Type of Fertilization

Reproductive System

Animals’ reproductive systems can be divided into the internal reproductive organs and the external genitalia. The gonads are the actual organs that produce the gametes. In the male, testes (singular = testis) produce sperm, and in the female, ovaries make eggs.

In most animals, individuals are either definite males or definite females. However, in some species, individual organisms are both male and female. Hermaphroditism is when one organism has both sexes. Earthworms and garden snails always have both male and female organs, and when, for example, two earthworms mate, they fertilize each other. A special variation on the theme is sequential hermaphroditism, in which an organism changes sex during its life. If an organism is female first and later changes to male, that organism is protogynous, and if the organism is male first and changes to female, it is said to be protandrous. In different species, sequential hermaphroditism can be influenced by the organism’s age or size or by various environmental/climatic factors.

While most higher animals reproduce sexually, there are some species in which the females can, under certain conditions, produce offspring without mating. Parthenogenesis is the ability of an unfertilized egg to develop and hatch. This seems to be especially prevalent among insects. Some of the giant walkingsticks at the Zoo are females who, without mating, lay eggs that hatch into more females generation after generation. Other insects, like some aphids, have complicated life cycles that involve sexually-reproducing generations alternating with parthenogenically produced generations. In honeybees, fertilized eggs turn into females (workers and queens), while unfertilized eggs, which are only produced in the spring, turn into males.

In sexual reproduction, there must be some way of getting the sperm to the egg. Since sperm and eggs are designed to be in a watery environment, aquatic animals can make use of the water in which they live, but terrestrial animals must, in some way, provide the wet environment needed for the sperm to swim to the egg. There are, thus, two major mechanisms of fertilization. In external fertilization, used by many aquatic invertebrates, eggs and sperm are simultaneously shed into the water, and the sperm swim through the water to fertilze the egg. In internal fertilization, the eggs are fertilized within the reproductive tract of the female, and then are covered with eggshells and/or remain within the body of the female during their development.

In species with external fertilization, at an appropriate developmental stage, the eggs hatch, and the new young simply swim away. However, females of species with internal fertilization must, at some point, expel the growing young. There are three general ways of doing this:

·      Oviparous organisms, like chickens and turtles, lay eggs that continue to develop after being laid, and hatch later.

·        Viviparous organisms, like humans and kangaroos, are live-bearing. The developing young spend proportionately more time within the female’s reproductive tract, portions of which are specially-modified for this purpose. Young are later released to survive on their own.

·         Ovoviviparous organisms, like guppies, garter snakes, and Madagascar hissing roaches, have eggs (with shells) that hatch as they are laid, making it look like “live birth.”