Inia: gnarly, heterodont, carries rocks for fun

More on weird odontocete skulls. This time, another river dolphin: this is the skull of the Amazon river dolphin or Boto Inia geoffrensis, also known as the tonina, bufeo or pink dolphin.

i-8f0738d5d1361fadfcb44a508064c4ff-Inia geoffrensis right lat C. McHenry.jpg

Three generally recognised Inia taxa exist, and views differ as to whether these are subspecies or species. I. g. humboldtiana inhabits the Orinoco basin, I. g. geoffrensis inhabits the better part of the Amazon basin, and I. g. boliviensis (or I. boliviensis) inhabits the Amazon tributaries of Bolivia. A few recent studies have supported species status for I. boliviensis (Banguera-Hinestroza et al. 2002, Martínez-Aguero et al. 2006). As you might recall from the series of articles on Marc van Roosmalen's new mammals, a dwarf Inia purported to be a new species inhabits the Rio Aripuanã. It has a proportionally shorter beak and less pronounced melon than other Inia populations.

Anyway, as in other river dolphins, the rostrum of Inia is long, but it's more robust than that of Platanista and the other river dolphins. In the nasal region, the premaxillae diverge widely such that they don't contact the nasals, and the maxillae form enlarged pneumatised crests over the orbits, though these aren't anywhere near as big as the immense maxillary crests of Platanista. Large bumps on those parts of the premaxillae just anterior to the nares combine with the maxillary crests to give the Inia skull a somewhat gnarly look, especially when it's viewed anteriorly as shown here. The supraoccipital is particularly narrow for a cetacean [the skull shown here is of the I. geoffrensis specimen BMNH 1169.b, image © Natural History Museum (London), courtesy C. McHenry].

i-4784d2da66b664d4b249048573816d72-Inia geoffrensis rostral view C. McHenry.jpg

You'll note that the jaws in this individual appear to be skewed to the left. This is in fact pretty common in Inia: I've seen pictures of individuals where the jaw are twisted so strongly along their length that their tips open perpendicularly to the jaw bases. Apparently, some individuals have more teeth on the right side than on the left (Watson 1981). The anterior teeth are conical while the posterior seven or eight tooth pairs are molariform, being massive compared to the anterior ones and sporting large shelves on their lingual surfaces. This is one of the most extreme examples of dental differentiation seen among extant cetaceans, most of which are far more homodont. The cusps of the posterior teeth are often heavily blunted by wear, and this detail, combined with the lingual shelves, indicates that Inia uses them for crushing prey, including crustaceans, armoured fishes and turtles. Apparently it's usual for the teeth to be chipped, discoloured, anomalous and diseased (Best & da Silva 1989, 1993).

There's some soft-tissue weirdness too: the melon has been described as small and flaccid (Best & da Silva 1989), and there are short bristles on the rostrum that probably serve a tactile function.

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Inia is a reasonably large dolphin, growing to 2.74 m (Best & da Silva 1989), and it ranges from dark to light grey, and of course to pink. It has large, broad paddles and flukes and a particularly flexible neck and body. In captivity it sometimes curls up and sleep on its back, and one of the neatest things known about it is that males 'beautify' themselves by carrying rocks around in their jaws. In case you don't believe me, here's the proof. Unlike in other river dolphins, males are bigger than females, and males bear scars and wounds way more frequently than do females (Martin & da Silva 2006). Males also have large toughened skin patches that might be special, heritable shields or weapons present specifically for use in combat (Martin & da Silva 2006).

And that is that. More soon...

Refs - -

Banguera-Hinestroza, E., Cárdenas, H., Ruiz-Garcia, M., Marmontel, M., Gaitán, N., Vázquez, R., García-Vallejo, R. 2002. Molecular identification of evolutionary significant units in the Amazon river dolphin Inia sp. (Cetacea: Iniidae). The Journal of Heredity 93, 312-322.

Best, R. C. & da Silva, V. M. F. 1989. Amazon river dolphin, boto Inia geoffrensis (de Blainville, 1817). In Ridgway, S. H. & Harrison, R. (eds) Handbook of Marine Mammals, Volume 4. Academic Press, pp. 1-23.

- . & da Silva, V. M. F. 1993. Inia geoffrensis. Mammalian Species 426, 1-8.

Martin, A. R. & da Silva, V. M. F. 2006. Sexual dimorphism and body scarring in the Boto (Amazon river dolphin) Inia geoffrensis. Marine Mammal Science 22, 25-33.

Martínez-Aguero, M., Flores-Ramírez, S. & Ruiz-Garcia, M. 2006. First report of major histocompatability complex II loci from the Amazon pink river dolphin (genus Inia). Genetic and Molecular Research 5, 421-431.

Watson, L. 1981. Whales of the World. Hutchinson, London.


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Completely OT, but I liked your piece in the latest Fortean Times. The idea that big-brained => humanoid always bugs me, so thanks.

Anyhoo, back to the dolphins...

Inia is called in Brazil boto-branco (white dolphin), boto-cor-de-rosa, boto-rosa (pink dolphin), boto-da-Amazonia (Amazonian dolphin), uiara, and in Amazonian folklore it's considered a magica creature, who turns into a man at ne night to impregnate women. Uiara (Iara, Ipupiara) is also used for mythical aquatic men and women; Iara is a kind of river siren, who enchanted men by her melodious voice.

So ladies must beware of men with rocks in their mouths, yes?

That one in the pic had better not follow his nose; he will end up going in circles.

They are pretty funny creatures, particularly in rainy season when they sally forth to explore the flooded woods, a strange habitat for a cetian indeed.

If they have a reduced melon, how do they echolocate? they are almost blind (not as much as the ganges example,) and I doubt theres much to see in the river.

No ziphidae yet, I see

I remember to find articulated skulls(and entire corpses too) of toninas in the beachs of Montevideo, Uruguay (That's the delta of Uruguay river, a mix of fresh water and Atlantic ocean) when I was a child. It may have been three or four of these evil-looking skulls decorating my room - a pretty common dolphin. Curiously, never seen one live and swimming!

By Luis Daniel (not verified) on 31 Jul 2008 #permalink

Is it possible there is a left side preference in feeding behavior?
I'm probably totally off the mark here, but don't the bottlenose dolphins that practice beach hunting favor the left side?

Very cool about the rocks!

By K. Capach (not verified) on 31 Jul 2008 #permalink

If these critters are effectively blind, the rocks must be carried to give a better sonar echo. You could track them by scattering beacon "rocks" in the water with a shape that gives a particularly strong echo. (That shape is formed by poking the corner of a cube into all the faces.) Make them more comfortable to carry than real rocks and they'll be more popular than iPhones.

By Nathan Myers (not verified) on 31 Jul 2008 #permalink

Is the heterodonty seen in Inia a retention of the primitive odontocete condition, or is it a reversal?

You haven't gone into mysticetes, but I'd like to note the remarkable similarities of the backfloating human nasal form to that of the blue whale's blowhole, remarkable convergence, seen also in the sea otter as compared to the rest of the weasel kin.

link and one I linked earlier:

Simply amazing when considering that humanity's closest kin are restricted to arboreal inland forests, that completely avoid water deeper than chest-high.

The single blowhole & echolocation of the odontocetes and sperm whale group has altered the skull so much, it can't be compared to others very well, but the mysticetes still follow typical hydrodynamic forms.

Swim softly and carry a big rock, and you too may get to curl up on your back some nights...

I didn't understand Nathan's comment, at first, to refer to rocks being easy for other individuals to detect & making the bearer more imposing; instead I thought he was saying that carrying a rock somehow helped the rock-carrier to detect other objects, which wasn't exactly intuitively obvious. But there might be sonmething to it as well, using rock-tooth-bone conduction of sound as an extra channel (jaw asymmetry might be part of the same scheme, while there appears to be not much asymmetry of the nasals in the anterior view above).

By John Scanlon FCD (not verified) on 31 Jul 2008 #permalink

That's another to add to the list of tool-using animals, then...

On what basis do they select the rocks, I wonder, and do they serve a functional reason in their own sonar-sensing, or a display purpose (to be sonar-sensed by others)? "Look (sic) what a dense object I found!" A sort of trophy?

Is it competitive between males, or males showing off to females, or just... Fun?

Or some other reason again?

(Reminded of Lewis Carroll's Mock Turtle's comment): rocks carried "With what porpoise?"