Delphinids: Dolphins in the Irrawaddy

Irrawaddy dolphin

While we normally think of dolphins as being sea-dwelling animals, there are no fewer than eight species referred to as such that are commonly found in rivers. Six of these, however, are not true members of the "dolphin family", or Delphinidae, their ancestors having split off from that group even before those of some of our modern whales did. Of the two exceptions, one is entirely freshwater, and lives in South America. The other is more varied in its habitat and lives in Asia.

The Irrawaddy dolphin (Orcaella brevirostris) was first described in 1866, from a specimen caught, not in a river, but off the northeast coast of India. We now know that this is at the far western edge of its range, and that it is also found all along the coast from northeast India, around the Malaysian Peninsula, to as far east as southern Vietnam. It is also found further south, around Borneo and along the north coasts of Sumatra and Java. In 1999, a very small population was discovered in the Philippines, living in a couple of isolated bays very far from the remainder of the animal's range, presumably the result of some having been swept away in a storm decades or even centuries before.

Oligocene (Pt 16): The First Monkeys in South America

A modern South American monkey

Much like the rodents, the presence of monkeys in South America has long been a puzzle. We know that monkeys evolved in Africa and that the monkeys still living in the Old World share a common ancestor distinct from, but related to, the common ancestor of the American sort. Genetic evidence shows that the split between the two lineages, which must have happened in Africa, happened a very long time ago. At some point then, early monkeys from what we now call the 'New World' group must have crossed the Atlantic, likely rafting on a floating patch of vegetation.

The Atlantic was narrower then than it was now, and ocean currents were different, but it's still a remarkable feat. It may also have been a lucky escape, since the African relatives of this first American migrant died out not long after, perhaps outcompeted by the ancestors of today's langurs, baboons, macaques, and apes. In South America, however, its descendants got almost free rein, diversifying into the five families we have today.

How to Drink Nectar

Orange nectar bat

When most people think of what bats eat, the first thing that likely comes to mind is insects. It probably doesn't make much further thought to remember that fruit bats also exist. And these are the two most common sources of food for bats, although 'fruit' in particular can cover quite a wide range of specific food types. But bats are the second largest order of mammals, after the rodents, and there is considerable variety amongst them.

This is particularly true of the leaf-nosed bats, or phyllostomids. While most formally recognised families of mammals have names almost everyone is familiar with - cats, bears, dolphins, horses, gibbons, etc. - and most of those that don't at least sound like they're actual names - binturongs, tuco-tucos, tenrecs, colugos - bat families tend to lack anything we could reasonably describe as a common name. Instead, we have bulldog-bats, and sucker-footed bats, and disc-winged bats, and so on. 

So it is with the leaf-nosed bats, which are the second-largest family of bats in terms of number of species, beaten only by the vesper bats. The family is usually divided into no fewer than eleven subfamilies, all of which have equally obscure-sounding names, and, in some cases, not even that much. It may not be obvious that, say, the spear-nosed bats are a subgroup of the leaf-nosed bats, but they are. And it's even less obvious that stenodermatines are phyllostomid, but kerivoulines are not.

Antlers and Ageing

Ageing is an inevitable fact of life. Without it, populations would rapidly expand to the point that insufficient resources existed to maintain them, unless we also do away with reproduction. And, if we do that, then the creature in question will never be very numerous, and will be wiped out by the first accident, natural disaster, or change in climate conditions to come along. This is something that has been the case since well before mammals existed, even if the nature and pace of ageing might be different for, say, an oak tree or a coral colony, or conceptually vague, as in a mycelial network.

When it comes to reproductive senescence, however, there is a difference in the way this affects male and female mammals. Females are born with a finite supply of eggs, although, in practice, this is far more than they will need, so they don't cease to be fertile simply because they run out. What actually triggers the menopause in humans is complex, even assuming no confounding health conditions, but the number of remaining egg follicles falling below a required level and thereby lowering the production of certain hormones is thought to be key.

Delphinids: The Freshwater Dolphins of Brazil

Tucuxi

In this series so far, I have generally been referring to the Delphinidae as the "dolphin family". That's a literal translation of the name and serves to distinguish it from, say, the porpoise family. However, as I mentioned in the first post, not all animals commonly referred to as "dolphins" belong in the family. Thus, when zoologists want to distinguish the family from those other animals, but want to avoid saying "delphinids", the more common term is "Oceanic dolphins". Oceans are, after all, where they are found.

With, it turns out, one exception.

The tucuxi (Sotalia fluviatilis) is unique among Oceanic dolphins is being an exclusively freshwater animal. It lives in the Amazon River and its major tributaries, mostly in Brazil, but also further upstream into Colombia, Peru, and Ecuador. Indeed, it was first formally described, by Paul Gervais in 1853, from an animal sighted in Peru, about 2,500 km (1,500 miles) from the mouth of the Amazon... and they are known to get further upriver than that, until they are stopped by features such as waterfalls.

Bast, Sekhmet, and the Egyptian Hyena-cats

Megistotherium, a Miocene hyena-cat

The majority of the land-based mammalian predators we are familiar with today belong to the order Carnivora. This is a diverse order, including such mammal families as the cats, bears, dogs, weasels, and seals. In fact, if we ignore the cetaceans and a few kinds of marsupial, they are the only large carnivorous mammals alive. But, as so often, this was not always the case, and they once shared the world with at least three other orders of predatory placental mammal (plus some marsupials far more fearsome than any Tasmanian devil). 

Two of these orders died out relatively early on, but one of them survived for much longer, producing multiple diverse species that lived across Eurasia, North America, and Africa. These were the hyaenodonts, named for Hyaenodon itself, first identified from a fossil all the way back in 1838. With so many species, they must have been successful in their day, but their numbers declined until the last two species died out in Africa and India around 9 million years ago, perhaps due to competition from the carnivorans, perhaps due to long-term climate change. Or, more likely, both.

I Ain't Afraid of No Owls

Northern bat

There is no particular reason why bats should be nocturnal. True, nocturnality has many advantages, but so does daytime activity; the real question is why so few bats fly during the day. I looked at this last year, where I mentioned that one of the main theories is that since birds evolved flight before bats did, the bats originally flew at night so that daytime predators, such as hawks and eagles, didn't try to eat them.

If so, it may be an effective strategy, since there isn't very much that eats bats on a regular basis. That isn't to say that there isn't anything, however. The bat hawk is, as its name implies, probably the single most specialised bat predator, but studies in Africa have shown that hobbies (which overwinter there), Wahlberg's eagles, and African goshawks also attack bats with some frequency, and they're probably not alone. In this part of the world, bats, as one might expect, take measures to reduce their risk of attack. Flying in large flocks may help, but it's also known, for example, that they avoid flying on moonlit nights, and, when they have to, they don't fly above the trees as they normally would, keeping themselves out of view.