Monday, November 04, 2013
The Problem with Pandas
by Paul Braterman
Same thumb, different family
Names can be deceptive. The red panda and the giant panda are not two different varieties of the same species; they are completely different species, and only distantly related. They do not even look very similar. The red panda is much smaller than the giant panda, coloured brown and cream, and has a long striped tail. The giant panda is, of course, black and white, with a very short tail, and black patches over its eyes. These patches help give it the cuddly appearance that makes it so popular in zoos.
Both animals are found in China, although the red panda spills over into Nepal and northern India; both are anatomically carnivores, but live on bamboo; and both have the same kind of false "thumb". This "thumb" is, really, nothing of the sort, but simply a modified wrist bone, while all five true digits are used in walking. The "thumb" is opposable, meaning that it can be moved to grip against the other digits, but has no joints or claw.
As early as 1825, Frédéric Cuvier (brother of the more famous Georges) described the red panda and proposed that was related to the racoon. The giant panda, however, did not become known in the West until considerably later. A French missionary in China described a skin in 1869; Teddy Roosevelt Jr and his brother Kermit, sons of President Teddy Roosevelt, saw a giant panda in China in the 1920s (true to family tradition, they promptly shot it); and it was not until 1936 that the first giant panda arrived in a Western zoo. Most zoologists considered it to be a kind of bear, on the basis of its anatomy, although a few thought that the two kinds of "panda" really were closely related. The matter was finally and conclusively resolved by comparing the DNA of both animals with that of other species. As expected, the giant panda belongs to the bear family, while it turns out that the red panda is in a genus all of its own, with skunks, raccoons and badgers as its closest relatives. But you do not find the false thumb in raccoons and skunks, and you do not find it in polar bears and grizzlies. So it is not a shared feature of this branch of the carnivore family tree, but a separate similar development in the two "pandas". A similar false thumb is also found in some species of mole. These are examples of what is called parallel evolution, in which the same modification arises independently in different species. To use technical language, the thumbs are analogous (similar, and performing the same function), but not homologous (not a feature inherited from a common ancestor).
The giant panda occupies a special place in US legal history. It was given a starring role in the 1989 "alternative textbook", Of Pandas and People, an attack on the science of evolution in general, and on common descent in particular. The book carries an attractive photo of a giant panda on its cover, and argues at great length that since the analogies between the "thumbs" of the red and giant panda are not due to common descent, therefore all anatomical arguments in favour of common descent should be regarded as suspect. One particular School Board, in Dover, Pennsylvania, voted to introduce this book into its high school, with a statement to be read to students. This statement described the book as a way of exploring "Intelligent Design", and declared that "Darwin's Theory … is not a fact. Gaps in the Theory exist for which there is no evidence… Intelligent Design is an explanation of the origin of life that differs from Darwin's view."
I should explain that "Intelligent Design" in this context means the view that biological complexity is due to the activity of an intelligent designer. It offers no suggestions as to how or why the designer operates, or how the design comes to be embodied, so I find it difficult to see how it is an explanation of anything. The reference to the origin of life is bizarre, given that Darwinexplicitly refused to discuss this topic. As for the expression "Darwin's Theory", and the creationist obsession with Darwin, I have already discussed these here and here.
The biology teachers, very courageously, refused to read out any such statement, because they said they could not tell their students what was simply not true, so the School Board told the Principal to read it to the class instead. A group of outraged parents promptly took the School Board to court, on the grounds that Intelligent Design is not a scientific theory but a religiously inspired doctrine. This would mean that propounding it in a publicly funded school is contrary to the establishment Clause of the First Amendment to the US Constitution. During the trial, the plaintiffs were able to demand drafts of the book as evidence, and discovered that it had its own evolutionary history. It was originally written in terms of "creation science" and "creationism", terms that for legal reasons were transformed by horizontal meme transfer into "intelligent design". One of the subpoenaed drafts even contained the expression "cIntelligent Designism". The Missing Link!
For this and many other reasons, the judge had no difficulty in concluding that "intelligent design" was nothing more than religiously motivated creationism. He set out his reasons in a detailed and powerful judgement, and while his decision is strictly applicable only to the States covered by that particular court, it seems most unlikely to be challenged unless the US Supreme Court falls into the hands of extreme religious conservatives.
For a work arguing in favour of Intelligent Design, the panda was a singularly unfortunate choice. The so-called puzzle of the two kinds of "panda" was solved over a century ago, and the results confirmed much more recently by modern molecular techniques. All the more reason to trust evolution science. The inflexible "thumb" is not what a designer would have chosen, but what the evolving animal was stuck with. All five of the panda's claws are used for walking or climbing, so it was not possible to coopt one of these to hold on to bamboo, even though the extra joints present inside a real thumb would have made it much better at this job. Finally, the panda's digestive system is very poorly adapted to its way of life, because it is descended from carnivores. An intelligent designer would have given the panda a digestive system rather like that of a cow. The panda is a wonderfully clear example of how an animal's form is constrained by its evolutionary history. In fact, the great essayist Stephen Jay Gould named one of his books in its honour way back in 1980, and www.pandasthumb.org, one of the world's top science blogs, follows his lead.
So what were the authors of Of Pandas and People thinking? They were neither stupid nor unqualified, and were well aware of, and indeed they themselves quote, the scientific literature that showed how the "problem" of panda species had long since been resolved. So how could they have imagined that they were putting forward a credible argument? There is a serious philosophical or psychological point here. It seems to me that for them and their supporters, the problem of the pandas really did illustrate an argument against evolution science. Some comparative anatomists had thought that the two kinds of panda were closely related. They were wrong. Therefore, the argument runs, comparative anatomy in the service of evolution is untrustworthy. Creationists think in absolutes. So they regard the awareness of fallibility, which lies at the very heart of the scientific method, as an admission of weakness. This could explain why creationism is so appealing to lawyers, who rely on cases being finally settled, and to conservative theologians, who regard their dogmas as established and revision as heresy.
Pandas have eating problems
It was the giant panda that graced the front cover of Pandas, that appears in the logo of the World Wildlife Fund, and that pulls in visitors to zoos. Panda diplomacy also played a role in the restoration of normal relations between China and the West, one of the major achievements of the much-maligned Nixon administration. The chubby-faced cuddly looking creature, with the big black patches round its eyes, has great emotional appeal. Unfortunately, its very survival in the wild is threatened, and attempts to maintain numbers in captivity have run into great difficulties.
Pandas[*] have painted themselves into a corner, adapting to a very narrow niche, for which their ancestry had not really equipped them. They live almost entirely on bamboo. Unfortunately, bamboo is of very low nutritional value to them. Pandas are, after all, members of the carnivore family, and have carnivore type digestive systems and enzymes, leaving them dependent on bacteria in their gut to break down their food. This is such an inefficient system that they need to eat around 20 kg a day, and in the wild just doing this can take them up to 14 hours. They need very powerful jaw muscles because of the chewing involved, and these, together with wide cheeks to accommodate large grinding molars, are what give the pandas their appealing round faces. We have fossil skulls and teeth showing that pandas had already adapted in this way 2 million years ago. But fossilised bones cannot tell us the history of those appealing black eye patches, which are probably favoured by evolution in the pandas' snowy habitat, because they reduce glare, like footballers' facepaint.
Pandas can only survive in areas where more than one species of bamboo is flourishing, so that when one species flowers and dies back, they do not lose their only food source. They extract so little nutrition from their food that they have little energy to spare, and find it a problem to make their way up steep slopes. This may not have mattered too much in their original habitat in the Chinese lowlands, but now that people have taken up all the land suitable for agriculture, they find themselves forced into shrinking and fragmented habitats in the mountains. Climate change is expected to make things even worse. There are probably fewer than 3000 surviving n the wild. So the number in captivity, which now exceeds 300, is a considerable fraction of the total population.
Sex problems too
In the wild, pandas are solitary animals, each one marking its own range by clawing and spraying urine, but they do gather together during the annual breeding season, when their endeavours are evidently successful, otherwise they would not be here. However, it has turned out to be enormously difficult to breed pandas in captivity, a feat not accomplished at all until 1963. What's the problem? Why not just put a male and a female panda together and let them get on with it?
Firstly, the female panda only comes into heat once a year, for about three days, during which she is only fertile for 12 to 24 hours (this fertile period can be detected by testing her urine). Then, most attempted romantic encounters proved very disappointing. The male (according to the BBC, no less) has a very short penis, so that accurate positioning is necessary, and they are not very good at doing this. Moreover, preliminaries in the wild involve fighting among males for the privilege of mating, and this spectacle seems to be an important part of what it takes to get the female interested. So despite measures ranging from sex education videos, to stimulating the males with sticks of bamboo carrying the female scent, to the use of Viagra, managed encounters in the zoo often end in disappointment or even violence. "Close, but no cigar" is how Edinburgh Zoo described the situation of their own two pandas, when in April 2012 Yang Guang (Sunshine) mounted female panda Tian Tian (Sweetie) several times, without full mating taking place.
The present successful breeding program in China has resorted to artificial insemination (don't ask!) However, that does not put an end to the problems. Pandas very often display pseudo-pregnancies, quite difficult to distinguish from the real thing, even by hormonal testing. Ultrasound can be helpful here, but requires considerable skill because of the smallness of the foetus, as well as the cooperation of the animal. (The mother weighs around 100 kg or more; the new-born offspring, a mere 100g.) In human and most animal pregnancy, we know when to start the countdown to birth, but not with pandas. These, like other bears, show delayed implantation of the foetus. So the time between fertilisation and birth can range from 11 weeks to 11 months. Pregnancy can end in miscarriage, as recently happened at Edinburgh Zoo. Finally, because of the small size of the foetus, and the common occurrence of pseudo-pregnancies, true pregnancy does not become obvious until shortly before birth.
Pandas usually give birth to two clubs at a time, but only make enough milk for one. So the expert panda breeders at the world's leading centre in Chengdu have resorted to trickery, caring for the abandoned cub in an incubator, swapping the two clubs around when their mother is not paying attention, and supplementing the mother's milk with imitation bear milk. Both cubs do, however, need their share of mother's attention. They have weak immune systems, so they rely on antibodies in their mother's milk. They also need help in defecating, which the mother supplies by stroking their lower abdomens with her tongue. However, this particular problem clearly resolves itself by adulthood, since a full-grown panda makes 40 panda-pats a day.
The panda breeding programme is big business, with the Chinese retaining ownership of the beasts, and renting them out to Western zoos for $1 million a year. This is over and above the cost of looking after them. Even so, major zoos consider this a worthwhile investment, because of added visitors and publicity. Centre-bred pandas are now also being reintroduced into the wild. Suitable habitat has been bought, and will no doubt in due course be a major attraction for eco-tourists. Keepers involved in the reintroduction have been dressing up as pandas, in the hope that their charges will feel more at home when released. However, critics of reintroduction point out that the entire exercise is meaningless unless these pandas are given enough protected habitat, and the first released panda died after only a few months in the wild.
And finally, the hard question
Biologists warn that we are on the threshold of a sixth great extinction, comparable to the demise of the dinosaurs. Unless we change our ways, or brute circumstance changes them for us, we could lose 75% of living mammals within the next 300 years. To say nothing of other life-forms, and with consequences for ourselves that we can only guess at. Amidst this mayhem, is the panda worth saving? If so, why? If not, why not?
[*] From here on, by "panda" I mean "giant panda".
Posted by Paul Braterman at 12:20 AM | Permalink