A Penguin Conveyor Belt in the South Atlantic
In our recent paper in Proceedings of the Royal Society B: Biological Science, Dr. Daniel Thomas and I attempted to unravel the biogeography of the extinct penguin species of Africa – that is, to figure out where they came from.
There are two main hypotheses for the history of Africa’s penguin fauna. One is that they represent an endemic radiation. In this scenario, a single founding population of penguins (perhaps just a few individuals) arrive in Africa to find it a wide open swath of penguin paradise. With plenty of prey available in the cold waters of the Bengali current and ample safe rocky islands to nest on, these colonizing individuals could have rapidly multiplied. Over time, the original species could have split off into multiple species as selective pressures pushed for different traits. Endemic radiations are well-documented in birds, most famously in the case of Darwin’s Galapagos finches. In that instance, a single species of finch split into more than a dozen distinct species over the course of a few million years. Arriving in a near ecological vacuum, the founding finches evolved a range of different beak shapes and behaviors to exploit different food sources. It is not too hard to image the same thing happening as penguins arrived on a continent new to them, without any direct competitors.
The second hypothesis is that the extinct penguin species arrived separately, in multiple waves of dispersal. Each species would thus have a separate ancestor on some other continent. Waves of dispersal are also common in island avifaunas. An amazing example is Hawaii’s assemblage of bizarre ducks and geese, now tragically almost entirely extinct. One of the few surviving species is the Nēnē, a descendant of wayward Canada Geese that became stranded on the islands. In the fossil record we find some stranger examples, including the giant “toothed” Moa-Nalos. These thundering flightless birds weighted up to 15 pounds and evolved from Mallard Ducks that gave up flight in favor of large size. Another intriguing example is Talpanas, a litter-foraging duck that was probably nearly blind and nocturnal, guiding itself with its powerful sense of smell. This species evolved from a Ruddy Duck-like ancestor.
In order to test which hypothesis was more likely, we constructed an evolutionary tree of the South Africa penguins and fossil species from elsewhere. What we found is that two of the extinct species, Inguza predemersus and Nucleornis insolitus, were NOT close relatives of the living Blackfooted Penguin (the only species that breeds in Africa today). This rules them out as being either ancestors of the Blackfooted Penguin or part of an endemic radiation. In fact, Inguza predemersus and Nucleornis insolitus were not even particularly close relatives of one another, and so must have arrived in Africa separately rather than splitting off from a single ancestor. The waves of dispersal hypothesis wins out. At least three separate dispersals must have occurred. There may have been even more, because two other species of extinct penguins are known from Africa’s fossil record. Unfortunately, we know too little about them to guess where they belong on the evolutionary tree. If we later find out they are also related to other non-African penguins, we could have up to five dispersals on our hands.
So, how did all these waves of penguins make it to Africa? It seems like ocean currents played a big role. One major circulation system in the southern oceans is the South Atlantic Gyre. This system of currents creates a huge counterclockwise flow that may have served as a “penguin conveyor belt” from South America to South Africa. Penguins have been in South America for at least 40 million years, and this continent was identified as the most likely area of origin for the ancestors of the African penguins in our analysis. One possible scenario involves penguins from the South American coast getting caught up in the Brazil Current while foraging out at sea, and swept away from the coast. From here they could become entrained in the east-flowing South Atlantic Current and after a long journey (penguins can survive at sea for months at a time) the Benguela Current could have swept them back up the coast of Africa to dry land. We propose that this type of current-aided dispersal happened may times, and that currents are the main reason why Africa has penguins today, while Madagascar, which is surrounded by unfavorable currents pushing southward and away from the coast, does not.