5.9 THE LIFE CYCLE OF A MAMMAL - THE DOG
Dogs have been our helpers and friends for thousands of years. Dogs guard our homes, herd our domestic animals, and help us to hunt food and pests. In many cultures, dogs are also kept as pets and companions. Like us, dogs are mammals. They are a good example of how mammals reproduce.
In mammals, the young develop in a special place called the womb or uterus inside the female’s body. Like reptiles and birds, mammals use internal fertilisation. When dogs mate, the male places his sperms in the birth passage of the female. The sperms swim to the uterus and into the tubes where the female releases her egg cells (ova). The sperms meet and fertilise several ova. The fertilised eggs (zygotes) become embryos, which grow and develop inside the uterus for about nine weeks. When the embryos have developed into puppies, they are born alive through the birth passage of their mother.
Dogs mating Embryos developing
When they are born, puppies are blind and helpless. They do not open their eyes for several days. Their mother looks after them and feeds them on milk from special glands called mammary glands. The puppies obtain the milk by sucking at teats on these glands. Feeding in this way is called suckling. As puppies grow up, they gradually become independent, and learn to find their own food.
Dogs have several puppies at the same time. But in human beings, and some other large mammals, the female usually releases only one egg cell at a time, so only one baby is born.
As you may have guessed, the word mammary comes from the word mammal. Only mammals have mammary glands for suckling their young. A good way to find out if an animal is a mammal is to see if the female has mammary glands.
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A summary of the life cycles of animals. We have seen how several different classes of animals reproduce. Adult male animals produce sperm cells and adult females produce egg cells. When a male and a female mate, the sperm cells swim to meet the egg cells. The sperms fertilise the egg cells by fusing with them and forming zygotes. The zygotes grow and develop by cell division. In different ways, they develop into young animals. Eventually the young grow up and become adults themselves. Then it is their turn to reproduce. So long as the life cycle keeps going round and round like this, an organism cannot die out.
mating fertilisation zygote
ADULT YOUNG
growth and development
- 1. List all the ways in which people use dogs.
- 2. What is the other name for the womb of a female?
- 3. Compare the life cycles of a reptile, a bird and a mammal.
- 4. If we see a baby animal drinking milk from its mother, what class of animals does it belong to?
- 5. Try to guess what unusual things might happen, just occasionally, to cause the birth of human twins.
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