1st Year. Honour Moderations in Biological Sciences: Organisms: Mammals
Synopses of lectures
Lecture 1: The origin of mammals
The closest living relative, or sister-group of mammals is the group consisting of modern reptiles and birds. The transition from the hypothetical common ancestor of these two clades, a very primitive sprawling-limbed, simple-toothed, ectothermic amniote, is spanned by the extraordinary fossil record of the synapsids, or ‘mammal-like reptiles’, which dominated the land before the origin of the dinosaurs. The most primitive are the Upper Carboniferous and Lower Permian pelycosaurs such as the large carnivorous finback, Dimetrodon and herbivorous and fish-eating species. These were replaced by the more advanced therapsids, which included a range of specialist herbivores such as dicynodonts, and carnivores such as gorgonopsids. They flourished in the Upper Permian but most disappeared at the end-Permian mass extinction. In the Triassic, the cynodonts, which were the most advanced mammal-like reptiles, radiated. Their mammal-like dentition and jaws, and the more advanced, slender limbs approach those of mammals.At the end of the Triassic, the first mammals such as Megazostrodon had arisen. These were tiny shrew-like animals. The process of the origin of mammals was the result of the increasing ability to regulate accurately the internal temperature and chemical environments of the animal.
The first two thirds of subsequent mammal history consisted of the "Mesozoic mammals", which were all small. Only after the extinction of the dinosaurs at the end of the Cretaceous did mammals start to radiate into large forms as well. At first they were archaic groups, but during the course of the 65 million years of the Tertiary, the modern groups emerged. The last phase was the extinction of a majority of the very large bodied mammals around 12,000 years ago. There is controversy over whether this was due to environmental change at the end of the last Ice Age, or to human over-hunting.
Lecture 2: Diversity of modern mammals
There are three groups of modern mammals, the monotremes, marsupials and placentals. Monotremes consist only of the egg-laying duck-billed platypus and echidnas of Australasia. Although oviparous, they were also mammalian in so far as the egg is very small, and is directly nourished in the uterus. Apart from a few other primitive characters such as an interclavicle bone in the shoulder girdle and the absence of nipples to the mammary glands they are just like other mammals.
The marsupials radiated exclusively in South America, as the didelphid opossums and the caenolestid shrew-opossums, and Australasia were there are carnivores (dasyurids) such as the extinct thylacine and the quolls and Tasmanian devil, bandicoots and bilbies (Peramelines), a single marsupial mole, and many diprotodontid herbivores such as kangaroos, wallabies, possums, koalas etc.
The placentals which include the great majority of modern species, fall into 18 orders. Recent molecular evidence points to four supraordinal groups. Afrotheria evolved in, though are no longer restricted to Africa, and includes elephants, sirenians, hyraxes, the aardvark, golden moles, elephant shrews, and tenrecs. The Xenarthra are all South or Central American and consists of the armadillos, anteaters and sloths. All the rest constitute two groups that originated on the northern continents of Eurasia and North America, though have since spread throughout the world.
The phylogeny of mammals shows there were independent radiations in the northern continents, Africa, and southern Gondwana, often producing similar adaptive types such as large herbivores, insectivores, and anteaters, and in two cases, fully marine mammals.
Lecture 3: Homeostasis
Homeostasis is using energy to maintain a constant, internal environment in the face of a fluctuating external environment. Benefits include the range of niches opened up and the possibilities of increased complexity, especially of the brain. An additional benefit of high basal metabolic rate is a high level of maximum sustainable aerobic activity. The cost is the very large metabolic rate needed, and therefore very high rate of food assimilation, compared to the reptiles’ ectothermic strategy.
Temperature regulation by endothermy: high internal heat production and fine control of rate of loss by varying surface conductance. Thermoneutral range of ambient temperatures, below which a temporary increase in metabolic rate occurs, and above which evaporation has to be used, again temporarily. Adaptations for cold climates include large body size, decreased conductance, regional heterothermy and seasonal or daily torpor. There is not a general reduction in body temperature though, and a need arises for occasional opening of heat windows during high activity levels. Adaptations for hot climates include large body size or burrowing, and again regional heterothermy, particularly with adaptations for maintaining the brain at the correct temperature.
Chemical regulation concerns particularly water. Mammalian kidney can produce very concentrated urine by the loops of Henle in the kidney tubules. (Birds produce less concentrated urine, but can extract further water in the rectum and secrete very high salt solution in the salt gland.) Regulation of levels of ions, solutes etc. is basically by differential secretion into the ultrafiltrate flowing through the kidney tubules, and finely controlled by hormones.
Lecture 4: Mammalian feeding mechanisms
Basic mammalian feeding is represented by the living marsupial Didelphis. The incisors are sharp for selecting food, and the sharp canines for disabling insects etc. The premolar and molar teeth are used for a generalized puncture-crushing action. Finally the opposing sharp crests between the cusps of the molars are for shearing, or cutting the food like scissor blades. The jaw muscles are generalized, with equal sized temporalis and masseter for closing, and digastric for opening. All specialized mammalian feeding mechanisms were derived from such a system.
Carnivores exaggerated the shearing function of the molars, and developed a larger temporalis muscle to allow a wide gape and prevent disarticulation of the jaw by struggling prey. The guts are simple. Herbivores exaggerate the crushing action of the teeth and develop bunodont and in some cases high crowned, continuously growing hypsodont teeth. The masseter is the largest jaw-closing muscle, giving a large bite force and allowing horizontal grinding movements, but restricting the gape. There is a large fermentation chamber, either evolved from the stomach as in many artidactyls, or from the caecum of the hind gut as in perissodactyls and rodents.
Other mammalian adaptations are formicivory (ants), with reduction of teeth, long snout, and long sticky tongue; piscivory, with a row of simple, sharp teeth; molluscivory, with pebble-like teeth for breaking shells; and planktivory in whalebone whales using sheets of keratin hanging from the upper jaw.
Lecture 5: Mammalian locomotory mechanisms
Again, Didelphis provides a modern illustration of basic mammalian biology, in this case locomotion. Lateral undulation of the vertebral column is lost, and even replaced by dorso-ventral flexion-extension. The forelimb is smaller than the hindlimb, and about half the movement is generated by movement of the shoulder girdle (scapula) on the rib cage. No net locomotory thrust is produced, and it acts analogously to the wheel of a wheelbarrow. The hindlimb is larger, the pelvic girdle rigidly fixed to the sacral vertebrae. The foot is an extra extensible unit, and the hindlimb produces all the animal’s locomotory thrust when walking. Overal, non-cursorial mammal locomotion is neither faster nor more efficient than in a similar-sized reptile. The benefits are increased agility and an ability to keep on breathing while running.
Fossorial locomotion is found in many burrowing and anteating mammal groups. The limb bones are short, stout and the muscle attachments well away from the joints. This provides powerful, but slow movements of the limbs.
Cursorial locomotion is achieved by increasing the limb length, both directly by digitigrady or unguligrady, and indirectly in carnivores by bounding. The frequency of the stride is increased by reducing the moment of inertia of the limbs. This involves reducing the weight such as by reducing the number of digits, simplifying the joints, and making the bones hollow. Also, the mass of the limb needs to be as high up as possible, which is achieved by concentrating the bodies of the muscles near the top of the limbs and using long tendons to apply their forces lower down. The efficiency of transfer of muscle work into kinetic energy can be increased by storing and re-applying elastic energy, and by gaits.
Other specialized forms of mammal locomotion are saltatory, aerial, arboreal (both above and below the branches), aquatic to various degrees, and bipedality in humans.
Lecture 6: Primates and the origin of humans
Within the order Primates, Homo sapiens is successively a haplorhine, an anthropoid, a catarrhine, and a hominoid. Within the hominoids (apes and humans) it is most closely related to the African great apes Gorilla and Pan, and specifically to the latter, which is our closest living relative as indicated by a large amount of molecular data. The last common ancestor as measured by molecular sequence differences was between 5 and 10 my.
The fossil record of hominids, the lineage that diverged from our last common ancestor with chimps is poor by absolute standards, but enormous efforts are put into interpreting what there is, and the record has greatly improved in the last few years. In simple terms, the earliest well-known genus is Australopithecus, with the first discovered specimen being the Taung infant skull, named A. africanus (3-2mya). Subsequently, a more heavily built, specialised herbivorous form A. robustus (2-1mya) was found that refuted the idea that there was only a single evolving lineage from 'apes' to 'man'. The famous 'Lucy' is a more primitive, smaller species, A. afarensis, that dates from about 3.5 mya. And is associated with bipedal footprints. Specimens with a cranial capacity of 650ccs and more are placed in the genus Homo. The first discovery was Java man, and along with many subsequent specimens, is referred to as H. erectus (1.5-0.5mya). This genus was world wide, with Peking Man, Heidelberg Man, and the Turkana Boy of Africa all at least close relatives. Brain size is around 1000ccs. The African H. habilis (1.5mya) was more primitive than H. erectus with a smaller brain. H. sapiens was more advanced with a brain of about 1400ccs. Archaic versions (0.5 mya) occur in Europe and Africa, and modern forms are represented by Cro-Magnon Man (0.3mya). The stockiliy built Neanderthal Man is sometimes regarded as a subspecies adapted for cold, Ice Age conditions, and sometimes as a separate species H. neanderthalensis (0.3-0.03my).
Even earlier dated specimens of hominids than A. afarensis have been appearing regularly in the last few years. Ardipithecus is about 5my old, and most recent discovery, Sahelanthropus, is 6-7 my old which is close to the believed divergence date of hominids from chimpanzees.
The hominid fossil record shows that of the major human characteristics, bipedality evolved first, in Australopithecus afarensis. Tool-making had evolved in H. habilis. Increase in brain size occurred more or less gradually throughout the sequence of grades from A. afarensis, through H. habilis, H. erectus to H.sapiens. The latest excitement is the discovery of apparently dwarf hominids on the Indonesian island of Flores that lived a few thousand years ago. There is much argument about this, Homo floresensis.