Warm Bodies and Big Babies Help Vertebrates Evolve Bigger Brains, Says New Study

A new study published in the Proceedings of the National Academy of Sciences (PNAS) by researchers at the Max Planck Institute of Animal Behavior reveals something fascinating about brain evolution across vertebrates: species that can keep their bodies warm and produce larger offspring tend to have larger brains relative to their body size. This discovery helps explain why mammals, birds, and even some sharks have such impressive brains compared to most other vertebrates like amphibians or bony fish.

Understanding the Big Brain Mystery

For years, scientists have been puzzled by the huge differences in brain size across vertebrates. Two animals might have the same body size, yet their brains can differ in mass by more than a hundredfold. Typically, mammals and birds sit at the top of the brain-size chart for their body size, followed by sharks and reptiles, while amphibians and most fish have the smallest brains.

One of the main questions has been why. Social factors were an early hypothesis—species that live in complex social groups often develop bigger brains to handle relationships and communication. But this pattern doesn’t explain everything. For instance, many solitary mammals or birds still have large brains, and some highly social fish have tiny ones.

That’s where the new study steps in. The researchers wanted to see whether basic biological traits—like body temperature and reproductive strategy—could explain brain size better than social behavior or environment.

The Study: Looking Across 2,600 Species

The research team, led by Zitan Song and colleagues, examined about 2,600 vertebrate species, including mammals, birds, reptiles, amphibians, and fish. They compared brain size, body size, body temperature, and offspring size across all these groups to uncover deeper evolutionary patterns.

They found two major predictors of brain size:

1. Body temperature – Species that maintain a stable and relatively warm body temperature tend to have larger brains.
2. Offspring size – Species that produce large babies (relative to the mother’s size) also tend to evolve larger brains.

These factors combined could explain why endothermic animals (those that generate their own body heat, like mammals and birds) have such large brains compared to ectothermic animals (those that rely on the environment for heat, like most fish and amphibians).

Why Warmth Matters

Brain tissue is an energy-hungry organ. Unlike muscles or other body parts, the brain cannot simply slow down or shut off to save energy. It demands a constant and high energy supply, even when the body is resting or sleeping.

Warm-blooded animals have an advantage here. Their ability to maintain constant high body temperatures means their brains operate in an efficient, stable environment, making it possible to sustain complex cognitive functions.

The study found that even among “cold-blooded” species, those living in warmer habitats (like tropical waters) tend to have relatively larger brains. Warm conditions, whether produced internally or obtained from the environment, seem to boost brain efficiency and make it easier to evolve and maintain larger brains.

Why Offspring Size Is Important

The study also revealed another key piece of the puzzle: the size of the offspring.

Producing a big baby is costly, but it allows more time and resources to be devoted to the young during development. Larger offspring can support a bigger developing brain, and in many cases, parents of such species provide more care, feeding, and protection after birth or hatching.

Species that can both maintain warm bodies and produce large young—such as mammals, birds, and some sharks—have the largest brains relative to body size. In contrast, species that produce many small, unprotected eggs—like most fish or amphibians—tend to evolve smaller brains.

This pattern aligns perfectly with the “Expensive Brain Hypothesis,” which states that large brains can only evolve when an organism can afford the energy costs, either through increased energy production (as in warm-blooded species) or reduced energy competition during development (as in species with fewer, larger offspring).

Evolutionary Chain Reaction

The researchers point out that a high, stable body temperature originally evolved for other reasons. In mammals, it likely developed to enable nighttime activity, while in birds, it helped them fly longer distances and maintain high energy output.

But once stable warmth evolved, it opened the door for larger and more complex brains. The same goes for reproductive investment—once a lineage started producing large, well-provisioned offspring, it became easier to support a growing brain.

It’s a reminder of how evolution often works in unexpected ways. Traits that appear for one purpose—like staying warm or having big babies—can end up enabling completely new capabilities, such as intelligence, complex communication, and problem-solving.

Surprising Examples

The study’s findings help explain some long-standing oddities in nature.

For example, many shark species—particularly those living in warm waters and giving birth to large, live young—have brains comparable in size to those of birds and mammals. Their biology and habitats meet both criteria: warm bodies and large offspring.

On the other hand, cold-water fish and amphibians, whose body temperatures drop with the environment and who lay thousands of tiny eggs, have small brains because their physiology simply can’t support the cost of maintaining more neural tissue.

Broader Insights into Brain Evolution

This study adds an important piece to our understanding of why brains evolve differently across species. It shows that energetic and developmental factors—not just social or ecological ones—play a crucial role in determining brain size.

By focusing on metabolic and parental investment constraints, it provides a unifying explanation for patterns that other theories only partially covered.

The findings also have implications beyond zoology. They suggest that the evolution of intelligence—like that of humans—required two critical preconditions:

1. The ability to generate and regulate body heat.
2. The ability to invest heavily in offspring, both before and after birth.

In humans, both conditions are fulfilled. We’re warm-blooded, our babies are relatively large, and we invest years in raising them—giving our species the metabolic and developmental foundation for an exceptionally large brain.

What This Means for Future Research

While the study provides a strong link between warmth, offspring size, and brain evolution, there’s still more to explore. Not all vertebrate groups fit the pattern perfectly. For example, within reptiles or amphibians, some species deviate from expectations, and researchers still need to understand why.

Moreover, body temperature isn’t just a fixed trait—it can be influenced by behavioral thermoregulation, like basking in the sun or living in warm microhabitats. Some “cold-blooded” species, such as certain sharks or tuna, even have regional endothermy, keeping specific body parts (like muscles or the brain) warm. These intermediate strategies might represent evolutionary bridges toward more sustained brain growth.

Future research could test these relationships more precisely by combining physiological data, environmental conditions, and direct measures of cognitive performance across species.

A Broader Lesson from Evolution

The study offers a beautiful example of how evolution often builds new features on top of existing ones. Warm-bloodedness and parental care didn’t evolve for the sake of intelligence—but once they were in place, they made big brains possible.

Humans are simply one of the most extreme examples of this evolutionary chain reaction: our warm bodies and long childhoods created the perfect conditions for our large and powerful brains.

So, next time you marvel at human intelligence—or even the cleverness of a crow or dolphin—it’s worth remembering that their brilliance started with something as simple as staying warm and raising big babies.

Research Reference:
Parental investment and body temperature explain encephalization in vertebrates – Proceedings of the National Academy of Sciences (2025)