Hedgehogs are not rodents!
Unlike another familiar quilled friend, the porcupine, hedgehogs are not rodents. A rodent is any mammal in the order Rodentia. They are characterized by their pair of continuously growing incisors, which has lend them their name: Rodentia comes from the Latin word rodere, “to gnaw”.
Rodents don’t have canines, and they have a large gap between their huge incisors and the premolars (most species at least). Those teeth were made for gnawing! And they have to gnaw, because their teeth never stop growing. In some species even the molars and premolars have to be worn down constantly by an appropriate fibrous diet or else they’ll become too long.
While rabbits and hares have similar continuously growing teeth (they have four incisors in the upper jaw instead of two), they’re not rodents but are members of a closely related sister group, the Lagomorpha.
Muskrat skull with a set of seriously impressive incisors. Go big or go home!
(Image source: landofstrange.com)
Most rodents are mainly herbivorous. Very few are exclusively herbivorous though, because they tend to eat the occasional eggs, meat, fish and/or insects as well. This means many rodents including popular pets such as rats and hamsters are actually omnivorous. However, for most of these rodents the majority of their diet still consists of plant matter and I prefer the name “herbivorous omnivore”.
Rodent dentition reflects this, as they do not have canines and the molars tend to be large and well-equipped for grinding fibrous plant matter.
Even less rodents are predators. One of them is the grasshopper mouse, an adorable animal which might be the honey badger of the rodent kingdom. It turns scorpion venom into a painkiller and howls like a wolf. A true badass!
But what about hedgehogs?
If we take a closer look at a hedgehog skull we see something entirely different. We see a mouth full of sharp teeth: large incisors facing forwards, small but pointy canines and (pre)molars. It’s clear these aren’t rodent teeth!
(Image source: viktorjezek.estranky.cz)
What stands out most (especially when viewed from the front) are those large incisors. While rodents have two enlarged incisors placed next to each other, hedgehogs have adorable fangs.
Hedgehogs: secretly vampires
Dentition, as we’ve seen in rodents, can tell us a lot about an animal’s dietary preferences. Hedgehog teeth are very typical for an insectivorous omnivore. Those vampire fangs are great for catching and holding onto squirmy invertebrates. The lower central incisors face forwards instead of upwards to pick up prey. They fit right between the gap of the top incisors.
The front teeth are followed by two more incisors in the top jaw and one more in the lower jaw. After these come the small canines and the premolars, which have sharp cusps made for crunching invertebrate prey.
The third upper premolar is relatively large and pointy, and paired with the first lower molar it mimics the carnassial tooth found in carnivores - a tooth which makes a shearing motion for faster and easier cutting (as opposed to tearing) through flesh. Ideal for eating large insects and the occasional reptile, bird or rodent!
The molars are broad and have relatively low cusps compared to stricter insectivores such as moles. This suggest a more varied, omnivorous diet than just invertebrates.
To put it in other words, hedgehogs are walking trash bins that will eat basically everything that moves. Or doesn’t move. Anymore. While they are primarily insectivorous they are considered opportunistic omnivores (I prefer the term “insectivorous omnivores”), which means they’ll eat whatever is available at that moment with a preference for insects and other invertebrates, carrion, eggs, small reptiles, rodents and birds, and a bit of plant matter (to a certain degree).
But teeth aren’t the only factor playing a part in animals’ dietary preferences. Another big difference between rodents and hedgehogs is the latter not possessing a caecum: the part of the intestine which helps digest cellulose (plant matter). Hedgehogs have an extremely simple gut system and do not digest plant matter well, even though they occasionally eat minor amounts of it. In fact they’re such walking trash bins most of the undigested plant matter found in hedgehog guts and faeces has likely been accidentally ingested.
Okay, so hedgehogs aren’t rodents and they are insectivorous omnivores. But where do they belong on the evolutionary tree?
I won’t go into too much detail or else this will get way too long (like it isn’t already I’m sorry, I like teeth). Just like their dietary preferences, their classification was somewhat of a mess. They were previously grouped in the now defunct wastebasket taxon Insectivora. The requirements to join this group of fun insect-crunching mammals were basically: are you a primitive placental mammal? Do you eat insects? You’re not a rodent? We have no idea what to do with you otherwise? Great, you may now join the Ancient Placental leftovers!
There are still vigorous debates between traditional palaeontologist and molecular phylogeneticists on how the grouping of modern placental mammals should be done, because (short version) there’s still a lot we don’t know but the general consensus, at least as far as the ol’ Insectivora goes, currently favours the molecular phylogeneticists.
The traditional palaeontologists placed the hedgehogs together with several other mammals such as shrews, tenrecs and (golden) moles in the order Insectivora, based on fossil records. Due to the very primitive features of the animals (both living and extinct) that got grouped in Insectivora it was viewed as an evolutionary grade, and early researchers assumed the Insectivora must contain the stock out of which all other placental mammals had evolved.
Cue molecular phylogenetics: by analysing molecular differences, mainly in DNA sequencing, researchers discovered that some of the animals grouped together in Insectivora aren’t even closely related to each other! This meant the entire order had to be split up and re-grouped. They were no longer viewed as the base of placental mammal evolution but got their own separate branches on the evolutionary tree instead. This is the current hypothesis on hedgehog and “Insectivora” evolution.
The family tree of placental mammals based on molecular phylogenetics looks like this. In red underlined are the members of the now defunct Insectivora; the golden moles, elephant shrews and tenrecs got regrouped in Afrotheria while the threeshrews and colugos were placed in Euarchonta.
In Laurasiatheria we find the remaining animals from Insectivora, which are now grouped in Eulipotyphla. The hedgehogs and their closest living relatives, the gymnures or moonrats stick together in their own little family Erinaceidae (formerly the order Erinaceomorpha, which is still the name on the image), and the moles, shrews and solenodons in three other families.
The order Rodentia is underlined in green. As you can see, while they shared a common ancestor millions of years ago rodents and hedgehogs aren’t closely related to each other.
(Image source: wikipedia)
TL;DR hedgehogs are not rodents + fun facts:
Hedgehogs don’t have continuously growing teeth
Hedgehogs do not chew or gnaw on things like a rodent does
Hedgehogs need animal protein to form the bulk of their diet
Hedgehogs need chitin-based fibre in their diet (chitin is found in invertebrate exoskeletons)
Hedgehogs cannot digest cellulose (plant matter) well
Hedgehogs have adorable vampire teeth
Hedgehogs’ closest living relatives are the gymnures aka moonrats (which, despite the confusing name, are not rodents either)
Hedgehogs are also related to shrews, moles and solenodons. None of which are rodents!
Eulipotyphla is the name of their order, which means “truly fat and blind”. These researchers don’t beat about the bush! Although I vote for “truly cute with bad eyesight”.
If you want to know if an animal is a rodent or not, look at its teeth. If it has a mouth full of tiny sharp teeth it definitely isn’t a rodent!
