The argument for structural inferiority rests largely upon differing modes of reproduction in marsupials versus placentals, bolstered by the usual smug assumption that different from us is worse. Placentals, as we know and experience, develop as embryos in intimate connection with a mother’s body and blood supply. With some exceptions, they are born as reasonably complete and capable creatures. Marsupial fetuses never developed the essential trick that permits extensive development within a mother’s body. Our bodies have an uncanny ability to recognize and reject foreign tissues, an essential protection against disease, but a currently intractable barrier to medical procedures ranging from skin grafts to heart transplants. Despite all the homilies about mother love, and the presence of 50 percent maternal genes in offspring, an embryo is still foreign tissue. The maternal immune system must be masked to prevent rejection. Placental fetuses have “learned” to do this; marsupials have not.
Marsupial gestation is very short—twelve to thirteen days in the common oppossum, followed by sixty to seventy days of further development in the external pouch. Moreover, internal development does not proceed in intimate connection with the mother, but shielded from her. Two-thirds of gestation occurs within the “shell membrane,” a maternal organ that prevents the incursion of lymphocytes, the “soldiers” of the immune system. A few days of placental contact follow, usually via the yolk sac. During this time, the mother mobilizes her immune system, and the embryo is born (or, more accurately, expelled) soon after.
The marsupial neonate is a tiny creature, equivalent in development to a rather early placental embryo. Its head and forelimbs are precociously developed, but the hind limbs are often little more than undifferentiated buds. It must then undertake a hazardous journey, slowly pulling itself along through the relatively great distance to mother’s nipples and pouch (we can now understand the necessity of well-developed forelimbs). Our embryonic life within a placental womb sounds altogether easier and unconditionally better.
What challenge can then be offered to these biogeographical and structural accounts of marsupial inferiority? My colleague John A. W. Kirsch has recently marshaled the arguments. Citing work of P. Parker, Kirsch contends that marsupial reproduction follows a different adaptive mode, not an inferior path. True, marsupials never evolved a mechanism to turn off the maternal immune system and permit a completed development within the womb. But early birth may be an equally adaptive strategy. Maternal rejection need not represent a failure of design or lost evolutionary opportunity; it may reflect an ancient and perfectly adequate approach to the rigors of survival. Parker’s argument goes right back to Darwin’s central contention that individuals struggle to maximize their own reproductive success, that is, to increase the representation of their own genes in future generations. Several highly divergent, but equally successful, strategies can be followed in (unconscious) pursuit of this goal. Placentals invest a great deal of time and energy in offspring before their birth. This commitment does increase the chance of an offspring’s success, but the placental mother also takes a risk: if she should lose her litter, she has irrevocably expended a large portion of her life’s reproductive effort for no evolutionary gain. The marsupial mother pays a much higher toll in neonatal death, but her reproductive cost is small. Gestation has been very short and she may breed again in the same season. Moreover, the tiny neonate has not placed a great drain upon her energetic resources, and has subjected her to little danger in a quick and easy birth.
Turning to biogeography, Kirsch challenges the usual assumption that Australia and South America were refugia for inferior beasts that couldn’t hang on in the placental world of the Northern Hemisphere. He views their southern diversity as a reflection of success in their ancestral homeland, not as a feeble effort in peripheral territory. His argument relies upon M. A. Archer’s claim for close genealogical relationship between borhyaenids (South American marsupial carnivores) and thylacines (marsupial carnivores of the Australian region). Taxonomists have previously regarded these two groups as an example of evolutionary convergence—separate development of similar adaptations (as in the marsupial and placental saber-tooths, mentioned previously). In fact, taxonomists have viewed the Australian and South American radiation of marsupials as completely independent events, following the separate invasion of both continents by primitive marsupials pushed out from northern lands. But if borhyaenids and thylacines are closely related, then the southern continents must have exchanged some of their products, probably via Antarctica. (In our new geology of drifting continents, southern hemisphere lands were much closer together when mammals rose to prominence, following the dinosaurs’ demise.) A more parsimonious view imagines an Australian center of origin for marsupials and a dispersal to South America following the evolution of thylacinids, rather than two separate marsupial invasions of South America—borhyaenid ancestors from Australia, and all the others from North America. Although the simplest explanations are not always true in our wondrously complex world, Kirsch’s arguments do cast considerable doubt on the usual assumption that marsupial homelands are refugia, not centers of origin.
Yet I must confess that this structural and biogeographical defense of marsupials falters badly before one cardinal fact, prominently featured above: the Isthmus of Panama rose, placental carnivores invaded, marsupial carnivores quickly perished, and the placentals took over. Does this not speak for clear competitive superiority of North American placental carnivores? I could sneak around this unpleasant fact by ingenious conjecture, but I prefer to admit it. How then can I continue to defend marsupial equality?
Although the borhyaenids lost big, I find no scrap of evidence to attribute defeat to their status as marsupials. I prefer an ecological argument predicting hard times for any indigenous group of South American carnivores, marsupial or placental. The real victims happened to be marsupials, but this taxonomic fact may be incidental to a fate sealed for other reasons.
R. Bakker has been studying the history of mammalian carnivores throughout the Tertiary. Integrating some new ideas with conventional wisdom, he finds that the northern placental carnivores experienced two kinds of evolutionary “tests.” Twice, they suffered short periods of mass extinction, and new groups, perhaps with greater adaptive flexibility, took over. During times of continuity, high diversity of both predators and prey engendered intense competition and strong evolutionary trends for improvement in feeding (quick ingestion and efficient slicing) and locomotion (high acceleration in ambush predators, endurance in long-distance hunters). South American and Australian carnivores were tested in neither way. They suffered no mass extinctions, and the original incumbents persisted. Diversity never approached northern levels, and competition remained less intense. Bakker reports that their levels of morphological specialization for running and feeding lie far below those of northern carnivores living at the same time.
H. J. Jerison’s studies of brain size provide an impressive confirmation. On northern continents, placental predators and prey evolved successively larger brains throughout the Tertiary. In South America, both marsupial carnivores and their placental prey quickly plateaued at about 50 percent of brain weight for average modern mammals of the same body sizes. Anatomical status as marsupial or placental seems to make no difference; a relative history of evolutionary challenge may be crucial. If, by happenstance, northern carnivores had been marsupials and southern carnivores placentals, I suspect that the outcome of isthmian exchange would still have been a rout for South America. North American faunas were continually tested in the fiery furnaces of mass destruction and intense competition. The South American carnivores were never strongly challenged. When the Isthmus of Panama rose, they were weighed in the evolutionary balance for the first time. Like Daniel’s king, they were found wanting.
8 | Size and Time
29 | Our Allotted Lifetimes
J. P. MORGAN, MEETING with Henry Ford in E. L. Doctorow’s Ragtime, praises the assembly line as a faithful translation
of nature’s wisdom:
Has it occurred to you that your assembly line is not merely a stroke of industrial genius but a projection of organic truth? After all, the interchangeability of parts is a rule of nature…. All mammals reproduce in the same way and share the same designs of self-nourishment, with digestive and circulatory systems that are recognizably the same, and they enjoy the same senses…. Shared design is what allows taxonomists to classify mammals as mammals.
An imperious tycoon should not be met with equivocation; nonetheless, I can only reply “yes, and no” to Morgan’s pronouncement. Morgan was wrong if he thought that large mammals are geometric replicas of smaller relatives. Elephants have relatively smaller brains and thicker legs than mice, and these differences record a general rule of mammalian design, not the idiosyncracies of particular animals.
But Morgan was right in arguing that large animals are essentially similar to small members of their group. The similarity, however, does not reside in a constant shape. The basic laws of geometry dictate that animals must change their shape in order to work the same way at different sizes. Galileo himself established the classic example in 1638: the strength of an animal’s leg is a function of its cross-sectional area (length × length); the weight that legs must support varies as the animal’s volume (length × length × length). If mammals did not increase the relative thickness of their legs as they got larger, they would soon collapse (since body weight would increase so much faster than the supporting strength of limbs). To remain the same in function, animals must change their form.
The study of these changes in form is called “scaling theory.” Scaling theory has uncovered a striking regularity of changing shape over the 25-millionfold range of mammalian weight from shrew to blue whale. If we plot brain weight versus body weight for all mammals on the so-called mouse-to-elephant (or shrew-to-whale) curve, very few species deviate far from a single line expressing the general rule: brain weight increases only two-thirds as fast as body weight as we move from small to large mammals. (We share with bottle-nosed dolphins the honor of greatest upward deviance from the curve.)
We can often predict these regularities from the basic physics of objects. The heart, for example, is a pump. Since all mammalian hearts work in essentially the same way, small hearts must pump considerably faster than large ones (imagine how much faster you could work a finger-sized, toy bellows than the giant model that fuels a blacksmith’s forge or an old-fashioned organ). On the mouse-to-elephant curve for mammals, the length of a heartbeat increases between one-fourth and one-third as fast as body weight as we move from small to large mammals. The generality of this conclusion has recently been affirmed in an interesting study by J. E. Carrel and R. D. Heathcote on the scaling of heart rate in spiders. They used a cool laser beam to illuminate the hearts of resting spiders and drew a crab spider-to-tarantula curve for eighteen species spanning nearly a thousandfold range of body weight. Again, scaling is regular with heart rate, increasing four-tenths as fast as body weight (.409 times as fast, to be exact).
We may extend this conclusion for hearts to a general statement about the pace of life in small versus large animals. Small animals tick through life far more rapidly than large animals—their hearts work more quickly, they breathe more frequently, their pulse beats much faster. Most importantly, metabolic rate, the so-called fire of life, increases only three-fourths as fast as body weight in mammals. To keep themselves going, large mammals do not need to generate as much heat per unit of body weight as small animals. Tiny shrews move frenetically, eating nearly all their waking lives to keep their metabolic fire burning at the maximal rate among mammals; blue whales glide majestically, their hearts beating the slowest rhythm among active, warmblooded creatures.
The scaling of lifetime among mammals suggests an intriguing synthesis of these disparate data. We have all had enough experience with mammalian pets of various sizes to understand that small mammals tend to live for a shorter time than large ones. In fact, mammalian lifetime scales at about the same rate as heartbeat and breath time—between one-fourth and one-third as fast as body weight as we move from small to large animals. (Homo sapiens emerges from this analysis as a very peculiar animal. We live far longer than a mammal of our body size should. In essay 9, I argue that humans evolved by an evolutionary process called “neoteny”—the preservation in adults of shapes and growth rates that characterize juvenile stages of ancestral primates. I also believe that neoteny is responsible for our elevated longevity. Compared with other mammals, all stages of human life arrive “too late.” We are born as helpless embryos after a long gestation; we mature late after an extended childhood; we die, if fortune be kind, at ages otherwise reached by warmblooded animals only at the very largest sizes.)
Usually, we pity the pet mouse or gerbil that lived its full span of a year or two at most. How brief its life, while we endure for the better part of a century. As the main theme of this essay, I want to argue that such pity is misplaced (our personal grief, of course, is quite another matter; with this, science does not deal). Morgan was right in Ragtime—small and large mammals are essentially similar. Their lifetimes are scaled to their life’s pace, and all endure for approximately the same amount of biological time. Small mammals tick fast, burn rapidly, and live for a short time; large mammals live long at a stately pace. Measured by their own internal clocks, mammals of different sizes tend to live for the same amount of time.
We are prevented from grasping this important and comforting concept by a deeply ingrained habit of Western thought. We are trained from earliest memory to regard absolute Newtonian time as the single valid measuring stick in a rational and objective world. We impose our kitchen clock, ticking equably, upon all things. We marvel at the quickness of a mouse, express boredom at the torpor of a hippopotamus. Yet each is living at the appropriate pace of its own biological clock.
I do not wish to deny the importance of absolute, astronomical time to organisms (see essay 31). Animals must measure it to lead successful lives. Deer must know when to regrow their antlers, birds when to migrate. Animals track the day-night cycle with their circadian rhythms; jet lag is the price we pay for moving much faster than nature intended.
But absolute time is not the appropriate measuring stick for all biological phenomena. Consider the magnificent song of the humpback whale. E. O. Wilson has described the awesome effect of these vocalizations: “The notes are eerie yet beautiful to the human ear. Deep basso groans and almost inaudibly high soprano squeaks alternate with repetitive squeals that suddenly rise or fall in pitch.” We do not know the function of these songs. Perhaps they enable whales to find each other and to stay together during their annual transoceanic migrations. Perhaps they are the mating songs of courting males.
Each whale has its own characteristic song; the highly complex patterns are repeated over and over again with great faithfulness. No scientific fact that I have learned in the last decade struck me with more force than Roger S. Payne’s report that the length of some songs may extend for more than half an hour. I have never been able to memorize the five-minute first Kyrie of the B-minor Mass (and not for want of trying); how could a whale sing for thirty minutes and then repeat itself accurately? Of what possible use is a thirty-minute repeat cycle—far too long for a human to recognize; we would never grasp it as a single song (without Payne’s recording machinery and much study after the fact). But then I remembered the whale’s metabolic rate, the enormously slow pace of its life compared with ours. What do we know about a whale’s perception of thirty minutes? A humpback may scale the world to its own metabolic rate; its half-hour song may be our minute waltz. From any point of view, the song is spectacular; it is the most elaborate single display so far discovered in any animal. I merely urge the whale’s point of view as an appropriate perspective.
We can provide some numerical precision to support the claim that all mammals, on average, live for the same amount of biological time. In a method developed by W.
R. Stahl, B. Günther, and E. Guerra in the late 1950s and early 1960s, we search the mouse-to-elephant equations for biological properties that scale at the same rate against body weight. For example, Günther and Guerra give the following equations for mammalian breath time and heartbeat time versus body weight.
breath time = .0000470 body0.28
heartbeat time = .0000119 body0.28
(Nonmathematical readers need not be overwhelmed by the formalism. The equations simply state that both breath time and heartbeat time increase about .28 times as fast as body weight as we move from small to large mammals.) If we divide the two equations, body weight cancels out because it is raised to the same power in both.
This states that the ratio of breath time to heartbeat time is 4.0 in mammals of any body size. In other words, all mammals, whatever their size, breathe once for each four heartbeats. Small mammals breathe and beat their hearts faster than large mammals, but both breath and heart slow up at the same relative rate as mammals get larger.
Lifetime also scales at the same rate as body weight (.28 times as fast as we move from small to large mammals). This means that the ratio of both breath time and heartbeat time to lifetime is also constant over the entire range of mammalian size. When we perform a calculation similar to the one above, we find that all mammals, regardless of their size, tend to breathe about 200 million times during their lives (their hearts, therefore, beat about 800 million times). Small mammals breathe fast, but live for a short time. Measured by the internal clocks of their own hearts or the rhythm of their own breathing, all mammals live the same time. (Astute readers, after counting their breaths or taking their pulses, may have calculated that they should have died long ago. But Homo sapiens is a markedly deviant mammal in more ways than braininess alone. We live about three times as long as mammals of our body size “should,” but we breathe at the “right” rate and thus live to breathe about three times as often as an average mammal of our body size. I regard this excess of living as a happy consequence of neoteny.)
The Panda's Thumb: More Reflections in Natural History Page 27