Hedgehogs versus Killer Robots

Hearing is a key sense for just about any mammal species. While humans, and primates in general, place particular importance on vision, this isn't true for many other mammals. Granted, that's usually because of the significance of scent, which we humans are particularly bad at, but hearing can hardly be discounted, even for us. 

The range of hearing, however, varies dramatically between different species. For humans, the typical range is from about 20 to 20,000 Hertz, which covers about 10 octaves (middle C is 261 Hz) although the upper range drops off with age, and there is some leeway under perfect conditions. Many mammals can hear outside this range, with smaller animals, in particular, being able to hear higher notes. This can be of more than academic interest, since it means that the soundscape a particular animal lives in may not be the same as our own, and that can have a bearing on conservation - we might think somewhere is quiet and peaceful, but other species might not agree.

Most research on the effects of human-created noise on animals has been conducted on whales and birds, which are likely to be particularly affected. If we want to extend it to other animals, it could be useful to know just what their hearing range is, and one recent study asked this question specifically of hedgehogs (Erinaceus europaeus).

I have discussed hedgehogs before, but here's a quick summary. There are about 18 species of living hedgehog found across Eurasia and Africa, but here we are talking about the one simply called "the hedgehog" in British English, because it's the only one found in that country - and, indeed, across the rest of western Europe. They are not at all closely related to porcupines, which are rodents, with the spines being a case of parallel evolution. Instead, the hedgehog family (which also includes some spineless species that aren't normally called "hedgehogs") is most closely related to the shrews, partly explaining their similar diet of insects and worms.

The European hedgehog is not an endangered species, but its population has declined dramatically in recent years. In the UK, for example, numbers are thought to have fallen by about 20% between 2010 and 2020, and similar figures apply to other countries. Premature deaths are largely due to road traffic, but poison bait intended for rats can be another factor, as can robotic lawnmowers. 

It was the last of the above that motivated the recent hearing study. The researchers were not interested in the general effect of human-created noise on hedgehogs (that they try to cross roads suggests this may concern them less than it might) but on whether lawnmowers, and similar garden devices such as strimmers, could be made safer for hedgehogs. They reasoned that fitting the devices with ultrasonic emitters creating an annoying sound would warn the animals, allowing them to flee before being killed.

For this to be acceptable, the sound emitted has to be too high-pitched for humans to hear, or nobody would ever use them. Ideally, however, you don't want it to alarm cats or dogs either, and that puts more constraints on the frequency. A cat, for example, can hear sounds up to at least 65,000 Hz, roughly two octaves higher than the highest-pitched sound a human can detect. If hedgehogs can't beat that, ultrasonic screamers won't be of much use to families with pets.

The study was conducted on 20 injured, sick, or orphaned hedgehogs delivered to a wildlife rescue sanctuary in Denmark. After recovering from whatever their original complaint was, and having their ears examined to rule out hearing problems, they were sedated and fitted with electrodes under the skin to monitor the activity in their auditory nerves and brainstems. Afterwards, they got to relax on warmed heat pads (of the sort you might use for your cat) before being fed and eventually released into the wild.

 All of this is designed to minimise the stress and discomfort of the animals being studied. But it does obviously have limitations. Most notably, although it should show what sounds hedgehogs are capable of hearing, it doesn't demonstrate what they respond to. If, for example, a sound is relatively quiet, the animal might ignore it even if it can hear it; in birds, the difference between what they can hear and what they'll respond to is about 20 to 30 dB, which is quite significant. 

To test that, we would need to keep the hedgehogs in captivity for much longer, but they become particularly stressed if that happens. Which, even leaving aside any issue of unintended animal cruelty, could affect the results; a stressed animal will not respond in the way that a comfortable one will. So we have to do what's practical, with the hedgehog's interests in mind.

The result of this was that hedgehogs' peak hearing sensitivity was around 40 kHz. This is definitely ultrasonic - a full octave beyond even the highest pitch that most humans can hear. But it is similar in pitch to a dog whistle and so clearly audible to them (regardless of breed and size, if you're wondering) and even more so to cats. Still, while this demonstrates that hedgehogs can hear ultrasound, which we already knew, it's merely the peak brainstem response. That is to say, it's the pitch at which you can play the sound at the lowest volume and still have the animal able to hear something. Raise or lower the pitch, and you'll have to up the volume before the hedgehog will hear you, but it still might.

There has to be some limit, some highest pitch the hedgehog can hear if you blast out the sound loud enough. The study was not able to find out what that was, but the good news is that the highest tone they could produce still registered in the hedgehog's auditory nerves. This was 85 kHz, roughly three notes on the scale above the highest tone a cat can hear. The sound had to be quite loud at that point, so the ceiling probably doesn't go much higher, but the point is that it should be high enough.

You should, in other words, be able to create a sound at, say, 70-75 kHz that will alarm a hedgehog and save it from a lawnmower, but not frighten a cat.

Furthermore, the researchers were also able to examine the ears of a hedgehog that had arrived at the sanctuary with unsurvivable injuries from a rat trap, and had had to be euthanised. This showed that the malleus bone of the middle ear was connected to the bony ring around the eardrum by a tough, fibrous structure. This would increase the overall stiffness of the chain of bones in the middle ear, and is associated with ultrasonic hearing in other mammals, such as rodents and bats. 

What we can't tell from this sort of study is why hedgehogs would want to hear ultrasound in the first place. They are solitary animals, so communicating with each other, while not impossible, isn't likely to be as important as it is in, say mice. Indeed, so far as we know, they don't make many sounds of any kind, although it's conceivable that we just haven't been listening at the right frequencies. 

Another possibility is that it enables them to hunt for insects that make sounds in this range. This has previously been suggested for four-toed hedgehogs (Atelerix albiventris), a species native to the savannah and open grasslands south of the Sahara. We don't know that it would also be true of the European sort, but it's plausible, and it doesn't rule out other uses for the same sense.

Either way, the research team now plans to see if they can use this information to develop ultrasonic repellents to fit on those killer robot lawnmowers.

[Photo by Michael Gäbler, from Wikimedia Commons.]

The Air Conditioning in Your Nose

The nasal cavities are not mere holes running through the head. Anatomically, at least, that's broadly true at either end - in the vestibule immediately behind the nostrils, and in the nasopharynx above the throat. But in between, in the area above the mouth and separated from it by the palate, the air instead must pass through defined channels. 

These channels are formed by the "conchae", projections from the outer side of each nasal cavity stretching almost to the inner surface, so that most of the air is forced through the narrow slots between them. These conchae are, in turn, formed by the turbinate bones, delicate, paper-thin, sheet-like structures rolled up like a scroll, and covered in the same sort of fleshy lining that we find in, say, the trachea (windpipe). 

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.

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.

Antelopine Antelopes: Antelopes with Trunks

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.

Moulting Marmots

Moulting is a feature of mammalian physiology that will be familiar to pet owners worldwide. While it's not present in all mammals, it is very widespread despite the fact that, when you think about it, it's obviously costly to the animal in question. Why shed and replace a large amount of hair in a short time when you could replace it bit by bit as humans do?

The fact that so many mammals, of widely different kinds, moult to at least some extent shows that it must be an evolutionary ancient phenomenon. In fact, it turns out that animals have probably been moulting since before they even had hair. We can tell this because it's not unique to mammals. For instance, birds moult their feathers, and the process is similar to hair moulting in mammals. More significantly, perhaps, moulting has the same underlying mechanisms as reptiles periodically shedding their skin and can be tied back to sloughing in fish and amphibians as well. 

Hair, Fur, Bristles, and Spines

Trinomys

Hair is one of the key defining features of mammals. Apart from cetaceans, such as dolphins, no mammals are entirely hairless, although, for example, hippos and some babirusa species come very close. The original purpose of hair was almost certainly to maintain body heat, something that matters to warm-blooded animals in a way that it doesn't to, say, reptiles. Since then, however, hair has adapted to fulfil many other functions as well.

To begin with, there is colouration, a function that hair needs to take over once it obscures the skin. The most obvious way that hair colour can help an animal is camouflage. While this is instinctively true, there have been statistical studies that demonstrate certain hair colours are, indeed, more common in animals living in certain environments. For instance, multi-species analyses of lagomorphs and cloven-footed mammals have shown that animals with grey fur are more likely to live in rocky environments. The same studies, as well as similar ones on carnivores, show that pale fur is associated with open environments, especially deserts, while dark fur is most common in those animals living in jungles, dense forests, or heavily vegetated swamps. And, surely to the surprise of almost nobody, white fur is associated with the Arctic, or with the winter coats of those living where it snows.

The Stinky Family: Skunks

Unlike raccoons, there was relatively little confusion on the part of early zoologists as to what general sort of animal the skunks were. Since the animals are not native to Europe, the first Europeans knew of them was when they reached the Americas. The closest analogy they could think of among familiar animals was a polecat, and in some parts of America, they are still referred to as such today. However, an indigenous name for the animal (probably Algonquian) won out in English, and "skunks" they became.

Skunks did make it on to the first recognised list of scientific animal names in 1758, as Viverra putorius - the latter half of which, again, means "polecat". The first half indicates that Linnaeus, probably having only a minimal description to go on at the time, considered them to belong with the civets and mongooses, as a sort of generic small, bitey, mammal. That did not last; in 1795, an encyclopedia jointly published by  Cuvier and Geoffroy gave them their own genus. The name for this, Mephitis, comes from a goddess of noxious underground gases worshipped in pre-Roman and Roman Italy - the sort of thing that only tribes living near a volcano are likely to come up with. 

The Raccoon Family

It has not always been easy to determine where exactly raccoons fit within the larger mammalian family tree. In part, this is because it's difficult to pin down exactly what defines them and makes them distinctive from their closest relatives. We can get some idea of this by looking at the taxonomic history of the group.

The scientific classification of living organisms we use today has its origins in the 1758 edition of Systema Naturae, by Carl Linnaeus. Raccoons live only in the Americas, but even in 1758, Linnaeus was aware of the existence of raccoons, having heard about them from his friend Peter Kalm, who had observed them in what were then the colonies of Pennsylvania and New Jersey. Based on Kalm's description, Linnaeus named this new species Ursus lotor - the "washing bear". It was one of four species of "bear" that he identified in that work, only one of which we'd still consider to be such today.

Leaf-Eating Monkeys: The Colourful Doucs of Vietnam

Red-shanked douc

Modern genetic analysis has shown that the leaf-eating monkeys of the Old World consist of three, related, evolutionary branches. Two of these, the colobus monkeys of Africa, and the langurs of Asia, had long been recognised as distinct, but the third was not always so. The monkeys belonging to this branch had, for the most part, previously been considered to be langurs, or at least part of the langur line, but it turns out that they split off from the others early on. Because of the comparative recency of this discovery, this third group doesn't have any distinctive common name and nobody seems to have come up with a scientific one, either. ("Rhinopithecinini" would seem the obvious choice to me, but one can see why nobody bothered).

So, instead, this group, when scientists need to refer to it, ends up being called the "odd-nosed monkeys". Even then, not all of them have noses that look (at least to me) especially odd, and this is most clearly true of the doucs. 

Monkeys with Many Stomachs

Rhesus monkeys have entirely
normal stomachs

The word "monkey" entered the English language relatively late, in the 16th century. It's not at all clear where it came from, and it doesn't have a direct counterpart in most other European languages. Instead, they, like medieval English, used the same word for both monkeys and apes (and that word was, in our case, "ape"). From a modern, scientific, point of view, these other languages are correct, because there is no scientifically definable group of animals that fits with what most people mean by "monkey".

This is due to the rule, often mentioned on this blog, that the members of any scientifically defined group of animals have to be more closely related to all the other members of that group than they are to anything else. The diagram further down this post showing how all the major groups of simian are related illustrates why this is. At the top, we have the macaques and the leaf monkeys, both of which are indisputably monkeys. But their closest relatives are the apes (which, of course, includes gibbons as well as "great apes"). Only then do we get to the other monkeys, which means that, for example, a macaque is more closely related to a gorilla than it is to a marmoset.

Why Animals Have Whiskers

One of the key defining features of mammals is that they have hair, or are at least descended from other animals that once had hair. The primary purpose of hair is to keep the animal warm, something useful for any warm-blooded animal, especially if it's small (elephants and rhinos, for example, while they do have hair, aren't really what you'd call "furry"). But, over millions of years of evolution, hairs have also evolved to carry out other functions, such as the protective spines of hedgehogs and porcupines.

Another example of specialised hair is that of whiskers. Technically known as vibrissae, whiskers are remarkably common in mammals. When we think of whiskers, it's likely that most people's thoughts jump immediately to the long, mobile, whiskers of cats and mice. But whiskers can also be shorter and less mobile than this, as we see in such animals as horses. Indeed, even if we look at a cat, whiskers are not restricted to the long ones on the snout; they also have whiskers on the eyebrows, on the cheeks and chin, and on their forelegs just above the paws. 

The Deer Family

Doe (a deer, a female deer)

A little over eight years ago (blimey...) I began a series on members of the goat subfamily, and started it off with a discussion of the difference between horns and antlers. In short, horns are permanent structures, comprised of a central bony core surrounded by a sheath of um... horny material. Antlers, on the other hand, are regrown every year, and once they reach full size and shed the velvet on their outer surface, they consist solely of dead bone, with no sheath of any kind. Antlers are also often branched, whereas 'true' horns never are.

Antlers are, of course, the key defining feature of the deer family, the Cervidae. They are found on (almost) every species in the family, although (almost) only on the males. A large stag with branching antlers is instantly identifiable as a deer, but it may be fair to say that some of the species with unbranched antlers do have a certain resemblance to some of the smaller species of antelope.

Porpoisely Quiet?

Way back in the early 19th century, the naturalist Karl Ernst von Baer, better known for his work on embryology, made an interesting discovery that seems fairly obvious to us in hindsight: the blowhole of a porpoise is, in fact, its nostril. Other anatomists followed up on this, leading to the further discovery that a number of air sacs appear to be connected to the nasal cavity. They can hold a fair amount of air when fully inflated, but nothing like as much as the lungs, so exactly what they were for wasn't clear.

Fast forward to 1956, and scientific research confirmed another interesting fact about such animals: they can echolocate by sending out sonar pings. In 1968, the two facts were put together when it was demonstrated how the unusual structure of the porpoise's head allows it to transmit the necessary sound pulses, which are initially generated somewhere in the nasal passages.

Like a Hole in the Head: What's the Point of Sinuses?

We're probably all generally familiar with the existence of what are technically termed paranasal sinuses. That's largely because the inflammation of them, known as sinusitis, affects as many as 1 in 8 Americans, and presumably a similar number of people elsewhere, at least in the west. The term 'sinus' just means an empty space in something, and in the case of the paranasal sort, those spaces are inside the bones of our skull, connected by narrow passages to the nasal cavities.

The existence of these passages is important, ensuring that the sinuses are drained of mucus and filled with air... and also allowing any germs that you might have breathed to get into them and make what would otherwise only affect your nose into something worse. But why do we have sinuses at all?

The Tale of the Tail

The defining feature of vertebrates is that they possess a vertebral column, or "backbone", a series of interlocking skeletal structures that run down the centre of the back and serve to protect the main nerve cord. Unless you're a shark or something of that sort, these structures are made of bone and, at least once we ignore fish and amphibians, there's a reasonably consistent pattern as to how these individual vertebrae are structured.

One of the distinguishing features of the mammal skeleton, however, is that the backbone can be divided into five distinct regions, based on the function and detailed structure of the vertebrae within it. At the front end (or top, if you're bipedal) are the cervical vertebrae, which together form the neck. The first two of these are further specialised, the frontmost one because it has to connect with the skull, rather than to another vertebra in front of it, and the second because it's the pivot that allows the head to turn. Including those, there are almost always seven cervical vertebrae, even in giraffes... although sloths and manatees are exceptions, because of course they are.

Primate Penis Bones

At a certain level, the skeletons of all mammals follow a broadly similar pattern. Most of the bones that we see in the human skeleton are also found in the majority of mammals, and often in the same numbers. Famously, for example, a giraffe has exactly as many bones in the neck (seven) as humans do - they're just rather longer. Well, there's a reason giraffes can't bend their necks like swans.

Of course, when we get into detail, there are many exceptions to this. For instance, the default pattern for the paws of mammals is that they all have four digits with three bones each, and one with just two bones (the thumb and big toe in humans). But, obviously, this isn't true of all mammals. For instance, dogs have no big toe on their hind feet, and, while they do have a full set of ankle bones, the metatarsal that would normally connect to the big toe isn't there, either. There are rather more dramatic alterations in, say, horses and two-toed sloths, let alone dolphins.

The Cat Family: Felidae

It is sometimes said, perhaps only half-jokingly, that the growth of the internet has been largely driven by two things: pornography and pictures of cats. I can't do much about the first of those here, but cats are a different matter.

In fact, the reason that I haven't looked at cats so far in my annual surveys of mammal families is precisely because, if there's one group of mammals that has substantial coverage on the internet already, it's cats. Lions, leopards, and so on are also amongst the most popular of subjects for TV wildlife documentaries, and it's hard to see that I have much to add. But, nonetheless, and while I have, of course, covered cats before in individual posts, exploring the family is a gap that I eventually had to fill. So here we go.

Compared with some, more diverse, mammal families, members of the cat family, Felidae, are all readily identifiable as such. Cats have short faces, lacking the long snout of dogs, with large eyes and ears, and prominent whiskers. Their bodies are generally sleek, usually (although not always) with long tails, and they have muscular limbs and sharp claws. Indeed, apart from size and coat colour, most species of cat are remarkably similar in physical appearance. If you removed the skins of a lion and a tiger, only a real expert would have any chance of telling them apart.

The Porcupine Sleeps Tonight

The book All Yesterdays presents, among other things, unusual, but scientifically plausible, depictions of dinosaurs and other prehistoric animals. One picture, for example, shows a T. rex asleep - something that they must logically have spent quite a lot of time doing despite the fact that typical dinosaur books never show it.

Well, it's not as exciting as fighting a Triceratops.

In general, though, even when it comes to living animals, it's probably fair to say that behavioural scientists don't spend a lot of time thinking about how they sleep. All animals advanced to have a brain sleep in some fashion, so far as we know, and obviously, that includes mammals. To be sure, this isn't terribly obvious in the case of dolphins and the like, since they can be both asleep and awake at the same time, but everything that lives on land has to take a nap from time to time.

The Pig Family: Peccaries Are Not Pigs

Over the the year so far, this series has described every living species of pig. Yet there are some animals that might seem to be missing. Found across much of Latin America, and up into the southwestern USA, we can find animals known as peccaries. These certainly look like pigs, and, unlike razorbacks, which can also be found in the US, they are genuinely wild animals, not the descendants of domesticated ones that happen to have escaped.

The reason I haven't mentioned them so far is, as the title of the post makes apparent, that they aren't actually pigs. So why the heck not, and what are they, if they're not pigs?