Bones of a giant manabou stork have been unearthed recently from the Pleistocene of Liang Bua cave in Flores, Indonesia. This new species, Leptoptilos robustus, is estimated to be 1.8 meters in length with an estimated weight of 16 kg. This stork has a reduced capacity for flight and would have been oriented more towards a terrestrial lifestyle. According to H. Meijjer and R. Due (2010), “The large body size and terrestrial lifestyle of L. robustus are responses to an unbalanced, insular environment with abundant prey items and a lack of mammalian carnivores, and emphasize the extraordinary nature of the Homo floresiensis fauna.”
The Liang Bua cave became famous when a team of paleoanthropologists discovered the diminutive Flores Man, H. Floresiensis, in this area. Since then researchers have continuously studied the Pleistocene fauna and flora of the island. According to the authors, the location of the island of Flores is an ideal place for understanding the phenomenon of island evolution–where smaller species tend to evolve into larger animals while larger species decrease in size:
The isolated position of the island is not only reflected in the limited number of species present, but is also evident from the unbalanced nature of the vertebrate fauna, which contains only some of the clades found on the mainland, and lacks specific groups such as perissodactyls and mammalian carnivores. Typical changes in morphology ensued, the most prominent being the dwarfing and gigantism of large and small mammals, respectively, that is, they follow the ‘island rule’. In view of the highly endemic nature of the Pleistocene fauna, Flores is considered a hotspot of insular evolution.
The designation of this Flores stork as a new fossil species is based on the osteological characters of the Liang Bua bones. L. robustus sp. nov.‘s tibiotarsus’ circumference is at 48 mm which puts the approximate weight of the bird at 16 kg. The authors remarked that “this is substantially heavier than any extant species of Leptoptilos, which reach maximally 9 kg.” The height of approximately 1.80 m also makes it twice as tall as the 1 m.H. floresiensis. Examining the tibiotarsus of the fossil specimen, the authors said that L. robustus has the “thickest bone wall of all living and extinct Leptoptilini.” This suggest that this giant manabou stork is well adapted to a lifestyle “with a reduced ability for flight and a greater reliance on terrestrial locomotion.”
H. Meijjer and R. Due thus posit that this particular terrestrial stork species may have evolved on the island from a flying ancestor. The similarity in osteological characters suggest that “L. robustus is closely related to L. dubius (another stork species), which implies that they share a common ancestor.”
In conclusion, they posit that,
A (flying) ancestral species of Leptoptilos colonizing the island must have been subjected to evolutionary forces very different from those on the continent. Carrion forms an important part of the diet for most Leptoptilos species. In mainland ecosystems with mammalian predators, scavenging birds compete for food with mammalian carnivores such as lions and hyenas. In the absence of such competition, food is more abundant. In combination with an abundance of middle and large-sized rats as well as juvenile Komodo dragons, a year-round food supply is provided, which favours habitat fidelity. The decreased interspecific competition for carrion and abundance of prey items would have reduced the need to fly large distances for food. As a consequence, the selection pressure on the flight apparatus was released, a well-known phenomenon in insular biotas (McNab, 1994). The insular nature, that is, the isolation and an unbalanced fauna lacking in mammalian carnivores, of Pleistocene Flores gave rise to the flightlessness and large body size of L. robustus.
HANNEKE J.M. MEIJER and ROKUS AWE DUE (2010). A new species of giant marabou stork (Aves:
Ciconiiformes) from the Pleistocene of Liang Bua,
Flores (Indonesia) Zoological Journal of the Linnean Society