Adaptations: case studies When looking at organisms in your local environment you will try to identify features which can be interpreted as adaptations. The two examples below may help you in your understanding of how organisms are suited to their environments. Case study 1: mangroves Mangroves form woody plant communities in warm, shallow tidal water over more than half of the Australian coastline. Twenty-nine species occur along the tropical north coast of Australia, but the number of species decreases further south. Around Sydney only two species are found: the grey mangrove, Avicennia marina, and the river mangrove, Aegiceras corniculatum. In South Australia and southern Victoria only the grey mangrove is found. There are none in Tasmania. Mangroves are found on sheltered muddy shores and along estuaries. Depending on the conditions and the species, they range from 2 m high shrubs to 30 m high trees. Support and movement Mangroves are upright woody plants. They are anchored by complex root systems in the shifting environment of tidal mud flats. They have vertical anchor roots to which are attached spreading cable roots. Together these form a dense mass which helps stabilise the mud. Some mangroves support themselves on ‘stilt’ roots that lift the plant out of the salty water Gaseous exchange There is a lack of oxygen in the water-logged soil, which is exposed at low tide but covered with water at high tide. Mangroves of the genus Avicennia have aerial roots or pneumatophores which push upwards through the mud and salt water. Their tips have pores or lenticels through which gaseous exchange can occur Control of water balance Mangroves are halophytes (salt-tolerant plants). The water available to them is salt water with a high ion concentration. They control their salt level in three ways: • accumulation: their cells maintain higher than normal concentrations of cell solutes • secretion: some mangroves have salt glands on their leaves which actively secrete salt • exclusion: some mangroves use energy to excrete salt. Control of internal temperature Mangrove leaves have a thick cuticle and are hard and leathery. These features help to control water loss and prevent wilting in hot weather. In dense mangrove swamps the thick canopy of leaves helps maintain a lower temperature in the lower layers. Obtaining light Mangroves are the dominant plant in their community. The shape of the plant and arrangement of leaves ensure abundant light is available for photosynthesis. The leaves are dark green because they are rich in chlorophyll. The leaves high on the plant are angled and those lower down are horizontal to best capture sunlight. Reproduction Mangroves have flowers and, following pollination and fertilisation, fruits containing one seed develop. These seeds begin germination before they drop from the parent plant (Figure 1.34). The seeds are buoyant and dispersed by the tide. Their initial development, particularly of the root system, ensures the new plant can rapidly anchor itself and grow rapidly once it is deposited in the mud. FIGURE 1.34 In some mangroves the seeds germinate while still attached to the plant. When fully formed these propagules drop from the plant and spear into the mud, where they begin to grow. (a) Ceriops, (b) Bruguiera. Case study 2: kangaroos Members of the genus Macropus are all physically very similar. Larger specimens (over 20 kg) are called kangaroos, and smaller species are known as wallabies and wallaroos. Species inhabiting steep, hilly areas rather than flat plains are usually called wallaroos. Few species are solitary—most congregate in groups or mobs. Kangaroos are widespread across Australia. They are all grazing herbivores that feed on grasses and herbs. The red kangaroo inhabits inland plains. The western and eastern grey kangaroos are found in grasslands, eucalypt woodlands and open forests from Tasmania to central Queensland and across to Western Australia. The antelopine wallaroo is found in the tropical north of Australia from Queensland across to Western Australia. The euro, or common wallaroo, is found in central and southern Australia. Support and movement Kangaroos have an internal bony skeleton. Their well-muscled hind legs are far larger than their forelegs (Figure 1.35). Only the hind legs are used when the animal is travelling at high speed. This method of bounding along is more efficient in terms of energy use when compared with animals which run on all four legs. When moving slowly, kangaroos ‘hop’. Their weight is pressed down on their forelimbs and tail and their large hind legs are swung forwards. FIGURE 1.35 The long tail of kangaroos and their relatives is an adaptation that helps the animals to balance when hopping. The long tail of the kangaroo is a useful structure. It is used as a balancing counterweight when bounding, as an extra limb when hopping, and it helps the kangaroo to remain upright when standing still. Gaseous exchange Kangaroos have lungs as internal respiratory surfaces. Control of water balance Kangaroos in arid areas, such as the red kangaroo and euro, can survive for long periods without drinking water, provided there is sufficient green plant material available. Kangaroos reduce water loss by sweating only during exercise. When they stop moving, sweating stops. Control of body temperature Kangaroos that live in hot, dry regions seek the shade of rock crevices and caves during the hottest part of the day. Kangaroos in other areas will bask in the sunshine, but when conditions are hot they seek the shade of trees and bushes. When the weather is hot, euros may lick their forelimbs where the blood vessels run close to the surface and heat is lost from the body. It is thought that the evaporation of the saliva has a cooling effect. Obtaining light Kangaroos have binocular vision. They are mainly nocturnal animals but may also be active in early morning and early evening. Reproduction Kangaroos are marsupial mammals. They have internal fertilisation and a very short gestation period in the uterus. At birth the young climb into the mother’s forward-opening pouch. They attach to a teat and continue development while suckling (Figure 1.36). When they leave the pouch there is a weaning period before parental care ends. In the red kangaroo the young are born after 33 days in the uterus and weigh less than a gram. They remain suckling in the pouch for 235 days. They leave the pouch weighing 4–5 kg and have a weaning period of up to 4 months, during which they suckle and eat grass. Kangaroo population numbers are controlled through reproduction. Under good environmental conditions numbers can increase rapidly, because female kangaroos can be almost continuously pregnant when adult. They mate again directly after giving birth. If the mother is still suckling her newborn young, the fertilised egg does not develop until the young leaves the pouch. This is known as delayed implantation. At any one time a female kangaroo may have a joey (young kangaroo) being weaned, a young one being suckled in the pouch and an embryo in the uterus awaiting development. Kangaroos have the amazing ability to produce two kinds of milk at the same time. The milk produced by the teat for the developing young in the pouch contains much less fat than the milk produced by the teat being used by the joey outside the pouch. When environmental conditions are not good, such as in times of drought, young joeys do not survive and any fertilised egg does not implant. Females do not begin reproducing again until conditions improve.