Long-tailed Planigale

MammalAustralia

Feb 2

Written By Alexander Julius Jensen

Planigale ingrami

The long-tailed planigale — the world's smallest marsupial — measures just 5 centimetres (2 inches) in length. Its extremely flat, wedge-shaped head allows it to squeeze into narrow cracks in the soil, offering refuge from predators and the daytime heat of northern Australia.

Land of Marsupials

Clockwise from top left; a yellow-footed antechinus (*Antechinus flavipes*), feathertail glider (*Acrobates pygmaeus*), southern ningaui (*Ningaui yvonneae*), brush-tailed mulgara (*Dasycercus blythi*), Tasmanian pygmy possum (*Cercartetus lepidus*), red-tailed phascogale (*Phascogale calura*), and a fat-tailed dunnart (*Sminthopsis crassicaudata*).

It's not a mouse, it's not a shrew, it's the tiniest pouch-bearing mammal in existence. Marsupials — characterized by their minuscule, undercooked young and the pouches they carry them in — make up a minority of the mammalian life on our planet. There are some 330 species alive today, scattered unevenly across three continents; North America has one species (the Virginia opossum), South America has around 100, while Oceania (including New Guinea and some surrounding islands) has the rest.

Australia is known as the 'Land of Marsupials' for a reason; some two-thirds of the world's marsupials live down there. This southern landmass hosts the most famed and charismatic members of this offbeat group, including the likes of koalas, wombats and, of course, kangaroos — with the 2-metre (6.6-ft) tall red kangaroo being the largest living marsupial.¹ But few people are familiar with the other end of the size spectrum; the world of miniature marsupials, a micro menagerie as rich and interesting as the macro. There are the squirrel-like phascogales with their bushy tails and arboreal lifestyles, and the antechinuses, whose breeding season is so stressful that males just die at the end of it, there are desert-living mulgaras that can forgo drinking water, and pygmy possums that crouch atop banksia and bottlebrush flowers to eat their nectar and pollen. There are dunnarts and ningauis and feathertail gliders. But no marsupial is as tiny and obscure as the long-tailed planigale.

Between the Cracks

Measuring between 55 and 65 millimetres (~2 inches) long and weighing some 4 grams (0.14 oz), the long-tailed planigale is the world's tiniest marsupial. It looks a lot like a mouse or a shrew; covered in grey-brown fur that's lighter along the belly, with a long bare tail, larger hind limbs than front, and a pointy whiskered face with big ears and shining black eyes. But, being unrelated (or very distantly so) to mice and shrews, the planigale has its unique quirks. For one, its head is noticeably wedge-shaped and extremely flattened — with a skull that's one-fifth as deep as it is wide. Additionally, its hind legs are unusually splayed to the sides of its body, allowing it to easily lie prostrate against a surface. Okay, so it's unusually flat. Why?

The long-tailed planigale lives across the grasslands of northern Australia, from the Great Sandy Desert in the west to the foothills of the Great Dividing Range in Queensland to the east. Much of its habitat experiences a seasonal cycle of floods and droughts. Australia’s "Top End," for example, is flooded by monsoons from December to March, then suffers an eight-month dry season during which the soils dry out and fissure into a puzzle-piece landscape riven with deep cracks. The days are hot and the predators — from various snakes to feral cats to cane toads — are many. Drying tussocks of grass offer shelter for the planigale, as they do for many small critters, but this marsupial's anatomy is specifically adapted to the parched earth, with its tiny, flattened body allowing it to squeeze between tight cracks in the soil where neither hunter nor sun can reach it. Needless to say, claustrophobia probably never evolved in the planigale. It spends its days in these tight, temporary valleys, conserving its energy by going into a daily torpor that lasts 2 to 4 hours. Come nightfall, it forces its way back to the surface to hunt.

Battling Bugs

What would it be like to fight a giant insectoid, with its multiple legs and eyes, its cutting pincers and gnashing mandibles? We don't have to imagine, because we've met them plenty of times in our own stories; from the Japanese yōkai known as the Ōmukade, a centipede massive enough to encircle mountains, to oversized spiders of fantasy like Shelob and Aragog, and a writhing galaxy of insectoid extraterrestrials from the metamorphosing xenomorph, to the Arachnids of Starship Troopers, and the Buggers of Ender's Game. The idea of giant, exoskeletoned fiends is a recurring fascination of the human psyche, despite the fact that we humans have never met any truly giant arthropods. But that's not to say they never existed.

Clockwise from top; *Meganeura*, a sea scorpion (*Jaekelopterus rhenaniae*), and an *Arthropleura*.

Bugs² once dominated the earth. There used to be giant scorpions, some two-and-a-half metres (8 ft) long, in our oceans. The skies thrummed with the beating of dragonfly wings that stretched to a span of 75 centimetres (2.5 ft). Monstrous centipedes slithered across the forest floor, leaving behind 50-cm (20 in) wide trackways. This was all some 400 to 300 million years ago, when we Homo sapiens (some 300,000 years old) were less than a bud on the Tree of Life. So long ago was this 'Age of Arthropods', that the first mammals, our first furry ancestors, had yet to evolve into existence.

Arthropods didn't disappear, of course — in fact, you could say that they're more successful than ever, since they currently make up some 80% of all animal species on Earth — but they did shrink.³ Today, we humans can only battle giant insect foes on the screen or page, but, compared to most of Earth's animals, we are fairly large creatures. The miniature majority still live in a world of "giant" arthropods.

The planigale does battle with such monsters on the daily (technically nightly) — against spiders and roaches, grasshoppers and crickets — creepy crawlies that are often close to the planigale's own size. It challenges centipedes several times its length, like Fujiwara no Hidesato who slayed the mountain-hugging Ōmukade. It fights and it wins. This isn't self-defence; the planigale is a vicious predator, pursuing insects and their larvae like they're living lunchboxes. It scrounges through leaf litter, sweeping around with its face-full of whiskers to find hidden grubs, and squeezes its flattened body into cracks and crevices so narrow, that the insects sheltered therein surely thought they were safe from predators. This mini-murderer pounces on its armoured victims, grabbing and pinning them with its front paws, biting their heads and bodies repeatedly. Once they're subdued, it begins the butchery; tearing off its prey's wings and head, then stripping the exoskeleton to get at the soft insides. And the planigale doesn't limit itself to invertebrates either — meat is meat, after all — it'll kill small lizards and young mammals too, if given the chance.

Microscopic Joeys

A newborn kangaroo joey is about the size of a grape. Miniature infants are a hallmark of marsupials — spending relatively little time developing within their mothers, young marsupials come out a little undercooked. The pregnancy of a long-tailed planigale likely lasts no more than 20 days.⁴ If a kangaroo comes out grape-sized, a planigale infant must be near microscopic. At 3 mm (0.1 in) long, a litter of planigale joeys are born as tiny flecks on their mother's fur and, like most marsupials, they must make the lengthy climb to their mother's pouch, where they will remain for some 6 weeks. They then spend another 6 weeks in a grassy nest, hidden within a tussock of grass or under some bark. Their sheltered childhoods end after only 90 days — not too short for an animal that lives, on average, 1.3 years. Weaned off of their mother's milk, the young planigales have grown "large" enough to begin independent lives of crevice crawling and insect slaughter.

A drawing of a *Diprotodon* — the largest known marsupial to ever exist. (© Anne Musser)

¹ The largest known marsupial to ever live is the now extinct — tragically, for it would have been a site to see — Diprotodon. It resembled its closest living relatives, the wombats, but around four times longer (4 m/13 ft) and 80 times heavier (2,500 kg/5,500 lbs).

The first people that populated Australia did get to see it, and they lived alongside it for thousands of years until it went extinct some 25,000 years ago. The reason for its extinction isn't certain. Humans, who've caused their fair share of megafaunal extinctions, don't seem to be the culprits here, as they co-existed with Diprotodons for around 20,000 years. It's possible we contributed to their decline — by hunting their young or altering their habitats via burnings — but climate might have been the major cause here. At the time of their extinction, Australia was experiencing a particularly rough patch; with severe droughts and an interior even more barren and inhospitable than it is today (if you could imagine such a thing). Unfortunately, these marsupial behemoths — whose "underdeveloped" joeys might have been near-normal sized — didn't make it through.

A phylogeny of arthropods (phylum Arthropoda).

² A note on the terminology of creepy-crawly things.

The term insect refers to any animal in the class Insecta — characterised by a three-part, segmented body, with six legs in total. Insects belong to a larger group known as the arthropods (in the phylum Arthropoda), which also includes arachnids (spiders and scorpions), myriapods (centipedes and millipedes), and crustaceans (crabs, shrimps, barnacles, etc.)

The term bug is usually used in a non-specific manner to refer to anything that is, well, bug-like — whether that be a cockroach (an insect), a spider (an arachnid), or a worm (not even an arthropod, but an annelid).

A graph of atmospheric oxygen levels over time (NSF).

³ The leading theory behind the large size of ancient insects/arthropods has to do with oxygen concentration. During the time of colossal dragonflies and centipedes, the Earth's atmosphere was over 30% oxygen, compared to 21% today. A higher concentration of oxygen might be necessary for insects to grow large due to their breathing anatomy; as opposed to lungs or gills, insects have a series of breathing tubes, with openings along the sides of their bodies that deliver oxygen directly to their tissues.

A related, but somewhat opposing theory is that oxygen, rather than allowing insects to become large, forced them to enlarge in order to survive. Oxygen in large concentrations can be toxic. Small insect larvae, which often live in water and absorb oxygen through their skin (and so cannot really regulate the rate of absorption), might be in danger of oxygen toxicity in a 30% oxygen atmosphere. The solution might have been to grow bigger since large larvae would absorb lower percentages of the gas, relative to their body sizes, than small larvae.

Comparing oxygen concentration with the insect fossil record does show a trend, with maximum insect size tracking oxygen. The trend was broken, however, at the end of the Jurassic and the beginning of the Cretaceous. Cretaceous air was rich with oxygen, but insects became smaller. Why? The best hypothesis is that during this time, some 150 mya, birds began to emerge — posing as both flighted competition and predators to insects, possibly pressuring them to become smaller and more manoeuvrable to avoid the new, feathered fliers (pterosaurs, evolving in the late Triassic about 230 million years ago, didn't seem to have this effect on insect evolution).

While giant insects are exciting and tend to be more often preserved in the fossil record, it's important to note that there were always small insects around — filling any niche they could adapt into. In a way, ever since they first evolved some 480 million years ago, Earth has been, and continues to be, a planet of insects.

A narrow-nosed planigale (*Planigale tenuirostris*) on the left and paucident planigale (*P. gilesi*) on the right.

⁴ I could find no record of gestation length for the long-tailed planigale, but I did for two of its close relatives; the narrow-nosed planigale (P. tenuirostris), which has a gestation period of 19 days, and the paucident planigale (P. gilesi), with a period of 16 days.

Speaking of relatives, the long-tailed planigale has six close ones, including the two just mentioned. The planigales (in the genus Planigale) are found across Australia, but aren't commonly seen due to their nocturnality, cryptic colouration, and habit of hiding in crevices — their most distinctive feature being their diminutive stature and flattened skulls. They belong to a larger family of marsupial carnivores (Dasyuridae), with relatives like the antechinuses, dasyures, dunnarts, quolls, and the Tasmanian devil.

**Where Does It Live?**

⛰️ Grasslands, black soil plains, and clay soil woodlands.
📍 Northern Australia, from the Great Sandy Desert in the west to the foothills of the Great Dividing Range in Queensland to the east.

‘Least Concern’ as of 15 June, 2015.

IUCN

Size // Tiny
Length // 55 - 65 mm (2.2 - 2.6 in)
Weight // 4.2 - 4.3 grams (0.15 oz)
Activity: Nocturnal 🌙
Lifestyle: Solitary 👤
Lifespan: 1.3 years on average
Diet: Carnivore
Favourite Food: Arthropods 🕷️
Class: Mammalia
Order: Dasyuromorphia
Family: Dasyuridae
Genus: Planigale
Species: P. ingrami

The long-tailed planigale is an Australian resident; ranging across the grasslands of northern Australia from the Great Sandy Desert in the west to the foothills of the Great Dividing Range to the east.
To escape the heat and predators — snakes, feral cats, and cane toads — this planigale either hides in dry tussock grasses or uses its flattened physique to squeeze between tight cracks in the soil.
It spends its days in these narrow hideouts — conserving its energy by going into a daily torpor that lasts 2 to 4 hours — and comes out to hunt at night.
The long-tailed planigale is a small but fierce hunter, taking on insects close to its own size and centipedes many times longer than itself. It pins them with its front paws and repeatedly bites them until they succumb.
Like all marsupials, planigale joeys are born underdeveloped and small — 3 mm (0.1 in) long — and must make a long (for them) climb into their mother's pouch where they will shelter and grow for some 6 weeks.
On average, a long-tailed planigale lives for only 1.3 years.
The long-tailed planigale is a carnivorous marsupial (a dasyurid), related to several other mouse-like marsupials (antechinuses, ningauis, dunnarts, etc.) as well as a few larger predators (cat-sized quolls and the Tasmanian devil).