The mammals to the dominant vertebrate group rose in the early reinforcing period. However, this did not make it possible for them thanks to the outstanding brain performance, on the contrary: their brains even even shrank, based on the body mass. Rather, the mammals prevailed in those millions by gaining body size and muscle mass. The researchers around Ornella Bertrand from the University of Edinburgh (Scotland) come to the conclusion. The team reports on this in the specialist magazine »Science«.
Around 66 million years ago, the Cretaceous ended and the Paleogene Age began. At this crossing, there was a worldwide mass extinction, to which, among other things, the dinosaurs (with the exception of birds) fell victim. Catastrophic environmental changes, probably triggered by a meteorite impact and possibly exacerbated by volcanic activity, had caused the death. The demise of the dinosaurs opened up ecological niches, which in the following millions of years took over the mammals for themselves. Thus, they developed into the dominant vertebrate group. Many experts suspect that the mammals benefited from their relatively large and powerful brains.
Apparently, however, this was not the case, Bertrand and her team write. The working group has studied mammalian fossils from more than 120 extinct species, including dozens of newly discovered fossil skulls from the Paleocene (66 to 56 million years before today) and the Eocene (56 to 34 million years before today). The team used X-rays to scan the skulls and took computed tomographic (CT) images of them. On the basis of the CT scans, it was possible to estimate how big the brains of the animals had once been and what dimensions those brain areas had that process sensory impressions such as smelling or seeing. The researchers related this to the body sizes of the animals.
Accordingly, the mammals in the Paleocene, which after the disappearance of the dinosaurs, increased significantly in size and muscle mass. Your brains also grew, but to a significantly lower extent. In other words: the relative brain size (related to the body mass). In addition, the investigations showed that the brain regions that were responsible for vision gave a relatively small proportion of the organ in the mammals of that time.
Only in the following age of the Eocene did this change. Now more and more mammalian species appeared, in which the relative brain size increased again. In these species, brain regions that process visual sensory impressions, are important for movement control and perform integrative functions had particularly larged. Brain areas, on the other hand, which contribute to the sense of smell, lost importance.
Bertrand and her team admit that this result is surprising. Studies have shown that mammals with larger brains are better at solving cognitively demanding problems and are more flexible in their behavior. Apparently, such skills were not in high demand in the millions of years after the dinosaurs had largely perished and free niches had to be conquered. Apparently, it had paid off at that time to be big and strong instead of intelligent. "Large brains are expensive to maintain and would probably have been a disadvantage for the early mammals of the modern era as a whole," says Bertrand.
In the Eocene, when the ecosystems had largely recovered and the niches were occupied again, competitive and distribution struggles increased, as the team postulated. Members with better sensory performance and expanded motor skills now had the better cards, which had created a selection pressure towards brain growth. Predators and omnivores in particular now increased in relative brain size - and the herbivores at that time fled this.
