In the beginning, size was obviously more important than brains: when the mammals “seized power” after the dinosaurs went extinct, they initially only expanded their bodies and kept modest brains, a study shows. The researchers explain that it was only when they had become widely established after about ten million years that many species began to invest in strengthening their cognitive abilities in order to gain competitive advantages.
They have particularly highly developed brains and mammals have also produced the cleverest of all living beings in the course of evolution: humans. The cognitive performance of this group of animals has to do with the relationship between brain size and body dimensions. The following rule applies here: The more brains compared to the mass, the more intelligence a living being can produce. Although elephants have a much larger brain than humans, we are still cognitively superior to them. Basically, however, one can ask the question why not all animals have simply developed larger and larger brains. The reason for the reluctance is the enormous energy consumption of nerve tissue. An increase must therefore be worthwhile, which is not always the case. After all, animals only have more chances of survival and reproduction if the benefits of more intelligence exceed the costs of the investment.
Targeting the successors of dinosaurs
Apparently, this was quite often the case in mammals – there is a corresponding trend in their evolutionary history. But there are still unanswered questions about the course of the so-called encephalization. Most notably, there was a gap at a crucial stage in mammalian “careers”: Little evidence of brain development existed in the immediate aftermath of the mass extinction event 66 million years ago—when surviving mammalian species began to fill the abandoned ecological niches that existed previously occupied by the dinosaurs.
In order to gain insight into the development in this era, the scientists led by Steve Brusatte from the University of Edinburgh have now examined fossils from a total of 124 extinct species, which date from the early period of mammalian development to the middle Cenozoic. Their study was particularly rich in data for the crucial period immediately after the end of the Cretaceous. As part of the investigations, the scientists recorded the body sizes of the animals and the characteristics of their brains. In addition to the general size, they also used CT scans to examine structures in the skulls that could provide clues to the developmental status of certain areas of the brain.
Great pioneers with little brains
The results showed that the size of mammalian brains compared to their body weight initially decreased in the first 10 million years after the asteroid impact 66 million years ago. The detailed results of the scans also indicate that these animals tended to rely heavily on their sense of smell and that their eyesight and other senses were comparatively underdeveloped, the scientists report. Only later did the trend reverse. Apparently, being tall was more important than being smart to begin with in order to be successful in the post-extinction period. In other words, “The mammals that replaced the dinosaurs were pretty stupid at first,” says senior author Brusatte.
According to the scientists, the findings make it clear once again that large brains and powerful sensory systems are not necessarily advantageous for achieving evolutionary success. “Large brains are expensive to maintain and, if not necessary to acquire resources, would likely have been detrimental to the survival of early mammals in the chaos and upheaval following the asteroid impact,” explains lead author Ornella Bertrand from the University of Edinburgh .
But this changed later on: As the research results of the scientists document, after the phase of the rather weak-headed mammals, which lasted about ten million years, some of the more modern representatives in particular developed ever larger brains as well as more complex sensory performance and motor skills. The scientists see the reason in the fact that there was increasing competition for resources. The investments in more brains could then apparently increasingly pay off and exceed the costs for the expensive nerve tissue.
Source: University of Edinburgh, professional article: Science, doi: 10.1126/science.abl5584