by Dr. John D. Pettigrew
Primates share a half-dozen brain pathways not found in any of the other 20 mammalian orders. These features are quantitative and are believed to reliably distinguish primates from non-primates. They provide a unique signature, enabling us to recognize a primate brain after a set of tests which involve labeling the pathways going from the eye to the brain.
Imagine my surprise when I found these features in the brain of a Gray-headed Flying Fox (Pteropus poliocephalus) during a routine investigation! Subsequent work by myself and others confirmed that this is true for all of the flying fox species thus far examined. Flying foxes belong to the order Megachiroptera, commonly referred to as megabats. The nearly 200 species of these bats are vegetarians that feed on fruit and nectar. They are mostly large, with wingspans measuring from two to six feet and navigate by their excellent vision.
Recently, working with Dr. Howard Cooper of the INSERM Laboratoire de Neuropsychologie in Lyon, France, we extended the comparison to include more visual nuclei, as well as more species of megabats and more prosimmian primates (lower primates, including lemurs and lorises). Under the microscope, the affinities between megabat and lemur brains are so striking that it is quite difficult to tell them apart! So far as one can tell from the intricate details of the wiring of thousands of nerve cells, primates and megabats shared a common ancestor not shared by any other group of mammals.
Origins of Bat Flight
These discoveries are especially intriguing, since unlike superficial anatomical features, which often prove deceptive, brains are known to be very conservative, capable of providing invaluable clues in the search for evolutionary relationships. I am now convinced that mammals developed powered, flapping flight on TWO separate occasions, not one, as has been traditionally believed.
My current hypothesis is that the first occasion was probably in the Cretaceous era when small, shrew-like insectivores developed the ability to fly while becoming more efficient at chasing insects. The legacy of that achievement is the highly successful group we now know as the Microchiroptera or microbat. Traditional taxonomic consensus has divided bats, the order Chiroptera, into two suborders, the Mega- and Microchiroptera. The microbats include nearly 800 species, all of which navigate by highly sophisticated echolocation, though they have vision as well. Most are small, with wingspans measuring from six to eighteen inches, and they are mostly insectivorous.
Much later, I believe in the Tertiary era, an early primate line improved its success in foraging for fruits by developing the ability to glide. The ancestors of these primate gliders are living today, either as gliders (the two species of flying lemurs, order Dermoptera) or as the even more successful megabats.
My belief that micro- and megabats evolved separately stems in part from the fact that the microbats that I and my colleagues have checked lack the unique primate visual pathways to the brain. The suggestion that megabats are primates or that they evolved independently from other bats came as a real shock. However, after extensive reading of the taxonomic literature on bats I was somewhat reassured.
I found obvious differences between micro- and megabats, including such things as diet, dentition, chromosomes, world geographic distribution, sperm, biochemistry, parasitology and numerous features of behavior. The only strong link found between the two groups was the presence of a forelimb modification enabling flight in both.
Evolution of Bats
The differences between megabats and microbats are so dramatic and numerous that I wondered why the two groups of bats had ever been linked together in the first place. The problem apparently stems from the great similarity of the handwings in the two groups of bats. The handwing of a megabat and that of a microbat are so similar to each other, and so different from pterosaur or bird wings that it is easy to fall into a perceptual trap: the assumption that similar structures necessarily share a common origin. The dangers of such an assumption are easily illustrated.
Everyone recognizes the image-forming device we call an eye at first glance, whether the bearer of the eye is a squirrel or a squid. The next step in working out the evolutionary origin of these two kinds of eyes is not so easy. In fact, a microscopical examination reveals that the retinal layers at the back of the squid's eye are inside-out with respect to those in the squirrel's eye. By this, and a host of other criteria, we can establish that the cephalopods "invented" their eye completely independently of the vertebrates.
The point here is that a casual inspection is no substitute for detailed quantitative investigation under the microscope. Such analyses, including measurements of wings in many species of bats, indicate that the microbat wing evolved from an antecedent which was small (around 40 mm forearm length or shorter) and had very long metacarpal hand bones in relation to the connecting finger bones. In contrast, the megabat wing evolved from an antecedent which was moderately large (around 100 mm forearm length) and had metacarpal hand bones more similar in length to its finger bones, as in the living flying lemurs.
One of the predictions of this scenario for the evolution of megabats is that flying lemurs will prove to be bonafide primates too, even though they are presently placed in their own order. Already published morphological and serological studies indicate this may be true. In fact, current work in our laboratory demonstrates that flying lemurs share the same primate visual pathways recently discovered in megabats.
Are Megabats Really Primates?
According to Allison Jolly (1985), a distinguished modern primatologist, primates are recognized by two sets of traits, one anatomical and the other behavioral. The anatomical traits were first set out by LeGros Clark (1960) and have been extended by modern neurobiological techniques. Conventional primates and megabats share all of these anatomical traits, whereas microbats are more like other, non-primate mammals. The behavioral traits listed by Jolly include, (1) free and precise movement of the hands and forelimbs; (2) a shift from reliance on smell to reliance on vision; (3) detailed spatial patterning of the world; and (4) prolonged care of dependent young, allowing time for learning the resources of the environment and the manners of the tribe. Although there is limited research about megabats in relation to these behavioral criteria, all are consistent with current knowledge, and none can be ruled out. Thus, to exclude megabats from the primate order would require redefining the presently accepted criteria. My own subjective impression of them after years of close interactions, at least with the genus Pteropus (the largest flying foxes), is that they are keenly aware of quite subtle changes in their environment in a way comparable with, or even superior to, the prosimian primates I have known. As I see it, the only way to deny that the megabats are flying primates is to claim that primates, by definition, do not fly!
(Dr. John D. Pettigrew is a Professor of Physiology at the Neuroscience Laboratory, University of Queensland in St. Lucia, Australia. His research has won him top recognition, including the American Association for the Advancement of Science Newcomb-Cleveland Medal for Excellence in Research in 1978. His major contributions have been in the field of neuroscience where he has clarified the way in which the brains of mammals and birds use two eyes to see in three dimensions and two ears to listen to particular directions in space.)
Background Reading and References
Pettigrew, J.D. (1 986). Flying primates? Megabats have the advanced pathway from eye to midbrain. Science, Vol.231, March 1986, pp.1304-1306.
Pettigrew,]. D. and B.].Frost(] 985). A tactile fovea in the Scolopacidae? Brain Behaviour and Evolution, 26:185-195 (Shows how the brain can be used to sort out evolutionary relationships of an unsuspected, unpopular or controversial kind. Superficially similar birds, in this case the shore birds we know as sandpipers and snipe, have very different brains. The separation into two separate groups suggested by the brain studies is supported by other studies with DNA and detailed skeletal measurements).
Hill, ].E. and ].D. Smith (1 984). Bats: A natural history. 243 pp., University of Texas Press, Austin, Texas. (includes a discussion of the idea that megabats and microbats are likely derived from separate evolutionary lines).
Jolly, A. (1985). The evolution of primate behavior. American Scientist, 73:230-239.
Dr. Pettigrew's discovery that flying foxes may be primates was published in the March 74, 7986 issue of SCIENCE. BCI invited him to share his findings with our readers.
A Grey-headed Flying Fox pollinates an important Australian timber tree, the Bush Box
(Lapostemum contestus). This is the species of flying fox in which Dr. Pettigrew first discovered uniquely primate brain pathways. Photo by Dr. Merlin Tuttle.
The Queensland Blossom Bat (Syconycteris australis), weighing only about one half of an ounce, is one of the world's smallest flying foxes. Dr. Pettigrew found that even this bat had the unique primate brain pathways. It is shown pollinating a Swamp Banksia (Banksia robur), in Australia. Photo by Dr. Merlin Tuttle.
BCI's first board member, Verne Read, and his wife Marion, funded Dr. Tuttle's recent trip
on behalf of threatened and endangered flying foxes in Australia and the Pacific islands. They accompanied him as field assistants and are shown here getting acquainted with orphaned baby flying foxes in Queensland, Australia. Photo by Dr. Merlin Tuttle.