school energy&environment

Animals’ reproduction

Most of animals, even the simplest ones, have a sexed reproduction, which allows to increase the genetic variability of individuals and organisms’ diversity. Their internal organs, which are different in males and females, are called gonads and specifically produce gametes (sexed cells: sperms and egg-cells). They are bound to join and form a single cell called zygote or fertilized egg, from which the embryo, that is the new organism, will develop.
Some animals, even invertebrates, are hermaphrodites, as they are able to produce both sperms and egg-cells. The chance to be able to behave as male and female individuals is an advantage for animals like earthworms, snails and slugs, very slow animals that in this way double their possibility to meet a mate. In this case both individuals will be able to produce new offspring.
Where does fertilization occur?
Animals that live in a water habitat release a high number of gametes into the water, where they will meet to form the zygote. This is called external fecundation. Those organisms that live on the land had to solve a serious problem: providing gametes with a liquid environment that allows their survival and fertilization. Some amphibians have adopted a simple solution: male and female exchange particular signs and go back into the water to release the gametes at the same time. Almost all terrestrial animals have adopted another solution: the male introduces the sperms into the female body, where the fertilization occurs (internal fecundation). This solution is advantageous as it protects the offspring during the first growth stages. The male individuals of insects, vertebrates and many animals that have a specialized organ (sexual organ) to insert the sperms inside the female organ and in particular into a hollow organ (vagina or cloaca). Other organisms have found particular solutions. For example some male arthropoda like mites and scorpions or some male amphibians like tritons build “bags” of sperms (spermatophore) and it’s the females who insert them into their own bodies. As for some spiders and squid, the male collects the bag he has formed and, by helping himself with his forelegs and tentacles, he inserts them into the female body.
Embryo protection/ The care for the new individual during his growth
From the zygote, the cell that forms after the insemination, the embryo develops, that is the new organism that at the beginning needs to be protected. In particular the embryo will have to be fed, oxygen will be needed for its breathing, and carbon dioxide will have to be kept away. Animals have solved this problem in two ways:

• Insects, reptiles, birds and mammals monotremata like echidna (oviparous animals) lay the eggs in the same environment as they live. The embryo grows inside the egg that contains all the nutritional substances it needs. The eggshell protect it from dehydration, but allows oxygen to enter and prevents carbon dioxide from disperse outside. Some parents, however, take care of the eggs by building a nest and hatching them. Once the new organism has developed and gets free from the egg protection, he will need more care.
• Mammals are viviparous animals since the females keep the embryo inside their body until a determined stage in its development. Marsupialia (kangaroo and opossum) deliver the babies when they have not completely developed yet. Then they are kept inside a special pouch until the growth has been completed. Mammals other than marsupialia and monotremata are called placentalia, as they can count on a structure (placenta) that allows the embryo to grow completely inside the female body, guaranteeing an efficient supply of nutrients.