How Animals Reproduce

Core Concepts

Asexual reproduction: one parent, clones

Asexual reproduction produces offspring without gametes or fertilization. Because the offspring arise by mitosis from a single parent, they are genetically identical to the parent (barring new mutations). Common modes include:

• Binary fission: The parent splits into two roughly equal offspring. This is typical of single-celled organisms (and is the same term used for bacterial division), but it also describes the division of some simple multicellular animals.
• Budding: A new individual grows out of the parent's body as a bud, then detaches. Hydras (small freshwater cnidarians) reproduce this way; the bud is a smaller copy that eventually separates.
• Fragmentation: The parent's body breaks into pieces, and each piece can grow into a complete new individual. Sea stars can regenerate a whole animal from a detached arm that includes part of the central disk, and some annelid worms regenerate from fragments.
• Parthenogenesis: A female produces offspring from unfertilized eggs — no sperm needed. This is common in aphids (which can alternate parthenogenetic and sexual generations) and occurs in some reptiles, fish, and other animals.

The big advantages of asexual reproduction are speed and efficiency: no need to find a mate, no energy spent producing gametes and courtship, and every individual can reproduce. The big disadvantage is lack of genetic variation: if the environment changes or a new pathogen arrives, a population of clones may have no individuals with a resistant combination of genes, and the whole population can be wiped out.

Sexual reproduction: two parents, new combinations

Sexual reproduction requires meiosis, which produces haploid gametes (gametes carry one set of chromosomes; body cells carry two). Fertilization — the fusion of an egg and a sperm — restores the diploid chromosome number and creates a genetically new individual. The genetic novelty comes from three sources: crossing over during meiosis (chromosome segments are exchanged), independent assortment (each gamete gets a random mix of maternal and paternal chromosomes), and the union of two different parents' gametes.

Most animals are dioecious — individuals are either male or female — but some are hermaphrodites, with both male and female reproductive organs in one individual. Simultaneous hermaphrodites (such as earthworms and many snails) can produce both eggs and sperm at the same time; many still exchange sperm with a partner rather than self-fertilize. Sequential hermaphrodites change sex during their lives — for example, some reef fish switch from female to male (protogyny) or from male to female (protandry) as social conditions change.

Sexual reproduction's main cost is that it is expensive: two parents are needed (or at least gametes from two sources), energy goes into courtship and gamete production, and each parent passes on only half its genes to each offspring. Its main benefit is genetic variation, which helps populations adapt to changing environments and resist pathogens.

Sex determination: how an individual becomes male or female

Sex can be determined in several ways. In many mammals (including humans), sex is determined by chromosomes: females have two X chromosomes (XX) and males have one X and one Y (XY); the sperm determines the offspring's sex because it can carry either an X or a Y. Birds and some other animals use the opposite system (ZW/ZZ, where the female has the two different sex chromosomes). Some insects (such as bees and ants) use haplodiploidy: females develop from fertilized eggs (diploid) and males from unfertilized eggs (haploid). In some reptiles, sex is determined by environmental temperature during egg incubation rather than by chromosomes — for example, in many turtles, warmer incubation temperatures produce one sex and cooler temperatures the other. Sex determination is diverse; there is no single universal mechanism.

Fertilization: external or internal

Fertilization is the union of egg and sperm. External fertilization occurs outside the parent's body — the female releases eggs into the environment and the male releases sperm over them. It requires a watery medium (sperm cannot swim in air) and usually requires the two parents to release gametes at the same time and place, often cued by environmental signals. It is common in fish and amphibians. Internal fertilization occurs inside the female's body; sperm are deposited during mating and swim to meet the egg. It is typical of reptiles, birds, mammals, and many insects. Internal fertilization protects the gametes and the developing embryo, allows fertilization on land, and is associated with more parental care of fewer offspring.

Reproductive strategies: many small or few large

Animals face a fundamental trade-off: resources spent on reproduction are limited, so a species generally produces either many small offspring with little parental care or few large offspring with extensive care. These are sometimes described as extremes of a continuum (the terms r-selected and K-selected are commonly taught, with the caveat that real species fall between the extremes): organisms that reproduce rapidly in unstable environments, producing huge numbers of offspring, most of which die young; versus organisms in stable environments that invest heavily in a few offspring that each receive substantial care and have a high chance of survival. Humans, elephants, and most birds fall toward the "few, well-cared-for" end; many fish, insects, and marine invertebrates fall toward the "many, mostly unprotected" end. The strategy that succeeds depends on the environment, not on any absolute superiority.

Common Confusions