Content warning: Contains information about reproductive organs and habits of mammals.

The typical mammals

As mammals, we (almost) all have some traits in common: seven cervical vertebrae in the neck, a relatively constant body temperature of 37°C/98°F, hair covering our bodies, three middle ear bones, a nice complex brain, and four limbs. Mammary glands produce the milk that our babies need and we almost all give birth to live young.

And then, there are the weird mammals

The odd animals that we have included on this list are here because they break the mammalian mold. Of course, we have to start with everyone’s favorite oddball:

1. Sloths

We really couldn’t start this list with anything else. First of all, our beloved sloths don’t have seven neck vertebrae. The two-fingered sloths (Choloepus spp.) have 5 – 6 (no one can really decide whether it is 5 OR 6. so we just say both) and the three-fingered sloths (Bradypus spp.) have 9! This extra-long neck allows three-fingered sloths to rotate their heads through 270° – a great way to reach those difficult leaves without having to move the whole body!

Sloths have a very low-calorie diet based on leaves (the equivalent of us eating lettuce all day), and it takes them an entire month to digest a single leaf. This is obviously not a very efficient way to acquire energy and explains why sloths are so slow. As a way to survive on so few calories, sloths have sacrificed the ability to thermoregulate and their body temperature can drop down to 28°C/84°F – not very mammal-like at all!

This is a great way to save energy, however, and as sloths are only found in hot, humid rainforests they really don’t need to worry about getting cold. Just like reptiles, they can bask under the tropical sun to heat up. Unfortunately, however, this means that they are not well equipped to deal with extreme weather events, and if it gets too cold, they can starve to death even with a full stomach!

As if things couldn’t get stranger, sloths also don’t store lots of milk like most other mammals. Storing milk takes a lot of energy, so instead a female sloth will produce it drop by drop throughout the day.  Since her baby will be snuggled on her chest for the first 12 months, they can simply suckle the milk as it is produced.  You can find more information and strange sloth facts by navigating the other pages on our website.

2. Blind, bald and almost immortal

The naked mole-rat (Heterocephalus glaber) is a weird mammal that develops a set of remarkable characteristics in order to live in the harsh conditions of an underground environment (think constant darkness and limited oxygen). They lack pain sensitivity in their skin, can tolerate oxygen deprivation and are very resistant to cancer.

They are also “cold-blooded” or “poikilothermic”, which means their body temperature fluctuates just like a reptile (and a sloth). The lifespan of these hairless creatures is 32 years, which is very unusual for rodents which typically only live for a couple of years. And as if all that isn’t enough, naked mole rats also seem to flout the ‘Gompertz law‘ – a mathematical equation that describes aging and mortality rates. In other words, the mortality rate is very low, even if they are very old.

The social structure of the naked mole-rat is also very rare in the mammalian world: they are the first eusocial mammals to be discovered. It means they live in colonies, just like termites, bees or ants, and each colony contains about 75 individuals. Only one female (the queen) and a couple of males reproduce: the rest of the colony are infertile “workers”. If the queen is removed or dies, another female will take over and become sexually active (after a big fight with other potential queens, of course).

3. Matriarchy and pseudo-penises

Female Spotted Hyena (Crocuta crocuta) is one of the seven species of mammal in which the group is lead by an alpha female, along with African elephants, lions, orcas, two species of lemur, and bonobos. But here we want to talk about the fake penis.

In female hyenas the clitoris has the position and shape of a penis – it is officially called a pseudo-penis and it can reach 7 inches long (almost as big as that of the male). It was originally thought that hyenas were hermaphrodites because of this unusual genitalia! Spotted hyenas are also the only female mammals that don’t possess a vaginal opening because the labia are fused to form a pseudo-scrotum as well.

Instead, the pseudo-penis has an urogenital canal alongside, and a female hyena will urinate, mate and even give birth through the pseudo penis (ouch)! Giving birth is clearly quite painful for the mother and often tragic for her offspring. Sadly, about 60 percent of the cubs suffocate on the way out.

Mating is not easy:  in an action similar to pushing up a shirtsleeve, the female retracts her reproductive tract on itself and creates an opening into which the male inserts his own genital apparatus. Males have to practice this activity in order to perform properly – rape is physically impossible in the hyena world.

4. A venomous primate

The Kayan Slow Loris (Nycticebus kayan) is pretty adorable and related to lemurs, but it is, in fact, the only venomous primate on the planet. They have glands near their elbows that produce oil, and when threatened, the Loris will lick this oil which mixes with their saliva to create a potent toxin.  The toxin won’t harm the Loris, but it does give them a secret weapon: a venomous bite. They might have a pretty face but don’t let their cuteness fool you – getting too close is dangerous (although you shouldn’t want to touch wild animals anyway)! Sadly these special little primates are heavily exploited for the pet trade thanks to some very unfortunate viral videos. We are sure that the owner of this famous video didn’t realize that their YouTube hit would drive these primates almost to extinction. To avoid the venomous bite, pet trade lorises (which have been ripped from the wild) then have their teeth pulled out prior to being sold.

5. Mating to death.

The Yellow-footed antechinus  (Antechinus flavipes) is a weird rodent-like marsupial from Australia. Once the male reaches sexual maturity at eleven months old, he will try to mate with as many females as possible for several weeks. He will mate with each female for up to 14 hours! Besides the starvation that occurs due to the frenzy of their sexual habits (they don’t have time to eat), males get so exhausted that their fur starts falling out, developing ulcerations and gangrene! But he won’t let this stop him, oh no. He continues on with his mating spree until he dies. 

6. Blue jewels

Blue pigments are quite rare in nature. In most cases, blue-colored animals, such as birds or butterflies, have microscopic structures in their feathers or scales that reflect the blue color of the light spectrum. It’s more like an optical illusion than an actual pigment. Can you think of a mammal that is blue? Hard to find one, right? Well, there are some exceptions, and the Vervet monkey (Chlorocebus pygerythrus) is one of them!  Males of this species are equipped with a rather alarming bright blue scrotum. Scientists are still trying to understand why they are so blue, and they have thankfully confirmed that it is not related to sexual frustration. Rather, it is more likely to do with sexual selection, with females preferring the males that boast larger and brighter “jewels”!

7. A list of weird mammals wouldn’t be complete without this one

Platypuses (Ornithorhynchus anatinus) are probably the strange mammal champions. They break almost all of the mammalian ‘rules’. In the modern era of fake news, they could probably pass as another internet hoax. In fact, the platypus was indeed considered to be a hoax animal originally: the first scientists to officially examine a platypus in the 18th century dismissed it as a fake, concluding that it was actually made of different animals sewn together. But the oddballs are real: they have the bill of a duck (that detects electrical signals of buried prey as a sixth sense), the tail of a beaver, the body of an otter, venomous spurs on the ankles in males and egg-laying females. Furthermore, the proteins in their milk might be helpful to develop new and powerful antibiotics. Females don’t have nipples, instead, they concentrate milk around the stomach and feed their young by sweating it out. And because the milk is exposed to the bacteria and germs of the environment, it is therefore highly resistant to them.

8. Immune to rabies and rattlesnake venom

Often confused with rats and the only marsupial of North America, the opossum  (Didelphis virginiana). While commonly misunderstood, they are in fact fascinating creatures. They boast two penises, two vaginas and two uteri! Marsupials don’t have a placenta, so these weird mammals give birth to undeveloped fetuses that crawl into a pouch on the mother’s underbelly and get attached to one of her nipples. Opossums are your friend – they eat thousands of ticks per week and consequently are a major controller of Lyme disease! When stressed or threatened, an opossum will “play dead” for hours while producing a smelly green mucus with a putrid odor that resembles a corpse in decomposition. Playing opossum every day keeps the predators away!

9. Cool cats

In comparison to the Margay (Leopardus weidii) most other wild cats may look pretty clumsy. Why? Because besides the clouded leopard, these are the only feline species that can climb down tree trunks headfirst. This is only possible because a margay can rotate its ankles through 180°. They can hang vertically upside down from their feet (just like a bat) and they move through the trees with the agility of a monkey. These arboreal predators have also been known to mimic the sounds of their prey to lure the victim into a false sense of security. If you aren’t impressed already, check out this video:

10. Artichokes with legs and sharp-scaled tails

All eight species of Pangolin (Manis spp., Phataginus pps, and Smutsia spp.) are covered with reptile-like scales instead of the typical mammalian hair. They look like little dragons, but rather than breathing fire they actually have no teeth and survive on a diet of ants and termites. When threatened, these scaly mammals will curl up into a tight ball and wait for the danger to pass.  All 8 species are now at risk of extinction as they are currently considered to be the most trafficked animal in the world. An IUCN report estimates that over a million pangolins have been illegally trafficked since the year 2000. Pseudo-therapies (without any scientific-based evidence) use pangolin scales (which are actually made of the same keratin like our fingernails) as “traditional medicine” and the meat is considered a luxury food in countries like Vietnam and China. A single plate of pangolin can cost hundreds of dollars.

Pangolins are secretive, and hard to study, just like sloths. As a highly endangered species, they also need our help. Check out savepangolins.org for further information about how this incredible mammal could disappear before many people notice their existence on this planet.

-Ckm Pamich