Among the most obscure and poorly known of the world’s living birds are the finfoots or heliornithids, a group of duck-sized, vaguely grebe-like swimmers of the American, African and south-east Asian tropics. There are just three extant species. Heliornithids are gruiforms (part of the crane + rail clade), recent studies indicating a close relationship with the Afro-Madagascan flufftails, a group conventionally included within the rail family but increasingly thought to represent a separate lineage termed Sarothruridae (García-R. et al. 2014).
There’s a lot to say about heliornithids – their biogeography and fossil record is confusing and fascinating – but here I want to focus on one particularly interesting aspect of their biology. Namely, that the tropical American Sungrebe Heliornis fulica (sometimes called the American finfoot) has pouches, and uses these pouches in the protection and transportation of its young. The chicks are altricial (that is, unable to walk or look after themselves) and hatch after a ridiculously short incubation period of just 10-11 days. Incubation is carried out by both parents, but it’s the male alone who has the pouches and is able to transport the chicks (Bertram 1996).
Waitaminute… pouches? In a bird? How exactly do these structures work? What do they look like? And where are they located? Books and articles that mention the pouches generally say that there are two of them and that they’re under the wings, but you don’t get much more detail than that. The primary source of detailed information on what’s going on here is Miguel Álvarez del Toro’s 1971 article, a semi-legendary paper that most interested people are aware of due to the summarised description of its contents provided by Brian Bertram in his heliornithid chapter in Handbook of the Birds of the World, Volume 3. Bertram (1996) didn’t provide photos or diagrams, basically because those that exist are not good enough or of the right style for HBW. But Álvarez del Toro (1971) did.
A far older publication – one of Prince Maximilian of Wied-Neuwied’s volumes of his Beiträge zur Naturgeschichte von Brasilien – also includes discussion and description of sungrebe pouches. I haven’t seen this reference but it was cited by Álvarez del Toro (1971).
If – for now – we ignore feathers, the pouches are essentially shallow, oval pockets formed by muscular pleats of skin that extend along the the side of the chest. The bird appears to have some muscular control over the shape and turgidity of the pleats (Álvarez del Toro (1971). A single chick can fit in each pouch, but it would likely fall out if the structure consisted of skin folds and a recessed area alone. Here, it seems, is where the feathers come in: a number of long, curved feathers grow upwards and backwards from the lower part of the side of the chest and form a feathery outer wall, this now meaning that the recess becomes a secure pouch. And let me state again that all of this is exclusive to males: females do not have any of these anatomical peculiarities.
When are the pouches used? I used to think that they were only there for emergency reasons (say, nest invasion by a predator), and possibly not even used by every sungrebe father. This appears to be incorrect. Álvarez del Toro (1971) disturbed a nesting male sungrebe and then saw it both swimming and flying with “two tiny heads sticking out from the plumage of the sides under the wings”, so this might support the idea of ‘emergency’ function. But his conclusion overall was that the chicks are (somehow) placed within the pouches immediately after hatching, kept there for days as they grow their plumage and become more capable, and are fed and kept clean by the male all the while: “While carrying the young, the male reaches beneath the wing to feed them and remove the droppings” (Álvarez del Toro’s 1971, p. 86).
And that, as far as I can tell, about sums up everything we know. The exact and precise details as goes what’s going on here haven’t been worked out, and a full and detailed study is still required. Presumably that will happen eventually. Are these structures definitely a novelty of Heliornis, or are they more widespread, and are there similar, related or relevant structures elsewhere in related species? It really doesn’t seem that there are, but Bertram (1996) noted that it might be difficult to say for sure given that much of our anatomical knowledge on these animals (as in, obscure gruiforms in general) comes from dried skins.
Those of you familiar with the arcane ornithology literature will know that the transportation of chicks by adults has been mooted for a reasonable range of species, including nightjars, woodcocks and other waders, various galliforms, anseriforms and others (see Ad Cameron’s illustrations below - the nightjar illustrations are almost certainly fanciful - from Perrins (1992)). Most of these cases are likely or almost definitely erroneous… and they don’t involve pouches…. but not all are. Jacanas definitely transport their babies by clasping them with their wings, but they can’t do this while flying.
I’d like to finish this article by taking you on a weird, speculative tangent in the vein of All Yesterdays (Conway et al. 2012). Regular readers of this blog will know that I often write about extinct dinosaurs, and – on occasion – about speculations pertaining to the behaviour or anatomy of extinct dinosaurs. If we were asked (as we occasionally are) how likely it might be that extinct dinosaurs – say, for example, hadrosaurs or therizinosaurs – had pouches in which they could carry and transport their young, we would typically respond by noting the total lack of evidence that might support such a possibility, and the sheer improbability of such a thing given the anatomy of the living animals that ‘bracket’ the relevant fossil animals on the family tree. There are no pouches in crocodylians or those living birds that diverged earliest in bird history, nor are there indications of them in stem-members of these lineages.
But novelty does arise – phylogenetic brackets can be violated, as I’ve said, to the amusement of some, at least once here in the past – and there’s no way we’d predict the anatomy of Heliornis if we only knew of it as a fossil. Could, then, that most bizarre novelty – a pouch for carrying babies – have arisen elsewhere in dinosaurs? To suggest such would be a grotesque and baseless novelty of the worst kind. Yup. Did I mention All Yesterdays?
My thanks to the anonymous colleague who kindly provided me with the key piece of literature that allowed the production of this article. And thanks to those who contributed to the poll at the Tetrapod Zoology facebook group and thereby forced this article to the top of the list.
Refs - -
Álvarez del Toro, M. 1971. On the biology of the American finfoot in southern Mexico. Living Bird 10, 79-88.
Bertram, B. C. R. 1996. Family Heliornithidae (finfoots). In del Hoyo, J., Elliott, A. & Sargatal, J. (eds) Handbook of the Birds of the World, Volume 3: Hoatzin to Auks. Lynx Edicions, Barcelona, pp. 210-217.
Garcia-R, J. C., Gibb, G. C. & Trewick, S. A. 2014. Deep global evolutionary radiation in birds: diversification and trait evolution in the cosmopolitan bird family Rallidae. Molecular Phylogenetics and Evolution 81, 96-108.
Perrins, C. 1992. Bird Life: An Introduction to the World of Birds. Magna Books, Liecester [sic].