The odd bird that is the musk duck

(writing in progress)

There is an unremarked similarity between the musk duck (Biziura lobata, https://www.inaturalist.org/taxa/7178-Biziura-lobata) and the platypus (Ornithorhynchus anatinus, https://www.inaturalist.org/taxa/43236-Ornithorhynchus-anatinus).
 
The following video shows the musk duck fairly well: https://www.youtube.com/watch?v=xWwoMvfTcGk .
 
Both the platypus and the musk duck are restricted to Australia (mainly the cooler parts). They are as peculiar as anyone might wish, as ‘Australian specialities’. The musk duck is the largest freshwater duck on Earth, and is peculiar relative to other waterfowl in various other ways.

Both platypus and musk duck are so specialised for swimming that they can hardly locomote on land (although of course the platypus burrows extensively at the water’s edge). In both the platypus and the musk duck, the male produces a musky odour at breeding time.
 
The body sizes, diets and foraging methods of platypus and musk duck are similar, and the two species look so alike in the water that they are sometimes confused by naturalists. The beak of the musk duck, perhaps more than other waterfowl, is similar to the beak of the platypus. The swimming methods are similar although it is the front feet with which the platypus paddles. Of course, both forms lay eggs.
 
It strikes me as surprising that evolution in Australia has produced such similar animals, sharing the same trophic guild and in some areas coexisting, but drawn from the mammals in one case and from the birds in the other case. How are these forms ecologically separated, given that they seem to compete for similar foods?

The musk duck is able to fly to water bodies too temporary to allow residence by the platypus, but I have yet to see this stated in the literature in the context of ecological separation within a guild. Are there permanent water bodies in eastern Australia that are inhabited continually by both the platypus and the musk duck?
 
With respect to brain size, what is interesting is that, just as the platypus is rather brainy for such a primitive mammal, so the musk duck is brainy for a waterfowl (Iwaniuk and Nelson 2001). This braininess is particularly intriguing in view of the fact that the musk duck is among the few waterfowl known to be able to mimic vocally.
 
The platypus has mean brain mass 10.1 g at body mass 1.4 kg, whereas the musk duck (n =9 individuals of unstated sex) has mean brain mass of 9 g at body mass 2 kg. Since the male musk duck is considerably larger than the female, I suspect that female brain mass in the musk duck is < 9 g. What this means is that there remains a difference in brain size in keeping with the general rule that large birds tend to have lighter brains than those of like-size mammals.

Although the musk duck is brainy for an anatid, and the platypus is ‘reptilian’ for a mammal, there remains a gap between them in brain size.
 
Most waterfowl of about the body mass of the musk duck have brain mass about 6.9 g. The Australian shelduck (Tadorna tadornoides, https://www.inaturalist.org/taxa/7070-Tadorna-tadornoides) happens to have the same body mass as the platypus (1.4 kg) but a brain of only 5.7 g. And the merganser (Mergus merganser, https://www.inaturalist.org/taxa/7004-Mergus-merganser), which lives far from Australia, also happens to have a body mass of 1.4 kg with brain mass only 5 g.

Note that the merganser has a brain only half the mass of that of the platypus, at similar body mass.

So the platypus has a far larger brain than those of the like-size shelduck and merganser, and a slightly larger brain than that of the coexisting musk duck, with other waterfowl of comparable body masses (e.g. Alopochen, Anser, Aythya, Melanitta, Somateria) in between.  
  
Musk duck (Biziura lobata):

Platypus (Ornithorhynchus anatinus):

Musk duck (Biziura lobata):
 
http://animaldiversity.org/accounts/Biziura_lobata/

Here are more facts about braininess in the musk duck (Biziura lobata). My source is Iwaniuk and Nelson (2001).
 
Firstly, there is a nice contrast between the musk duck and another ‘Australian speciality’, namely the Cape Barren goose (Cereopsis novaehollandiae, https://www.inaturalist.org/taxa/7150-Cereopsis-novaehollandiae). Whereas the musk duck is perhaps the most encephalised anatid in Australasia, the Cape Barren goose is the least encephalised of all the anatids sampled by these authors worldwide.

What this means is that simply being an ‘Australasian speciality’ predicts little w.r.t. braininess.

Nobody should be surprised to find that a grazing goose, ecologically comparable with the emu (Dromaiu novaehollandiae, https://www.inaturalist.org/taxa/20504-Dromaius-novaehollandiae) and bearing precocial offspring, and furthermore associated with islands off an island continent, is small-brained – even relative to the standards of anatids, which are among the smaller-brained of birds.

However, it is surprising that a species as peculiarly Australian as the musk duck is large-brained.
 
Secondly, it just so happens that the absolute brain masses are similar in musk duck and Cape Barren goose: about 9 g. (Note that Iwaniuk and Nelson 2001 give brain sizes in ml, and I have used a factor of 1.03 to convert these volumetric data to masses.) What is nice, in illustration of the extreme difference between the musk duck and the Cape Barren goose w.r.t. encephalisation, is that they share a single brain mass (about 9 g) despite the fact that their body masses are more than two-fold different (2 kg for musk duck compared to 4.5 kg for Cape Barren goose).
 
Thirdly, another ‘Australian speciality’, the magpie goose (Anseranas semipalmata, https://www.inaturalist.org/taxa/6905-Anseranas-semipalmata) just so happens to have similar brain mass again: about 9 g. Because its body mass is 2.4 kg, it is close to average in brain/body mass for an anatid – despite any peculiarities it may have in terms of diet etc.
 
The only other anatid in the data-set with brain mass very approximately 9 g, namely the white-fronted goose (Anser albifrons, https://www.inaturalist.org/taxa/7019-Anser-albifrons), is also worth mentioning because it is below-par in braininess but not as extremely so as is the Cape Barren goose. Both the white-fronted goose and the Cape Barren goose are ‘grazers’, but the former bird, grazing the tundra in summer and migrating far south within the Northern Hemisphere in winter, does not live in the kind of isolation, virtually protected from predators, that the insular Cape Barren goose enjoys.
 
Fourthly, the discussion by Iwaniuk and Nelson (2001) of the braininess of the musk duck is worth reading. The musk duck is odd among anatids in minimising the number of eggs per clutch and in having parental feeding of hatchlings and juveniles.

Almost all species of anatids have precocial offspring which, although guarded by parents, forage for themselves from the start. The peculiarity of parental care in the musk duck, although unique in detail, seems to echo a theme in the Australian fauna: odd reproductive habits.

It could even be framed as some sort of convergence between musk duck and platypus that both provide for their offspring, the former by feeding its chicks small invertebrates and the latter by oozing milk. The niche of invertebrate-eating diver in freshwater might have been expected to be filled by ‘normal’ diving ducks in Australia, but instead the forms sharing this niche on the island continent are both aberrant in showing more parental provisioning and more encephalisation than expected in waterfowl.

It seems odd that one of these niche-occupiers is an aberrantly non-buoyant duck, and the other is a duck-billed, egg-laying mammal (monotreme). However, it leads to various questions once one overcomes the traditional reluctance to compare across the bird-mammal divide.
   
The following zoom-in shows a horizontal line-up of spp. at the value of about 9 g for brain mass in four spp. of anatids. The left-most is musk duck, which is encephalised. The right-most is Cape Barren goose, which is either decephalised or primitively unencephalised owing to island life, relatively free of predation. The two intermediate points are magpie goose on the left (only a tad brainier than expected for the average anatid) and white-fronted goose (which is like a Northern Hemisphere version of the Cape Barren goose). The white-fronted goose is a tad less brainy than expected for the average anatid.
 
Cape Barren goose (Cereopsis novaehollandiae):
  
(writing in progress)

Publicado por milewski milewski, 22 de junho de 2022, 12:21 AM

Comentários

The musk duck, encephalisation and precociousness of offspring/parental care: it makes sense that the more parental care offspring require the greater the braininess required to effectively keep them alive. I imagine mothers of the musk duck need considerable multi-tasking abilities. I wonder how this compares to 'marsupials, because even in species that lack a marsupium, offspring are naturally more contained (in a pouch or latched on to a teat, and then later, in the Phalangeridae and smaller dayurids, on the back, or in a den in the larger dasuyrids). It makes sense to me that marsupial babies require less parental supervision than, say, ducklings, which are waddling everywhere.  But if Anseriformes require relatively high EQs to keep track of numerous highly mobile, semi-aquatic offspring (particularly without the aid of a pack, but just a single or pair of parents), why is the musk duck's EQ considerably higher than that of others within its order, such as the magpie goose? Several other factors may also be at play. For example, the musk duck not only is rather altricial for an anatid, but also has extremely few offspring per clutch. Many precocial birds, such as anatids and galliforms, are not only precocial but also fecund, with many offspring per clutch. It makes sense that birds with many offspring per clutch might be less careful parentally, and relative carelessness would reasonably be correlated with lesser braininess.

Publicado por milewski cerca de 2 meses antes (Sinalizar)

Adicionar um Comentário

Iniciar Sessão ou Registar-se to add comments