Weird and wonderful
Two icons of the Aussie bush, the platypus and echidna, have a multitude of monotreme-specific quirks that mark them out as unique among mammals.
1 ELECTRORECEPTION:
Electroreceptive sensors in platypus bills help them to find prey under water. Echidnas have a less sensitive version of this in their beaks, which may aid locating ants and termites in moist soil.
It’s a trait they likely inherited from platypus-like ancestors.
2 BEAK:
As adults, both the platypus and echidna lack teeth. Echidnas’ long, narrow beaks can’t open like the jaws of other mammals.
The diameter of the mouth is only about 5mm, limiting the size of prey they can take.
3 TONGUE:
The
Tachyglossus part of the short-beaked echidna’s Latin name means
‘fast tongue’ and they can flick these 18cm-long appendages in and out up to 100 times a minute. This helps them lick up as many as 40,000 ants or termites/day.
4 SPRAWLING GAIT:
With their limbs sticking out at right angles to their bodies, the lizard-style gait of the platypus and echidna is more akin to crocodiles and goannas than other mammals.
5 LEG SPURS:
Male platypuses have venomous spurs on their hind feet, which can deliver a debilitating sting. Male and female echidnas are also born with spurs, which females typically lose in adolescence. Rather than delivering venom they may secrete chemicals used for communication.
6 BACKWARDS FEET:
Echidnas’ hind feet are twisted around to point backwards, which helps them push soil away as they burrow.
7 NEOCORTEX:
The neocortex, which takes up 80 per cent of the brain in humans and is linked to personality and reasoning, fills about 50 per cent of an echidna brain, nearly twice the size of even some of the most intelligent mammals. What they use this for is a complete mystery.
8 OLFACTORY BULB:
Echidnas have a highly developed sense of smell, and are the only animals with olfactory bulbs so enlarged in their brains that they are highly folded. This suggests smell is of great importance to them, perhaps for communication with other echidnas.
9 STOMACH:
Monotreme stomachs are not the usual highly acidic environments that secrete enzymes called pepsins to break down proteins. This raises intriguing questions as to how they can kill viral and bacterial pathogens in food.
10 CLOACA:
Rather than separate orifices for reproduction and excretion, the platypus and echidna have a single multi-purpose opening called a cloaca. “Monotreme” is Latin for “single hole”.
11 SEX CHROMOSOMES:
All mammals but monotremes have two sex chromosomes (XX in females; XY in males). Female echidnas have 10X; males have 5X4Y.
12 POUCH:
The echidna pseudo-pouch, created from swollen mammary glands, disappears when a female is not sexually active.
13 FOUR-HEADED PENIS:
Unique among mammals, the echidna penis has four heads, only two of which are used in each mating, swelling to fit the female’s dual-branched reproductive tract.
The two heads alternate between matings.