Reproduction: How Life Continues | Class 9 Science Notes

11.3 Sexual Reproduction in Animals

Sexual reproduction in animals involves the fusion of male and female gametes, but the mode of fertilisation varies among species. In many aquatic animals like frogs and fish, fertilisation occurs externally; females lay eggs in water, and males release sperm over them. This external fertilisation results in a large number of eggs, but survival rates are low due to predation and environmental factors. In contrast, reptiles, birds, and mammals exhibit internal fertilisation, where sperm fertilises the egg inside the female body, providing greater protection to the developing embryo and higher survival chances. The chapter compares reproductive strategies across animals, highlighting differences in habitat, fertilisation mode, egg production, and offspring survival. For example, fish and frogs produce hundreds to thousands of eggs with low survival, while birds and lizards produce fewer eggs with moderate to high survival. Some animals have larval stages that feed externally before metamorphosing into adults, such as butterflies and frogs. Mammals nourish embryos internally and provide post-birth care, including breastfeeding. These variations reflect adaptations to different environments and reproductive challenges.

📊 Diagram: Fig. 11.17: Life cycle of a butterfly

🔗 Connection: Prepares for detailed study of human reproductive system and processes.