This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
On the island of Madagascar lives a group of millipedes that can roll into balls as large as a small orange. Although that may seem alarming, they have no poison glands. They can, however, activate your cuteness sensors.
See what I mean?
Am I not adorable? That's what I thought. Sphaeromimus andrahomana. Fig 1c from Wesener et al. 2014. Click image for source.
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And they can even chirp. Really!
There are certain organisms you know you have to write about when you discover them -- and the giant, ancient chirping pill-millipedes of Madagascar cannot be avoided at any cost. Their name in the local language -- tainkintana -- means "star droppings". You can't make this stuff up.
In essence, these guys are stumpy millipedes that evolved the ability to roll up pill bug-style. Here is another view of a few in their coiled-up glory. Note the striking coloring. The pill-millipedes in the upper left, upper right, and lower right are all color variants of the same species, Sphaeromimus musicus. The pill-millipede in the lower left is a look-alike, only distantly-related species, Zoosphaerium blandum. Just as humans have many hair and eye colors, other species can vary greatly in their appearance as well.
Here's one in action in the rainforest of Malaysia. Notice how articulate their antennae are -- they appear to be as sensitive and dexterous as the trunks of elephants. Their probing appendages look almost cartoonish in the curious way they move and explore their environment.
Here's a close-up of one of those antennae showing the sensory cones at their business end. In this case, it's the antenna of Sphaeromimus lavasoa.
Sensory cones on the antenna of Sphaeromimus lavasoa. Fig. 10 from Wesener et al. 2014. Click image for source.
Scientists announced in June the discovery of seven new Malagasy (the word to use for things from Madagascar) species of pill-millipede -- discovered on an expedition back in 2007 -- in the genus Sphaeromimus. Strangely (to me), many of the new species have green eyes. And although the largest of this bunch is just bigger than a ping-pong ball, I think we can all agree that is still impressive. When sporting-equipment sized pill-millipedes are still wandering about undescribed on your island, you know you have a ways to go in getting your biodiversity documented.
Worryingly, many of the new species were found only in tiny scraps of rainforest a few hundred meters wide, some threatened with destruction by a nearby titanium strip mine. One pill-millipede was restricted to a 100+ hectare rainforest isolated on Lavasoa Mountain. Another two species, though found living on rainforest scraps quite close to each other, were only distantly related. Another species from a group of rainforest pill-millipedes was found somewhat inexplicably stranded in a cave surrounded by unappealing-sounding "dry spiny forest". The little arthropods were apparently marooned inside the cave at the end of the last ice age 3,000 to 5,000 years ago, when the rainforest that formerly surrounded the cave vanished (lemur skeletons found inside the cave testify to the formerly posh neighborhood).
One question that has long puzzled biologists is how much of the odd animal life found on Madagascar was present before the island broke away from Africa, and how much of it swam, rafted, or flew there across the Mozambique Channel. Studies of other animals show that Malagasy vertebrates and invertebrates able to fly or float seem to largely be African migrants. But the DNA of these pill-millipedes showed that the Sphaeromimus found on Madagascar are actually more closely related to pill-millipedes in India than they are to the other pill-millipedes on their own island. That means the pill-millipedes of Madagascar qualify for the "Native" bumper sticker and were living together at least 88 million years ago when India and Madagascar were still one piece of real estate and dinosaurs still had 15 million years on the clock.
Pill-millipedes are unique in some other ways. They often have special chirping organs, and those of Sphaeromimus -- called the male "harp" and female "washboard" -- are the largest of any millipede. Chirping -- or stridulation, as scientists like to call it -- is the rubbing of a body part (sometimes a field of nubs, other times a single solid object) over a second ridged body part. Think crickets. But why might a pill-millipede chirp?
I have not been able to find out why the female Sphaeromimus might chirp, and I'm not sure anyone knows. But in a related genus of pill-millipedes -- Sphaeortherium from South Africa -- the chirps of the male seem to prevent the female from reflexively curling into a ball, or coax her to unfurl if she's already clammed up. It's a bit like a mating password. Indeed, within the various Sphaerotherium species, the timing and pattern of chirps -- though not their acoustic frequencies -- seemed to be species-specific, much like the flashing pattern of fireflies. Even more mind-bogglingly, pill-millipedes are apparently not even capable of hearing their own chirps, since the scientists who study them have not yet been able to find any hearing organs. Instead, they believe that the animals are feeling the vibrations of the stridulating organs.
Pill-millipedes are alone among millipedes in chirping, and scientists believe based on studying their DNA that chirping evolved in the group on at least four separate occasions (a phenomenon called convergent evolution). It seems to be a side-effect of ball-rolling, as it's an effective way to convince a reflexively coiling female pill-millipede that it might be worth her time to uncoil and have a look at the cute, trundling goods ringing her doorbell.
References
Wesener T. & Stephanie Loria (2014). Integrative revision of the giant pill-millipede genus Sphaeromimus from Madagascar, with the description of seven new species (Diplopoda, Sphaerotheriida, Arthrosphaeridae), ZooKeys, 414 67-107. DOI: http://dx.doi.org/10.3897/zookeys.414.7730
Wesener T., Stefan Fuchs & Didier Spiegel (2011). How to uncoil your partner—“mating songs” in giant pill-millipedes (Diplopoda: Sphaerotheriida), Naturwissenschaften, 98 (11) 967-975. DOI: http://dx.doi.org/10.1007/s00114-011-0850-8
Wesener T. & Petra Sierwald (2010). The origins of the giant pill-millipedes from Madagascar (Diplopoda: Sphaerotheriida: Arthrosphaeridae), Molecular Phylogenetics and Evolution, 57 (3) 1184-1193. DOI: http://dx.doi.org/10.1016/j.ympev.2010.08.023