brown antechinus aside, unless they die young, most female mammals can expect to give birth to several litters over their lifetime. This is a useful strategy, ensuring that you don't put all your eggs in one basket, so to speak. However, as human mothers may have noticed, looking after young can be a stressful and exhausting task, especially in species where the male can't be bothered to help out (which is most of them). As a result, in the wild, raising a litter of young is not always as successful as it might be, and, at times, the mother doesn't even survive the experience.
In general, it seems that mothers raising their first litter of young are the least likely to manage; those who have done it at least once before cope much better. That might not sound very surprising, but consider that there at least three different reasons why this might be the case. Perhaps the most obvious is that they just aren't very good at it the first time round. That may be partly due to a lack of experience, but raising young is fairly instinctive for most species, so it could also be that older mothers are larger, and better able to survive the experience of diverting some of their food resources to their offspring.
But another possibility is that the mothers might be deliberately not doing such a good job the first time round. This explanation, which I also discussed for the brown antechinus, is "terminal selection theory". The idea is that it's sometimes better for the mother to lose a litter and live to breed again than it is for her to die raising the first one. As she gets older, the less likely she will have another litter anyway, and the less she has to lose by putting all of her effort into raising the current one (assuming that is, that your primary objective is to have as many healthy young as possible, which in evolutionary terms, it usually is). So, if you don't completely wear yourself out on the first litter, maybe they'll survive anyway, but, even if they don't, at least you'll get to grow bigger and raise more young in the future.
The third possibility is that some mothers are better than others. If the 'bad' mothers are going to die trying to raise young, its far more likely to happen on their first attempt. If this theory is right, the only reason that older mothers do a better job of raising young is that they've proven they're good at it - all the bad mothers are already dead. They survive to raise more young, not because they were lucky, but because they have some advantage that continues to stand them in good stead as they age. Finally, of course, we should acknowledge that there's no reason why there can't be some truth in all three of these options, or that some might be more important for some species than others.
The northern elephant seal (Mirounga angustirostris) is one of the largest of all seals, with the adult males reaching over two tons in weight. They breed along the coast of California, spending the rest of the year offshore and travelling at least as far as southern Alaska. They belong to the true seal family, which is to say that they don't have visible ears or the ability to waddle about on land as fur seals and sea lions do; their hind legs are so well adapted to swimming that they can't be used for much else, and they have to drag their bodies with their front flippers when out of the water. Their closest relatives all live in Antarctic waters, from which the ancestors of the northern elephant seals probably headed north about three million years ago.
Leopard Seal Crabeater Northern Southern
etc. Seal Elephant Seal Elephant Seal
^ | | | Monk
| | (move north) | Seals
| | | | ^
--------------- ------------------ |
| | |
| | |
(migration to Antarctic) |
Interestingly, monk seals today are found in only two locations - the Mediterranean and Hawaii, which aren't exactly noted for being close to each other. Since seals as a whole seem to have originated in the North Atlantic, the Hawaiian ones must have travelled west by swimming in between North and South America, before they collided. Indeed, fossil monk seals have been discovered in the Caribbean, which would support this idea. For that matter, this is probably also the route that the Antarctic seals themselves took, much earlier, since we know of fossils of them from the Pacific, but not from the South Atlantic.
In a recently published study, Derek Lee of Dartmouth College in New Hampshire, looked backed over twenty years of records of northern elephant seals breeding at the Farallon Islands near San Francisco. There are a number of reasons why first-time elephant seal mothers might find it harder to raise their young than their more experienced relatives. Seals have to come ashore to give birth, but the rest of the time they are at sea. Since mating also occurs on land, it makes sense for it to happen at the same time, so they courtship takes place as soon as the previous litter are weaned, if not before. The rookeries where they breed become raucous and violent places with enormous males charging about all over the place, so the females can't afford to leave their young alone, which, since the males certainly aren't going to help, means they have to starve themselves for all the time that the pups are growing up.
This makes the life of a female elephant seal a pretty tough one, at least once they reach sexual maturity at three or four years of age. They spend eight months of the year pregnant, another two months raising a pup, and then two months stuffing themselves with food to make up for all the weight they have lost during their enforced crash diet. First time mothers, being smaller than their older relatives, would be expected to have a harder time surviving that catastrophic weight loss, and maybe also defending their patch of land and protecting their pup against aggression.
Its unsurprising, then, that first time mothers do not do as well than more experienced ones. They are less likely to raise their young to the point that they can be weaned and go off to live on their own, and also less likely to survive the experience long enough to return the following year. Not that this stops them trying; the study found that they were just as likely to mate after giving birth as experienced mothers.
Notice that, if this study is correct, it does rule out the possibility that this population of elephant seals, at least, are purposely expending less effort in looking after their first pup to increase their odds of raising a second or a third later on. If that were the case, while first time mothers would be less effective at raising young, they would be at least as likely to survive to breed again as experienced ones, and they weren't. So if they are trying to maximise their own survival at the expense of their pups, it clearly isn't working.
Its tougher to decide between the other two possibilities. Do experienced mothers do better and live longer because they got lucky the first time round, or because they have always been more skilled or fit than those which died? By examining the statistics, the study concluded that it was fitness as a mother, not sheer luck, that made most of the difference. Apparently, there is a Darwinian cull of unfit mothers, ensuring that only the best get to raise multiple pups in their lifetime.
The study doesn't end there, though, because it also looked at the effects of El Niño over the twenty year period. The main fish that northern elephant seals eat are Pacific hake, and, when El Niño brings its periodic warming of the eastern Pacific waters, the hake do less well, and the elephant seals suffer. Of course, hake aren't the only thing they eat; they are quite happy to consume squid, and even some small sharks and rays, but it is clear that the elephant seals do not put on so much weight in years when El Niño strikes.
We might expect that first-time mothers would do worst, especially if their smaller size is a handicap for them, but the study seemed to show that they did no better or worse than experienced mothers during warm weather. Which isn't to say that climate change, with increasingly warm waters and reduced numbers of tasty hake, doesn't affect elephant seals, just that it affects all of them equally, and that there are some things that being a good mother just can't help you with.
So, if you're an elephant seal, and you want to live long enough to have a second child, and more after that, you don't have to rely on luck, and you don't have to callously withhold all your energy from your first-born. Thanks to Darwin, you just have to be a good mother.
[Picture from Wikimedia Commons. Cladogram adapted from Fyler et al. 2005]
Sunday, 26 June 2011
Sunday, 19 June 2011
|Aelurodon, a borophagine dog|
But 65 million years is an incredibly long time. In comparison, mastodons and sabre-tooth cats both died out round about 10,000 years ago, not even 0.1% of that great timespan. Over the course of those millions of years, many groups of mammals (and, of course, of other animals, too) came and went, and the faunas and landscapes of the world changed time and time again. If sabre-tooth cats prowled the countryside just a few thousand years ago, you can see that you don't have to go back very far to find a world that looks, in many respects, quite different to that of today, and the further back you go, the greater the differences become.
For example, compare Africa and North America today. Africa has lions, elephants, leopards, dozens of different kinds of antelope, rhinos, hyenas, giraffes, hippos... the list of large and cool-looking animals just goes on. North America, while not truly impoverished, has far less in comparison. There's pronghorns, a few types of deer, three bears, wolves and pumas and, in terms of big animals, not a lot else. But it used to be like Africa just a few tens of thousand of years ago, never mind millions. America had elephants - or at least mastodons - along with many more large and fierce cats, and plenty of odd-looking antelope like creatures, and more besides. If you could go on a safari trip to the North America of millions of years ago, there would certainly be plenty to see.
This 65 million year span is conventionally divided into seven epochs. The last of these, in which we currently live, only starts with the end of the last Ice Age around 10,000 BC, and is therefore so short that you can't even see it on the chart I've used here. Most of the others are so long that they need to be divided into much shorter chunks for them to be of any real use when we try to get a picture of what appeared when.
The Miocene is the fourth of these epochs, and its name means "less recent" because there were less familiar looking animals alive then than in the three epochs that follow it. It is, in a sense, the last epoch before the sorts of animals we see today began to really gain a foothold. One illustration of the way that this happened is the story of the dogs, and a recent review of local fossils by Eric Ekdale and Timoty Rowe of the University of Oregon provides a snapshot that feeds into the bigger picture.
Oregon today has coyotes, wolves, and two species of fox - the red and grey. The wolves have only recently returned to the state, and are still struggling to gain a foothold, but the other three species are reasonably secure. Like all living dogs, all four of these animals are true "canines", in the true technical meaning of the word (which includes foxes, which, in more common parlance, we would call "vulpines"). But these are not the only sort of dogs that have ever lived.
Dogs Foxes Borophagines
^ ^ ^
| | |
| | | Hesperocyonines
------------- | ^
| | |
Canines | |
| | | Bears,
---------------------- | etc.
| | ^
| | |
(First dogs) |
The group of "true" dogs (technically "canins" - note the missing 'e') includes the grey fox, among others that we traditionally think of as foxes. The red fox of Europe and North America really is a fox, though!
As the chart above shows, the canines - modern dogs and foxes - lived alongside the borophagines ever since both groups diverged from the more primitive hesperocyonines at around the end of the Eocene. During the Miocene, however, the borophagines were by far the dominant group, with there being (so far as we know) relatively few species of true dogs or foxes around at that time. The fossils in the review date from around 10 million years ago, just as the balance was starting to switch, and the borophagines began to go into decline. The hesperocyonines had already gone by this point, apparently out-competed by their relatives during the previous Oligocene epoch.
I've mentioned before that many fossils are very incomplete, and give scientists very little to go on. This was the case with many of the fossils in this review, which included five teeth, one foot bone, and one leg bone, all from different animals. The researchers were fairly confident that these all belonged to borophagines of some sort, but had no way of saying any more than that. For example, one tooth was a premolar. It was the wrong shape to belong to a cat, too well-developed to belong to the only other sort of large carnivore around at the time, and too large to belong to any sort of fox. That only leaves the borophagines, but beyond that, who can say?
Another pair of teeth, however, both seemed to come from the same animal, and had been described as belonging to a fox (Vulpes sp) in a previous study back in the 1960s. The authors of the review thought that that was a bit over-optimistic, not least because that sort of fox isn't thought to have appeared for another three million years. They do agree, however, that it isn't a borophagine, and therefore is a true "canine" dog. Although there's not really any way to know, it might have belonged to Leptocyon, which lived across much of North America at this time. Leptocyon was smaller than a red fox, and slightly larger than a kit fox, and probably looked quite like both of these animals.
Its significant that these remains were the only ones that belonged to something resembling a modern canine. With so many borophagine specimens, its may be that this animal - whatever it was - was rarer than they were, confirming the story that it was the borophagines that were the top dogs of their day.
The other three fossils all belonged to different species of borophagine. That means that, 10 million years ago, like today, Oregon had at least four species of dog, although there's every chance that there were more we don't know about. The most complete fossil belonged to a coyote-sized animal called Carpocyon, that had previously not been found this far north and west. Unfortunately, it was missing the skull. Palaeontologists like skulls; they're usually the most distinctive bit of an animal, and in this case, it meant that we can't tell exactly what sort of Carpocyon it was - there are at least two possibilities.
It's the other two animals that really show how the dogs of modern Oregon are no match for those of ten million years ago, and they're also the only ones we can identify the exact species of. One, Epicyon saevus, was perhaps about the size of a rottweiller, but the other, its close relative, Epicyon haydeni, was a really, really, big dog. It was larger than a black bear, and had powerful, bone-crushing jaws that could presumably have demolished the entire carcass of anything it ate, skeleton and all. Unlike Africa, North America never had hyenas, but it had the great pack-hunting Epicyon haydeni instead, which lived in the same way and was considerably larger.
It must have been one of the most fearsome predators of its day, and it's hard to imagine anything would have chased it away from a kill, except possibly a sabre-tooth. Apart from the large size, it probably looked more like a wolf than a bear or hyena, but was surely a muscular and impressive animal. These are the animals that once terrorised the wilds of Oregon.
[Picture from Wikimedia Commons. Cladogram adapted from Wesley-Hunt & Flynn, 2005]
Saturday, 11 June 2011
Cetaceans, of course, don't even come ashore to do that (although they aren't quite alone in this respect, even among mammals). These animals are superbly adapted to an aquatic life, even more so than seals, and much, much more so than we decidedly non-aquatic humans. This means that studying cetaceans in their native habitat is a difficult, and rather specialised field - and, depending on what you want to know, studying them in the laboratory may not even be an option.
Sunday, 5 June 2011
|Palestinian mole rat|
The blind mole rat family (more properly called the Spalacidae) is usually said to represent the very first branch within the "muroid" rodents - the superfamily to which the familiar rats and mice belong. This is true enough, according to the most common modern classification scheme, but its worth remembering that quite where you draw the lines between different groups can be fairly arbitrary. It would be just as valid to define the muroids more widely, and say that the jerboas were the first branch, or more narrowly, and say that the blind mole rats are weird enough to have their own superfamily.
Indeed, there have been a number of debates about the origin of the group, and whether it's even real - rather than consisting of a bunch of unrelated animals that just happen to look similar because of their shared lifestyles. It now seems likely that they are a genuine family, and that they first split from their relatives around 28 million years ago, before the sudden and dramatic radiation of the many, many, forms of mice, rats, voles, hamsters, gerbils and so on.
Root Rats Bamboo Rats Blind Mole Zokors
^ ^ Rats ^ (All other
| | ^ | muroids)
| | | | ^
--------------- | | |
| | | |
| | ? |
Note: so far as I can tell, the exact position of the zokors within the group seems uncertain, and may never have been properly analysed.
As the above chart shows, the family contains more animals than just the blind mole rats themselves. All of the rodents in the group spend most of their lives underground, as burrowing animals feeding largely on plant roots and tubers. Compared with the African mole rats, they have been little studied, and rather less is known about them. Of them all, the blind mole rats are noticeably the best adapted to a fully underground life, while the bamboo rats seem to be the least.
There are around forty recognised species in the family, but, as so often with animals that live underground, its very likely that there are more that haven't been specifically identified yet. They are widespread, with the root rats (often also called 'mole rats', just to confuse issues) living in east Africa, the bamboo rats in southeast Asia, the zokors in China, Mongolia, and southern Siberia, and the blind mole rats in and around the Balkans and the Middle East.
They all tend to have relatively cylindrical bodies for moving through burrows, short limbs, large teeth, and small eyes and ears - although the zokors are unusual in digging with their feet, rather than their teeth. It is, however, the blind mole rats themselves that show the most dramatic adaptations. Like the African mole rats, their lips are behind their teeth, which therefore remain visible even when their mouth is closed - allowing them to gnaw their way through the soil without getting a mouthful of dirt. Apart from the large teeth, they do look quite like moles, although they can be much larger, with the biggest species being over a foot long.
Perhaps the best studied member of the group is the Palestinian mole rat (Spalax ehrenbergi), although this almost certainly consists of multiple separate species that have yet to be officially named because they all look more or less the same.
The blind mole rats are so named because their eyes are not visible; while African mole rats never open their eyelids, the eyes of the blind mole rats are entirely covered in hairy skin. The eyes beneath the skin are small and degenerate, and may be the most rudimentary of any known mammal. They have no true lens, and often no pupil, with an irregular pigmented mass filling the front of the eye. Nonetheless, the retina at the back of the eye does have a relatively normal structure and contains what appear to be abnormal rod cells, normally used by mammals to see during low light conditions. Furthermore, the eyes are fully connected to the optic nerve, and are clearly sending the brain signals about something.
In fact, there does seem some evidence that blind mole rats are able to detect light (they run away from it), although given their anatomy, its clear they can't do any more than that. Intriguingly, however, they appear to have co-opted structures normally involved with vision to the other functions. The odd retina, for example, has some similarities to the pineal gland of birds.
The pineal gland is a small organ, located well inside the skull of both birds and mammals, and is believed to represent a "third eye" once present in our evolutionary ancestors. (This third eye, incidentally was always very small compared to the other two, and is covered by skin even in the few reptiles that still have one - this is why you don't see pictures of three-eyed dinosaurs). The pineal gland's function, at least in part, is controlling our body clocks in response to external light, and is responsible for the jet lag we get when we get out of tune with the timing of sunrise and sunset. Thus, blind mole rats may use their eyes, if not to see in the conventional sense, at least to synchronise their body clocks with the day-night cycle, and perhaps to determine the time of year from the changing day length.
Interestingly, the visual cortex in the brain appears relatively normal, rather than being shrunken. However, it is not connected to the optic nerve, as it would be in other mammals, but instead, via other parts of the brain, to the auditory pathways. Although it's not entirely clear how, there is evidence that blind mole rats can communicate with one another by making vibrations in the ground, and its unsurprising that hearing would, in any case, be important to an animal that can't see. That the auditory pathways have co-opted parts of the brain used for vision in other mammals suggests that, in a sense, blind mole rats can 'see' using the vibrations of the earth around them, allowing them to navigate through their strange environment in complete blackness.
Another oddity of the blind mole rats is their remarkable tolerance for low oxygen levels. Especially in the rainy season, when the soil becomes clogged with water, their tunnels have relatively little free oxygen, down to less than a third what it would be in the open air, and the rodents seem to have evolved to cope with this. This is partly due to modified muscles, containing an unusual mix of fibre types, and reservoirs for oxygenated blood, but it is also due to changes at the biochemical level. This is one of those areas where figuring out how non-human mammals work can have direct relevance to our own species - if we can work out how blind mole rats pull off this trick, it might be helpful in combating human diseases where oxygen can't get to the tissues, or perhaps finding a way to stop cancer cells doing the same thing. (Tumours tend to choke off their own blood supply, and yet this doesn't always stop them growing).
Unlike the African mole rats, the blind mole rats do not live communally, with only one living in each tunnel system. These tunnels can reach hundreds of feet in length, as the animals burrow in search for food. They breed in the spring, and it seems they can't wait to get away from home, leaving only a few weeks after being born. Digging their tunnels can create numerous mole hills, and because they eat very little except underground roots, they can be a pest when they reach agricultural lands.
For the most part, the blind mole rats are not endangered, although, since we don't know much about them, that may be due to a lack of proper information in any cases. Indeed, since it seems probable that there are rather more species than we've noticed so far, it's entirely likely that some of them are threatened by human activity without us having realised. There are a few we know are in trouble, for example the giant blind mole rat (Spalax giganteus) is officially rated as vulnerable, and seems already to have been extirpated from Chechnya, with the civil war there probably not helping matters any.
[Picture from Wikimedia Commons. Cladogram adapted from Steppan et al, 2004]