Size doesn't matter for mammals with more complex brains

In many mammal species, the males can be bigger than the females (or vice versa), a trait called sexual size dimorphism (SSD). For example, male elephant seals are around three times bigger than females. In contrast, dolphins have no difference in sizes between the sexes. Humans are somewhere in between, with the average male being larger than the average female, but across the population there is an overlap.

To understand how this trait is associated with genome evolution, scientists from the Milner Centre for Evolution at the University of Bath in the UK looked at similarities between the genomes of 124 species of mammals.

They grouped the genes into families of similar functions and measured the size of these gene families.

They found that those species with a big difference in size between the sexes had bigger gene families linked to olfactory functions (sense of smell) and smaller gene families associated with brain development.

Therefore, this could also mean that those species with very little difference in sizes between males and females (termed monomorphic) had bigger gene families associated with brain development.

Publishing in Nature Communications, the authors suggest that in species with a large SSD, traits such as the sense of smell could be important for identifying mates and territories.

In contrast, mammals with a smaller SSD are potentially investing in their brain development and tend to have more complex social structures.

This means they compete for mates using other means than simply using size to select who to breed with.

Dr Benjamin Padilla-Morales, from the Milner Centre for Evolution at the University of Bath led the research.

He said: "We were surprised to see such a strong statistical link between a large SSD and expanded gene families for olfactory function. Even more interestingly, the gene families under contraction were linked with brain development.

"This could mean that those species with a small SSD have bigger gene families associated with brain function and tend to show more complex behaviours such as biparental care and monogamous breeding systems.

"It shows that while size in some species is an important sexual selection pressure for evolution, for others it doesn't matter so much.

"It makes us ask the question how traits like SSD are shaping the evolution of our brains and genomes."

In future work, the researchers want to investigate how testes size impacts the evolution of mammals' genomes.

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Brain size riddle solved as humans exceed evolution trend

The largest animals do not have proportionally bigger brains -- with humans bucking this trend -- a new study published in Nature Ecology and Evolution has revealed.

Researchers at the University of Reading and Durham University collected an enormous dataset of brain and body sizes from around 1,500 species to clarify centuries of controversy surrounding brain size evolution.

Bigger brains relative to body size are linked to intelligence, sociality, and behavioural complexity -- with humans having evolved exceptionally large brains.

The new research, published today (Monday, 8 July), reveals the largest animals do not have proportionally bigger brains, challenging long-held beliefs about brain evolution.

Professor Chris Venditti, lead author of the study from the University of Reading, said: "For more than a century, scientists have assumed that this relationship was linear -- meaning that brain size gets proportionally bigger, the larger an animal is. We now know this is not true. The relationship between brain and body size is a curve, essentially meaning very large animals have smaller brains than expected."

Professor Rob Barton, co-author of the study from Durham University, said: "Our results help resolve the puzzling complexity in the brain-body mass relationship. Our model has a simplicity that means previously elaborate explanations are no longer necessary -- relative brain size can be studied using a single underlying model."

Beyond the ordinary

The research reveals a simple association between brain and body size across all mammals which allowed the researchers to identify the rule-breakers -- species which challenge the norm.

Among these outliers includes our own species, Homo sapiens, which has evolved more than 20 times faster than all other mammal species, resulting in the massive brains that characterise humanity today.

But humans are not the only species to buck this trend.

All groups of mammals demonstrated rapid bursts of change -- both towards smaller and larger brain sizes.

For example, bats very rapidly reduced their brain size when they first arose, but then showed very slow rates of change in relative brain size, suggesting there may be evolutionary constraints related to the demands of flight.

There are three groups of animals that showed the most pronounced rapid change in brain size: primates, rodents, and carnivores.

In these three groups, there is a tendency for relative brain size to increase in time (the "Marsh-Lartet rule"). This is not a trend universal across all mammals, as previously believed.

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T. Rex not as smart as previously claimed

Dinosaurs were as smart as reptiles but not as intelligent as monkeys, as former research suggests.

An international team of palaeontologists, behavioural scientists and neurologists have re-examined brain size and structure in dinosaurs and concluded they behaved more like crocodiles and lizards.

In a study published last year, it was claimed that dinosaurs like T. rex had an exceptionally high number of neurons and were substantially more intelligent than assumed. It was claimed that these high neuron counts could directly inform on intelligence, metabolism and life history, and that T. rex was rather monkey-like in some of its habits. Cultural transmission of knowledge as well as tool use were cited as examples of cognitive traits that it might have possessed.

However the new study, published today in The Anatomical Record, involving the University of Bristol's Hady George, Dr Darren Naish (University of Southampton) and led by Dr Kai Caspar (Heinrich Heine University) with Dr Cristian Gutierrez-Ibanez (University of Alberta) and Dr Grant Hurlburt (Royal Ontario Museum) takes a closer look at techniques used to predict both brain size and neuron numbers in dinosaur brains. The team found that previous assumptions about brain size in dinosaurs, and the number of neurons their brains contained, were unreliable.

The research follows decades of analysis in which palaeontologists and biologists have examined dinosaur brain size and anatomy, and used these data to infer behaviour and lifestyle. Information on dinosaur brains comes from mineral infillings of the brain cavity, termed endocasts, as well as the shapes of the cavities themselves.

The team found that their brain size had been overestimated -- especially that of the forebrain -- and thus neuron counts as well. In addition, they show that neuron count estimates are not a reliable guide to intelligence.

To reliably reconstruct the biology of long-extinct species, the team argues, researchers should look at multiple lines of evidence, including skeletal anatomy, bone histology, the behaviour of living relatives, and trace fossils. "Determining the intelligence of dinosaurs and other extinct animals is best done using many lines of evidence ranging from gross anatomy to fossil footprints instead of relying on neuron number estimates alone," explained Hady from Bristol's School of Earth Sciences.

Dr Kai Caspar explained: "We argue that it's not good practice to predict intelligence in extinct species when neuron counts reconstructed from endocasts are all we have to go on."

"Neuron counts are not good predictors of cognitive performance, and using them to predict intelligence in long-extinct species can lead to highly misleading interpretations," added Dr Ornella Bertrand (Institut Català de Paleontologia Miquel Crusafont).

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No, the human brain did not shrink 3,000 years ago

Did the 12th century B.C.E. -- a time when humans were forging great empires and developing new forms of written text -- coincide with an evolutionary reduction in brain size? Think again, says a UNLV-led team of researchers who refute a hypothesis that's growing increasingly popular among the science community.

Last year, a group of scientists made headlines when they concluded that the human brain shrank during the transition to modern urban societies about 3,000 years ago because, they said, our ancestors' ability to store information externally in social groups decreased our need to maintain large brains. Their hypothesis, which explored decades-old ideas on the evolutionary reduction of modern human brain size, was based on a comparison to evolutionary patterns seen in ant colonies.

Not so fast, said UNLV anthropologist Brian Villmoare and Liverpool John Moores University scientist Mark Grabowski.

In a new paper published last week in Frontiers in Ecology and Evolution, the UNLV-led team analyzed the dataset that the research group from last year's study used and dismissed their findings.

"We were struck by the implications of a substantial reduction in modern human brain size at roughly 3,000 years ago, during an era of many important innovations and historical events -- the appearance of Egypt's New Kingdom, the development of Chinese script, the Trojan War, and the emergence of the Olmec civilization, among many others," Villmoare said.

"We re-examined the dataset from DeSilva et al. and found that human brain size has not changed in 30,000 years, and probably not in 300,000 years," Villmoare said. "In fact, based on this dataset, we can identify no reduction in brain size in modern humans over any time-period since the origins of our species."

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