5.8.1 Roles of carnivores past and present
Carnivores and the larger omnivores play an important role in nutrient cycling and, as illustrated in Fig. 5.13, are probably crucial in regulating the size and nature of herbivore populations. In most European countries, and many other parts of the world, including much of North America, large carnivores have either been very greatly reduced in number or entirely eliminated. A number of factors such as change in habitat, food supply and competitors may partly account for the resultant highly undesirable increase in deer numbers seen around the world, but lack of predators is certainly a major contributory factor. Deer control now involves either fencing off the habitat to be protected, or shooting animals whose populations have become excessively large. This was not a problem when large carnivores were common to almost all forests. Those of earlier times were often quite remarkable. In the early Tertiary the fossil sabre-tooth tiger Smilodon of the northern hemisphere, which struck with its whole head, had a marsupial equivalent, Thylacosmilus in South America, whose enormous stabbing teeth continued growth, unlike those of true cats, and whose lower jaw had a large flange to protect them. Amazingly, many of the other marsupial hunters which fed on both placental and marsupial herbivores, dominating the South American forest for some 30 million years, were superseded by very large, fast-running ground birds which used their massive beaks to stab and rip their prey as did the extinct carnivorous dinosaurs.
It had been considered that the early mammals living in the Mesozoic era, when the dinosaurs flourished, were insect-eating or herbivorous animals no larger than modern rats, mice and shrews. Very recently two much larger and carnivorous early mammals, 'Mesozoic dogs' as they have been termed, were discovered in the Yixian formation of China. Both belong to the genus Repenomamus, lived 130 million years ago, had powerful jaws, and fed on the young of small dinosaurs. At a metre long and weighing around 14 kg, the largest, R. giganticus, was a quite formidable member of the forest fauna and in effect a forerunner of the hunting dogs, a few of whose troops still cooperatively hunt antelope in African forests.
The reptiles in their time gave rise to the early mammals, the dinosaurs, and two types of flying animal, the pterosaurs and the birds. In contrast modern reptiles, though including tortoises, turtles, lizards, alligators, crocodiles and caimans, are a much reduced group, of which the most interesting are the limbless snakes. All these animals occur in the forests of the world, often near to waterways. The crocodiles and their immediate relatives remain powerful hunters though showing many early features. O'Shea and Halliday (2001) provide an excellent recognition guide to the major reptiles of the world and also the amphibians, including the newts, salamanders, frogs, toads and the limbless caecilians which also frequently occur in forests and often fall prey to reptiles, especially snakes.
There has been more than one trend for reptiles to lose their limbs in the past, but the snakes, many of which live in forests, have their origin in a single line which began around 100 million years ago and has given rise to some 2800 named species which vary greatly in size and ability. The giant green anaconda Eunectes murinus can reach a length of 10 m and weigh 200 kg, 7000 times as much as the tiny Brahminy blindsnake Ramphotyphlops braminus, while various snakes, despite their lack of limbs, can run, jump and swim. Many snakes kill their prey by means of the poison produced by their modified salivary glands, others by crushing and suffocating their victim. Both methods are extraordinarily efficient. A bite from a black mamba Dendroaspis polylepis can kill a human in 30 minutes, while a giant green anaconda, the largest living snake, can wrap itself round an animal as large as a caiman and crush it to death. The problem of swallowing comparatively large prey has, in the course of evolution, been solved in a quite remarkable way. Not only can the lower jaw be displaced from its point of articulation with the upper jaw, but the two sides of the lower jaw also come apart at the front when the animal swallows a large object. The green anaconda, an aquatic arboreal species that lives along forest waterways, can swallow and digest a complete caiman, bones and all, resuming its activities after a period of rest.
Fossil evidence regarding the ancestry of the various groups of snakes is almost non-existent, so relationships between and within some groups have been investigated by genetic studies of their DNA. Results from three groups of highly poisonous bushmaster snakes Lachesis spp. in central and southern America yielded results of considerable interest. Though all three groups are now geographically isolated, they show major resemblances. The southernmost bushmasters, however, show considerable differences from the others, from which they have been separated for 12 million years. Differences between the two more northerly populations, which have been isolated for each other for a shorter period, were considerably less. Snakes can live in a wide variety of places, even the sea, but being poikilothermic find northern winters very difficult. In North America red-sided garter snakes Thomnophis sirtalis parietalis have solved this problem by hibernating deep underground in limestone sinkholes. In spring up to 15 000 have been seen emerging from a single locality, the greatest concentration of snakes ever known.
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