PETER C. H. PRITCHARD
Florida Audubon Society
460 Hwy 436, #200, Casselberry, Florida 32707
Presented at the British Chelonia Group Symposium, May 1992 as The Oliphant Jackson Memorial Lecture
Of all the creatures that have lived on earth, surely few are more unlikely candidates for survival than tortoises. I use the word "unlikely" because the natural world has always been a competitive one, murderous on those who fail to meet its standards. It is difficult to conceive that a creature with no offensive capability, with locomotion so compromised by the massive body and the awkward stance of the limbs that any predator could capture it without even giving chase, and that offers such a predator a substantial quantity of excellent meat, could have survived so long, and be part of the moderm fauna of most continents and many islands. How did they do it?
The first thought I would offer is that the early chelonians, presumably terrestrial species in that a shelled, land-laid egg makes no sense otherwise, soon found themselves under strong selective pressure to invade the water. Their uniquely compact body form proved surprisingly pre-adapted to life in lakes, ponds, rivers and oceans. Swimming by serpentine undulation was obviously impossible for them, but it proved relatively easy for the limbs to become re-shaped as webbed feet or as flippers. Today the most aquatic of the chelonians - the softshells and the sea turtles - are superb swimmers indeed.
But some small percentage of chelonian species remained on land. In the past, various families of turtles included at least some members that showed terrestrial adaptation - the bizarre horned turtles of the family Meiolaniidae, for example; or some of the early African pelomedusids; or the Mongolian dermatemyid Zangerlia testudinimorpha. But, for the most part, we are talking about testudinids, the familiar, slow-moving, stump-footed, inoffensive tortoises, as the group of turtles that remained on the land even as their relatives swam away.
Before the arrival of man, development of elaborate body armour may have been the single most important component of testudinid survival strategy. Some tortoises developed extensive gular¹ projections, or reduced the shell openings to foil predators bent upon scooping out the meal within. Others developed movable sections of the shell, that could be pulled in like a drawbridge when danger struck- although the masters of this approach have been the box turtles, members of a separate family (the Emydidae) that only relatively recently invaded the land. Still others developed great size and massive body armor, literally outgrowing their predators. A few weeks ago, a friend showed me a huge nuchal bone, almost a foot wide and at least an inch and a half thick, that he had recovered from the bottom of the Withlacoochee River. This was a fragment of Florida's giant tortoise, Geochelone crassiscutata, and the whole plastron and much of the remainder of the skeleton of the four-foot animal is still on the river bottom. Such a creature would have done well in Florida before man arrived, but a specimen from Charlotte County, a huge shell of an animal clearly killed with a wooden spear and roasted intact over a fire, is poignant evidence of why giant tortoises are not found in Florida today.
Today, the forty species of tortoise represent the survivors, not only of the predatory forces that they have faced since the Eocene, but of the entirely new suite of forces- presented by human predation, to which the shell itself, no longer a lifesaver, now offers nothing more than a minor inconvenience. Moreover, even very small tortoises, that might offer only an ounce or two of meat and thus be hardly worth the hunt, may be valued by man for their beauty or curio value, whether it be for buchu-pouches for Kalahari bushmen or for pets for Western hobbyists.
The tortoises that have survived fall into certain categories. As every zoo-goer knows, giant tortoises are not extinct, they have simply retreated from their almost worldwide range to some of the most remote islands in the world, where traditional predators were absent, where populations could reach phenomenal density, and where defences could be relaxed in ways that would have been catastrophic elsewhere. For example, some populations - the saddleback tortoises of Rodrigues and some of the Galapagos Islands - increased the exposure of the shell openings fore and aft in the interest of greater agility and feeding range. Such a modification would be disastrous in, say, Africa or South America. where lions or jaguars would make short work- of such an exposed animal.
Other tortoises - the gophers, that still live within a few hundred yards of my house, or the pancake tortoise of East Africa - survived by building or finding specialized retreats. The gopher only emerges from its 20-foot burrow for a minimal percentage of its daily cycles, coming forth cautiously to feed in the early morning or late afternoon, or even more cautiously and rarelv to mate or lay eggs. but racing towards and down its burrow with astonishing speed when danger is sensed. And the pancake tortoise seems to be able to maintain its numbers despite very low reproductive potential and a shell reduced to insubstantial thickness by spending almost all of its time deep within cracks and crevices in ancient, granite outcrops or kopjes.
Other tortoises are masters of crypsis. I have spent many months of my life wandering through the forests in the Guianas, for example, yet very rarely have I seen a tortoise; and while the yellowfoot tortoise reaches a large size and is cherished as a food item wherever it occurs, villagers maintain that they know of no effective way to hunt them, and find them quite accidentally while in the course of other business. On the other hand, on a few islands of the Caribbean where redfoot tortoises are reasonably abundant, they may be hunted by local people with trained dogs. On the island of Barbuda, I was amazed at the skill of tortoise-dogs in locating redfoots that were so hidden that I could have looked right at them without seeing them.
Even tortoises that seem to have developed dramatic and conspicuous shell patterns may in fact be highly cryptic. For example, the radiating pattern of the so called star tortoises of Asia, Madagascar, and southern Africa, viewed against a plain background, would seem to make the animal extraordinarily obvious. But backgrounds in nature are not plain, and, when a star tortoise has retreated deep into a tussock of dry savanna grass, it becomes very hard to see. Other tortoises the padlopers of the genus Homopus - have become miniaturized and flattened, and live under rocks or in rocky crevices from which they may only emerge, the locals will tell you, when it thunders. The newly-reinstated Namibian species Homopus bergeri appears to have mastered this life style so thoroughly that not only has it been overlooked by tortoise specialists for eighty years, but it seems to be undergoing an evolutionary reduction of the shell bones comparable to that of the pancake tortoise itself.
Man is the key question in tortoise survival today - if they can keep out of his way, they may have a chance. Our own species is certainly not monolithic in its perception of tortoises, either as objects of utility or as sentient beings worthy of humanitarian consideration. In tropical South America, a tortoise seen is a tortoise doomed; it will be taken to village or market, probably trussed up so it cannot extend its head and limbs, and ultimately chopped open alive, with no more thought than if it were a log. On the other hand, I have talked with tribal people in remote parts of East Africa who disdained the very thought of eating a tortoise, and who suggested that anyone who would even contemplate such a thing must be from a very primitive background indeed.
Urban western man is often charmed by tortoises. taking them into captivity, and at times exuding love and anthropomorphism to a degree that only a trained psychoanalyst could fully interpret. Dressing them up with pink bows, kissing them, and claiming they are "happy" with their new indoor terrarium are clearly acts that please the keeper rather than the kept. Yet one feels also that the hard-line academic view, that regards tortoises entirely as objects, that may show reactions worthy of study but that are incapable of suffering or of fulfilment, and that may be killed (if non-endangered) to satisfy even the most trivial scientific whim, seems inappropriate also. We have, as biologists,been taught to eschew anthropomorphism, yet I believe a case can be made for some degree of philosophical pondering of what does go on within that little, scaled head, behind those enigmatic black eyes.
I had thoughts of this kind when I was on holiday in Maine a few weeks ago. I was picking blueberries in the middle of a extensive, slightly rolling, heath-like area. The berries, growing at ground-level, were very abundant, and I was sitting comfortably on the ground, picking those within reach. Eventually, I had reduced the population of berries within arm's reach to a detectable degree, and was presented with the question of whether to get up and move a yard or two, or whether to go on picking from the same location. The same question, I realized, is faced daily by a Galapagos giant tortoise in the lush uplands of Santa Cruz, reaching out with its long neck to eat the grass in front of it. Eventually, the grass becomes cropped to a degree, and the tortoise is faced with the same question. Shall I move or shall I go on feeding here?
It would seem that the penalty of making a wrong decision is small, and it may be. Ultimately, if no move is made at the "optimal" time, it progressively becomes clear that a move is necessary; the accessible grass is too reduced to be worth cropping any more. But a question that arises every day that an animal feeds may be worth answering accurately nonetheless. One could cover a blackboard with calculus to develop the appropriate equation for deciding on the right moment to make a move, from the point of view of optimizing energy input versus output. The tortoise does not do this - it has difficulty holding a piece of chalk - but somehow. within that little head, a more-or-less correct integration of all factors occurs, and an adequate decision is made.
Aldabra tortoises are faced with comparable, but more serious, decisions. On the eastern end of Aldabra, where most of the tortoises live, the ground cover - the food - has been reduced by tortoise grazing pressure to a degree that the tortoises now have to make a long trek from the few good shade trees in order to find adequate sustenance. They wake at dawn, and sally forth with some urgency to the somewhat distant grazing areas. There, they have to keep an eye on the rising sun, and they have to start their return journey to shade well before the sun gets so hot as to present a threat to survival. Those that miscalculate will quite possibly die and regular miscalculation is evidenced by the numbers of tortoise carapaces that litter the landscape. But it is quite an advanced concept for a reptile to stop feeding voluntarily in anticipation of a future problem. Something is going on in that little head.
The human analogy I would draw here is not one of picking blueberries. Rather, I would allude to my relatives from England and Ireland who occasionally visit me in Florida during the summer.They spend what I would consider an unconscionable amount of time sunning around the swimming pool - a practice I abandoned years ago. But, as with the Aldabra tortoises, they have to anticipate when to stop and seek shade; they have to retreat before they feel burned, or they will be in agony the next day. Their tender, reddened skin may not be as permanent a punishment as that which awaits a tardy Aldabra tortoise; but it is a real one, as their periodic miscalculation indicates.
So, if you will forgive me for asking a very unscientific question, what is it like to be a tortoise? For starters, we know that their low-slung body form gives them a singularly low viewpoint on the world, a viewpoint dominated by details of a few inches of ground and low vegetation that a taller animal would literally overlook. For some, too, the viewpoint may be further compromised by their viewing the world through a narrow tunnel, as is the case with Kinixys or Chersina; or the restrictions on view of the outside world may be purely geological, as with a pancake tortoise deep within a rocky crevice. Others may spend their life staring at their own gular scutes, a situation that reaches its apogee in a adult male Astrochelys yniphora, whose progressively extending and upcurving gular prong forces the animal to reach laterally to feed.
Tortoise life must be dominated too by the paucity of available responses when danger does threaten. Three centuries ago, Francois Leguat noted that vast herds of the now extinct Rodrigues tortoise seemed to include certain individuals with the responsibility of sentinels, but he wryly commented that it wasn't clear what predator they were looking for, or how the alarm would be sounded, or what effective response the herd might take. In continental areas, a tortoise is programmed to minimize its exposure to danger by hiding almost all of the time, so that it may survive unseen even when a predator approaches. It has few behavioural alternatives to lying low, just the ancient tortoise response of retraction of head and limbs and hoping for the best. This reaction still works in many cases against smaller nonhuman predators, but has severe limitations in the case of those that swallow tortoises whole. or smash them against rocks. And the very act of retraction, in the case of giant tortoises, may breach their concealment to a predator with good hearing, as gallons of air are loudly displaced from the lungs to make room for the long, folded neck and the limbs within the fixed volume of the shell.
The adjustments that tortoises make to life in captivity have resulted in many an observer, from Gilbert White onwards, concluding that they are creatures of remarkable sagacity. One writer 60 years ago even found that wood turtles could equal the expected accomplishment of a rat in a maze, although Archie Carr dismissed this conclusion with a phrase of characteristic elegance - "one must conclude that Tinklepaugh had known only feeble-minded rats." My own favourite example of this genre pertains to a young Aldabra tortoise that I have kept in my garden (or yard, as we say in America) for about nine years now. We have a warm climate, but for a few nights, or at most a few weeks, every winter we have to bring the big tortoises into the house when temperatures drop low. The house is also far from warm at such times, and the 10 tortoises are most comfortable when clustered around an electric heater with a fan that emits warmed air close to floor level. When the heater is off and the room cold, the Aldabra will move directly over to the appliance, and bang it with his carapace until it starts up. I do not know whether the tortoise is contemplating remedies for stuck thermostats or simply knows that this behavioural demonstration usually stimulates someone to turn the machine on; but to have learned that a certain piece of hardware across the room, presently emitting no heat whatsoever, may do so if appropriately manipulated suggests to me that tortoises have powers of learning and contemplation that we should not underestimate.
And there are times when even the most circumspect of creatures throw caution to the winds. When feeding is good - and particularly when their colour vision indicates that delicious yellow or orange blossoms are interspersed amongst the adequate but basically monotonous greenery - they will feed with an energy and a singleness of purpose that leaves little time for caution. And if a female with a well-turned supracaudal scute should pass by, the chase is on. When it comes to sex. tortoises are truly driven creatures. I have watched captive groups of redfoot tortoises where it seemed that no male could walk by a female without climbing aboard and taking care of its genetic posterity.
A component of successful plans for genetic posterity, of course, requires not only that females be found but also other males challenged. In the case of the South African bowsprit tortoise, the male is equipped with a crowbar on the front of the plastron that seems to offer little everyday utility except for attacking and inverting other males. This they do with great abandon. The prostrate male can sometimes get back over-, and I have heard of a case where a male actually returned to a defeated rival and seemed to right it deliberately. although I suspect that it was just going back for another bash rather than being kind.
And sometimes the turned tortoise will die. Bowsprit tortoises are very abundant, but the larger Madagascar plowshare tortoise, even better equipped with a gular crowbar, is on the verge of extinction. Wistfully, one might wish that, in such situations, tortoises could lay aside considerations of sexual selection and apply themselves to the problem of overall survival of their species. But this is beyond their capability, as it is beyond ours.
And when giant tortoises mate, far from feeling they have to hide or be cautious, the triumphant roar of the mounted male, audible from a great distance, is one of the memorable expressions of reptilian passion, and one of the oldest voices in the world. At this point, they are no longer "rocks, nor stones, nor worse than senseless things," but rather they take their place as animated beings, worthy of protection and respect not just because they are interesting or curious, but because they are sentient fellow travellers on Planet Earth, who came aboard a great deal earlier that we did.
¹Gular Scutes - the frontmost plates of the plastron or underside of shell, sometimes extended into projections.
Testudo Volume Three Number Four 1992