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Sunday, 2 December 2018

Growth in Animals

In higher animals, most cells except the nerve cells retain their ability to divide. Due to wear and tear, cells in animals become damaged all the time. These cells are broken down and replaced by new cells through cell division. Animal cells undergo rapid cell division and cell differentiation, but unlike plant cells, they undergo very little cell enlargement.

Growth patterns in animals

Most invertebrates and some vertebrates, like turtle and fishes , show unlimited growth in which the rate of growth slows down with increasing age. Growth in such animals take the form of an S shaped or sigmoid curve.

     Arthropods like insects and crustaceans have exoskeleton which cannot expand; so they moult from time to time in order to grow. Immediately after moulting, before the new exoskeleton hardens, the insect grows rapidly and takes in plenty of water to expand its exoskeleton. This means it can still grow slightly after the exoskeleton hardens. If its wet mass is used as growth parameter, an unusual ' step like' growth pattern is obtained. This type of growth is called intermittent growth. A normal sigmoid curve is obtained if dry mass is used as a growth parameter.

    Birds and mammals show limited growth, I.e. they grow to a maximum size and then stop growing. Usually, their growth is rapid until they become sexually mature. Then, very little growth occurs. Thus, their growth curve is a typical sigmoid. During old age or senescence, they show 'negative' growth because they loose weight. In humans, there ate two periods of rapid growth: during infancy and adolescence when they mature sexually.

Actors affecting growth

The growth of an animal is greatly affected by the availability of food, since animals cannot make their own food. Certain hormones also affect the growth of animals.

   In animal, hormones are secreted by endocrine glands. The hormones mainly concerned with growth are secreted by the anterior pituitary gland, the  thyroid gland and the gonads. Abnormal growth in humans, like stunted growth or excessive growth in height , is due to errors in the secretion of one of these hormones.

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