Category Archives: Animals

When did dogs figure out pointing?

In “Transposition,” a sermon delivered during World War II and published in 1949 in Transposition and Other Addresses, C. S. Lewis refers to dogs’ inability to understand pointing.

You will have noticed that dogs cannot understand pointing. You point to a bit of food on the floor; the dog, instead of looking at the floor, sniffs at your finger. A finger is a finger to him, and that is all.

If you’ve ever owned a dog, you will no doubt find this a rather extraordinary thing to say. Dogs obviously understand pointing, even without any training, and it’s quite common to train dogs to respond to pointing as a command (for example, pointing to a doorway to tell the dog to go into the room indicated). No dog I’ve ever met would waste time sniffing my finger when I’d just pointed out a bit of food it could eat. Cats, yes, but certainly not dogs.

However, Lewis had already had no fewer than six dogs by the time “Transposition” was published (details here), so it’s hard to dismiss what he says about them. This isn’t Pliny the Elder we’re dealing with, reporting hearsay about animals he’d had no personal contact with. Lewis knew dogs well and must surely have known from direct experience how they respond to pointing.

Is it possible that Lewis was right, and that dogs have changed in the half-century since he wrote?

We know that dogs’ ability to understand pointing is a relatively recent evolutionary development. According to dog expert Stanley Coren (as quoted in a 2009 Bloomberg article), domestic dogs understand pointing but their wild conspecifics do not.

“Suppose I point at something — the dog recognizes that I’m indicating something in that direction and looks,” Coren said, referring to a 2004 experiment carried out by Harvard anthropologist Brian Hare, which focused on the increase in dog IQ from domestication. “They do this even if they’re eight to ten weeks old, whereas a wolf, reared since puppyhood in a human environment, would look at my hand,” explained Coren.

Is it possible that the change Coren alludes to could have happened within living memory, sometime after the Second World War? It would be interesting to comb old books for references to dogs’ understanding or not understanding pointing and try to infer when the change took place.

I suppose it’s also possible that geography is a factor. Perhaps the North American dogs studied by Hare and Coren have abilities which English dogs do not. (Iain McGilchrist, a Scot, also refers to dogs’ ability to understand pointing, but he seems to be drawing on the same American research as Coren, not on his own experience.) Most of my own experience with dogs has been in America, but I often see stray Taiwan Tugous (a local breed far removed from anything in Europe or America) and should be able to test their responsiveness to pointing.

If you have any direct experience with dogs and pointing, or if you know of any references to it in books, please leave a comment.


Filed under Dogs, Evolution

The New World is an older country

All the biggest land animals — elephants, rhinos, hippos, giraffes — live in Africa. Asia also has elephants and rhinos, but they are both smaller and less abundant than their African cousins, and no other continent is even a contender in terms of megafauna.

Even if we define megafauna more liberally and look only at abundant species, Africa still wins hands-down. I made a list of wild land mammal species which weigh 100 pounds or more and which are listed as “least concern” on the IUCN Red List. Even though many of Africa’s trademark megabeasts (elephants, rhinos, hippos, gorillas, and lions, among others) are excluded by the latter criterion, Africa still comes out on top. A full 50% of the species on the list (37 out of 74) live only in sub-Saharan Africa. By comparison, Eurasia (including North Africa) has 20 abundant megamammals, the Americas have 17, and Australia has 5. (The total is only 74 because 5 of the species are common to Eurasia and America.) The four heaviest species on the list (giraffe, Cape buffalo, and two species of eland) are all African. If I relaxed the conservation criteria a little, Africa’s dominance would be even more overwhelming.

Africa has not always been so exceptional, though. Until quite recently (geologically speaking), the other continents had significant megafauna of their own. North America in particular was home to several species of elephant, ground sloth, horse, and big cat. In his hunting memoir Green Hills of Africa, Hemingway comments on this and offers an explanation:

Looking at the way the [elephant] tracks graded down through the pleasant forest I thought that we had the mammoths too, a long time ago, and when they travelled through the hills in southern Illinois they made these same tracks. It was just that we were an older country in America and the biggest game was gone.

Hemingway surprises us a little here, turning our familiar ideas upside down and calling the New World “an older country.” What he presumably means is that humans have been hunting there for a longer time than in Africa and have killed off most of the big game — which is the exact opposite of the truth. Our species evolved in Africa, and the Americas were the very last continents we colonized. So why, if Africa has been subject to human hunting for so much longer than America, does America seem to be “hunted out” while Africa still abounds in big game? Later in Green Hills Hemingway writes:

A continent ages quickly once we come. The natives live in harmony with it. But the foreigner destroys . . . . Our people went to America because that was the place to go then. It had been a good country and we had made a bloody mess of it and I would go, now, somewhere else and as we had always gone. You could always come back. Let the others come to America who did not know that they had come too late. Our people had seen it at its best and fought for it when it was well worth fighting for. Now I would go somewhere else. We always went in the old days and there were still good places to go.

Nothing surprising here, at least at first glance. “The natives live in harmony with it. But the foreigner destroys” sounds like the familiar cliché about how every country was an ecological paradise until the white man showed up and ruined everything — a cliché to which there is admittedly some truth, since whites did in fact decimate the megafauna of both America and Africa. Whites couldn’t have killed off America’s mammoths, though, or her ground sloths and horses and the other Pleistocene megafauna, all of which went extinct millennia before Columbus sailed the ocean blue. If humans are to blame for those extinctions (and circumstantial evidence certainly points to us), the culprits were the Clovis people — Paleo-Indians, the ancestors of “the natives.” They were still “foreigners” when they killed off the mammoths, though, having recently arrived from Siberia. When “natives” and “foreigners” are understood in a general sense, rather than as referring to specific ethnic groups, Hemingway’s point makes sense and is consistent with his earlier reference to America as “an older country” — that is, a country with a longer history of depredation by foreigners — than Africa.

In the big picture, Africa is the only land that can properly be said to have natives, the only place on earth where humans are not an invasive species. Humans and the African megafauna evolved in tandem, adapting to each other; the game animals evolved the instincts they needed to survive in an environment which included human predation, and the humans developed sustainable hunting practices.

In America, on the other hand, the Clovis suddenly showed up in a land whose wildlife had no evolutionary history of living with humans, and the results were disastrous. Over time, the Paleo-Indians learned to live in harmony with their new habitat and became naturalized “natives,” and the surviving megafauna presumably evolved as well, developing an instinctive fear of humans and other adaptations which would help them survive in the new (humanized) America. Millennia later, when the next wave of “foreigners” arrived in the New World, there was to some extent a repeat of the Clovis apocalypse, but on a much smaller scale. After all, the animals had already adapted to living under human predation in a general sense and only had to deal with the somewhat different behavior and technology of the Europeans. Though the Europeans greatly reduced the numbers of several species (bison, wolves, etc.), there were few all-out extinctions. In comparison to the totally foreign Clovis invaders, the Europeans were only somewhat foreign and therefore easier to adapt to — like adapting to a new strain of flu, as opposed to a completely novel pathogen.

It would be hard to overstate the difference between Africa and America in terms of big game. America (North and South) is over twice as large as Sub-Saharan Africa in terms of land area; it includes a wider variety of climates, which should be conducive to the evolution of more species, and it includes the colder climates which Africa lacks and which (based on Bergmann’s rule) should tend to produce physically larger species — and yet Africa has over twice as many abundant species of large mammal as America. Perhaps the reason lies in the fact that Africa, alone among continents, has never been invaded by a truly foreign human population like the Clovis in America, only by foreign strains of a locally evolved species.


Filed under Animals, Evolution

Bladder stones in sugar gliders: update

In a previous post I discussed the bladder stones three of my sugar gliders have suffered and said that the stones were calcium oxalate, which they certainly appeared to be based on the color, shape, etc. However, the vet had them analyzed in a lab and has just told me that they were actually calcium phosphate, meaning that they weren’t caused by spinach or any other high-oxalate food. The most likely culprit is now the calcium phosphate powder which we had been giving our mealworms as a dietary supplement (recommended by a different vet). Since mealworms are already quite high in phosphorus, this was probably a very bad idea. The vet also says that the problem may not be dietary at all, that the gliders may just not have been drinking enough water — but I doubt that, since one of the gliders who had a stone really drinks a lot of water.


Filed under Sugar Gliders

Bladder stones in sugar gliders

Two of our sugar gliders recently underwent surgery to remove calcium oxalate bladder stones which were obstructing their cloacae, and which would have been fatal had they not been removed. Some years previous, we lost a glider to what, though it was misdiagnosed at the time, was also (in hindsight) almost certainly cloacal obstruction caused by a bladder stone.

There is virtually nothing on the Internet about bladder stones in gliders, and even the very experienced exotic-animal vets I consulted had never encountered them before, so I’m posting what we’ve learned (such as it is), in the hope that it will prove useful to other sugar glider owners and perhaps even save a life or two.


  • General listlessness is the first symptom to appear: ears down, eyes dull, no interest in exercise. The glider will often avoid other colony members and sit by itself.
  • Loss of appetite, soon developing into a refusal to eat or drink anything at all
  • No urination or defecation, even when the cloaca is stimulated. The glider will make the panting/hissing sound which usually accompanies defecation, but nothing will come out.
  • Instead of normal excretion, a foul-smelling brownish or whitish liquid may leak out of the cloaca. The fur around the cloaca may be wet even though there is no urination.
  • You may (or may not) be able to feel a hard mass in the glider’s abdomen.
  • One of our gliders suffered cloacal prolapse. The other two did not.
  • A few days before the end, the glider who died began experiencing seizures and sudden attacks of rigor mortis-like stiffness. Once this starts happening, it’s probably too late.

Diagnosis and treatment

Gliders are too small for ultrasound, so the vet will have to x-ray it. Be sure he takes a side-view x-ray as well, since the tail bones may obstruct the view of a bladder stone lodged near the mouth of the cloaca.

For some reason, x-rays of sugar glider bladder stones look quite different from those of other animals such as rabbits and tortoises. A mass will be visible, but the fact that it is a bladder stone may not be obvious. When the vet operated on the first of our two recent cases, he was expecting to find an intestinal obstruction, which quickly leads to inoperable necrosis of the intestines. He told us that the chance of saving the glider was very very low, but fortunately we opted to go ahead with the surgery anyway. We sent him into the operating room fully expecting that we would never see him again, and when the vet came out with the announcement that it had actually been a bladder stone (much easier to operate on), it seemed like a miracle.

Surgery is, as far as I know, the only effective way to treat — and, in some cases, even to diagnose — a bladder stone in a sugar glider. It is a relatively low-risk procedure, and full recovery takes about a week.

Cause and prevention

Obviously, after having three gliders with bladder stones, we’ve been reconsidering the diet we’ve been feeding them. The stones are calcium oxalate, and gliders necessarily eat a lot of calcium (calcium deficiency can lead to paralysis and bone damage), so the most important thing is probably to limit their intake of oxalic acid (oxalate). This is the same advice given to humans who suffer from calcium oxalate kidney stones, and there are various “kidney stone diet” sites out there which give lists of high-oxalate foods. The main culprit in our case seems to have been spinach, which our gliders love but will not be allowed to eat anymore. (Low-oxalate green vegetables which can be used instead include lettuce and cucumber.) Other foods to avoid are potatoes, nuts and beans of all kinds, celery, and many kinds of berries. Of course, making sure they drink enough water is also important.

We’ve just started our gliders on a low-oxalate diet, so I can’t report on the results yet, but I certainly hope this will solve the problem and that no more operations will be necessary. I’ll be updating this post with any new information that comes my way, and I encourage comments from anyone else who has experience with this.


Filed under Sugar Gliders

Cold-blooded in cold water

I’ve recently watched several BBC clips about the marine iguanas of Galapagos, and all of them make a big deal about the danger the cold water poses to the cold-blooded iguanas. This clip goes into the most detail:

Down on the shoreline live the most extraordinary of the island’s many strange inhabitants: marine iguanas, the only saltwater lizards in the world.

They eat algae — seaweed — growing on rocks between the tides, so they have to wait for the water to go down before they can feed. They live only on shores exposed to cold currents. The arrival of the cold water is a double-edged sword. Its nutrients stimulate the growth of the algae they eat, but because the lizards are cold-blooded, cold water slows them down and could even kill them. The best algae grow lose to the low tide mark, so it’s a race to eat all they can before the rocks are covered again and their bodies are chilled to danger point. Strong claws and a good grip are essential if you’re not to be swept away.

For most iguanas, life is ruled by the tides, but the big males have another option. Below the low tide mark, the growths of algae are more luxurious because the rocks are always covered. The males use the heat of the tropical sun to exploit them. They expose the greatest possible surface to its warming rays. Because their bodies are larger, they can store more heat and don’t chill down so quickly. When they’ve warmed to an optimum of 25 [or 35? unclear pronunciation] degrees, they take to the water. Down here they can take advantage of a food supply that’s out of reach for the smaller iguanas. They can hold their breath for 20 minutes or more, but they have to feed fast. Every minute they spend here, the heat is draining from their body. If their temperature falls too low, they’ll be unable to move, and they’ll die. It’s time to go.

As the voiceover  goes on about how dangerous the cold seawater is to the iguanas (on account of they’re cold-blooded, you see), the camera shows plenty of equally cold-blooded fish swimming around without any problem. The obvious question this raises is never addressed. The program talks as if the iguanas’ way of life were almost unheard of — “Get this!” it seems to say, “a marine animal that’s cold-blooded!” — when in fact ectothermy is the norm for marine life, even in the coldest parts of the sea. (What do penguins eat? Fish, squid, and crustaceans.) Warm-bloodedness is a terrestrial trait which has never evolved in aquatic animals. The only endotherms in the sea are whales, seals, penguins, and other air-breathing animals whose warm blood is a legacy of their terrestrial ancestors. (That’s actually pretty strange, when you think about it, given that the sea — especially the deep sea — tends to be much colder than the land. My guess at an explanation would be that the ambient temperature doesn’t change as much in the sea as it does on land, so marine animals can just adapt to whatever the ambient temperature happens to be. On land, there are seasons and weather to cope with.)

So why is cold-bloodedness such a big problem for iguanas but not for fish? At first I thought it might have something to do with the antifreeze proteins some fish (but not reptiles) have — but antifreeze proteins are only for dealing with water so cold that it would otherwise literally freeze a fish’s blood, and, according to this map, the sea around the Galapagos, while certainly a bit chilly by equatorial standards, isn’t anywhere near cold enough for freezing to be an issue. According to the range map here, cold-blooded sea turtles seem to do just fine without antifreeze both in the Galapagos area and in waters a good 10 degrees cooler.

So far I haven’t figured this out — this post has been in my drafts folder for quite some time now waiting for me to find the answer — but I’m going to go ahead and post it in hopes that some knowledgeable person will happen upon it and leave an enlightening comment. I’ll keep reading and thinking and post again if I find anything that sheds any light. In the meantime, here are some more entertaining marine iguana clips.

With an annoying sea lion:

With lots of spitting:

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Filed under Iguanas, Oddities

Darwin vs. Jared Diamond, part 2

I’m still reading The Origin of Species and found another passage that made me think of Jared Diamond’s Guns, Germs, and Steel.

It is a remarkable fact, strongly insisted on by Hooker in regard to America, and by Alph. de Candolle in regard to Australia, that many more identical plants and allied forms have apparently migrated from the north to the south, than in a reversed direction. . . . I suspect that this preponderant migration from north to south is due to the greater extent of land in the north, and to northern forms having existed in their own homes in greater numbers, and having consequently been advanced through natural selection and competition to a higher stage of perfection or dominating power, than the southern forms. And thus, when they became commingled . . . the northern forms were enabled to beat the less powerful southern forms. Just in the same manner as we see at the present day, that very many European productions cover the ground in La Plata, and in a lesser degree in Australia, and have to a certain extent beaten the natives; whereas extremely few southern forms have become naturalised in any part of Europe. . . (The Origin of Species, pp. 370-71 in the Penguin Classics edition).

What Darwin observes in the plant kingdom — namely, that it is generally the northern (and specifically Eurasian) forms that have successfully invaded the south, rather than vice versa — has its parallel within the human species. It is disproportionately those races that developed on the great northern continent of Eurasia that have been successful in invading other continents and displacing other peoples, a phenomenon which Diamond attempts to explain in his book Guns, Germs, and Steel.

Diamond, an enlightened anti-racist, casts his explanation mostly in terms of culture and technology rather than biological evolution, invoking genes only to explain racial differences in resistance to particular diseases, and would of course never dream of using Darwin’s language about advancing “to a higher stage of perfection” — but for all that, his ultimate explanation is essentially the same as Darwin’s: that Eurasia is simply bigger. Diamond also notes that, in addition to being larger in absolute terms, Eurasia has the further advantage of being oriented east-to-west, which means that any given climatic zone on the Eurasian continent is likely to be wider than the corresponding zones on north-to-south continents such as Africa and the Americas. For Diamond, Eurasia’s size and orientation facilitates the wide dispersion of domestic animals and technological advances. Based on Darwin, we can add that a larger population and easy migration between regions would mean more mutations, more intense competition, and therefore accelerated evolution.

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Filed under Animals, Evolution, Race