Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 46.2 Fertilization Depends on Mechanisms That Bring Together Sperm and Eggs of the Same Species
Fertilization unites sperm and egg and may occur externally or internally depending on the species. Animals have evolved a wide range of reproductive systems and behaviors to ensure that gametes successfully meet and that offspring survive.
1. External vs. Internal Fertilization
- External fertilization: Eggs are released into the environment, where sperm fertilize them externally (e.g. in many amphibians, fish)
- Requires moist environments to prevent desiccation and enable sperm motility
- Often involves spawning, a synchronized release of gametes triggered by cues like temperature or moon phases
- Internal fertilization: Sperm is deposited in or near the female reproductive tract, fertilizing the egg within
- Enables reproduction in dry environments and usually involves specialized reproductive anatomy and mating behavior
2. Behavioral and Chemical Coordination
- Some species use courtship behaviors to synchronize gamete release, improving fertilization success
- Pheromones: chemical signals that influence mating behavior; used widely in insects and some vertebrates
- Pheromones are active at low concentrations and dispersed through air or water
3. Gamete Production and Delivery
- Gamete-producing cells often differentiate early in embryonic development
- In animals without gonads (e.g. palolo worms), gametes develop from coelomic lining cells
- Gonads (testes and ovaries) in most animals produce and deliver gametes through accessory ducts and glands
- In insects, spermathecae allow females to store sperm for later use—ensuring fertilization occurs under favorable conditions
4. Ensuring Offspring Survival
- Species with internal fertilization generally produce fewer gametes but have higher zygote survival
- Protection strategies include:
- Shells and membranes in reptile and bird eggs
- Internal gestation in marsupials and placental mammals
- Parental care, even in some invertebrates like giant water bugs, increases survival rates
- Mammals show a range from short internal development (marsupials) to long-term placental nourishment (eutherians)
5. Anatomical Diversity
- Insects: Males use claspers and accessory glands; females store sperm in spermathecae
- Vertebrates: Reproductive anatomy varies—some have cloacas, others have separate reproductive and excretory openings
- Complexity in reproductive structure often reflects the reproductive strategy and environment
6. Reproductive Competition and Choice
- In species where females mate with multiple males, sperm competition occurs
- Experiments in fruit flies show that females can influence which sperm fertilize their eggs
- Females may eject or replace previously stored sperm, showing active selection mechanisms
In a Nutshell
Fertilization strategies in animals reflect adaptations to environmental conditions and reproductive success. External fertilization requires timing and moisture, while internal fertilization supports embryo protection and parental care. Complex behaviors and anatomical features—like sperm storage and courtship—enhance the chances of successful reproduction and survival of offspring.