The subjects of this book are the animals that I will designate as megaherbivores. I define this term to include those plant-feeding mammals that typically attain an adult body mass in excess of one megagram, i.e. ten to the power six grams, 1000 kg, or one metric tonne. This demarcating criterion conveniently encompasses elephant, rhinoceros and hippopotamus among living forms, while giraffe slip marginally into the category. Such animals have been colloquially designated pachyderms; but a thick skin is a minor feature, and it is their very large body size that sets these few species apart from the numerous smaller species of unguligrade herbivore that occupy a wide variety of terrestrial ecosystems today. Paleontologists such as Martin (1967) have used the term ‘megafauna’ to encompass those species attaining a body mass exceeding about 45 kg (100 pounds). However this division is arbitrary and has no functional basis. In this book I show that there are distinctions between animals reaching a mass in excess of 1000 kg, and those of smaller size, in almost all aspects of ecology.

Of course, many whales attain a larger size than any terrestrial mammal, but all whales are carnivorous, feeding on other animals ranging from tiny shrimps to seals. However, among marine mammals there are also the strictly herbivorous sirenians (manatees and dugongs), which feed on submerged plants growing in shallow lagoons and coastal waters. Manatees may weigh up to 1600 kg, while the recently extinct Steller's sea cow weighed up to 6000 kg.