Vulture Ecology and Evolution

This part looks at the evolution of vultures. This introduction to the next three chapters provides a background, by describing a few of the larger animals, mainly pachyderms, armored mammals and large cats, that provided food for the vultures. It seeks to answer the question; where did the vultures, both Old and New World, come from, and what was the context within which they evolved. Vulture evolution followed the evolution of birds, which parallel to mammals followed that of reptiles and other terrestrial and marine life forms.

The birds we now call New and Old World Vultures, which are an example of convergent evolution, developed in a very different world from the current context. Palaeontogical evidence shows that the vultures developed during the peak of the Age of Mammals, similar to a more developed version of the modern African savanna with vast herds of large mammals, but found in North America and Europe as well. The megafauna consisted of huge herbivores that provided bountiful food for avian scavengers (probably a factor for the huge size of some vultures) and the huge carnivores that killed their huge prey, opening the carcasses of thick skinned giants and enabling the vultures to feed.

The result was that vultures developed in a much more plentiful world than the current scenario, where they are reduced to feeding on small livestock, road kills and garbage. Such ecosystems possibly were a factor for the convergent evolution of avian scavengers, as different Orders, Families and Genera of birds may have capitalized on the abundant meat to evolve similar forms: large, carrion eating, soaring, bare headed birds, that did not need to hunt smaller prey in the midst of carrion abundance, but increasingly found it difficult to adapt as the Age of Mammals shifted to smaller forms of life.

The evolution of prehistoric ecosystems is the key to understanding the development of vultures. The time sequences, Eons, Eras and Periods are fundamental to such understanding (in order of duration, Eons are the longest, followed by Eras and then Periods). To evaluate the ecosystems of the time, it is first necessary to create time-series scenarios involving the main carnivores, herbivores and scavengers of the various periods. In this section, the first focus is on the geologic time scale (GTS), during which the different animals roamed the earth.

The geologic time scale (GTS) links stratigraphy to time. Stratigraphy is the study of sedimentary and volcanic rock layers and layering (stratification), and has two main branches; lithologic stratigraphy or lithostratigraphy, which looks at physical and chemical differences in rock layers, and biostratigraphy or paleontological stratigraphy, which addresses the fossil record revealed in the rock layers. The main concern in this book is biostratigraphy, and how this reveals the animals that lived during the various geologic time periods. The first period of the Earth's existence was the Precambrian, which is believed to have lasted from the formation of the Earth approximately 4600 million years ago, to the start of the Cambrian Period, about 540 million ago. During the Precambrian, life forms appear to have evolved from unicellular to multicellular forms. The Cambrian Period was the first period of the current Phanerzoic Eon. The Phanerzoic Eon, from about 540 million years to the present is the main period for the evolution of species. It is divided into the Paleozoic, Mesozoic and Cenozoic Eras.

The Paleozoic lasted from 541 to 252.2 million years ago (ICS 2004), and was the longest of the eras within the Phanerozoic period. The Paleozoic was divided into six geologic periods, from the Cambrian (541.0 ± 1.0 to 485.4 ± 1.9 million years BP), Ordovician (485.4 ± 1.9 to 443.4 ± 1.5 million years BP), Silurian (443.4 ± 1.5 million years BP), Devonian (419.2 ± 3.2 Mya (million years BP), Carboniferous (358.9 million years ago, to the beginning of the Permian Period, about 298.9 ± 0.15), and Permian (298.9 ± 0.2 to 252.2 ± 0.5 million years BP).

The Cambrian Period saw the extensive development of animal life, also termed the 'Cambrian Explosion.' Life changed from mostly unicellular forms to mostly aquatic, multicellular forms of molluscs and arthropods in the sea, while the land was possibly still dominated by unicellular animals and plants. During the next period, the Ordovician, life was more sophisticated, mostly sea dwelling mollusks and arthropods and a few fish ancestors. During the Silurian, bony fish appeared in the seas, as did terrestrial arthropods and small moss-like land plants. During the Devonian, there was further development of vascular plants, with leaves and roots in forest communities, with spores later giving way to seed bearing plants. Fish developed further, and tetrapods began the colonization of land, with the fins changing into legs. During the succeeding Carboniferous Period, terrestrial life was dominated by amphibians, from which reptiles later evolved, and larger arthropods (for example the Meganeura), were also common in the forest dominated landscapes. The succeeding Permian period was the end of the Paleozoic, and saw more development of large reptiles and also flying reptiles and the ancestors of mammals, and insects.

The next Era, the Mesozoic lasted from 252 to 66 million years BP, and has been termed the Age of Reptiles, and included three geological periods: the Triassic (252.2 to 201.3 million years BP); the Jurassic (201.3 to 145 million years BP) and the Cretaceous (145 to 66 million years BP). The end of the Permian and the beginning of the Trassic, which was also the boundary of the Paleozoic and Mesozoic Eras, was marked by the Permian-Triassic (P-Tr) extinction event, sometimes termed the Great Dying, which happened 252.28 million years BP and was the largest known extinction event.

It is estimated that 70 percent of terrestrial vertebrates and 96 percent of all marine species were extinct after this event, and that the recovery of life on Earth took up to 10 million years after this event. During this period, and possibly a little earlier, the archosaurs appeared. These are recognized as the ancestors of birds and crocodilians, and include the dinosaurs, the two main clades being Avemetatarsalia, including birds and their extinct relations; and Pseudosuchia, which includes crocodilians and their extinct links. The Avemetatarsalia, named by the palaeontologist Benton (1999) for those archosaurs more related to birds than crocodiles, included the subgroup, Ornithodira, which included the Pterosauromorpha, among which were the first vertebrates considered capable of flight. Also appearing during this period were the first true mammals, a specialized subgroup of Therapsids which possessed many of the traits that later developed in more advanced mammals.

During the next period, the Jurassic, the first real birds or Avialans, like Archaeopteryx, evolved from the forms of the Triassic. The large dinosaurs, the herbivorous sauropods and the largely carnivorous Theropods dominated the land. The succeeding Cretaceous period saw further development of birds and mammals (firstly marsupials and then placentals), and the extinction of the very large reptiles. As noted by Olson and Parris (1987) 'fossils of Cretaceous birds are scarce and usually difficult to interpret.' The Cretaceous-Paleogene extinction event began during this period (about 66 million years BP), possibly due to a massive comet/ asteroid impact during which the dinosaurs, and large numbers of birds (including all non-neornithean birds, such as the toothed enantiornithines and hesperornithiforms), mammals and marine life became extinct (Alvarez et al. 1980; Martin et al. 1996). This period may be regarded as the main background to current life forms, and forms the transition to the Cenozoic Era, which continues today.

The Cenozoic Era, the current geological era covers the period following the extinction event to the present, and is also known as the Age of Mammals (possibly due to the extinction event that ended the Age of Reptiles of the Mesozoic). It is divided into three periods: the Paleogene (66 to 23.03 million years BP, including the Paleocene, Eocene and Oligocene Epochs), the Neogene (23.03 to 2.588 million years BP, including the Miocene and Pliocene Epochs) and the Quaternary (2.588 to years BP to the present, including the Pleistocene and Holocene epochs). Birds and mammals proliferated, and increased in size possibly because of the extinction of the large reptiles. Most of these developments happened during the Paleogene, and by the Neogene bird and mammal species were almost similar to the modern species. Compared with the previous epoch, mammals and birds in some cases increased in size. Hominids, believed to be ancestral to humans, appeared at this time. During the Pleistocene Epoch of the Quaternary Period (2,588,000 to 11,700 years BP), many mammals and birds were much larger than their modern equivalents, and human/hominids forms were more similar to modern Homo sapiens.

The Cenozoic Era is the primary concern in this book, as it includes the evolution of avian scavengers, their changes in size and ecology, and the evolution and extinction of their avian and mammalian competitors, which also served as their predators and food sources. The main issues are the provision of food by the carcasses of the megafauna, the carnivores that killed the megafauna and hence provided food for the vultures, and also competed with them, and scavenging competitors, including birds, mammals and reptiles. Prehistoric megafauna refers to the large species of mammals that dominated some places during this period. Most of these animals became extinct in may be termed a Quaternary extinction event, this altering the food sources of scavengers and carnivores, both aerial and terrestrial. The main extinctions, which have been blamed on increased human hunting, climatic change, disease and/or an asteroid or comet (Scott 2009), occurred in the Americas, northern Eurasia and Australia, while subSaharan Africa and southeast Asia retained some larger mammals. The actual extents of change have been challenged.

For example, in North America, 35 genera of primarily large mammals became extinct by the end of the Pleistocene (Grayson 2006). The common assumption is that most of these extinctions happened between 10,000 and 12,000 BP. However, some writers have noted that only 16 of these extinct genera have conclusively been dated to this period, and there is the possibility that the extinctions took place over a longer period (Grayson and Meltzer 2002). The first birds appeared in the late Jurassic/early Cretaceous (150 million years BP). There is evidence of massive bird extinctions during the late Mesozoic. The beginning of Cretaceous/Tertiary (about 65 million years BP) also saw extensive diversification of bird forms (Feduccia 1995; Wink 1995). Fossil evidence from the Eocene and Oligocene (35 million years BP) reveal the presence of all orders of birds (including raptors, except Passerines). The expansion of bird species to most of the orders, 'an extraordinary explosive evolution', may have occurred in only 5 to 10 million years (Feduccia (1995).

Mammal groups that became extinct during this period included the Genera Pampatheriuma and Holmesina (family Pampatheriidae), Glyptotherium (family Glyptodontidae), Megalonyx (family Megalonychidae), Eremotherium and Nothrotheriops (family Megatheriidae), and Paramylodon (family Mylodontidae). These all of the Order Cingulata (SuperOrder Xenarthra), were large, armored, placental, vegetarian mammals related to modern armadillos Dasypodidae Gray, 1821. Glyptodonts, among the largest, weighed as much as two tonnes and measured up to 3.5 m, with a covering protective shell of hundreds of 2.5 cm-thick bony plates or osteoderms or scutes. Other large mammals were of the Order Artiodactyla.

These included Mylohyus and Platygonus (family Tayassuidae, similar to modern peccaries, only larger); Camelops and Hemiauchenia (family Camelidae, similar to modern camels, but some species larger); Navahoceros and Cervalces (family Cervidae, related to modern deer); Capromeryx, Tetrameryx and Stockoceros (family Antilocapridae, similar to the modern pronghorn (Antilocapra americana Ord, 1815); and Saiga, Euceratherium, Bootherium, Oreamnos harringtoni (family Bovidae, similar to the modern musk ox (Ovibos moschatus, Zimmermann 1780) and mountain goat (Oreamnos americanus, Blainville 1816). Also present were the American mastodon (Mammut, Blumenbach, 1799; family Mammutidae) and the mammoth (Mammuthus primigenius, Blumenbach, 1799; family Elephantidae).

These herbivores, generally similar or larger than their modern relatives, coexisted with large carnivores, which were capable of killing them and leaving remains for scavengers. Carnivore species that became extinct in Pleistocene North America included: Brachyprotoma, a skunk species (family Mustelidae); and the dire wolf Canis dirus (family Canidae, similar to the modern smaller dhole Cuon alpinus, Pallas 1811), the only surviving member of the Genus Cuon. The dire wolf was larger and fiercer than the modern wolf (Canis lupus, Linnaeus 1758). The bears of this period were the huge short faced bear (Arctodus simus, Cope 1897 and the Florida spectacled bear (Tremarctos floridanus, Gidley 1928; family Ursidae). The large cats of the family Felidae, comprised the subfamily Machairodontinae. These were an extinct subfamily of the family Felidae (true cats). They were endemic to Asia, Africa, North America, South America, and Europe from the Miocene to Pleistocene living from about 23 million until about 11,000 years ago.

The Machairodontinae line started in the early or middle Miocene of Africa, possibly from the early felid Pseudaelurus quadridentatus, which developed large, elongated upper canines (Augusti 2002). The earliest recorded genus of machairodont was Miomachairodus from middle Miocene Africa and Turkey (Ostende et al. 2006). During the earlier Miocene, other large carnivores with long upper canines, called barbourofelids coexisted with the machairodontines (Ostende et al. 2006). Three groups of machairdontines are recognised: the massively built Smilodontini, with the largest upper canines (sometimes referred to dirk toothed, as a dirk is long dagger about twelve inches long) including Megantereon and Smilodon; the Machairodontini or Homotherini, including the Machairodus or Homotherium, with smaller upper canines (sometimes referred to as scimitar toothed); and the Metailurini, with yet smaller teeth including Dinofelis and Metailurus (also sometimes classified as scimitar toothed) (Martin 1989; Barnett et al. 2005; Van Valkenburgh 2007; Slater and Van Valkenburgh 2008; Meloro and Slater 2012). Some recent writers put the Metailurini within the Felinae (the subfamily of modern cats) while others dispute this classification (Wesley-Hunt and Flynn 2005; Ostende et al. 2006; Meloro and Slater 2012; Salesa et al. 2012). The last machairodontine genera were Smilodon and Homotherium (Fabrini, 1890), which existed until the late Pleistocene in the Americas, about 10,000 years BP (Berta 1985; Kurten and Werdelinb 1990; Rawn-Schatzinger 1992; Turner and Anton 1997; Anton et al. 2005; Ascanio and Rincon 2006; Andersson et al. 2011).

Megantereon, with a size range from 60 to 150 kg (130-330 lb) which is commonly seen as the ancestor of the most well known sabertoothed cat, the Smilodon, lived during this period in Africa, North America and Eurasia, with the oldest fossil evidence dated to 4.5 million years from the Pliocene of North America (Turner and Anton 1997). The African records date to about 3-3.5 million BP, and Asian records date to 2.5 to 2 Million years BP. The oldest European records are from Les Etouaries (France), about 2-2.5 million years BP (Martin 1989; Turner and Anton; Barnett et al. 2005; Slater and Van Valkenburgh 2008). At the climax of the Pliocene, Megantereon evolved into the larger Smilodon in North America, but in the Old World it remained until the Middle Pleistocene (Turner and Anton 1997; Hemmer 2002).

The three species of Smilodon were S. populator (Lund 1842); S. fatalis (Leidy 1869); and S. gracilis (Cope 1880). Smilodon gracilis was the first Smilodon, living from 2.5 million-500,000 BP and weighing about 55 to 100 kg (120 to 220 lb) (Christiansen and Harris 2005). It is considered to have evolved from the North American Megantereon in North America. Smilodon fatalis, which lived 1.6 million-10,000 years ago, replaced S. gracilis in North America and also occupied western South America during the Great American Interchange (Kurten and Werdelinb 1990; Rincon et al. 2011). It weighed 160 to 280 kg (350 to 620 lb) (Christiansen and Harris 2005; Turner 1997; Christiansen 2007; Jefferson 2001; Shaw and Cox 2006; Van Valkenburgh and Sacco 2002). The largest, Smilodon populator, lived 1 million-10,000 BP, in eastern South America (Kurten and Werdelinb 1990; de Castro and Langer 2008). It is considered possibly the largest known felid, weighing 220 to 400 kg (490 to 880 lb) (Christiansen and Harris 2005; Sorkin 2008). The Smilodons survived in Africa until 1.5 million years BP, Eurasia until 30,000 years BP and North America until 10,000 years BP (Turner 1997; Turner and Anton 1997).

The larger cats, by opening the megafauna carcasses with very thick skin, provided avian scavengers with access to the internal meat. Kurten and Werdelinb (1990) note that an association of Homotherium species with proboscidean (elephant and mastodon) and rhinoceros remains, suggests that Homotherium preyed selectively on these tough-skinned animals. Smilodon, a massive apex predator, hunted large bison, camels, ground sloths, horses and mastodons, and pig-like Platygonus and the llama-like Hemiauchenia, possibly in competition with the American Lion (Panthera leo atrox) and the Dire Wolf (Vanvalkenburgh and Hertel 1993; Coltrain et al. 2004; Fennec 2005; Christiansen and Harris (2005).

The constructed scenario of megafauna provides a justification for vulture evolution. These evolved into very large birds, the largest and strongest probably capable to tearing through the skin of the huge carcasses left from the carnivores. Each massive meal could feed large flocks of large vultures much more easily than could an alternative predatory life style. There is little doubt that such a proliferation of large carcasses could fuel the evolution of large avian scavengers, possibly from different sources within the avian kingdom. The main question is when vultures appeared, or when birds may be classified as the ancestors of either the present New World or Old World vultures. Is the ancient bird discovered a vulture, a vulture ancestor or a different species altogether? Furthermore, what is a vulture?