Wednesday, January 30, 2013

Broodfellas: Nest parasitism and Mafia involvement among cuckoos

Ah, mother birds: so caring, so dedicated to the art of raising healthy, successful young. Surely there are few better examples of committed mothers than a delicate bird returning to its nest with a beak full of worms for its eagerly waiting chicks.

And then there are mother birds that produce infanticide-committing, constantly-eating, greedy monsters and destroy the nests of those who don’t agree to raise them.

The common cuckoo Cuculus canorus, mother of the most evil baby birds you'll ever see. Photo by Jari Peltomaki.
Such is the wonderful life of a nest parasite. Nest, or brood, parasitism is a much more interesting phenomenon than we imagine it to be: it takes much more than just a sneaky mama bird and an unsuspecting host. In fact, neither of those needs to be true for a brood parasite to be successful. Though brood parasitism has been recorded in several species, including ducks, whydahs, and honeyguides, there is no better example of the complexity and interesting adaptations of parasitism than the cuckoo.

Babies and Gentes
While only about 40% of cuckoo species are brood parasites, those that are display a variety of ingenious traits allowing them to ensure the safety and growth of their offspring, even when they are absent as parents. Since before the egg is even laid, its host is predetermined. Female cuckoos belong to several different genetic varieties, known as “gentes.” These different lines of females produce eggs which match the eggs of their chosen hosts.

Random males can mate with a female of any gente. However, females will always produce eggs of their own gente, suggesting that the genetics for egg coloration are inherited maternally. The mystery of how gentes are maintained is puzzling, as cuckoos are known to parasitize a wide variety of hosts, ranging from warblers to robins. Gentes may reflect the hosts which raised the mother cuckoo in her own infancy, though the mechanisms controlling them are still mysterious.

A newly-hatched cuckoo making short work of reed warbler Acrocephalus arundinaceus eggs. Photo by Mike Richards.
As soon as the egg hatches, the young cuckoo gets to work disposing of its nestmates. In some species of cuckoo, the egg spends up to 24 hours longer in the mother’s body, allowing it to hatch faster than its nestmates. Using a shallow depression in its back, the newly-hatched, naked cuckoo dumps the other eggs over the side of the nest, ensuring that it gets all the attention it needs. Which is a lot.

The video below shows just how burdensome the task of raising a young cuckoo can be for the host. In this clip, a young common cuckoo receives an insect from its host parent, a reed warbler. The size comparison between the two species is enormous: even a young cuckoo can weigh five times as much as a warbler, and takes even longer to fledge.

And so, the life of the cuckoo goes on: the chick eventually fledges and leaves the nest and, one day it will produce its own brood of home-wrecking parasites. But what causes birds to simply accept a parasite in their nest to begin with? Why care for the cuckoo’s chick at all?

Don Cuckooleone: An offer you can’t refuse
Amotz Zahavi, a behavioral ecologist at Tel Aviv University, suggested a theory which sounded rather outlandish at first. He related the cuckoo’s powers of persuasion to that of the Mafia: the cuckoos, he suggested, get other birds to comply not with brains, but with brawn. It’s hard to imagine birds, some of earth’s most graceful and fragile of animals, engaging in gang-like violence against one another.

A male and female great spotted cuckoo Clamator glandularius in the process of creating even more little parasites.
The idea, appropriately labeled the “Mafia hypothesis,” was largely unsupported until Manuel Soler of the University of Granada put it to the test. On the plateau of Hoya de Guadix in Spain, Soler documented the interaction between great spotted cuckoos Clamator glandularius and magpies Pica pica over three breeding seasons (1990-92). Soler recorded the number of instances in which magpies removed cuckoo eggs from their nests, and the resulting number of attacks.

The results of Soler’s observations revealed some chilling statistics: 86% of magpie nests were attacked by cuckoos when their eggs were removed. When magpies accepted the eggs, only 12% of nests were attacked. About 22% of unparasitized nests were attacked. Arm- or, rather wing-twisting on the part of the cuckoos apparently worked successfully: when the magpies attempted to build nests and breed again, they readily accepted the cuckoo’s eggs.

A Burchell's glossy-starling Lamprotornis australis feeding a young great spotted cuckoo in Africa.
Fortunately, the breeding success of the magpies was not drastically effected. For the cuckoo Mafia to destroy all the magpie nests in the area would be impractical and biologically impossible. Researchers continue to search for similar behavior among other species of cuckoo throughout the rest of the world, but nothing to the extent of Soler’s experiment has been observed.

Nature can often be cruel, but as Michael Corleone says, “it’s not personal… it’s strictly business.”

Monday, January 28, 2013

The Ecological Importance and Awesomely Gigantic Reptiles of the Cerrejon Formation, part II

Dyro Straits
While T. cerrejonensis evolved to rule with brawn, crocodylomorphs known as dyrosaurids championed in adapting to a completely new environment. The majority of dyrosaurids dwelled in shallow, coastal environments, preying on small fish; however, the two species of dyrosaurid uncovered from the Cerrejon display some remarkable adaptations for living in calm, freshwater shallows.

A. guarjiraensis swims below T. cerrejonensis. Reconstruction by Danielle Byerley.
Acherontisuchus guajiraensis, a relatively large species measuring up to 6.5m (~21ft), was found with a bone found in most dryosaurids and crocodylomorphs known as the ischial shaft. This bone anchors the muscles which allow the animal to control its buoyancy and pitch in the water, an indispensable task for life in the rolling waves of the coasts. However, in A. guajiraensis, this bone is much more slender than in other species, indicating a lesser need and ability to control its movement in rough waters. The placid waters of the Cerrejon wetlands were free of waves or strong currents, leading to the shrinking of this bone. A. guajiraensis was also found to have various physiological adaptations for living in an inland freshwater environment, as well as adaptations found in extant crocodilians which help the limbs support the animal’s entire weight. A. guajiraensis, unlike other dyrosaurids, could therefore easily navigate amongst the river systems in which it thrived, both on land on in the water. The long, slender snout of A. guarjiraensis, a feature typical of most dyrosaurids, indicates a specialized piscivorous diet; it likely fed on eels and lungfishes in the sluggish rivers. (Bloch et. al, 2011)

The holotype skull of C. improcercus. From Hastings et. al, 2010. Scale bar represents 10cm.
Unlike A. guarjiraensis, which was large-bodied, slender-snouted, and specialized for a diet of fish, Cerrejonisuchus improcerus was a small, short-skulled generalist. This little dyrosaurid, the shortest-bodied species yet discovered, reached lengths of only 2.2m (~7ft) which, while still large for a reptile, is small compared to the rest of the family. Its short snout and uncurved teeth, both of which are very unusual for a dyrosaurid, indicate a diet much more general than that of A. guarjiraensis. C. improcerus likely fed on small amphibians, reptiles, and mammals.

While T. cerrejonensis and other huge reptiles get media attention for their extraordinary size, C. improcerus was touted as simply being “snake food.” Various articles about the discovery of this dyrosaurid merely mention that it was “devoured” by the fifty-foot-long super-serpent. C. improcerus, as we know, was much more than just a prey item. It evolved in an environment unexploited by crocodilians or other aquatic carnivorous reptiles, and its real beauty lies not in its edibility, but its adaptability.

Carbonemys: the football-headed car-turtle
While giant snakes and dyrosaurids are all unique and interesting in their own right, no species of the Cerrejon Formation is as downright bizarre as Carbonemys cofrinii.

Edwin Cadena of the University of Florida, discoverer of C. cofrinii. The huge thing on the table is its shell.
Before we get into just how awesome C. cofrinii was, we need to delve into the phylogeny of turtles. The order Testudines is divided into two suborders: Cryptodira and Pleurodira. The former subfamily includes turtles which are able to retract their heads and necks backwards into their shells in classic turtle fashion. The latter, the pleurodires, cannot retract in this way: instead, they tuck their necks into their shells horizontally, and include species known as side-necked turtles. C. cofrinii is a pleurodire of the family Podocnemididae, which includes extant side-necked turtles found in Madagascar and South America.

What nightmares are made of when you're a baby snake or dyrosaurid. Reconstruction by Liz Bradford.
With a streamlined shell the size of a Smart Car and a skull as big as a regulation American football, C. cofrinii was an absolutely massive animal, a product of the hothouse climate which led to the evolution of the other massive reptiles with which it shared its environment. Side-necked turtles today are notorious for eating just about everything they can fit in their mouths, equipped with powerful jaws that can break open the shells of snails, claws that can tear apart carrion, and necks strong enough to drag birds into the water. An animal the size of C. cofrinii would have no trouble chowing down on the hatchlings of other reptiles, including dyrosaurids and snakes, as well as preying upon fish and browsing on vegetation.

Disappearance of the Cerrejon wetlands
The hot and humid environment of the Paleocene did not endure. Within a few million years, the climate became cooler and drier, and this led to the disappearance not only of the Cerrejon swamp, but of rainforests in North America, Europe, and Asia. Just as the hot climate favored large reptiles, cooler temperatures favored smaller ones, and eventually the tremendous reptiles went extinct. The entire family of dyrosaurids went extinct, while boids closely related to Titanoboa still include the largest extant snakes, the anacondas. Certain species of South American pleurodires still grow to large sizes, but none compare with the vast Carbonemys. Thankfully, the coal swamps of ancient Colombia preserved a remarkably detailed account of life on the stagnant wetlands of the Paleocene, and we are constantly discovering more about this alien environment.

Bloch, J.I.; Hastings, A.K.; and Jaramillo, C.A. 2011. A new longirostrine dyrosaurid (Crocodylomorpha, Mesoeucrocodylia) from the Paleocene of north-eastern Colombia: biogeographic and behavioural implications for New-World Dyrosauridae. Paleontology 54, 1095–1116.

Gomez-Navarro, C.; Herrera, F.; Jaramillo, C.A.; Labandeira, C.C.; Wilf, P.; and Wing, S.L. 2009. Late Paleocene fossils from the Cerrejon Formation, Colombia, are the earliest record of Neotropical rainforest. Proceedings of the National Academy of Sciences of the United States of America.

Herrera, F.; Jaramillo, C.; and Wing, S. 2005. Warm (not hot) tropics during the late Paleocene: first continental evidence. AGU Fall Meeting Abstracts, 608.

Niinemets, U.; Peppe, D.J.; Royer, D.R.; and Wheeler, E.A. 2012. Roles of climate and functioning traits in controlling toothed vs. untoothed leaf margins. American Journal of Botany 99, 915-922.

Saturday, January 26, 2013

The Ecological Importance and Awesomely Gigantic Reptiles of the Cerrejon Formation

We all know the story: 65.5Ma, the dinosaurs became extinct (except for the ones that survived), and the world was at last liberated from the cold, scaly monarchy of reptiles. Hooray!

This wasn’t the case. At all. First of all, dinosaurs are warm-blooded, and secondly, in the ten-million-year period following the extinction of the non-avian dinosaurs, the world was still, in some areas, home to truly enormous reptiles. The Cerrejon Formation of Colombia, dating back to the Paleocene epoch, preserves an otherworldly environment in which giant snakes, turtles, and crocodilians ruled the landscape, and this formation has drawn much attention from its abundance of tremendous scaly beasts.

While the wildlife of the Cerrejon Formation is no doubt amazing, the ecological importance of the 58-million-year-old formation is often overlooked. The various fossilized plants, leaves, and evidence of herbivorous insects represent the earliest known record of modern Neotropical rainforests. The families of plants that have been found, including various palms and legumes, can still be found in the Amazon Basin. Though they were present in the Paleocene, they are far more diverse today; this may be due to the fact that the earth was still in recovery from the latest mass extinction at the time the Cerrejon was formed. It may also be due to the fact that the Cerrejon represents a wetland, rather than a rainforest, environment; wetlands generally host a lower diversity of plants than forests. (Gomez-Navarro et. al, 2005)

Titanoboa cerrejonensis, a dyrosaurid, and a giant turtle: the scaly, cold-blooded overlords of the Cerrejon Formation. Reconstruction by Jason Bourque.
Mean annual temperature (MAT) and mean annual precipitation (MAP) estimates are somewhat varied. Some authors suggest a balmy temperature of greater than 28C (~82F), with an average of 2,500mm (~8ft) of rainfall annually. (Gomez-Navarro et. al, 2005) Others suggest a slightly cooler MAT of 24C (~75F) and still higher rainfall levels of 3240mm (~11ft). (Herrera et. al, 2005) These figures were calculated using a method known as leaf-margin analysis: the number of angiosperm species with toothed leaves is inversely proportional to the MAT. Therefore, the greater percentage of untoothed leaves that are present in any environment, the higher the temperature is likely to be. (Niinemets et. al, 2012) Untoothed angiosperms make up anywhere from 74-78% of fossilized leaves recovered from Cerrejon, implying a high MAT for the local environment.

Scaly Monsters and Serpent Freaks
These early Neotropical wetlands of Colombia, at the northern tip of the ancient island of South America, were home to a host of reptiles. The high temperatures and humid environment favored the cold-blooded creatures and, because no large mammalian or avian predators had evolved yet, some of these grew to outrageous sizes.

A life-sized model of T. cerrejonensis swallowing an unfortunate dyrosaurid, with artist Kevin Hockley. Photo by Robert Clark.
In 2009, one particular species made headlines, breaking all kinds of records as the world’s largest, longest, and heaviest snake: Titanoboa cerrejonensis, a name that needs no explanation. Twenty-eight specimens of T. cerrejonensis were uncovered from the coal mines of the Cerrejon Formation, and the largest of these measured up to 15m (~50ft) long and tipped the scales at 1,135kg (~2,500lb).

This huge serpent was the apex predator of its environment, and, like the closely-related anacondas, hunted a variety of animals, including fish and crocodilians. As a boid, the family of snakes which includes boa constrictors and anacondas, T. cerrejonensis used its bulk and muscle to subdue its prey. With an abundance of large-bodied reptiles in the area, it was likely a specialist in hunting local dyrosaurids, as well as various species of lungfish and other large fish in the waterways.

Check back soon for the next installment of this small series, in which we will take a look at the specialized dyrosaurids of the Cerrejon. 

Thursday, January 24, 2013

The Leptoptilos Storks: Why You Have Every Right to Fear Them

We usually associate storks with the widespread and familiar white stork Ciconia ciconia. Most members of the family Ciconiidae, found on every continent with the exception of Antarctica, are similarly plumed, with some tropical species displaying more vibrant patterns of feathers, beaks, and skin. Most of these long-legged and –billed birds feed on small prey, ranging from fish to frogs to molluscs. However, one genus betrays the grace and beauty so often associated with these birds.

Greater adjutants L. dubius in a garbage dump in Guwahati. Photo by Yathin on Flickr.
Enter the Leptoptilos storks: bald-headed, ungainly, carrion-loving animals ranging from sub-Saharan Africa to southern Asia. Comprising of three extant species, the Leptoptilos storks lack any and all forms of purity, grace, and fragility associated with their baby-carrying relations. They are scavengers by nature, appearing and behaving like vultures on stilts. Adding to their already unpleasant description, they are seemingly fond of garbage dumps, and groups of the critically endangered Asian species are commonly seen alongside humans scrounging for scraps. Photos of such scenes, primeval-looking scavengers perched menacingly on mounds of human garbage, are chilling reminders that dinosaurs are alive and well in the modern world.

Marabou L. crumeniferus in Kenya. Photo by Lip Kee Yap on Wikimedia Commons.
The most common species is the African marabou stork L. crumeniferus. The English name “marabou” is a reference to the bird’s rather reserved demeanor, deriving from the Arabian “murabit,” meaning “quiet.” The Portuguese word “marabuto,” of similar roots, translates to “[a] hermit.” Ranging across the continent south of the Sahara, it is a common sight alongside various species of both avian and mammalian scavengers. Measuring up to 1.5m (~5ft) tall and equipped with a formidably heavy bill, they have a distinct advantage over other scavengers against which they compete at carcasses. Though they are brilliantly adapted for scavenging, they will also accept both live fish and scraps from humans, as well as small reptiles, mammals, and insects kicked up by herds of large mammals. The breeding habits of this species vary with local habitats and latitudes; breeding occurs in the dry season in populations closer to the equator. Colonies number anywhere 20 breeding pairs to thousands of them, and small groups usually stick together when feeding.

Lesser adjutant L. javanicus in flight. Unlike most storks, the Leptoptilos genera fly with their necks tucked close to the body. Photo by Lip Kee on Flickr.
The remaining two extant species are restricted to southern Asia. These are the adjutants, referring to their likeness to men in “stiff dress pacing slowly on a parade-ground.” The adjutants are comprised of two species: the greater L. dubius and lesser L. javanicus. These charismatic birds have been described in literature and travel accounts as early as 1773, when John Latham’s illustration of the “gigantic crane” was published. An adjutant even appears Rudyard Kipling’s The Second Jungle Book, appearing in a grim trio alongside a crocodile and a jackal. Both species breed in the dry season, when prey is concentrated into smaller pools of water. L. dubius, now classified as endangered, was once widespread, though the current population may number up to 1,800 birds, based on surveys of various breeding colonies. The largest modern breeding colony can be found in Assam, India, consisting of up to 800 individuals, though recently this colony has experienced various nesting failures. This may be due to disease, as well as the reduced use of garbage dumps as placed to dispose of food scraps and livestock carcasses. Competition for nesting space with lesser adjutants may pose another threat to this species, as well as their local reputation as nuisances. The smaller L. javanicus is far more common, with a worldwide population of up to 8,000 individuals spread from Sri Lanka to Indonesia. They are less dependent on garbage dumps and human-produced waste than the greater adjutant, spending more of their time in forested wetlands, with coastal populations inhabiting mangrove swamps and tidal flats.

L. robustus and Homo floresiensis. L. robustus measures approximately 1.8m (~6ft) tall; H. floresiensis approx. 1.06m (~3.5ft). Illustration by I. van Noortwijk, from Due & Meijer, 2010.
While the extant species of Leptoptilos may be intimidating, both in habits and in appearance, the various extinct species were truly monstrous. Most of these fossil species were similar in size to modern species, and filled the niche as large avian scavengers. However, some species grew to enormous proportions. L. robustus shared its habitat on the Indonesian island of Flores with a variety of endemic species, all products of the wonderful forces of insular evolution. On this island, giant monitor lizards and rats roamed alongside dwarf stegodonts and hominids. With no other scavengers, and only monitor lizards to serve as predators, Flores’ Leptoptilos stork evolved into a gargantuan bird, measuring a whopping 1.8m (~6ft) tall. An estimated weight of 16kg (~35lb) implies a largely reduced ability to fly. (Due & Meijer, 2010) With its size and heavy bill, L. robustus could have easily preyed on anything on which it could clamp its bill. L. falconeri, which lived in the Pliocene, was a widespread species, ranging from eastern Africa to Europe to southern Asia. It was similarly gigantic, and probably had a much more varied diet. L. falconeri coexisted with some of our ancestors in its African range. (Brunet et. al, 2005)

Marabou L. crumeniferus and a lappet-faced vulture Torgos tracheliotus. The marabou's height and long bill give it an advantage over shorter scavengers. Photo by Lip Kee on Flickr.
The Leptoptilos storks are unsightly, quiet animals, more than deserving of their nickname as “undertaker birds.” However, their grim appearance often overshadows how remarkably well-adapted and unique they are. They completely betray their family history as fishers and hunters of small invertebrates, and have instead evolved to compete alongside aggressive vultures and jackals, as well as eke out a living from the trash and waste produced by humans. The giant species which once roamed Eurasia and Africa are haunting reminders of the terror-birds which once stalked the plains of the Americas, and, in the eyes of their prey, were just as fearsome. As grotesque and bizarre as they may be, their natural history proves how variable just one family can be.

Brunet, M.; Likius, A.; Louchart, A.; Vignaud, P.; and White, T.D. 2005. A large extinct marabou stork in African Pliocene hominid sites, and a review of the fossil species of Leptoptilos. Acta Palaeontologica Polonica 50, 549-563.

Due, R.A. and Meijer, H.J.M. 2010. A new species of giant marabou stork (Aves: Ciconiiformes) from the Pleistocene of Liang Bua, Flores (Indonesia). Zoological Journal of the Linnean Society 160, 707–724.