The biggest underground living animal

Question

It's easy to find information about the biggest animals at land / at sea / in the air. But what was the biggest underground-living animal that ever existed?

• No animals that were forced to live underground (e.g. pit ponies)
• The animal should live at least 95% of its life under the earth (e.g. foxes)

There are two possible answers:

1. By length
2. By mass

My first guess was a mole

Answer 1

Microchaetus rappi Microchaetus rappi, the African giant earthworm, is a large earthworm in the Microchaetidae family, the largest of the segmented worms (commonly called earthworms). It averages about 1.36 m (4.5 ft) in length, but can reach a length of as much as 6.7 m (22 ft) and can weigh over 1.5 kg (3.3 lb).(https://en.wikipedia.org/wiki/Microchaetus_rappi)

Giant Gippsland earthworm These giant earthworms average 1 metre (3.3 ft) long and 2 centimetres (0.79 in) in diameter and can reach 3 metres (9.8 ft) in length; however, their body is able to expand and contract making them appear much larger. On average they weigh about 200 grams (0.44 lb). They have a dark purple head and a blue-grey body, and about 300 to 400 body segments.(https://en.wikipedia.org/wiki/Giant_Gippsland_earthworm)

Answer 2

Key Words:

To aid in your search, you might want to try searching the term "Fossorial."

• Fossorial animals are animals that are adapted to digging and life underground.

  • Though note: Lessa et al (2008) use fossorial to describe species that spend a substantial fraction of their lives outside their burrows, while they use subterranean to describe species that perform most activities underground.

You might also want to examine a list of troglobites.

• Troglobites are animals that live entirely in the dark parts of caves.

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Big Species

I think large specimens of @JayCkat's suggested species (Microchaetus rappi) will be tough to "beat", with large specimens reaching 6.7 m and 1.5 kg.

However, here's a list of other species anyways:

Some extant large species of fossorial animals:

• Cape dune mole-rat (Bathyergus suillus): 27-35 cm (up to 39cm including tail); 570-1350 g

  • Supposedly, some blesmols can reach a weight of 1800 g.

• Russian Desman (Desmana moschata): 18-21 cm (up to 41 cm including the tail); 400-520 g

• Giant Armadillo (Priodontes maximus): up to 1.5 m; up to 50 kg

  • It spends all day in underground burrows though it hunts above ground at night.

• Olm (Proteus anguinus): up to 40 cm long.

^{Cape dune mole-rat}

^{Giant Armadillo}

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The only info I could find via a quick search for prehistoric fossorial animals:

• A series of 240 million year old underground tunnels/chambers suggest some burrowing species lived there. [Source: Seeker].

  "You should imagine the tracemaker as a stout, short-bodied, four-legged animal with a short tail and short neck...The trunk was about 20-25 centimeters (8-10 inches) in length.

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Bonus: How about the deepest living animal?

• That award goes to the "Devil Worm" (Halicephalobus mephisto), a nematode that has been found living at 3.6 km below the surface!! ...(tied w/ Plectus aquatilis).

  • Though at 0.5 mm, it's definitely not winning the largest trophy :p.

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^Citations:

^{- Lessa, E. P., Vassallo, A. I., Verzi, D. H., & Mora, M. S. (2008). Evolution of morphological adaptations for digging in living and extinct ctenomyid and octodontid rodents. Biological Journal of the Linnean Society, 95(2), 267-283.}

Answer 3

I don't know if there is a bigger animal, but the biggest mole is the Russian desman with 400-520g and a length of 18-21 cm (35-41cm including the tail)

Answer 4

Can badgers qualify? They shelter underground and get a significant proportion of their food underground as well, I don't know if they stay there 95% of the time though. Wikipedia gives them as going up to 17kg and 90cm in length.

I thought a snake might get the record for length but there don't seem to be many burrowing snakes and they seem to be small. Given the sizes of other non-mammalian tetrapods and how few of them are burrowers whichever the largest underground animal is may well be a mammal.

Answer 5

If you accept far-fetched argument, I've got an individual that weight more than a ton! But let's start with a simple example of a large troglodyte.

Blind cave eel

The blind cave eel (Ophisternon candidum) is a troglodite. It is a pure white fish growing to 40 cm, with an eel-like body, no eyes, and a thin rayless membrane around the tip of the tail. I could not find its weight but it probably does not weight much! Just like many troglodite species, its distribution range is very restricted. It only occurs in a single location in Western Australia.

Fungus

Well a fungus is not an animal (but is closely related), so it does not answer your question but I still wanted to mention it. There's an individual fungus that covers 965 hectares. See this popular article.

Ant supercolonial individual

The individual ant is of course not that big but it is not uncommon to consider a colony as a superorganism. You might want to have a look at major transitions in evolution: revisited for a discussion about the concept of individual for social species. Note also that most ant casts spend most of their time underground in the nest.

From wikipedia

Until 2000, the largest known ant supercolony was on the Ishikari coast of Hokkaidō, Japan. The colony was estimated to contain 306 million worker ants and one million queen ants living in 45,000 nests interconnected by underground passages over an area of 2.7 km2 (670 acres). In 2000, an enormous supercolony of Argentine ants was found in Southern Europe (report published in 2002). Of 33 ant populations tested along the 6,004-kilometre (3,731 mi) stretch along the Mediterranean and Atlantic coasts in Southern Europe, 30 belonged to one supercolony with estimated millions of nests and billions of workers, interspersed with three populations of another supercolony. The researchers claim that this case of unicoloniality cannot be explained by loss of their genetic diversity due to the genetic bottleneck of the imported ants.[citation needed] In 2009, it was demonstrated that the largest Japanese, Californian and European Argentine ant supercolonies were in fact part of a single global "megacolony".

Considering a weight of 3.5 mg per individual, a colony of 306 millions ant weight more than a ton (1071 kg exactly) and of course this excludes their constructed habitat which could arguably be considered as part of the individual.