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 Thrush-beaked Consort Bird

A modern thrush

This post will be the latest as of 1st April, so it's that time again...

About 40% of all living mammal species are rodents. Over a third of those that aren't are bats, leaving just 38% belonging to any other group - primates, cloven-hoofed mammals and all the rest. These are, at least in terms of speciation, clearly very successful body plans.

But the bias is even stronger with birds. A whopping 60% of living bird species belong to just one order: the Passeriformes. The chances are that, if you think of a "typical bird" the image that pops into your head is of a passerine. There are about 140 different families of passerine (the exact number being a matter of taste among ornithologists), of which 123 constitute the songbirds.

Viverrids: Long-tailed Palm Civets (and coffee...)

Common palm civet

While the palm civets do, arguably, form a single group of related animals, they are spread between two subfamilies that diverged at least 23 million years ago, at the dawn of the Miocene epoch. This is only about 5 million years after the split between the true civets and the genets, so it's fair to say that the two subfamilies are distinct. The subfamily that includes the animal originally known in the West as a "palm civet" is that of the paradoxurines.

The animal in question is now known simply as the common palm civet (Paradoxurus hermaphroditus). The alternative name of "Asian palm civet" is also often seen, although it has the disadvantage of not clearly distinguishing it from all the other palm civets that live in Asia. Which, since there are no palm civets outside of Asia, isn't really narrowing it down much.

The Patagonian Homunculus

In 1891, Argentinian naturalist Florentino Ameghino, the founding father of South American palaeontology, described a new species of fossil primate. Naming it Homunculus ("little man"), he recognised that it resembled a lemur almost as much as a monkey and must therefore be very primitive, but he was unable to categorise it further.

To be fair, all he had at the time was a section of the lower jaw. Over the next seven years, working with his brother Carlos, he uncovered a few further specimens. These included part of a skull and some limb bones, but the exact details of what they had discovered remained obscure, beyond the fact that it was a primate of some sort.

It didn't help that, after 1898, the next discovery of a fossil belonging to the genus didn't happen until the 1980s. And that was only a few isolated teeth. Since then, nothing until the current century.

Black Bears and the Uncertain Apex

The concept of the food pyramid is a central one in ecology. The idea is that since consumption cannot ever be 100% efficient, every type of animal must necessarily be less common than whatever it is that it feed on, at least in terms of its total biomass. Plants are more common than herbivores are more common than small carnivores are more common than larger carnivores.

The actual picture is more complicated than this. Many "carnivores" are at least partly omnivorous, and they often eat large herbivores more regularly than they eat small carnivores. Plus, we also need to consider the detritivores and parasites. But the general pattern holds, and at the top of the pyramid, we find the apex predators. 

It's possible to argue as to what exactly constitutes an apex predator. The general idea, however, is that they feed on other animals without being preyed upon themselves. At least among terrestrial mammals, an average body mass of more than about 15 kg (33 lbs) is generally about enough that predators need to manage their own population (through competition, territoriality, infanticide, etc.) rather than having to worry about something larger and scarier managing it for them. 

Viverrids: Half-Weasel Palm Civets

Banded palm civet

The word "civet", as currently used in English, is a rather broad term, referring to a wide range of vaguely similar-looking animals. Not all of these are even members of the "civet family" as we currently understand it, and even those that are don't form a natural biological group within that family. 

The word was originally Arabic (pronounced something like "zabad") and would have referred to the animals that medieval Arabs were familiar with, which, given how far they traded, would have included both African and South Asian species. These are still regarded as "true civets" today, but the word now also appears in the name of the "palm civets", long thought to be merely a variant of the true sort.

Beavers in the Wetlands

The North American beaver (Castor canadensis) is one of two living species of beaver, along with its Eurasian counterpart. They are, of course, rodents - they are related to gophers - and relatively large ones at that. They are found across all but the most treeless parts of the US and Canada, as well as the border regions of northern Mexico; they have even been introduced to Finland, Belgium, and Argentina. They are a relatively common species across much of this range, something that has been helped in recent decades by restrictions on hunting.

Nonetheless, while not endangered themselves, they can be key to maintaining ecosystems, not least because they are one of the few nonhuman species that substantially modifies the land around them. Their ability to alter wetland habitats by dam-building has been identified as a key factor in maintaining other species at greater risk, such as amphibians in the Rocky Mountains. On top of which, their habit of cutting down trees affects the composition of the forests in which they live.