Speculative evolution
Abstract
The wildlife of the future, the human-deprived African continent is largely dominated by a variety of differentially specialized primates. Being the evolutionary most successful group, they covered all possible ecological niches, ranging from enormous grazers to voracious predators. This is why, and by analogy to the Planet of the Apes, this unique biosphere was called the Planet of the Primates. The African plains throughout the continent and especially the Serengeti ecosystem became a scene of annual herd migrations because of the changing, drier and warmer climate. These migrating herds on the other hand became a constant source of food to predators, but also carrion eaters. The evolutionary process also shifted toward larger scavengers, most of which happened not to be primates.
The scavenging colossus
The well-known contemporary African fauna is not deprived of mean beasts, but hyenas may be regarded as some of the meanest. Although successful predators, hyenas are well adapted to find, steal and eat carcasses. They do not fear even lions, but the evolving of wolverine baboons in savannahs and killer chimps in African forests represented a far more dangerous rival. Descending from the spotted hyena Crocuta crocuta, the elephant-sized Neocrocuta gigas (Fig. 1) dealt with the competition by increasing its size. The otherwise agile and comparatively small ancestor evolved into a 6 m tall and nearly 4 tonnes heavy beast with no natural enemies in the plains. The new species developed longer front limbs to allow for a more upright position and better view.
Neocrocuta gigas is also characterized by extremely sensitive scent, crushing bite like its ancestors, and newly evolved sharp, long claws on the middle finger of the front limbs. The function of this claw was primarily to slash through the dead flesh of the carcasses, but they are also a nice addition to the arsenal of defensive weapons. The larger size, however, forced this beast to give up two features of the ancestor hyenas. The new species lost agility and became a comparatively slow-moving animal, which also prefers a solitary lifestyle to avoid food competition.
Birds of prey – how large birds quit flying once again
The classic carrion eaters of contemporary Africa are beyond doubt the vultures. These sinister birds had dominated the African plains for centuries, able to find animal carcasses from kilometers thanks to their exceptional sight and high point of view. Flying, however, is an energy-demanding and size-limiting activity. Although carrion was abundant, flying vultures started to experience difficulties in their competition with fellow scavengers and needed to develop more efficient adaptations to outcompete rivals. The vulture evolution followed two parallel routes.
Figure 1. Giant hyena Neocrocuta gigas and giant non-flying vulture Neogyps gigans, rivaling over a carcass.
The first and most obvious option was to become larger. The larger size, however, interrupted flying, which also meant that wings are not required anymore. As result, the genus Neogyps evolved with non-flying, ostrich-like representatives. The most dominant and largest of them, Neogyps gigans (Fig. 1) is a direct descendant of the white-backed vulture Gyps africanus. Unlike its ancestor, barely reaching a 2 m wingspan and 6-7 kg weight, the non-flying N. gigans is an enormous, over 100 kg and nearly 3 m in height bird. While not so efficient in finding carcasses as their flying ancestors, these ostrich-like vultures are much better adapted to fight off other scavengers and specialized in following the herds from a close distance, waiting for exhausted or sick animals.
The other evolutional direction was followed by the descendants of the Egyptian vulture Neophron percnopterus. The new species, Neophron implumem developed into a flightless and featherless, even smaller bird. Both the back limbs and the rudimentary wings are modified into hooked devices for gripping, usually to the fur of the giant hyenas. This vulture is simultaneously a symbiont – keeping parasites away from the hyena and a cleptoparasite itself – using the motility of the hyena to reach carrion and eat it along with his host. The transport host of course is not limited to hyenas and these vultures could be also found on numerous large grazers, using them for transport and simultaneously exhausting them, ensuring potential carcasses for food.
Neophron implumem also hides a unique weapon of both defense and intentional (and slow) killing of potential prey. Like most of the carrion eaters, vultures do have mechanisms to resist the deadly bacteria, found in their food like cholera, anthrax, and botulinum toxin. Their acidic stomach content and immune system are crucial for this to be successful. This particular species, however, evolved a specialized pouch (Fig. 2), an appendix of its esophagus, where all these bacteria freely develop in the substrate of rotting meet into a deadly, highly contagious fluid, which could be vomited and pulverized under pressure in the face of animals, regarded as potential rivals or future meal. This fluid is well-known and feared because of both its pungent smell and its high infectious potential.
Figure 2. The head of Neophron implumem, showing the (1) pouch, where pathogenic bacteria develop and the modified nostrils (2), used to pulverize the contagious fluid.
Minor large scavengers
Besides the enormous hyenas and ostrich-like nonflying vultures, several other animal groups developed into large carrion-eaters. In a convergent evolution event, the Nile monitor lizard Varanus niloticus evolved into the twice bigger African dragon Varanus venenifer, very similar in lifestyle to the Komodo dragon. Just like his cousin, this lizard attacks with his bite and contagious bacterial-enriched saliva, then patiently follows his prey and waiting for the inevitable death of infection. More commonly, however, the African dragon feeds on carcasses.
Unlike the logical transformation of a big, predatory lizard into a bigger, carrion-eater, another animal may have challenged all evolutionary logic. A descendant of the air-breathing African sharp tooth catfish Clarias gariepinus, the carrion catfish Neoclarias terrestris just improved several features of its ancestor. Unlike other fishes, adult catfishes are inactive during the rainy season. Its whole lifecycle is completed during the start of the dry season, when adult males and females start to crawl across the drying water reservoirs, feasting on dead or dying fishes and finally laying their eggs and digging themselves into the almost completely dry basin bed, long after the last common fish or other animals had fallen into lethargy. Accordingly, it is not uncommon for the carrion catfish to leave its water habitat and feed on nearby carcasses of larger animals, if available.
A vegetable diet for the scavengers
A unique adaptation in a plant species was driven by this particular abundance of large carrion-eaters. The common wild watermelon Citrullus lanatus, native to Africa evolved into the carrion watermelon, Citrullus cadaverinus. Because it requires, like many plants, the passage of its seeds through the digestive system of an animal for seed dormancy to be broken (by hydrolysis of the seed coat by digestive enzymes), it developed a special strategy to attract carrion-eaters. Its fruit, when fully ripped, produces large amounts of carrion-smelling cadaverine and putrescine. This is not uncommon in the plant kingdom, especially in the flowers of numerous plant species, an adaptation to attract pollinating carrion flies. In the carrion watermelon, however, this is a fruit adaption to attract seed dispersers. The watermelon itself produces fruits about the size of a football, with fleshy dark red inside. With ripening, the fruits split off and reveal their inside, resembling and smelling like rotting meat. The seeds then pass through the gastrointestinal system of the carrion eaters (mainly hyenas). Digestive proteases are needed for the breaking of seed dormancy, while the lignin-cellulose seed coat is not completely hydrolyzed because of the absence of cellulose and/or lignin-degrading enzymes. Then the seeds readily germinate when dispersed with the hyena’s feces. This adaptation proved to be beneficial to the carrion eaters either as the carrion watermelon fruit is rich in vitamins, otherwise hard to obtain from animal carcasses and the polyphenols in the seed coat are also beneficial with their antibacterial and antioxidant activities.