Cranes are members of Gruiformes - an ancient order of birds. One chicken-sized gruiform (fossil to the right) lived 30 million years ago on the grasslands of present-day France1. The famous ornithologist Alexander Wetmore identified fossil leg bones from the Paleocene prairies of Nebraska and Kansas as those of Sandhill Cranes from 7 million years ago2, an argument for cranes as the oldest bird species alive today. There are no fossil skulls found for these Paleocene Sandhills, but their brains, like those of modern birds, must have carried a legacy of adaptations acquired through natural selection over hundreds of millions of years.
Birds and mammals are descended from a four-legged little beast that looked something like a lizard. In Triassic times about 250 million years ago, these lizardy vertebrates (Solenodonsaurus3 to the left) made a major evolutionary breakthrough. They acquired a special sac, called an amnion, that protected embryos and thus allowed eggs to be laid on land.
As the "stem amniotes" invaded terrestrial ecosystems, they split into two lineages: the mammal branch and the reptile-bird branch. Reptilian dinosaurs reigned on the land, in the sea, and even flying through the air. Inconspicuous mammals furtively scuttled around as they dodged voracious reptiles. By 100 million years later, the famous bird-dinosaur Archeopteryx had appeared4. But after a gigantic meteor slammed into the Yucatán peninsula 70 million years ago, the earth's climate cooled and the huge reptiles died off.
Of all the major groups of dinosaurs, only the flying feathered lineage survived and prospered to give rise to our present-day birds. Flight enables birds to colonize vertical niches for foraging and safer nesting and, perhaps just as important, to travel great distances in order to exploit seasonal food bonanzas. Today, the migratory lifestyles of billions of birds allow access to lush summer food sources at middle and high latitudes and then escape to warmer climes during winter.
With the passing of generation after generation, birds and mammals independently improved upon the brain of their stem amniote ancestor. The descendant groups retained the basic brain components (cerebrum, cerebellum, and so forth) but each lineage added neuro-architectural enhancements. For example, the profound increases in the numbers of nerve cells of the mammalian cerebral cortex (light blue in the drawing)5 caused it to become deeply infolded. Early anatomists linked intelligence to the mammalian brain's 6-layered cortex. Since the bird's cerebral hemispheres weren't so deeply wrinkled, these anatomists concluded that birds lacked higher cognitive functions. Early 20th century psychologists made a similar presumption, and dismissed birds as mere reflex machines. In some scientific circles, that view still persists. However, an increasing body of data is convincing more and more scientists that birds have been underestimated. In surprising ways, birds are quite bright.
The 21st century is often called the "Century of the Brain". As discoveries about human brain mechanisms cascade from university laboratories, new lines of evidence help strengthen the case for avian intelligence. Bird scientists now use molecular biology, neuroendocrine physiology, cognitive ethology, imaging technologies, field biology, computer modeling, and other diverse approaches to reveal the secrets of brain function.
Research on bird brains and bird cognition might seem somewhat esoteric at first blush, but it has proved to be important to understanding ourselves. Experimentation with birds has led to better comprehension of the human brain. Birds showcase some widespread aspects of brain function. In subsequent Blogs, we will discuss some of those data and relate them to crane biology.
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1. Mayr G, 2005. A chicken-sized crane precursor from the early Oigocene of France. Naturwissenschaften 92:389-393.
2. Wetmore A, Martin HT, 1930. A fossil crane from the Pliocene of Kansas. Condor 32:62-63.
3. Dimitri Bogdanov's drawing of a stem amniote (Solenodonsaurus) is from Wikipedia Commons.
4. The drawing of Archaeopteryx is from NASA.
5. The brain diagram is from www.crowstarver.com.