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On Aptenodytes

December 1, 2012

 

00000018421Penguins, besides being one of the most popular birds without any established cultural symbolism, are unambiguously among the most specialised and divergent of all Neornithes. Basically a bird’s answer to a plesiosaur or a dolphin, they apropriated the theropod anatomy into an unique set of adaptations for life in the southern oceans. Their feathers look more like fluffy fish scales than feathers, their wings are quite rigid, flat, lost the “thumb” and incorporated the ungual as part of the flipper-like structure, their metatarsals are highly reduced, their pygostyle is atavistically “primitive”, et cetera.

With all these anatomical modifications, their relationships with other birds had been poorly understood. Consistently, petrels and albatrosses have been considered the penguins’ closest living relatives, and indeed this position has been proven correct. Recent genetic studies, though, also show that loons belong to this petrel+penguin clade as well, and indeed the earliest penguins are noted as very loon-like. The fact that the oldest Gaviiformes like Polarornis have been founded in Gondwanna seems to cement this idea. The question one indeed could be asking is if penguis and loons developed their adaptations for diving independently, or if petrels evolved from loon/Pelecanoides like ancestors.

This understanding of the evolutionary relationships of penguins was seemingly foreshadowed by studies on the most iconic living extant taxon, Aptenodytes.

The Royal Penguins

Aptenodytes ridgeni, a Piocene fossil from Nw Zealand.

Aptenodytes ridgeni, a Piocene fossil from New Zealand.

Aptenodytes currently includes two of the most famous penguin species: Aptenodytes forsteri, the Emperor Penguin, and Aptenodytes patagonicus, the King Penguin. Another species, Aptenodytes ridgeni, is known from the Pliocene of New Zealand. Genetic studies show that Aptenodytes has diverged from other penguins as far back in time as 40 million years, a view previously considered based on their behaviour and morphology. Compared to other penguin clades, Aptenodytes‘ line has a relatively poor fossil reccord; this, as well as the fact that New Zealand was submerged for a good part of the Cenozoic, seems to suggest that the clade had an Antarctic origin, and that A. ridgeni was most likely not ancestral to the modern species, but rather an unsuccessful offshot. The exact date of divergence between the modern species is unknown, though a Pliocene date seems likely. It’s somewhat safe to say that the King Penguin is the more conservative species, being less rotund, having a more temperate distribution, more vivid colouration, longer wings and jaws, smaller size and a more pelagic method of foraging rather than the distinctively benthic hunting style of the Emperor Penguin.

As stated before, morphological and behaviour studies pretty much support the status of Aptenodytes as the most basal living penguins (Jouventin P (1982): “Visual and vocal signals in penguins, their evolution and adaptive characters”). Personally, I’ve always found King and Emperor Penguins more loon-like than other penguin species, which in turn makes sense considered the rather loon-like beaks of more basal penguins like palaeeudyptines. They are noted as being the birds capable of the deepest dives, with Emperor Penguins having been registered of being capable of diving to depths of 535 meters or more. Accordingly, they are also among the most pelagic penguin species; aside from the complicated breeding cycle on land, these birds spend most of their time well into the Southern Ocean, away from any landmasses; incidents where birds end up far north from their usual range are not uncommon.

Musings on ecological role

Under threat: King penguins off the coast of Macquarie Island.Although much smaller than the giant penguins of the Eocene and Oligocene, Aptenodytes penguins are positively massive when compared to other modern penguins. Part of the reason for them retaining larger sizes might be due to the relative rarity of delphinids and porpoises on the Antarctic waters, which seem to be the main penguin competitors, or else such an abundance of food that competition with marine mammals is less intense than on northern waters. The extreme specialisation of the Emperor Penguin to benthic feeding might also be help it avoid competition from Antarctic seals. Compared to other Antarctic predators, Aptenodytes is also notable for feeding rarely on krill; Emperor Penguins build the bulk of their diet on benthic and ice-dwelling fish and crustaceans, while King Penguins feed mostly on squid and lanternfish.

Aptenodytes penguins are most well known for their self-sacrificing breeding habits. The female, exhausting her resources on producing a single massive egg, trusts it to the male, which developed an unique cloacal pouch in order to incubate the egg. This incubation style, no doubt developed on environments where nest building is useless – as well as energy wasting -, seems to solidify their status as a product of Antarctica’s freezing. Like other large seabirds, breeding takes a long time; 14-16 months from egg laying to offspring fledging are the norm. As such, while annual breeding attempts still occur, most birds preffer to have gaps of two or three years between breeding seasons, as in other large seabirds like albatrosses. The birds usually reach sexual maturity at 6 years of age, returning to land after several years in the open ocean. With such an enormously slow breeding cycle, population recovery can be very hard, and currently Aptenodytes is only capable of sustaining it’s massive populations after thousands of years of stable conditions. Something that will change soon enough, what with global warming and everything.

What does the future hold?

Dougal Dixon's futuristic penguins, the Vortex and the Porpin. Considering the inflexible avian spines, the fact that birds are incapable of ovovivipary and that beak holes are just silly, I doubt animals exactly like these would ever evolve. Similar forms could, though.

Dougal Dixon’s futuristic penguins, the Vortex and the Porpin. Considering the inflexible avian spines, the fact that birds are incapable of ovovivipary and that beak holes are just silly, I doubt animals exactly like these would ever evolve. Similar forms could evolve, though.

Unless the effects of global warming suddenly disappear, the future seems dim for the Emperor Penguin, which depends on the polar icecaps for survival. The King Penguin, however, might have better chances at survival, especially given it’s less crustacean-dependent diet.

Should the King Penguin survive, it’s future seems quite bright indeed. The cloacal pouches mdoern penguins have would be a very advantageous exadaptation; birds cannot develop ovovivipary, given their reliance on the calcium from the egg shells to devleop their bones, but the clocal pouches could easily be developed further, allowing the animal to carry the egg while underwater. If so, this would certainly speed up the birds’ breeding cycle a lot, which in turn would lead to a more rapid breeding cycle, and thus making it far less vulnerable to extinction.

In the oceans of the future, fully aquatic penguins might thus evolve.

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