ScienceWeek

EVOLUTION: ON THE EVOLUTION OF BIRD INTELLIGENCE

The following points are made by N.J. Emery and N.S. Clayton (Current Biology 2005 15:R946)

1) In Western society, the term "bird brain" is often used as a derogatory term for a person of diminished intellect, partly because many people tend to think of birds as pecking machines, responding reflexively to stimuli in their environment, and partly because birds seem so different from us, with their beady eyes and small heads. But over 40 years ago William Thorpe, who was the leading authority on bird learning at that time, pointed out: The poor development in birds of any brain structures clearly corresponding to the cerebral cortex of mammals led to the assumption among neurologists not only that birds are primarily creatures of instinct, but also that they are very little endowed with the ability to learn...This misconceived view of brain mechanisms hindered the development of experimental studies on bird learning .

2) In the 1960s little was known about the cognitive capacities of birds, but recent studies lend support for Thorpe's view: we now know that some bird species make and use tools, can count, remember specific past events and reason about the mental states of individuals, behaviors that some have considered to be unique to humans. Despite the apparent cognitive similarity between humans and some birds, neuroscientists have tended to view bird brains as interesting curiosities with little relevance to the workings of the human brain. Recently, however, the Avian Brain Nomenclature Consortium published a series of papers attempting to re-address the issue of the importance of the bird brain to neuroscience by investigating how the avian brain evolved, how the structure of the avian brain relates to that of the mammalian brain, and how names have had a negative influence on how birds are perceived.

3) Negativity surrounding the avian brain began in the late 19th century, when Ludwig Edinger provided names for the various parts of the vertebrate brain. His form of nomenclature was based on the naïve view that evolution occurs in a linear progression, so that each new species is an elaboration of an older species. This "scala naturae" is often represented as a ladder. With respect to intelligence, Arthur Jensen, one of the key recent figures in studies of human intelligence, has argued that single-cell protozoans, such as amoeba, rank at the bottom of the scale, followed in order by the invertebrates, the lower vertebrates, the lower mammals... and finally the primates, in order: New World monkeys, Old World monkeys, the apes, and at the pinnacle, humans.

4) With respect to brain evolution, Edinger applied this scala naturae suggesting that the brains of living vertebrates retained ancestral structures, but that new brain areas were added onto older ones, or older areas increased in size and complexity to form new areas. According to this view, evolutionarily older brains are simple, and so produce simple instinctive behavior, and evolutionarily newer brains are complex, and therefore can control learned and intelligent behavior. The oldest brain regions -- those present in all vertebrates -- were prefixed with the term "paleo-", the next oldest brain regions were given the prefix "archi-", whereas the new brain regions -- those present in the species closest to the top of the "ladder" -- were assigned the prefix "neo-".

5) We now know that, as with other parts of the body, the brains of distantly related species tend to be derived from the same basic elements found in the common ancestor -- they exhibit homology. So although the common ancestor of birds and mammals lived approximately 300 million years ago, studies of extant reptiles have revealed that the reptilian (therapsid and sauropsid) forebrain is pallial in origin, and so the common ancestor should also have shared this trait. If so, then the forebrain of modern birds and mammals will also be pallial. This seems to be the case.[1-5]

References (abridged):

1. Avian Brain Nomenclature Consortium, (2005). Avian brains and a new understanding of vertebrate brain evolution. Nat. Rev. Neurosci. 6, 151-159

2. Emery, N.J. (2005). Cognitive ornithology: The evolution of avian intelligence. Phil. Trans. Roy. Soc. Lon. Biol. Sci., in press

3. Emery, N.J. and Clayton, N.S. (2004). The mentality of crows: convergent evolution of intelligence in corvids and apes. Science 306, 1903-1907

4. Pepperberg, I.M. (1999). The Alex Studies: Cognitive and communicative abilities of grey parrots. Harvard University Press, Cambridge, MA

5. Reiner, A., Perkel, D.J., Bruce, L.L., Butler, A.B., Csillag, A., Kuenzel, W., Medina, L., Paxinos, G., Shimizu, T., Striedter, G. et al. (2004). Revised nomenclature for avian telencephalon and some related brainstem nuclei. J. Comp. Neurol. 473, 377-414

Current Biology http://www.current-biology.com

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ON TOOL-MAKING BY CROWS

Notes by ScienceWeek:

In this context, a "stepped tool" is a tapered tool whose tapering involves a series of steps that sequentially narrow the short-axis diameter to make the tool end in a point.

The following points are made by G.R. Hunt et al (Nature 2001 414:707):

1) New Caledonian crows fashion tapered tools from either the left or the right edge of the long narrow leaves of pandanus trees or screw pines, the crows using the tools to extract invertebrates in rainforest vegetation. Although right-handedness is thought to be uniquely human, the authors demonstrate that crows from different localities display a widespread laterality in making their tools, indicating that this behavior is unlikely to be attributable to local social traditions or ecological factors. The authors state that to their knowledge this is the first demonstration of a species-level laterality in manipulatory skills outside humans.

2) The use of left or right leaf-edges by crows depends in part on the direction in which the leaves spiral. Clockwise- spiraling leaves provide easier access to left edges, and anti-clockwise spiraling provide easier access to right edges. This access effect was overridden, however, by an island-wide preference for manufacturing tools from left edges.

3) It has been proposed that right-handedness in humans may be a consequence of the evolution of language, which is also predominantly left-hemispheric. The authors suggest their results favor the more general possibility that species-level lateralization is an adaptation for the efficient neural programming of complex sequential processing, of which language and right-handedness in humans, and stepped-tool manufacture in crows are examples.

Nature http://www.nature.com/nature

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NATURAL HISTORY: ON THE MAGNETIC COMPASS OF SONGBIRDS

The following points are made by W.W. Cochran et al (Science 2004 304:405):

1) Billions of songbirds migrate between continents twice each year, but their orientation capabilities are almost exclusively studied in the laboratory. The authors presented birds with experimentally altered orientation cues and followed their subsequent migratory flights in the wild. Avian navigation capabilities are very precise (1), with many individuals returning to the same breeding sites year after year (1-3) after a voyage of up to 25,000 km (4, ).

2) Migratory songbirds can orient on the basis of compass information from the sun and its associated polarized light patterns, the stars, the earth's magnetic field, and the memorization of spatial cues en route. However, the interactions and relative importance of these cues remain unclear and a source of much debate. Our knowledge about the orientation mechanisms of songbirds relies almost exclusively on data from cue-manipulated captive migrants tested in various orientation cages, on vanishing bearings based on the first few hundred meters of flight, and to a much lesser degree on field data (ringing and radar and visual observations) from unmanipulated natural migrants.

3) On clear evenings, the authors fitted Catharus thrushes with radio transmitters and placed them in outdoor cages in an artificial eastward-turned magnetic field from about sunset until the sun was 11 deg or more below the horizon when they were set free. The authors then radio-tracked the birds in flight to obtain heading data. Because Catharus thrushes do not compensate for wind drift but individuals maintain nearly constant preferred headings from night to night, the authors used measured headings for orientation analyses.

4) In summary: Night migratory songbirds can use stars, sun, geomagnetic field, and polarized light for orientation when tested in captivity. The authors studied the interaction of magnetic, stellar, and twilight orientation cues in free-flying songbirds. The authors exposed Catharus thrushes to eastward-turned magnetic fields during the twilight period before takeoff and then followed them for up to 1100 kilometers. Instead of heading north, experimental birds flew westward. On subsequent nights, the same individuals migrated northward again. The authors suggest that birds orient with a magnetic compass calibrated daily from twilight cues, and that this could explain how birds cross the magnetic equator and deal with declination.

References (abridged):

1. P. Berthold, E. Gwinner, E. Sonnenschein, Eds., Avian Migration (Springer, Berlin, 2003)

2. J. P. Hoover, Ecology 84, 416 (2003)

3. P. O. Dunn, D. W. Winkler, Proc. R. Soc. London Ser. B. 266, 2487 (1999)

4. D. C. Outlaw, et al., Auk 120, 299 (2003)

5. W. L. Engels, Biol. Bull. 123, 94 (1962)

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