not.

The study cuts through the noise of social and ecological variation. By comparing thousands of species within an evolutionary framework, it reveals how the fundamental economics of energy shape the architecture of the brain.

The takeaway is practical and humbling. Warmth and parental investment are not just lifestyle traits, they are preconditions for building complex nervous systems.

The study is published in the journal Proceedings of the National Academy of Sciences.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–