Nuts About Hornbills: A Fascinating Evolutionary Story


The extraordinary habit of the male, in plastering up the female with her egg,
and feeding her during the whole time of incubation,
and until the young one is fledged, is common to several of the large hornbills,
and is one of those strange facts in natural history
which are “stranger than fiction.”

-Alfred Russel Wallace
Wrinkled Hornbill, Aceros corrugatus, from Southeast Asia

Wrinkled Hornbill, Aceros corrugatus, from Southeast Asia

I first became acquainted with hornbills at the zoo. Amazing beautiful creatures, graciously hopping from branch to branch inside their artificially created environment.

southern ground-hornbill, hornbill, birds

Southern Ground-Hornbill, Bucorvus leadbeateri, endemic to the Sub-Saharan Africa

Like many people very familiar with American birds (South, Central and North American) but not so much with the African or Asian at the time I thought these gorgeous birds where relatives of the South American toucans! How could not they be? After all they are both very colorful and have enormous bills!! Isn’t the African ostrich a very close relative of the South American rhea, after all both continents were part of one giant Gondwana and Pangea some many million years ago, sure they must have evolved from the same ancestor and diverged some time after the continents separated.

Abyssinian Ground-hornbill (Bucorvus abyssinicus), also known as the Northern Ground-Hornbill, is also endemic to Africa but while its Southern relative can be found from Kenya to South Africa the Abyssinian can only be found to the North of Kenya as well S Mauritania and Guinea E to Ethiopia, NW Somalia,  and Uganda.

Abyssinian Ground-hornbill (Bucorvus abyssinicus), also known as the Northern Ground-Hornbill, is also endemic to Africa but while its Southern relative can be found from Kenya to South Africa the Abyssinian can only be found to the North of Kenya as well S Mauritania and Guinea E to Ethiopia, NW Somalia, and Uganda.

Well nothing could be more untrue! Like the penguins and the puffins, the toucans and the hornbills are another fascinating example of convergent evolution, where both species have adapted to survive in similar ecological niches divided by thousands of miles. Toucans reside in Central and South America, while hornbills are found only in Africa and Southeast Asia. Both groups feed on fruits, insects and small creatures, including bird eggs, lizards and young mice, and their similar bills come in handy when foraging for food.

Hornbills and toucans nest in cavities and their large bills also play a role in protecting eggs and nestlings from potential predators but this is where similarities end and differences begin, toucans belong to the Ramphastidae family and seat in the order of the Piciformes along with the barbets, the honeyguides, and the woodpeckers. While hornbills, seat in the order of the Coraciiformes along with the kingfishers, tories, motmots, rollers, wood hoopoes and other families.

Red-billed Hornbill, Tockus erythrorhynchus, Endemic to Africa

Red-billed Hornbill, Tockus erythrorhynchus, Endemic to Africa

It is quite possible that at some point in the evolutionary history of birds the Coraciiformes and the Piciformes might have shared a common ancestor, who knows, systematics are not only confusing, but there are still many unresolved issues in the classification of modern birds. There is even disagreement in how the hornbills are represented in the avian evolutionary tree.

There are at least two different ways of classifying these birds and on behalf of simplicity I’ll use the one that I think is the clearest. Under this classification we are elevating the hornbills to the order of Bucerotiformes with two families the Bucorvidae, composed by two species of ground-hornbills and the Bucerotidae composed by the other 52 extant species and divided in 14 genus.

Hornbill Classification According to Poonswald, Kemp and Strange: Hornbills of the World a Photographic Guide.Draco Publishing. 2013

Hornbill Classification According to Poonswald, Kemp and Strange: Hornbills of the World a Photographic Guide.Draco Publishing. 2013

One of the most interesting aspects of hornbills are their evolution and later their explosive radiation in Asia.

So how these fascinating birds came to be? Well, available evidence indicates that hornbills originated in Africa in the mid Eocene at least 50 Ma. Unfortunately the earliest credible fossil records for ground-hornbills is from the mid-Miocene around 15 Ma, an extinct species called Bucorvus brailloni, found in Morocco.

Today, there is little doubt that the Bucorvus ground-hornbills in Africa represent the earliest divergence of the crown-group hornbills. The other early lineage to diverge, which is also an Afrotropical species, is the Tockus hornbill. This lineage divided early, at 45 Ma, into two distinctive clades, one semi-terrestrial and the other mainly arboreal.

According to the article About Hornbills – From the book Ecology and Conservation of Asian Hornbills, the evolutionary history of these birds begins in Africa with an ancestral ground-hornbill. Later a bird that shared the characteristics of both the modern ground-hornbills and the Tockus must have made its appearance. This hypothetical hornbill is referred as a “proto-buceros” to emphasize its position as the first of the “typical” hornbills. Proto-Buceros was a large-bodied, territorial carnivorous bird that carried single food items in its bill. Part of the Proto-Buceros group stayed in Africa where it evolved as modern Tockus and Tropicanus. Other proto-Buceros wandered across to Asia where they invaded the forests and began to evolve specializations for forest life. One fundamental specialization was adaptation to a largely frugivorous diet. In fact, the Asian great-hornbills, the frugivorous Rhinoplax and Buceros, are among the earliest species on that continent, and they share many things including their large size with the African Bucorvus a fact that suggests they might indeed be Asian survivors from the same lineage.

Pangea, Gondwana, india, continental drift

Indian Microcontinent as part of Gondwanaland and colliding with Asia

The ‘explosive’ radiation of the remaining hornbill lineages, around 48 Ma, coincided with a mixing of diverse tropical floras as rainforests from the microcontinent of India successfully invaded and dispersed across Southeast Asia when India collided with this continent after drifting away from Gondwanaland-Morley 2000.

Female Wrinkled Hornbill, Aceros corrugatus

Female Wrinkled Hornbill, Aceros corrugatus

Once our ancestral hornbills invaded the broad frugivore niche, they began to specialize and radiate. At some later point, a more advanced Buceros, probably resembling a modern Asian hornbill, reinvaded Africa and gave rise to the African forest hornbill genera, Ceratogymna and Bycanistes.

How is that for a great evolutionary story?

Unfortunately several species of Hornbills are extremely threatened, particularly in Asia, where forest destruction is rampant. Hornbills rely on the availability of natural tree cavities for reproduction and depend very much on fruit as their food resource, any loss of habitat means loss not only of breeding sites but also of food resources. Poorly planned human activities within forests, coupled with explosive population growth and indiscriminating exploitation of natural resources have considerably altered the environment resulting in a massive depletion of forest resources and hence of hornbill habitat -Pooswad p. et al 2013.

Luckily great work is being done to protect and save these gorgeous birds from extinction, and I’ll be writing about those efforts in future posts.

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