Reproduction Strategies in Animals

Introduction

Reproduction is the ability of an organism to produce a new generation of itself.
Reproductive effort involves producing the maximum number of surviving offspring using the least amount of energy.
Asexual reproduction requires less energy than sexual reproduction.
Sexual reproduction requires more energy.

Courtship

Courtship involves behavior and/or signals designed to attract other animals for mating and breeding.
Simple courtship:
- Uses chemicals (pheromones).
- Visual cues (brightly colored body parts).
- Auditory signals (sound to find each other).
Example:
- Moth females produce species-specific pheromones to guide males.
- Frogs return to water for mating and breeding, using singing (grunt and croak) to attract females.
- Male birds use species-specific songs and special plumage (bright, long tail feathers).
Complex strategies:
- Unique to each species.
- Females favor males that are:
  - Larger.
  - Display more energy during courtship dance.
Examples:
- Springboks engage in an annual rut (period of sexual excitement) involving defending territories with loud grunts, attacking vegetation with horns, and depositing urine and dung in ritualized displays.
- Advantages of rutting: it times breeding when mothers are in good condition with enough food present when young are born. It allows breeding only if conditions are favorable.
Courtship feeding in some birds:
- Male African hoopoes feed females with insects to allow them to save energy for breeding and brooding.

How Courtship Maximizes Reproduction

Finding suitable mates, such as the strongest male.
Timing to ensure males and females are ready for reproduction at the same time.
Males usually spend the energy, while females conserve energy for breeding.

External Fertilization

Occurs in water (fish and frogs).
Reasons for it not being ideal:
- Many eggs are wasted, requiring a large amount to be produced.
- Many eggs (and young) are eaten.
- Fertilization is not certain.
How external fertilization maximizes reproduction:
- Huge number of eggs and sperm increases the probability of fertilization, with sufficient amounts left after predation.
- Example: Female frog lays 2000-3000 eggs.
Courtship rituals:
- Many fish swim side by side when releasing eggs and sperm to ensure male and female gametes are close together.

Internal Fertilization

Occurs in terrestrial vertebrates.
No water is involved; sperm cells (male gametes) are released directly into the body of the female (mating or copulation).
Eggs are fertilized inside the female body.
Birds and reptiles mate using a cloaca, a single opening at the bottom of the abdomen.
During mating, both males and females line up cloaca for transfer of sperm.
Male mammals use a penis to introduce sperm into the female during copulation.
Once inside the female, sperm swims to the egg in fluid (seminal fluid and mucous membrane fluid).
How internal fertilization maximizes reproduction:
- More certain, as gametes are placed as close together as possible.
- Fewer gametes are required.
- Energy saved on gamete production can be used on other purposes to maximize reproduction.
- Example: Internal development via the placenta (or protective shell and increased yolk in birds).

Ovipary, Ovovivipary, and Vivipary

Terms used to describe the moment at which the future offspring separates from a parent.
Include:
- Location of embryo development.
- Nourishment of embryo and fetus.

Ovipary

Eggs develop outside the animal.
Fertilization may be external or internal.
Egg yolk is the only source of nourishment that developing embryos receive until they hatch.
How ovipary in aquatic environments maximizes reproduction:
- Most fish and amphibians.
- Eggs are released into water with external fertilization.
- Large amounts of eggs with a small amount of yolk.
- Eggs and embryos are vulnerable to predation.
- Vast number ensures survival.
- High energy input from the female.
- Less energy required for yolk production and parental care.
Catsharks:
- Females lay eggs in mermaids' purses.
- Egg cases contain yolk and embryo.
- No large amounts of eggs required because the egg case protects the embryo.
How ovipary in land environments maximizes reproduction:
- Birds and most reptiles.
- Few eggs are produced (especially by birds).
- Saved energy can be used for:
  - Producing eggs with more food (nutrient-rich yolk and protein-rich albumin), so the animal is more fully formed at birth.
  - Protecting and incubating eggs before hatching.
  - Parental care for young.
The shell protects the developing embryo from pathogens, predators, physical damage, and dehydration.
Examples: Python and green mamba.

Ovovivipary

Internally fertilized eggs are kept inside the female body until they hatch.
Offspring are born “live.”
Nourishment for the embryo:
- Large amounts of yolk.
- Not dependent on the mother, only for protection and gaseous exchange.
Example: Ragged tooth sharks.
Embryos feed on yolk and later on other developing embryos (intra-uterine cannibalism), resulting in only 2 pups being born per litter.
Gestation period is 9-12 months.
How ovovivipary maximizes reproduction:
- Few eggs are produced, leading to less energy expenditure by the mother.
- Developing embryo is less vulnerable to predation and cold temperatures.
- Young are born fully developed, able to get their own food and escape predators.

Vivipary

Internal fertilization.
Eggs do not have a shell.
Retained egg forms an embryo, then a fetus, which develops inside the mother.
Nourishment is obtained through a placenta.
Born live.
Example: Placental mammals (eutherians) and 55% of sharks and stingrays.
How vivipary maximizes reproduction:
- Number of eggs is reduced.
- More energy is spent on:
  - Nourishing and protecting the embryo and fetus.
  - Providing parental care after the young are born.
- Greatly increases the chance of offspring reaching reproductive age.
Example: Hammerhead shark. The egg sac joins the mother and developing young, forming a connection with oviduct tissue, receiving nourishment from here. Young are born live and fully functional.

Summary of Embryo Development:

Vivipary:
- Definition: The embryo develops and is nourished within the mother's body.
- Examples: Most mammals, including humans, dogs, and cats.
- Characteristics: Fertilization and embryo development occur inside the female, and the mother provides nutrients directly to the embryo.
Ovipary:
- Definition: The embryo develops within an egg outside the mother's body.
- Examples: Most amphibians and reptiles, and all birds.
- Characteristics: Internal fertilization, and the eggs are provided with a lot of yolk.
Ovovivipary:
- Definition: The embryo develops within an egg inside the mother's body until it hatches.
- Examples: Some species of fish and reptiles (certain snakes).
- Characteristics: Internal fertilization, and the eggs are nourished by yolk from the egg sac.
- Other features: The mother provides gas exchange, and the young remain inside the mother's body for a time after hatching.

Amniotic Egg

Early vertebrates (fish and amphibians) lay eggs in water, and eggs have no shells.
Later vertebrates, the amniotes (reptiles and birds), lay eggs on land and there's no need to return to water for breeding.
Mammals are also amniotes, but no eggs are laid; eggs are retained in the body.
Amniotes: After fertilization, extra-embryonic membranes develop in the egg.
Embryonic membranes are membranes filled with fluid, allowing the embryo to survive and develop on land (terrestrial).
Embryonic membranes:
- Amnion
- Allantois
- Yolk sac
- Chorion
Eggs are known as amniotic eggs (formed due to evolution).
How amniotic eggs maximize reproduction:
- Embryo is able to withstand harsh terrestrial conditions.
- Functions:
  - Fluid-filled amnion surrounds the embryo and protects against dehydration and mechanical injury.
  - Allantois is a reservoir for nitrogenous waste (in reptiles and birds). Aquatic animals' waste passes directly into water.
  - The yolk sac holds nutritious food for embryo development.
  - The chorion facilitates gaseous exchange together with the shell (birds and reptiles) and forms the placenta with the endometrium (mammals), which is evolutionary and allows fuller and safer embryo development.

Precocial and Altricial Development

Common in birds and mammals.
Refers to post-natal development (once they are born).
Provide nourishment to developing embryos and young.
Protect from predators.
Offspring able to survive to reproductive age.
Precocial development:
- Offspring is fully developed when hatching or being born.
- Parental energy expenditure is focused on pre-natal developments.
- Females are less involved after birth.
- These species at birth or hatching:
  - Have open eyes and hair/down.
  - Brains are large relative to their size.
  - Immediately active and mobile (may still be unsteady on feet).
  - Not confined to nests (birds have well-developed feet).
- How precocial development maximizes reproduction:
  - Offspring can find food on their own (able to feed themselves).
  - Look after themselves against predators, able to flee, defend, or camouflage themselves.
  - Able to run and keep up with herd.
  - Example: Ostrich chick
- Examples of precocial species:
  - Ground-nesting bird species (ducks, ostriches, penguins, domestic chickens).
  - Large mammals (elephants, grazing animals, and hares).
Altricial development:
- Not well-developed when born/hatched.
- Parent energy expenditure is focused on post-natal development.
- Altricial species at birth:
  - Naked with no hair/down.
  - Unable to walk/fly (poorly developed legs and wings).
  - Closed eyes.
  - Have colored mouth lining or gape-edge (birds).
  - Rely on parents for warmth (no thermal regulation), food, transport, and protection.
- Post-natal care spreads over a long time.
- How altricial development maximizes reproduction:
  - Parental care is involved, leading to more energy expenditure on breeding, feeding, keeping warm, and protection.
  - Reach reproductive maturity faster:
    - Easier to feed (stay in nest).
    - Rapid growth from being fed large nutrient-rich food items (insects and regurgitated meat).
    - Safer from predation because nests are inaccessible and only leave nest when fully grown and ready to evade predators.
- Examples of altricial species:
  - Small mammals that produce big litters, such as rodents, dogs, and cats.
  - Humans.
  - Tree-nesting birds that have nests away from danger and predators.
  - Marsupials: young are cared for in a pouch (kangaroos).

Parental Care

Definition: Any pattern of behavior where a parent spends time and energy on improving the survival, condition, and future reproductive success of their offspring.
High energy input both before and after birth.
Parental care means fewer eggs need to be produced, and all energy can go into care and not egg production.
Stages of care-giving:
- Pre-natal care: Guarding eggs, building nests, carrying broods, incubating eggs, placental nourishment (mammals).
- Post-natal care: Providing food, protecting offspring, teaching offspring.
Fish:
- No parental care (no feeding of young).
- 20% do show parental care by building nests, fanning eggs to increase oxygen supply, guarding eggs and hatchlings from predators, cleaning eggs to eliminate fungi, having a brood pouch.
- Example of parental care in fish: Sea horse.
Amphibians:
- Return to water for reproduction.
- Parental care minimizes loss of eggs by predators.
- Accomplished by:
  - Guarding eggs.
  - Males carrying eggs of midwife toads on their rear ends.
  - Building breeding sites away from water. A female frog secretes a fluid that she churns up with her back legs to form a ball, which is then hung on a tree over water. Fertilized eggs remain protected and moist, and hatching tadpoles drop into the water.
Mammals:
- Intense parental care is required, with an innate (natural) drive for caring for new-born.
- Maternal care involves:
  - Lactation (feeding): Milk composition is best possible for the development of each species (specific to each species).
  - Protection from predators and cold.
  - Teaching to gather food.
  - Licking new-borns directly after birth to establish a relationship. If no licking, the new-born rejects the mother.
- Paternal care: Only a few mammals provide feeding and protection against predators.
- Special nests (burrows) are made for warmth and protection. Example: African wild dog.

Effectiveness of K-strategy and r-strategy for Survival in Reproductive Strategies

K-strategy: Example, elephants.
- Have a long life (70 years).
- Sexually mature in their teens but only mate at about 20 years.
- Typical characteristics of K-strategy:
  - Few offspring produced - elephants only have one baby (3 years apart).
  - High level of parental care - the entire herd looks after and protects young.
  - Usually climax species - produce slightly more offspring than the maximum that the environment can hold (population density is close to carrying capacity).
r-strategies: Example, turtles.
- Leatherback and loggerhead turtles.
- Long lifespan (50-100 years).
- Sexually mature at 17 years.
- Energy saved for egg production.
- Typical characteristics of r-strategy:
  - Numerous eggs are laid (multiple clutches of 90-130 eggs each).
  - No parental care - few offspring and eggs survive (10 per 1000 eggs reach adulthood).
  - Low survivorship due to predation of eggs, hatchlings, and in the ocean itself. Safe when they reach currents.
- Predation decreases with increasing body size.

Survivorship Curves

Expresses the pattern of mortality for a species.
Graph shows the numbers of survivors out of 1000 during the life cycle of three different species.
Convex curve (A):
- Explanation:
  - Most offspring become adults.
  - Mortality mainly occurs among older individuals.
- Reason: High levels of parental care lead to a higher rate of survival.
- Examples: K-strategy species (elephants).
Straight line (B):
- Explanation:
  - Mortality decreases steadily with time.
  - Young organisms are just as likely to die as older organisms.
- Examples: Birds and plants.
Concave curve (C):
- Explanation:
  - Few offspring reach adulthood.
  - Higher mortality rate when young.
  - When they become established, life expectancy improves.
- Reason: No parental care leads to a low survival rate.
- Examples: Turtles, r-strategy species - a large number of eggs are produced, but only a few survive.