What is it about?
Vertebrates differ enormously in brain size—even species with the same body size can have brains that vary by a hundredfold. Mammals and birds typically have the largest brains relative to body size, followed by sharks and reptiles, while amphibians and most fishes have the smallest. Why? Many ideas point to social complexity or lifestyle (e.g., resident birds vs. migrants), but a general explanation has been elusive. Our study from the Max Planck Institute of Animal Behavior (Konstanz) shows that body temperature and offspring size are key. Species that keep a consistently warm body can usually afford larger brains, because neural tissue works more efficiently at higher temperatures. This pattern even appears in “cold-blooded” lineages that live in or choose warmer environments. We also find that larger newborns/offspring ease the energetic bottleneck of building a costly brain early in life, enabling larger adult brains. Lineages that combine warm, stable body temperatures with larger offspring evolve the biggest brains for their size. In humans, warm-blooded physiology plus prolonged, well-provisioned development helped make our brains exceptional.
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Why is it important?
Brain tissue is costly and needs steady energy. Our study identifies two broad, testable levers that make big brains feasible across vertebrates: thermoregulation (warm, stable body temperatures that raise neural efficiency) and life history (larger offspring that ease the early-life energy bottleneck). This unifies patterns across fishes, amphibians, reptiles, birds and mammals, reframing long-standing debates about sociality and ecology by showing the energetic prerequisites for encephalization.
Perspectives
This work reframes brain evolution as an energy-allocation problem with two levers—thermoregulation and parental provisioning. It unifies patterns across vertebrates, explains long-standing anomalies, and yields clear predictions: warming environments should raise feasible brain size in ectotherms, while reductions in parental care or offspring size should constrain encephalization. Methodologically, it encourages pairing macro-comparative models with developmental energy budgets and experimental tests of temperature–neural performance links.
Zitan Song
Max Planck institute of animal behavior
Read the Original
This page is a summary of: Parental investment and body temperature explain encephalization in vertebrates, Proceedings of the National Academy of Sciences, November 2025, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2506145122.
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