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Part A – Follow-up of Mini-project
Packetids lived in riverine areas that were frequent flooding occurred. Also, the osteosclerosis on packetids’ bones makes them become heavier and indicates that packetids are not good at running (Thewissen & Williams, 2002). It is a disadvantage of them to compete with other terrestrial animals on the land. The adaptation of aquatic environment helps them to expand their food choices.
Osmoregulatory system of whales was changed to adapt the environment from land to freshwater and ocean. In order to adapt seawater, the excess salts will be excreted via urination (Thewissen & Bajpai, 2001). There is a transition period for whale to accommodate the marine water, they increased their seawater consumption gradually. Packetids depended on freshwater; Ambulocetids and Remingtonocetids consumed both freshwater and seawater; Protocetids and modern cetaceans can live in ocean (Thewissen & Williams, 2002).
Part B – New Topic
Title: The evolution of bat from land to flight
The transition of bat from land to flight is controversial within the scientists. Darwin suggested that the ancestor of bat is a small, quadrupedal terrestrial animal, then changed into gliding and finally transited to flight. In contrast, some researches proposed that no gliding state in this evolution since there was no evidence can prove gliding is an intermediate stage of flight (Adams & Shaw, 2013). The reason of bats need to fly and the adaptations of flight in bats will be discussed in the following paragraphs.
The lower competitive ability of bats may prompt them to evolve. Several studies found that body size is highly associated with competitive dominance. Animals with larger body size has better competitive ability than the smaller animals (Glazier & Eckert, 2002). The small body size of bat is a drawback for them to contend with other larger animals in the same living habitat. Hence, the bats are likely to increase their competitiveness via flight. Flight empower bats to access larger range of habitat. Some studies shown that bats can fly greater distance at lower energetic cost (Fenton, 1997). The larger distance they can be travelled, the lower dependence on a specific environment and more prey they may discovered.
Elongation of forelimb digits supports wing membrane formation and flight in bat. Bone morphogenetic proteins (Bmps) is one of the signal that promotes bone elongation on third, fourth and fifth digits. However, the Bmps may cause cell death in bats. Gremlin is an antagonist of Bmps which prevents the Bmp-mediated apoptosis occur (Cooper et al., 2012). Therefore, the digits can be proliferated successfully with the secretion of Bmps and gremlin. The wing membrane is formed between digits III and V which contains muscles, hairs and blood vessels. Each of them play an important role on bat’s flight. The membrane allows the bat to adjust the stiffness of the wings, increase the surface area of wings and determines the pattern of wing muscles (Saikia, 2007; Tokita et al., 2012). The muscles regulate the curvature of wing during flying (Myers et al., 2015). The hairs in wing membrane act as a sensor which used in control the flight velocity (Sterbing-D'Angelo et al., 2011).
Apart from the elongation of bat’s digits, the hindlimbs of bats are also modified. In order to reduce the weight, the modern bats have shorter hindlimbs and lower bone mass than the ancestral bats (Adams & Pedersen, 2000; Cooper et al., 2012). The hindlimbs also play an essential role on bat flight. The angle of bat hindlimbs can be rotated is much greater than other ancestral mammals. Owing to the bat wings are attached to the hindlimbs, the hindlimb movement may change the wing shape and angle of attack, thus allows hindlimbs to change the three dimensional shape of bat wings (Cheney et al., 2014). In addition, the hindlimbs also influence the reposition of tail membrane, which is considered as a crucial structure in pitching moment, by adjusting the hindlimbs’ angle (Gardiner et al., 2011).
Energy metabolism genes of bats have been changed to adapt the new mode of locomotion. As the metabolism rate for bat flight is 3 to 5 times higher than terrestrial animals exercising on land, more energy requires for bat flight. Oxidative phosphorylation is one of the stage in cell respiration that produces the most adenosine triphosphate (ATP). Mitochondrial DNA is involved in this metabolism pathway and meet the extra energy requirement due to the increased energy metabolism. Concerning the digestive process, the encoded secretory proteins, such as carboxyl ester lipase, involve in lipid hydrolysis (Shen et al., 2010). Bat muscles are mainly depended on fats as the fuels because the storage of fats is more efficient than glycogen and fats have higher energy density than carbohydrates and proteins, as well as bats can utilize the fat rapidly to support flight after consumption (Phillips et al., 2014).
Proper oxygen management is important for bat during the cold night and flying at high altitudes, therefore its respiratory and cardiovascular system have been strengthened. The lung volume of bats is approximately 72% higher than the non-flying mammals with similar weight and bats have a thin alveolar-capillary barrier, leading to the high oxygen diffusion capacity. Except for having the strong respiratory system, bats also have the greater cardiac output and higher level of haemoglobin in mammals with similar mass, which help to transport oxygen more efficient (InTech, 2011). Thus, the modification of respiratory and cardiovascular system enable bats to adapt the flight and survive in extreme environment.
In conclusion, the ancestor of bats may has lower competitive ability in their living habitats. Evolution from land to flight is a way for them to survive rather than become extinct. The adaptations of bat flight are elongation of forelimb digits, modification of hinblimbs, mutation of energy metabolism genes and strengthening of respiratory and cardiovascular system. These adaptations make bats more suitable for flight.
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