Why Dinosaur Eggs Stayed Small as Bird Brains Grew

The biggest egg ever laid was not produced by a dinosaur. It was produced by a bird.

Although some dinosaurs were among the most massive creatures ever to roam Earth, their eggs remained unexpectedly small.

A new explanation links this apparent contradiction to an unlikely place: the skull.

Huge dinosaurs, unexpectedly small eggs

An ostrich egg exceeds every egg known from a non-avian dinosaur in size. At first, that difference seems counterintuitive.

Many dinosaurs were far larger than any bird living today. Yet their eggs never matched the dimensions of the largest bird eggs.

One sauropod could have weighed more than an entire herd of elephants. Nevertheless, its eggs were small enough to sit easily in two hands.

Why offspring sizes differ

Animals use remarkably different reproductive strategies. Some hedge their chances by producing numerous tiny offspring, whereas others invest heavily in only a few.

A blue whale has one calf every two to three years. Its newborn may weigh about 2,500 kilograms (2.76 short tons). By contrast, a hawksbill sea turtle can lay approximately 158 small eggs in one clutch.

Both strategies can be successful. They simply represent opposing ends of the same broad spectrum.

Most reptiles fall towards the many, small offspring end. Birds and mammals generally sit nearer the few, large offspring end.

Comparing clutches between species

Previous research commonly examined mammals and birds separately. This made it difficult to identify what drives these patterns across the entire evolutionary tree.

Stephanie Lechki at Princeton University and Roger Benson at the American Museum of Natural History (AMNH) took a broader view.

Their team compiled reproductive and anatomical information for 2,857 living species and included 35 fossil dinosaurs.

The modern dataset covered more than 500 bird species, over 800 mammals and more than a thousand reptiles. Collectively, these animals represent more than 300 million years of evolution.

They applied an approach that adjusts for shared ancestry among species. This prevents closely related animals from distorting the results.

The brain requires greater investment

A consistent trend emerged. Species with comparatively larger brains generally had fewer offspring, larger offspring, or both. Building and maintaining a large brain is energetically expensive.

Developing a brain of that kind appears to require greater investment in every individual young animal. In both birds and mammals, relatively larger brains were associated with larger eggs and larger newborns.

“At first glance, this seems counterintuitive,” said Lechki, the lead author of the work. “Many non-avian dinosaurs were enormous, yet even the biggest dinosaur eggs were smaller than the largest bird eggs.”

“Our results suggest that the answer lies in brain evolution. As birds evolved larger brains, they also evolved larger offspring, which required larger eggs.”

Metabolism did not explain the pattern

Brain size was not the sole possible explanation. Researchers had suggested that metabolic rate - the speed at which an animal uses energy - could account for the same reproductive patterns.

However, the evidence provided limited backing for this theory. Metabolic rate had weak and inconsistent associations with how animals allocated their reproductive investment.

Dinosaurs provide a natural test

Dinosaurs offered a valuable way to test the question. Their metabolisms were relatively high, more like those of birds and mammals than modern reptiles.

Their brains, however, remained small and reptile-like. Their reproductive pattern also resembled that of reptiles, with many small eggs.

This combination is almost absent among living animals. It allowed the researchers to distinguish the influence of brain size from that of metabolism.

Had metabolism determined offspring size, dinosaurs would have resembled birds. They did not, strongly implicating the brain instead.

Eggs enlarged along the bird lineage

The move towards larger eggs was gradual rather than sudden. It slowly developed along the evolutionary branch that led to birds.

Non-theropod dinosaurs produced many small eggs relative to their body size. Some carnivorous dinosaurs, including oviraptorosaurs and troodontids, laid somewhat bigger eggs.

The largest eggs relative to body size appeared in the earliest true birds. Their emergence also coincided with an increase in relative brain size.

Larger brains required larger eggs

“This work places those remarkable fossils into a much larger macroevolutionary picture,” said co-author Roger Benson, the Macaulay Curator of Dinosaur Paleobiology.

He said the connection between brain size and offspring size could have produced cascading changes throughout the transition from dinosaurs to birds.

“If larger brains required larger offspring, and larger offspring required larger eggs, then other aspects of anatomy and behavior may have evolved in response,” explained Benson.

Bigger eggs altered birds’ bodies

Producing a larger egg is not straightforward. Its effects extended to anatomy, nesting behaviour and parental care.

A broader egg requires a broader route out of the body. Across time, the pelvis widened to allow larger eggs to pass through.

Birds also evolved to retain just one functioning ovary. Rather than producing an entire clutch at once, they lay a single egg at regular intervals.

Nesting behaviour changed as eggs grew as well. Larger eggs exchange gases less effectively when buried, potentially shifting nests from covered mounds to the open nests birds make today.

Raising a small number of large young also demands more from parents. More substantial nests and greater attention suit a lineage increasingly investing heavily in each chick.

Mammals followed a similar route

Birds were not the only animals to undergo this shift. The lineage that led to mammals experienced a notably similar transition.

Early mammal relatives likewise changed from producing many small young to having fewer, larger offspring. Once again, this change accompanied increasing brain size.

What still needs to be established

One part of the question remains inaccessible. Fossils seldom show how often an animal reproduced each year.

That figure is important for understanding the complete reproductive pattern. Future research will examine whether the same relationship appears in animals that breed multiple times annually.