Fossil evidence sheds light on why whales and dolphins have large brains

An international team of palaeontologists, led by AMRI and UNSW’s Dr Matthew McCurry, shed light on why whales and dolphins have such large brains – with the help of baleen whale fossils.

The reasons why whales and dolphins have evolved large brains – which are only rivalled by those seen in primates – have long been debated. A new study by a group of international scientists, from Australia, New Zealand, Italy, the UK, the US and Japan, and led by the AM’s Dr Matthew McCurry helps answer the hotly debated topic. Their findings have just been published in the prestigious scientific journal, the Biological Journal of the Linnean Society.

Cetaceans (whales and dolphins) are renowned as having the largest and one of the most highly developed brains in the animal kingdom – for example, the sperm whale has a brain roughly 7-9 kg in size¹. Many species also have brains that are relatively large compared to their body size. Despite the amount of scientific research undertaken in this area, the reason why this group evolved large brains remains abstract. One of the most prevalent arguments is tied with echolocation; a sense used to navigate environments and hunt prey². Dolphins and other toothed whales use a series of high-pitched clicking sounds, which are ‘beamed’ out through their heads and reflected back off objects such as prey.

The argument is that toothed whale brains evolved to be large in size (even relative to their body size) due to the need to process complex sensory information that is gathered during echolocation. Tied to this, it has generally been thought that baleen whales had small brains compared to toothed whales (the group that includes sperm whales and dolphins) because they did not echolocate. This study debunks this hypothesis.

Our international group of scientists measured and CT scanned a number of mysticete (Baleen whale) fossils and then calculated the size of the cavity where the brain sits, to estimate brain size. The results of the study subsequently showed that the early baleen whales had surprisingly large brains, therefore contradicting the idea that echolocation was the primary driver of brain size in the group. Dr McCurry has said that the large brain sizes arose before echolocation and therefore, cannot be the only cause of evolution in brain size.

Instead, Dr McCurry suggests that sociality could be a more important driving factor, but it is difficult to tell how social a fossilised species was. The data gathered in this study indicated that non-echolocating whales also possessed large brains. Although baleen whales are typically thought of as solitary animals, they are still socially complex – for example, Humpback whales communicate via songs and feeding behaviours.

The driving factors behind the complexity and size of brains are multifaceted and are still debated in relation to primates, ungulates and carnivores. Echolocating whales, such as toothed whales and dolphins, do have a larger brain size which confirms the cognitive demands in echolocation. However, we cannot contribute the evolution to a single factor – echolocation, foraging complexity and sociality likely played varying and influential roles.

Dr Matthew McCurry, Curator of Palaeontology, Geoscience and Archaeology, The Australian Museum; Lecturer, School of Biological, Earth and Environmental Science, The University of New South Wales.