This post is about guinea pigs. Partly. It’s also being written in the teeny box of the app because it’s kind of a spontaneous thang.
There seem to be two major reproductive strategies, although this is apparently disputed. I don’t know why that is, but my understanding is that at one extreme, organisms produce millions of offspring in one go and die immediately after. Almost all of those offspring die before being able to reproduce, there’s no parental care but a few do make it through and do the same thing. At the other, organisms produce very few young and devote a lot of time to parenting having done so, so that isn’t the end of their lives, and when this happens they may produce litters, or single young (on the whole – I think most animals who usually have a baby at a time also occasionally have twins or multiple births), but from the viewpoint of replacement animals who reproduce sexually must be able to produce at least two offspring given that they are not in some way invasive. In extreme cases, the reproductive period may even end long before the animal dies of old age, as happens in humans.
Okay, now you’re gonna get your guinea pigs.
Apparently nobody knows why they’re called guinea pigs because they have no association with either Guinea or New Guinea, and I wonder whether at some point they cost a guinea each or whether they’re like turkeys, considered to come from somewhere considered exotic by Western Europeans and therefore associated with an arbitrary distant land. The German name is Meerschweinchen, which translates as “little porpoise”, and I don’t know why they’re called that either. The other name, cavy, is from the Tupi saujá, which means “spiny rodent”, and which they definitely aren’t, and in fact even the name isn’t that close for no known reason (to me). I’m going to call them cavies because the name “Guinea pig” freaks me out a bit, since I don’t know where it comes from, although “cavy” is almost equally weird.
There are, as far as I know, three main suborders of rodent. There are the mouse-like muriomorphs, the squirrel-like sciuromorphs and the cavy-like caviomorphs. This has probably changed since I learned this division due to the revolution in taxonomy which resulted from advances in genetic sequencing, but it remains the case that in terms of number of species and population, rodents are the most successful order of mammals and if mammals survive the current mass extinction at all, they will, and will therefore could end up being the last mammals of all. The first mammals were not rodents at all but very rodent-like in form, because in a way the default body plan for mammals is to be rodent-like, as seen in many marsupials and also shrews, golden moles and others. Multituberculates, arguably the most successful mammals ever, were also rodent-like and it’s theorised that they partly became extinct because their litter sizes were smaller rodents’.
Cavies themselves were never wild as the species they currently are. They are closely related to a wild cavy found in the Andes and I presume they have rapidly evolved to become reproductively incompatible with them. They can reproduce at the age of five weeks and produce two to four precocial pups, but the gestation period is fairly long at around two months. I should probably explain “precocial” vs “altricial” at this point. Cavies clearly do practice parental care, and when an animal does this the evolutionary option exists for young to be born before they are anything like fully developed. These are known as “altricial”. Humans do this, and it’s also common among birds, but ratites such as ostriches and tinamous don’t do this so probably the first real birds didn’t. Egg-laying mammals, however, do, and so do most rodents. Guinea pigs are unusual, compared at least to muriomorphs, in that they produce “precocial” young who are already furry and a little more independent than, say, baby mice or hamsters. They cannot, however, churn out massive litters of children over and over again like muriomorphs, and probably for this reason whereas they do sometimes eat their pups, this is relatively rare. There is a stark disposability to many glires offspring. Okay, I’ll explain glires again too.
The glires are the superorder including lagomorphs (rabbits, hares and pikas), rodents and scandentia (treeshrews) and are close to primates. I have a thing about the insistence that lagomorphs are technically not rodents because there is no definition of what constitutes any clade other than species (a breeding population), so all those families, orders, classes and the like are individually defined but there is no criterion at all which determines which level any of those is at. Therefore, either get rid of the idea of rodents or plonk lagomorphs together with ’em. I have slightly more sympathy with the idea that tupaias (treeshrews) are separate from rodents.
But anyway, rabbits breed like rabbits, and consequently it makes biological sense for them to end up eating their young. Not all archontoglires (including primates) do this. To quote Willy Wonka, “But that is called cannibalism, children, and is in fact frowned upon in most societies.” Humans generally have a taboo about eating babies. Not so most other archontoglires.
Caviomorphs, who I understand we’re now supposed to call “hystricomorphs”, were originally from Afrika like a lot of other animals, and got to South America by floating across on vegetation back when the Atlantic was narrower, during the Eocene. At the time, they were the only placental mammals in South America other than bats and xenarthrans (sloths, anteaters and armadillos, who are the sister clade of all surviving placental mammals and differ from the rest of us in interesting ways, e.g. they tend to be bulletproof, have a lower metabolic rate and an unusually large number of ribs), so they were able to radiate into all sorts of forms which would have been unfeasible in the rest of the world, such as becoming capybaras, who are basically rodent hippos. The largest capybaras were the size of small cars, but those died out a long time ago, probably when the Isthmus of Panama formed (that’s a guess).
Like all placental mammals, cavies lack abdominal ribs, don’t lay eggs and suckle their young through nipples. In their case they only have a single pair of nipples and if they don’t become pregnant when they’re fairly young the pubis can fuse, making it impossible to give birth vaginally. This situation is actually quite similar to that in non-eutherian mammals generally, who have bones making it impossible for them to give birth to live babies. And so we ask ourselves, how did we get here? How did we get into a situation where humans give birth vaginally to mainly singleton altricial babies and suckle them from a pair of pectoral nipples? What does it mean about our society? I haven’t filled in all of the second bit yet.
Humans are anapsids. We are descended from animals in the late Carboniferous who arose more or less directly from amphibians, or rather a vague group of vertebrates who included examples ancestral to living forms who are now called amphibians. We are not synapsids – “reptiles” or birds. Anapsids have tended to go to considerable measures to regulate their temperature, for instance by having large fins on their backs to absorb the sun’s heat and radiate it back again by letting the wind blow past it. The descendants of these animals began to use chemical reactions which created more heat than they absorbed, even to the extent that they were sometimes working in an apparently pointless cycle, because it was able to raise their temperature above the ambient. It’s worth bearing in mind, incidentally, that over most of the history of the synapsids the struggle would often have been to keep cool rather than warm, because of the climate, so in a very real sense it’s the mammals and their ancestors who were cold-blooded and the “reptiles” who were warm-blooded.
There are various ways in which synapsids regulate their temperature, one of which is sweating. That sweat carries antibodies, which are made of protein. The small babies hatching out from the eggs with a need to generate their own heat or keep themselves cool would need to be curled around by parents. One of the distinctive things about synapsid spines is that they can roll up – they can bend backwards and forwards as well as sideways. Hence they can keep themselves warm and their young can be too, at a point where the small size of their bodies means they get hot or cold very easily.
Imagine, then, sweaty things down burrows in the Permian, at a time when practically all the land formed a single continent almost from pole to pole, three times the size of today’s Eurasia – Pangaea. Such a vast continent would be mainly desert simply because so much of it would be so far from the single ocean, Panthalassa. Deserts away from the poles are hot during the day and cold at night, due to lack of cloud cover. Thus these sweaty things down burrows would have to huddle very close, and in doing so the young would lick the sweat for salt and to educate their immature immune systems with antibodies against the infections their parents had previously acquired immunity to. Later on they’d derive protein and other nutrition from the perspiration as well, and so was born suckling. Duck-billed platypodes and echidnas still suckle by licking skin secretions from their mothers.
It’s easy to think of those last two mammals as primitive, but in fact monotremes, for such are they dubbed, have unusually large brains for their size. They are, however, unusual for the mammals living today and they and other mammals did split off very early. From today’s perspectives, it makes sense to look at monotremes as one group, and marsupials and placental mammals as another, as the last two share much more recent ancestors than they do with the first.
One of the unusual things about monotremes is that they have poisonous spurs on their feet – they are venomous mammals. There is a sense in which other mammals are venomous, because our saliva can infect and kill other animals we bite. This is actually quite like the venom of snakes and even more like that of Komodo dragons, all of whom tend to get their toxins from bacteria living in their mouths. Human bites are, after dogs and cats, the most common bites leading to medical emergencies, and are usually inflicted by children. One hand infection in three is caused by a human bite. Cases of limb amputation and necrotising fasciitis have been reported, and death from infections. Nonetheless, platypus venom is another matter and is unlike snake venom or the toxins produced by salivary bacteria. It derives from modified immune system genes and causes a drop in blood pressure with no necrotic effects. It also contains a right-handed amino acid and although females have the spurs when young, and echidnas also, they’re vestigial. The venom is also secreted in the tears. Since similar spurs have been found in Mesozoic mammals, it seems reasonable to assume that they too were venomous. It can kill smaller animals and cause pain for months in humans. Questions of gender role arise in my mind here. Like mammary glands, venom glands are related to the immune system but whereas milk is nurturing, venom is destructive and defensive. I’m imagining male animals going out and hunting or fighting over females using their venom. Some multituberculates had spurs, as did Zhangheotherium quinquecuspidens and Gobiconodon, and although so far it hasn’t been possible to conclude that these spurs are venomous (other mammals do sometimes have spurs today which are not), they are thought to share their origin with those of monotremes, and just as a small animal today such as a wasp or a weaver fish might need to have venomous defences, at a time when mammals tended to be underdogs it’s easy to imagine that they might too.
Looking at their genomes, the common ancestors of monotremes and therian mammals seem to have lived around 210 million years ago, meaning that the early fossils of mammals such as Morganucodon and Megazostrodon are about twenty million years younger. Prior to that, synapsids were non-mammalian. Growth rings in the teeth of the earliest fossil mammals reveal another surprise: they probably weren’t “warm-blooded”. Today a mammal the size of Morganucodon could be expected to live a year or two, but they seem to have had a much longer life expectancy, of up to about fourteen years, which is similar to a living reptile of the same size. This is all the more surprising given that they were already producing sweat and apparently regulating their temperature that way, so maybe some mammals actually lost their ability to generate their own heat internally. This of course contradicts the “whig prehistory” assumption that everything is trying to turn into a human. In fact endothermy requires small animals to work like anything to get enough food. There are shrews who need to eat their own body weight in insects and other protostomes more than once a day. It’s a very efficient way to run a body, particularly if there are easy external sources of heat as there would’ve been in many parts of the planet in the late Triassic.
To monkey sensibilities such as our own, particularly in the richest parts of that same planet in the early twenty-first century of the Common Era, the disposability of muriomorph rodents seems most disturbing. A house mouse can be expected to live well under a year in the wild, although a genetically-modified mouse living in captivity could live up to five and a mouse whose genes have not been directly tinkered with could live up to four given a sufficiently friendly environment. They produce up to fourteen litters a year of up to ten young, although the average of both is much lower. This means a two year old house mouse could have produced as many as a hundred and forty children, and assuming half of those had only been reproducing for a year, that could mean almost ten thousand descendants. This usually means, of course, that most of them would’ve died by that point, which in turn means that life is cheap and they may well have died because their parents have eaten them. Although as humans we do find that harder to handle, even conditions for us used to be a lot harsher. I am one of seven children, three of whom died, and two being adopted in, and this is unusual for a mid-twentieth century British family but not so much a few generations ago or in another part of the world today, and this can lead to a certain lack of emotional engagement with the youngest children for one’s own emotional protection. Nonetheless the need for that emotional protection implies that children matter a lot to us. They also matter to other mammals and birds, and it’s important neither to anthropomorphise nor floccinaucinihilipilificate that in other species.
Another notable aspect of most mammals alive today is that we are born with at most only a thin set of membranes surrounding us and are in the case of placentals retained within our mothers’ bodies for a relatively long time. This is actually less than it might be for humans due to our proportionately large heads, necessitating fontanelles even then, which is one reason we’re altricial at birth. But the question arises of how most mammalian embryos started to be retained rather than being laid in eggs – viviparity.
Viviparity has evolved independently many times in vertebrates, such as in sharks, bony fish, amphibians and reptiles. Birds always lay eggs, I presume because of flight although it also applies to flightless birds. The closest to an exception are kiwis, who lay the largest eggs in proportion to their size and whose eggs hatch out very quickly afterwards. In the case of humans and other placental mammals, the origin of the placenta seems to be viral.
As viruses work by using their host cells to help them reproduce, they sometimes write their own genes into host DNA to do so. If this happens to a spermatozoön, ovum or zygote and the cell concerned survives, this will be present in the genome of most or all of the nucleated cells (or their mitochondria I imagine) of the organism concerned. Around eight percent of the human genome consists of viral genes, and in fact it’s theorised that ultimately the entire genome of most living organisms may originate from viruses, though it will clearly have evolved since. Leaving this possibility aside, most viral DNA in our genomes has stopped working completely, but the way placentae work is different and resembles an infection. The ball of cells all mammals start off as implants itself in the wall of an internal organ and bathes itself in the mother’s blood. This would normally provoke a successful immune response, but doesn’t because the fetal cells fuse, preventing white blood cells from getting a purchase on them. This is known as a syncytium, also found for example in respiratory syncytial virus infections, which protects dodgy cells from being attacked and enables them to produce more viruses. This is made possible by a viral protein called syncytin, previously used by viruses to bind with cells. Given that both marsupials and placental mammals have placentas, in the former case a rudimentary one formed from the yolk sac, and in some marsupials such as bandicoots even a more sophisticated placenta shortly before giving birth, this must have happened before they split from each other about 160 million years ago towards the end of the mid-Jurassic. Hence at some point, as if it wasn’t bad enough to scratch a living dodging the feet of thirty-ton Diplodoci, some ovum or zygote was infected with a virus. Instead of developing a shell and being pushed out, this egg started to invade the wall of the mother’s reproductive system and got stuck, only later being ejected. It probably happened to a lot of mammals, and a lot of them probably died in childbirth or underwent retention of dead fetuses which prevented them from reproducing, or maybe they just died straightforwardly of viral infections, but after this pandemic, a new kind of reproduction had begun to evolve and those mammals are our direct ancestors.
Bringing this back to the present, it’s interesting to note that we owe our existence as placental mammals who bond intergenerationally and invest our time in parenting to the extent we do, to a viral pandemic, and the hope remains that the current viral pandemic will lead to a similar leap in social evolution in the near future. A costly leap, but considering the price we are paying, we may as well get value for money out of it.