Chapter 3
The Mammals
1. General Introduction
About 1050 contemporary genera of mammals are named by WALKER (1964) in
his monograph 'Mammals of the World'. Three of these belong to the order Monotremata (egg-laying mammals), ninety to the order Marsupialia (typically undeveloped young kept in pouches), and the remainder are placentals. Monotremes
are only known from the Australian region and are represented by the echidna and
platypus. This order probably represents a separate line of mammalian evolution,
beginning in the Triassic period. The marsupials and placentals probably diverged
from a common stock early in the Cretaceous period, about 85 million years ago.
The Australian (75 genera) and American (15 genera) marsupials subsequently diverged late in the Cretaceous. The placental mammals today contain about 950 genera, of which 360 are rodents (Rodentia) and 180 are bats (Chiroptera). The other
generically numerous groups are the Insectivora (hedgehogs, shrews), 66 genera;
Primates (man and monkeys) 62; Cetacea (whales) 35; Carnivora 103; Pinnipedia
(seals) 20 and Artiodactlya (pigs, cattle, sheep) 82. The mammals have a wide geographic and osmotic distribution; they are predominantly terrestrial and are even
found in the driest desert regions, though some are aquatic and live in rivers and
lakes or in the sea. Only one genus of the monotremes (Ornithorhynchus, the
platypus), and one marsupial (the South American water opossum, Chironectes)
live in fresh water and these groups have no marine representatives. Most mammalian orders have representatives that live in desert regions.
The ability to osmoregulate in such contrasting environments depends on physiological and behavioural adaptations, many of which are uniquely suited to the
particular needs of mammals. In the absence of such specializations most temperate
terrestrial species find it difficult to survive in areas where water is more restricted,
though there are some exceptions such as the dog and European rabbit which thrive
in the desert conditions of Australia. Dispersal of mammals is, nevertheless, relatively restricted by deserts, and especially limited by salt-water barriers (DARLINGTON, 1957). Bats, due largely to their aerial capabilities, have the widest distribution
among the mammals reaching isolated oceanic islands, such as the Azores, New
Zealand and Hawaii, that are otherwise inaccessable to all but marine species. The
rodents also have often managed to cross salt-water barriers (probably on rafts),
but if the aid of man can be excluded, this occurs only across the short distances
to islands lying adjacent to the continents. Such journeys, whether as aviators or
mariners, nevertheless constitute an osmoregulatory feat of some substantial magnitude considering the rather unfavourable conditions experienced by terrestrial
mammals in oceanic regions.
2. Osmoregulation
Two evolutionary events have had profound effects on the osmoregulatory problems of tetrapods. The first was the adoption of a terrestrial, instead of an aquatic,
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The Mammals
1. General Introduction
About 1050 contemporary genera of mammals are named by WALKER (1964) in
his monograph 'Mammals of the World'. Three of these belong to the order Monotremata (egg-laying mammals), ninety to the order Marsupialia (typically undeveloped young kept in pouches), and the remainder are placentals. Monotremes
are only known from the Australian region and are represented by the echidna and
platypus. This order probably represents a separate line of mammalian evolution,
beginning in the Triassic period. The marsupials and placentals probably diverged
from a common stock early in the Cretaceous period, about 85 million years ago.
The Australian (75 genera) and American (15 genera) marsupials subsequently diverged late in the Cretaceous. The placental mammals today contain about 950 genera, of which 360 are rodents (Rodentia) and 180 are bats (Chiroptera). The other
generically numerous groups are the Insectivora (hedgehogs, shrews), 66 genera;
Primates (man and monkeys) 62; Cetacea (whales) 35; Carnivora 103; Pinnipedia
(seals) 20 and Artiodactlya (pigs, cattle, sheep) 82. The mammals have a wide geographic and osmotic distribution; they are predominantly terrestrial and are even
found in the driest desert regions, though some are aquatic and live in rivers and
lakes or in the sea. Only one genus of the monotremes (Ornithorhynchus, the
platypus), and one marsupial (the South American water opossum, Chironectes)
live in fresh water and these groups have no marine representatives. Most mammalian orders have representatives that live in desert regions.
The ability to osmoregulate in such contrasting environments depends on physiological and behavioural adaptations, many of which are uniquely suited to the
particular needs of mammals. In the absence of such specializations most temperate
terrestrial species find it difficult to survive in areas where water is more restricted,
though there are some exceptions such as the dog and European rabbit which thrive
in the desert conditions of Australia. Dispersal of mammals is, nevertheless, relatively restricted by deserts, and especially limited by salt-water barriers (DARLINGTON, 1957). Bats, due largely to their aerial capabilities, have the widest distribution
among the mammals reaching isolated oceanic islands, such as the Azores, New
Zealand and Hawaii, that are otherwise inaccessable to all but marine species. The
rodents also have often managed to cross salt-water barriers (probably on rafts),
but if the aid of man can be excluded, this occurs only across the short distances
to islands lying adjacent to the continents. Such journeys, whether as aviators or
mariners, nevertheless constitute an osmoregulatory feat of some substantial magnitude considering the rather unfavourable conditions experienced by terrestrial
mammals in oceanic regions.
2. Osmoregulation
Two evolutionary events have had profound effects on the osmoregulatory problems of tetrapods. The first was the adoption of a terrestrial, instead of an aquatic,
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