Of Pregnancy and Progesterone

Female mammals, like those of many other creatures, are not permanently fertile. Instead, they go through regular cycles, ovulating at intervals under the control of hormonal signals produced by the pituitary gland and the ovaries. In humans, this manifests as the menstrual cycle, but this is a relatively unusual feature of our species.

Whether or not other mammals menstruate may depend on your exact definition of the term. Chimpanzees certainly do (and, indeed, rarely experience menopause), and it's present to a variable extent in other apes and Old World monkeys. In New World monkeys it's microscopic and it's completely absent in lemurs. At least some bats menstruate, as do sengis (elephant shrews) and, so far as we know, just one species of rodent.

The Earliest Landfowl

Gallinuloides

This post will be the latest as of 1st April, so if you've been following this blog for a while, you'll know what that means... 

The humble chicken (Gallus domesticus) is a member of the pheasant family. This is a moderately-sized family, with around 180 species. Alongside the chicken and its wild ancestor, this also includes, not just pheasants, but many similar ground-dwelling birds, such as grouse, partridges, true quails, turkeys, and peacocks. 

It is, in turn, a part of a larger taxonomic group technically referred to as the Galliformes, or more commonly the "landfowl". The other four living families in this order have fewer species and are generally less well-known, but they share the same features of being generally plump, often quite large by avian standards, and having short, rounded wings unsuited for long-distance flight. 

Delphinids: Humpback Dolphins

Indo-Pacific humpback dolphin

The genus Stenella, to which many of the closest relatives of the well-known bottlenose and common dolphins belong is, genetically speaking, a mess. A combination of interbreeding and rapid speciation have made it very difficult to determine how its family tree should be constructed or if, indeed, there is even a clear pattern to find. 

Our best evidence suggests that it probably isn't a "real" group, in the sense of one consisting of a common ancestor and all of its descendants. That's because three other genera of dolphin appear to be descended from that common ancestor, forming part of the same cluster of what we might describe as "typical-looking" dolphins. The common and bottlenose dolphins form two of these interspersed groups, while the third is represented by the humpback dolphins.

As it turns out, their classification has also had to undergo significant revision in recent years, albeit for different reasons.

Oligocene (Pt 14): The Southern Horses That Weren't

Scarritia

30 million years ago, South America obviously lacked horses, since those (still three-toed at the time) originated in the north, and the southern continents had long been isolated from their homeland. What it did have, however, were a group of mammals called the notohippids, a name that literally translates as "southern horses". While they might not be closely related, the name might lead one to suspect that they had at least a resemblance to the modern animals.

They didn't.

Well, not much. When the name was originally coined, for the Miocene genus Notohippus, back in 1891, it was assumed that they really were horses, or at least closely related. This is because of the shape of their teeth which, did indeed resemble those of equines. It only took until 1914 to realise that, teeth aside, they weren't very horse-like. That their teeth were similar suggests a similar diet with plenty of tough vegetation, and their head was elongated in an almost horse-like fashion to accommodate them... but that's pretty much where the resemblance ends. For one thing, they had claws, not hooves; their bodies were also stockier, albeit with long limbs that may have given them a certain agility.

Gorilla Communities

It is, I would hope, well-known that the closest living genus to our own is that of the chimpanzees and bonobos. The second-closest is, of course, that of the gorillas. There are two species of gorilla, each with two subspecies, and both are considered "critically endangered" - literally on the verge of extinction. For one species, this is, as one would expect, largely due to their tiny surviving population and restricted range. 

There are many factors that we need to consider when attempting to reverse this, and some of them also have a bearing on the evolution of our own species. Among these is the question of how gorilla groups are socially constructed and how they interact. It turns out that here, we can't just consider "gorillas" en masse because the two species behave in very different ways. For example, while one species can have multiple silverback males in the same troop, this is rare (but not unheard of) in the other.

Delphinids: Spotted, Striped, and Spinning Dolphins

Pantropical spotted dolphin

The closest relatives of the common and bottlenose dolphins, which are perhaps the most familiar species to the western public, are those in the genus Stenella. Or at least, that might be true, because there has been a lot of confusion about the genus over the years.

It was first created in 1866 as a subgenus within Steno, the "narrow-beaked" dolphins, and contained just one species. It became a full genus in 1934,and by the end of the 20th century was agreed to contain five species - all of which had, in fact, been named before 1866. Since then, our understanding of genetics has greatly improved, and it has become clear that these various species cannot be so neatly arranged on a family tree as we might like.

Wolves, Foxes, and Food

Large scary predators are, by their nature, rare. There simply can't be too many of them, or they would run out of food to eat, something typified by the trophic pyramid often seen in biology textbooks: plants are eaten by herbivores are eaten by carnivores are eaten by bigger carnivores, with each step necessarily having a smaller total mass. The reality is, of course, somewhat more complex than this simple chain would suggest, most obviously because it ignores omnivores and decomposition.

But the basic idea holds, and apex predators - those that are large enough that nothing else normally eats them - exist in much smaller numbers than herbivores or smaller carnivores. This means that, relative to their numbers, they have a disproportionate effect on the ecosystem within which they live. Take away the apex predators and, even though there weren't very many of them to begin with, you will radically change the local ecology.

And, because they are relatively few in number, apex predators tend to be especially vulnerable to being wiped out. That's even assuming that humans don't focus on them deliberately out of fear, whether for their own lives or for the good of their livestock. Globally, apex predators are declining. (On this blog, we're mostly interested in mammals, but consider, for example, that at least eight of the 23 species of crocodile/alligator are currently thought to be endangered).

When Snow Leopards Reached Portugal

While the various species of "roaring cat" all share a great many points of similarity, most are easily distinguished from each other. Arguably, one of the most distinctive is the snow leopard (Panthera uncia). Not only does this have unusually thick fur for a wild cat, but it also has a longer tail than we would expect, and the face is shorter and wider than that of other roaring cats.

The snow leopard was first scientifically described by Johann Schreber in 1775 as a member of the genus Felis. The differences from other cats were sufficient that, in 1854, John Edward Gray proposed that it be given its own genus, Uncia. His original definition of the genus did not stand (it also included at least one species of "purring cat"), but it was resurrected again in the early 20th century, and used solely for snow leopards up until 2006. In that year genetic evidence placed it alongside the other "roaring cats" in Panthera, something that has been amply confirmed since.

Delphinids: Common and Bottlenose Dolphins

Common dolphin

When the genus Delphis was first named in 1758 at the dawn of taxonomy, it included three species of small to medium-sized toothed cetacean. By the time the dolphin family, Delphinidae, was named in 1821, one of those species (the porpoise) had been moved elsewhere, but five new ones had been added. Many more followed, but, from the mid-18th century onwards, naturalists began to notice subtle differences between animals that were probably more mysterious to them than land-dwelling mammals, and many species of dolphin began to be separated out into newly created genera.

So much so in fact, that by the time we reach the 21st century, only one species remains in the genus originally created to contain all dolphins and dolphin-like animals. That species is, of course, the one that we believe Linnaeus happened to be thinking of when he named the genus, and therefore is the defining (or 'type') species not only for its genus but for the dolphin family as a whole. This is the aptly named common dolphin (Delphinus delphis).

Call of the Mole Vole

There are over 170 recognised species of vole. The exact number is hard to determine because new species keep being described and not all will necessarily stand the test of time but we can certainly say that there are a lot. Although we might typically think of voles as being all rather similar to one another - small, mouse-like animals with short tails and rounded snouts - within such a large group there is inevitably some variation.

Most obviously different from regular voles are the giant species. Although genetic data shows us that these are voles, in the sense of being offshoots of the vole family tree rather than something distinct, we don't normally refer to them as such in English. Instead, we call them "lemmings" or, in the case of the very largest species, "muskrats". To avoid potential confusion, in more scientific language we would therefore use the taxonomic name when referring to the subfamily as a whole and say that voles, lemmings, and muskrats collectively are "arvicolines". 

Delphinid Dolphins

Of all mammal groups, those that have undergone the most dramatic change from their original form are perhaps the cetaceans. The most thoroughly adapted of all mammals to life in the water, there is little else they could possibly be mistaken for. The cetaceans have been considered a distinct order of mammals since the dawn of taxonomy in 1758, and today are divided into fourteen living families. The largest of these families, in terms of the number of species, is the dolphin family.

Technically known as the Delphinidae, this was named by John Edward Gray in 1821 as part of one of the earliest formal lists of mammal families. Gray's original definition basically included all toothed cetaceans other than narwhals and sperm whales, encompassing four genera, only one of which is still placed in the family today - Delphinus, from which it takes its name. He didn't list how many species he thought that genus contained, but it would certainly have included the two named by Linnaeus in 1758, and probably at least three others that had been described in the interim. Over the following decades, the number expanded considerably, with Gray himself identifying several of them, notably when he catalogued the observations and specimens collected by the Ross Expedition of 1839-43.

Oligocene (Pt 13): The First Porcupines

Prosotherium

The dawn of the Oligocene is marked, in part, by the formation of a deep water channel between South America and Antarctica. To what extent there had been a true land bridge between the two continents in the preceding epoch is debatable, but the separation was certainly more complete than it had been previously, beginning a long period of isolation that allowed unique animals to evolve and flourish. Furthermore, most of the large mammal groups we are familiar with today originated on the northern continents, so much of the fauna of South America was already strange to modern eyes.

 

Much of it, but not all. The exact date of the oldest rodent fossil from South America has been disputed, but recent analysis suggests that it may have lived shortly after the beginning of the Oligocene. How it reached the continent is also unclear. Logically, it must have made a long sea journey, presumably clinging to some storm-tossed piece of debris, either from North America or Africa (the Atlantic being narrower then than it is now). The latter seems the more likely, since South American rodents are more closely related to the African sort than to their equivalent northern counterparts.

The Struggles of a Pollinating Bat

Note the dusting of pollen...

When we think of flowers being pollinated, for most people, the first things that are likely to come to mind are insects, especially bees. But other creatures, including mammals, can also be a part of the pollination process, and this is particularly true of bats. Nor are we talking about some occasional event that merely gives the flower some sort of backup - for some the presence of pollinating bats is crucial to their survival.

Even aside from the obvious importance that this gives bats to the wider ecosystem, this can also have direct economic importance to we humans. For example, sour pitayas are an important cash crop in parts of Mexico. Similar to the much sweeter dragonfruit (although not closely related), they grow on a particular type of cactus that is native to the country but is also commonly cultivated. As it turns out, this cactus relies on bats for pollination. While they are not essential, crop yields drop by over a third when the bats are prevented from reaching them, which would clearly be devastating for a Mexican farmer who may be living on the edge of profitability to start with.

Miniature Mediterranean Mammoths

Insular dwarfism is a phenomenon that has occurred many times throughout evolution. What happens is that a population of some large animal becomes trapped on an island, out of contact with its mainland kin., Because the island has a limited size, it also has a limited amount of food on it, and this is a problem for a large animal that needs plenty to eat,

If the island is particularly small, of course, the animals in question are likely to die out, if not immediately, then dwindling over a few generations until they lack the genetic diversity to sustain themselves. On the other hand, if the island is a large one (such as, say, Britain) then there may not be a problem at all, and nothing happens beyond the usual genetic drift between isolated populations.