Bligh's Bounty

by Stephen Jay Gould

( For more Information about this great author and biologist, see Voyager CD ROMS by category)

Natural History, September, 1985 (Vol. 94 ; Pg. 2; ISSN: 0028-0712)

Bligh's bounty; a captain who followed the rules meticulously found an animal that broke them brilliantly; the echidna.

In 1789, a British naval officer discovered some islands near Australia and lamented his inability to provide a good description:
Being constantly wet, it was with the utmost difficulty I could open a book to write, and I am sensible that what I have done can only serve to point out where these lands are to be found again, and give the idea of their extent

As he wrote these lines, Capt. William Bligh was steering a longboat with eighteen loyal crew members into the annals of human heroism at sea--via his 4,000-mile journey to Timor, accomplished without loss of a single man, and following the seizure of his ship, The Bounty, in history's most famous mutiny.

Bligh may have been overbearing; he surely wins no awards for insight into human psychology. But history and Charles Laughton have not treated him fairly either. Bligh was committed, meticulous, and orderly to a fault--how else, in such peril, could he have bothered to describe some scattered pieces of new Pacific real estate.

Bligh's habit of close recording yielded other benefits, including one forgotten item to science. Obsessed by the failure of his Bounty mission to bring Tahitian breadfruit as food for West Indian slaves, Bligh returned to Tahiti aboard the Providence and successfully unloaded 1,200 trees at Port Royal, Jamaica, in 1793 (his ship was described as a floating forest). En route, he stopped in Australia and had an interesting meal.

George Tobin, one of Bligh's officers, described their quarry as a kind of sloth about the size of a roasting pig with a proboscis 2 or 3 inches in length. . . . On the back were short qills like those of the Porcupine. . . . The animal was roasted and found of a delicate flavor.

Bligh himself made a drawing of his creature before the banquet. The officers of the Providence had eaten an echidna, one of Australia's most unusual mammals-- an egg-laying anteater closely related to the duck-billed platypus.

Bligh brought his drawing back to England. In 1802, it appeared as a plate accompanying the first technical description of the echidna's anatomy by Everard Home in the Philosophical Transactions of the Royal Society (G. Shaw had published a preliminary and superficial description in 1792).

Home discovered the strange mix of reptilian and mammalian features that has inspired interest and puzzlement among biologists ever since. He also imposed upon the echidna, for the first time, the distinctive burden of primitivity that has continually hampered proper zoological understanding of all monotremes, the egg-laying mammals of Australia. Home described the echidna as not quite all there in mammalian terms, a lesser form stamped with features of lower groups:

These characters distinguish the echidna in a very remarkable manner, from all other quadrupeds, giving this new tribe a resemblance in some respects to birds, in others to the Amphibia; so that it may be considered as an intermediate link between the classes of Mammalia, Aves, and Amphibia.

Unfortunately, Home could not study the organ that most clearly belies the myth of primitivity. "The brain,' he wrote, "was not in a state to admit of particular examination.' Home did have an opportunity to infer the echidna's anomalously large brain from the internal form of its skull, well drawn on the plate just preceding Bligh's figure (and also reproduced here). But Home said nothing about this potential challenge to his general interpretation.

And so the burden of primitivity stuck tenaciously to echidnas, and continues to hold fast in our supposedly more sophisticated age. Some great zoologists have struggled against this convenient fallacy, most notably the early French evolutionist Etienne Geoffroy Saint-Hilaire, who coined the name Monotremata (see last month's column) and labored unsuccessfully to establish the echidna and platypus as a new class of vertebrates, separate from both mammals and reptiles and not merely inferior to placentals. By his own manifesto, he chose his strategy explicitly to avoid the conceptual lock that assumptions of primitivity would clamp upon our understanding of monotremes. He wrote in 1827:

What is defective, I repeat, is our manner of perception, our way of conceiving the organization of monotremes; that is, our determination, made apriori, to join them violently to mammals by violemment,
Geoffroy means, of course, "without any conceptual justification' , to place them in the same class and, after our disappointments and false judgments, then to make our unjust grievances heard, as when we speak of them as mammals essentially and necessarily outside the rules. But Geoffroy's legitimate complaint, so eloquently expressed, did not prevail, and the myth of primitivity continues, despite its blatant flaw. As I argued in last month's column on platypuses, the myth of primitivity rests upon a logical confusion between early branching from the ancestors of placental mammals (the true meaning of reptilian characters retained by monotremes) and structural inferiority. Unless geological age of branching is a sure guide to level of anatomical organization --as it is not--egg laying and interclavicle bones do not brand platypuses and echidnas as inferior mammals.

Beyond this general defense, echidnas can provide ample specific evidence of their adequacy. They are, first of all, a clear success in ecological terms. Echidnas live all over the Australian continent (and extend into Papua-New Guinea), the only native mammal with such a wide range. Moreover, the echidna, as a single struggling relict, ranks with the rat and the monkey (those meaningless synecdoches of the psychological literature) as an absurd abstraction of nature's richness. Echidnas come as two species in two separate genera and with quite different habits. Tachyglossus aculeatus (the Australian form with Papuan extensions) rips apart ant and termite nests with its stout forelimbs and collects the inhabitants on its sticky tongue. The larger and longer-snouted Zaglossus bruijni of Papua-New Guinea lives on a nearly exclusive diet of earthworms. Moreover, three other species, including the "giant' echidna, Zaglossus hacketti, have been found as fossils in Australia. Echidnas are a successful and at least modestly varied group.

But echidnas hold a far more important ace in the hole as their ultimate defense against charges of primitivity. The same cultural biases that lead us to classify creatures as primitive or advanced have established the form and function of brains as our primary criterion of ranking. Echidnas have big and richly convoluted brains. Scientists have recognized this anomaly in the tale of primitivity for more than a century--and they have developed an array of arguments, indeed a set of traditions, for working around such an evident and disconcerting fact. Large brains undoubtedly serve echidnas well; but they also help to instruct us about an important issue in the practice of science--how do scientists treat factual anomalies? What do we do with evidence that challenges a comfortable view of nature's order?

The echidna's brain refutes the myth of primitivity with a double whammy--size and conformation. (I discuss only the Australian species, Tachyglossus aculeatus; its larger Papua-New Guinea relative, Zaglossus, remains virtually unknown to science --for basic information about echidnas, see the two books by M. Griffiths: Echidnas, Pergamon Press, 1968; and The Biology of Monotremes, Academic Press, 1978.) Since mammalian brains increase more slowly than body weight along the so-called mouse-to-elephant curve, we can use neither absolute nor relative brain weight as a criterion. (Big mammals have absolutely large brains as an uninteresting consequence of body size, while small mammals have relatively large brains because brains increase more slowly than bodies.)

Biologists have therefore developed a standard criterion: measured brain weight relative to expected brain weight for an average mammal of the same body size. This ratio, dubbed EQ (or encephalization quotient) in amusing analogy with you know what, measures 1.0 for mammals right on the mouse-to-elephant curve, above 1.0 for brainier than average mammals, and less than 1.0 for brain weights below the norm.

To provide some feel for the range of EQ's, so-called basal insectivores--a selected stem group among the order traditionally ranked lowest among placental mammals--record a mean of 0.311. Adding advanced insectivores, the average rises to 0.443. Rodents, a perfectly respectable group (and dominant among mammals by sheer number), display a mean EQ of 0.652. (Primates and carnivores rank consistently above 1.0.) Monotremes are not, by this criterion, mental giants --their EQ's range from 0.50 to 0.75--but they rank way above the traditional primitives among placentals and right up there with rodents and other "respected' groups. Monotremes continue to shine by other standards of size as well. Some neurologists regard the ratio of brain to spinal cord as a promising measure of mental advance. Fish generally dip below 1:1 (spinal cord heavier than brain). We top-heavy humans tip the scale at 50:1; cats score 4:1. The "lowly' echidna waddles in front of tabby at approximately 6:1.

By conformation, rather than simple size alone, echidnas are even more impressive. The neocortex, the putative site of higher mental functions, occupies a larger percentage of total brain weight in supposedly advanced creatures. The neocortex of basal insectivores averages 13 percent of brain weight; the North American marsupial opossum records 22 percent. Echidnas score 43 percent (platypuses 48 percent), right up there with the prosimians (54 percent), basal group of the lordly primates. (All my figures for brain sizes come from H.J. Jerison's Evolution of the Brain and Intelligence, Academic Press, 1973, and P. Pirlot and J. Nelson, "Volumetric Analysis of Monotreme Brains,' Australian Zoologist, vol. 20, 1978.)

The neocortex of echidnas is not only expanded and nearly spherical as in primates; its surface is richly convoluted in a series of deep folds and bumps (sulci and gyri), a traditional criterion of mental advance in mammals. (Curiously, by comparison, the platypus neocortex, while equally expanded and spherical, is almost completely smooth.)

Many famous nineteenth-century neuroanatomists studied monotreme brains, hoping to understand the basis of human mental triumph by examining its lowly origins. Echidnas provided an endless source of puzzlement and frustration. William Henry Flower dissected an echidna in 1865 and wrote of "this most remarkable brain, with its largely developed and richly convoluted hemispheres.' He admitted: "It is difficult to see in many of the peculiarities of their brain even an approach in the direction of that of the bird.' And Grafton Elliot Smith, the great Australian anatomist who later fell for Piltdown in such a big way, wrote with evident befuddlement in 1902:
The most obtrusive feature of this brain is the relatively enormous development of the cerebral hemispheres . . .. In addition, the extent of the cortex is very considerably increased by numerous deep sulci. The meaning of this large neopallium is quite incomprehensible. The factors which the study of other mammalian brains has shown to be the determinants of the extent of the cortex fail completely to explain how it is that a small animal of the lowliest status in the mammalian series comes to possess this large cortical apparatus.
One might have anticipated that scientists, so enlightened by monotreme mentality, would simply abandon the myth of primitivity. But prompt submission to items of contrary evidence is not, despite another prominent myth (this time about scientific procedure), the usual response of scientists to nature's assaults upon traditional beliefs. Instead, most students of monotreme brains have recorded their surprise and then sought different criteria, again to affirm the myth of primitivity.

A favorite argument cites the absence in monotremes (and marsupials as well) of a corpus callosum--the bundle of fibers connecting the right and left hemispheres of "higher' mental processing in placental mammals. In a wonderful example of blatantly circular logic, A.A. Abbie, one of Australia's finest natural historians, wrote in a famous article of 1941 (commissures, to a neuroanatomist, are connecting bands of neural tissue, like the corpus callosum):

Since in mammals cerebral evolution and with it any progressive total evolution is reflected so closely in the state of the cerebral commissures it is clear that the taxonomic significance of these commissures far transcends that of any other physical character.

In other words, since we know (a priori) that monotremes are primitive, search for the character that affirms a lowly status (lack of a corpus callosum) and proclaim this character, ipso facto, more important than any other (size of brain, convolution, or any other indication of monotreme adequacy). (I shall have more to say about commissures later on, but let me just mention for now that lack of a corpus callosum does not preclude communication across the cerebral hemispheres. Monotremes possess at least two other commissures-- the hippocampal and the anterior--capable of making connections, though by a route more circuitous than the pathway of the corpus callosum.)

This tradition of switching to another criterion continues in modern studies. In their 1978 article on monotreme brain sizes, for example, Pirlot and Nelson admit, after recording volumes and convolutions for echidnas: "It is very difficult to isolate criteria that clearly establish the "primitiveness' of monotreme brains.' But they seek and putatively find, though they honorably temper their good cheer with yet another admission of the puzzling size of the monotreme neocortex:

This cortex could be considered to be among the most primitive mammalian cortices on the basis of the low number and low density of large, especially pyramidal neurons. It is surprising to find that a very high proportion of cortex is neocortex. This does not necessarily mean an advanced degree of progressiveness, although the two are usually related.

The basic data on size and external conformation of echidna brains have been recorded (and viewed as troubling) for more than a century. More sophisticated information on neural fine structure and actual use of the cortical apparatus in learning has been gathered during the past twenty years--all affirming, again and again, the respectability of echidna intelligence.

In 1964, R.A. Lende (Journal of Neurophysiology, vol. 27) published the first extensive map of localized sensory and motor areas on the echidna's cerebral cortex. (I discussed the general procedures of such study in August's column on platypus brains. P.S. Ulinski has recently confirmed and greatly extended Lende's work in a series of elegant experiments--Journal of Comparative Neurology, vol. 229, 1984.) Lende discovered a surprising pattern of localization, basically mammalian in character but different from placental mappings. He identified separate areas for visual, auditory, and sensory control (the motor area overlapped the sensory region and extended forward to an additional section of the cortex), all demarcated one from the other by constant sulci (fissures of the cortex) and located together at the rear of the cortex.

Most surprisingly, these areas abut one another without any so-called association cortex in between. (Association cortex includes areas of the cerebral surface that do not control any specific sensory or motor function and may play a role in coordinating and integrating the basic inputs. For this reason, amount and position of association cortex have sometimes been advanced as criteria of "higher' mental function. But such negative definitions are troubling and should not be pushed too hard or far.) In any case, Lende identified a relatively enormous area of unspecified (perhaps association) cortex in front of his mapped sensory and motor areas. Lende concluded, in a statement oft-quoted against those who maintain the myth of primitivity:

Ahead of the posteriorly situated sensory and motor areas established in this study there is relatively more "frontal cortex' than in any other mammal, including man, the function of which remains unexplained.

Other studies have tried to push the echidna brain to its practical limits by imposing upon these anteaters all the modern apparatus of mazes, levers, and food rewards so favored by the science of comparative psychology. Echidnas have performed remarkably well in all these studies, again confuting the persistent impression of stupidity still conveyed by textbooks and even by the most "official' of all sources--the Australian Museum's Complete Book of Australian Mammals, edited by R. Strahan (Sydney: Angus and Robertson, 1983), which insists without evidence:
In this last respect brainpower , monotremes are inferior to typical placental mammals and, probably, to typical marsupials. The paucity of living monotremes may therefore be due to their being less bright, less adaptable in their behavior, than other mammals.
To cite just three studies among several of similar intent and conclusion:

1. Saunders, Chen, and Pridmore (in Animal Behavior, 1971) ran echidnas through a simple two-choice T-maze (down a central channel, then either right or left into a bin of food or a blank wall). They trained echidnas to move in one direction (location of the chow, of course), then switched the food box to the other arm of the T. In such studies of so-called habit-reversal learning, most fish never switch, birds learn very slowly, mammals rapidly. Echidnas showed quick improvement with a steady reduction in errors-- and at typically mammalian rates. Half the experiments (seven of fifteen) on well-trained echidnas yielded the optimal performance of "one-trial reversal' (you switch the food box and the animal goes the wrong way--where the food used to be--the first time, then immediately cottons on and heads in the other direction, toward the chow, each time). Rats often show one-trial reversal learning, birds never.

2. Buchmann and Rhodes (Australian Zoologist, 1978) tested echidnas for their ability to learn positional (right or left) and visual-tactile (black and rough versus white and smooth) cues--with echidnas pushing the appropriate lever to gain their food reward. As an obvious testimony to mental adequacy, they report that "unsuccessful (unrewarded) responses were often associated with vigorous kicking at the operanda.' Echidnas learned at a characteristic rate for placental mammals and also remembered well. One animal, retested a month later, performed immediate one-trial reversals.

Buchmann and Rhodes compared their echidnas with other animals tested in similar procedures. Crabs and goldfish did not show improvement (did not learn) over time. Echidnas displayed great variation in their speed of learning--one improved faster (and one slower) than rats and mentally retarded humans: all echidnas performed better than cats. Take these results (not to mention the reward for success) with a grain of salt because numbers are limited and procedures varied widely among studies--but still, the single best performer on the entire chart was an echidna.

Buchmann and Rhodes conclude: "There is no evidence that the performance of echidnas is inferior to eutherian placental or metatherian marsupial mammals.' They end by ridiculing the "quaint, explicitly or tacitly-held views that echidnas are little more than animated pin-cushions, or, at the best, glorified reptiles.'

3. Gates (Australian Zoologist, 1978) studied learning in visual discrimination (black versus white, and various complex patterns of vertical and horizontal striping). His results parallel the other studies --echidnas learned quickly, at typical mammalian rates. But he added an interesting twist that confutes the only serious, direct argument ever offered from brain anatomy for monotreme inferiority--the claim that lack of a corpus callosum precludes transfer between the cerebral hemispheres, thereby compromising "higher' mental functions.

Gates occluded one eye and taught echidnas to distinguish black from white panels with the other eye. They reached "criterion performance' in an average of 100 trials. He then uncovered the occluded eye, bandaged the one that had overseen the initial learning, and did the experiment again. If no information passed from one cerebral hemisphere to the other, previous learning on one side of the brain should offer no help to the other, and the 100-trial average should persist. But echidnas only needed 40 trials to reach criterion with the second eye.

Gates conjectures that information is either passing across one of the other two commissures in the absence of a corpus callosum, or via the few optic fibers that do not cross to the other side of the brain. (In vertebrate visual systems, inputs from the right eye go to the left hemisphere of the brain, left eye to the right hemisphere; thus, each eye "informs' the opposite hemisphere. But about 1 percent of optic fibers do not cross over, and therefore map to their own hemisphere. These few fibers may sneak a little learning to the hemisphere dependent upon the occluded eye.) In addition, direct evidence of electrical stimulation has shown that inputs to one hemisphere can elicit responses in corresponding parts of the other hemisphere --information clearly gets across in the absence of a corpus callosum.

The solution to the paradox of such adequate intelligence in such a primitive mammal is stunningly simple. The premise--the myth of primitivity itself-- is dead wrong. To say it one more, and one last, time: the reptilian features of monotremes only record their early branching from the ancestry of placental mammals --and time of branching is no measure of anatomical complexity or mental status.

Monotremes have evolved separately from placentals for a long time--more than enough for both groups to reach, by parallel evolution in independent lineages, advanced levels of mental functioning permitted by their basic, shared mammalian design. The primary evidence for parallel evolution has been staring us in the face for a century; it forms part of the standard literature on echidnas, well featured even in primary documents that uphold the myth of primitivity. We know that the echidna's brain attained its large size by an independent route. The platypus has a smooth (if bulbous) brain. The echidna evolved the complex ridges and folds of its cerebral surface as a special feature of its own lineage. These sulci and gyri cannot be identified (homologized) with the wellknown convolutions of placental brains. The echidna brain is so different, by virtue of a separate evolution to large size, that its convolutions have been named by Greek letters to avoid any misplaced comparison with the different ridges and folds of placental brains. And Grafton Elliot Smith, the man most puzzled by echidna brains, did the naming--apparently without realizing that the very need for such separate designations embodied the direct evidence that could refute the myth of primitivity.

In his eloquent plea for monotremes (1827), Geoffroy Saint-Hilaire wrote brilliantly about the subtle interplay of fact and theory in science. He recognized the power of theory to guide the discovery of fact and to set a context for fruitful interpretation. ("To limit our efforts to the simple practicalities of an ocular examination would be to condemn the activities of the mind.') But he also acknowledged the flip side of useful guidance, the extraordinary power of theory to restrict our vision, in particular to render "obvious' facts nearly invisible, by denying them a sensible context. ("At first useless, these facts had to remain unperceived until the moment when the needs and progress of science provoked us to discover them.') Or as Warner Oland, the Swedish pseudo-Oriental Charlie Chan, once said in one of his most delightfully anachronistic pseudo-Confucian sayings (Charlie Chan in Egypt, 1935): "Theory like mist on eye-glasses. Obscure facts.'

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