Synapsida

During the 20th century, when we were trying to group animals together into some kind of evolutionary tree, we often found that there were some that were sufficiently strange that they didn't quite fit. This could be at the level of families (e.g. the red panda) or even higher groupings (aardvarks), but often it was apparent what general kind of animal something was, just not exactly where it fit. Such was the case with the saiga (Saiga tatarica). 

Saigas once lived across the steppes of central Asia, in lands stretching from Moldova to China. Hunting, for both meat and horns, caused a dramatic decline in their numbers throughout the 20th century, accelerating after the collapse of the Soviet Union and leaving them virtually extinct by the dawn of the 21st. A mass outbreak of infectious haemorrhagic septicaemia in 2015 threatened to finish the job, but since then there has been a truly remarkable recovery, with what's thought to be an eleven-fold increase in their numbers between 2015 and 2022. This is so great, in fact, that the species was removed from the international endangered species list in April 2023.

Apterodon

In most parts of the world today, all large, carnivorous, land-dwelling mammals are members of the order called, appropriately enough, the Carnivora. This is the group to which cats, dogs, and bears belong, along with many other animals from weasels to seals. If we go back far enough in time, we find that this group originated on the northern continents and was unable to reach the then-isolated southern ones until much later. During the Oligocene, therefore, there were no carnivorans in Africa.

Which isn't to say that there were no carnivorous mammals at all; absent some non-mammalian predator to out-compete them, it's just too useful a niche for nothing to evolve to make use of it. But those mammals were not close relatives of the lions and hyenas that live there today, belonging to different branches of the mammalian family tree.

Around 5 million years ago, as North and South America finally moved close enough together for a land bridge to form, a species of cat took the opportunity to move south. It found the southern continent devoid of any similar mammalian predators and prospered, leaving descendants from the Caribbean coast to the Magellan Straits.

We do not have a name for this lost species, or know much about where it lived prior to its journey south. However, we can tell that it existed because all of the small cats of South America are missing a pair of chromosomes found in every other species across the world - including the jaguars and pumas with which they share their continent. As confirmed by more detailed genetic analysis, this means that they all shared a single common ancestor in which this oddity first arose.

While for humans, speech is our primary means of communication, for many mammals scent-marking is more important. This is not to say, however, that audible signals are irrelevant. This can be especially true for social mammals (such as ourselves), which often have a need for something more immediate and complex than the sort of long-lasting "keep away" or "I'm ready to mate" signals that scent marks can provide. Communication through sound can help keep a herd together, signal aggression, provide instant warnings, and so on. It's key to many primates, for instance, because most of them live up in the trees where it may be difficult to keep sight of all troop members when they are out foraging.

It's also important for many hoofed herd animals and, at least in the wild, few North American mammals are more sociable than the bison (Bison bison). While herds are no longer as vast as they were 200 years ago, recovery plans for the species are underway, and, in many cases, may rely on some degree of fencing or other containment at least for the time being. Understanding bison behaviour, including communication, could help with that, making it easier to assess how comfortable the animals are feeling - and, perhaps, the likelihood of a 750 kg (1,600 lb) bull deciding it's had quite enough of that fence and heading off somewhere it doesn't realise is less safe.

Kirk's dik-dik

The "antilopine" antelopes are, as their name might suggest, mostly typical examples of their kind. I suspect that when many people think of antelopes in general, gazelles and springbok are among the first to come to mind (although, to be fair, so might say, impalas). Traditionally, the antilopines have been divided into two main groups: the gazelle-like antelopes and the dwarf antelopes, with the latter surely being the less familiar to those of us living outside of Africa. Both groups have at least some species that don't fit the "typical" image of the subfamily. For the gazelle-like animals, that's probably the gerenuk and its relative, the dibatag.

Among the dwarf antelopes, we have the dik-diks.

While some researchers subdivide them further, four species of dik-dik are widely recognised, at least three of which are reasonably common within their respective homelands. This may partly be because they are too small to be worth hunting for meat - although, inevitably. this still happens from time to time.