Mating Systems, Courtship, and Parental Investment

A mating system is the pattern by which males and females in a species pair up to reproduce, and the main types are monogamy, polygyny, polyandry and promiscuity; the pattern depends largely on parental investment, the time and resources that each sex spends on offspring. In brief: the sex that invests more in each offspring (usually the female) is choosy, while the sex that invests less (usually the male) competes for mates. Courtship displays let females assess the quality of males, and sexual selection explains traits such as antlers and bright plumage.

Animals differ widely in how they find mates and raise young: albatrosses pair for life, elephant seal males control harems of dozens of females, and in a few species, such as seahorses, the male carries the young. This guide defines the mating systems, explains parental investment and sexual selection, describes courtship, compares the systems in a table, works through a numerical example of reproductive success and corrects common mistakes.

What are mating systems?

• Monogamy: one male and one female form a pair, for a breeding season or for life. It is common in birds (about 90 percent of bird species are socially monogamous), where both parents are needed to feed the chicks. Social monogamy does not mean genetic monogamy: DNA studies show that many “monogamous” birds have offspring from extra-pair matings.
• Polygyny: one male mates with several females. It occurs where males can monopolize resources or females, as in red deer and elephant seals, where dominant males hold harems. In some birds, males hold territories with good resources.
• Polyandry: one female mates with several males. It is rare. In jacanas and phalaropes, females compete for males, and the males incubate the eggs and care for the chicks.
• Promiscuity: males and females mate with several partners, with no lasting bond, as in chimpanzees and many fish and insects.
• Lek systems: males gather in an arena and display, and females choose one to mate with. Males give no parental care. Examples include sage grouse, peacocks and some birds-of-paradise.

What is parental investment?

Robert Trivers defined parental investment in 1972 as anything that a parent does for an offspring that increases its chance of survival at the cost of the parent’s ability to invest in other offspring. This includes producing gametes, feeding, protecting and incubating.

Females generally produce few large, costly eggs, and males produce vast numbers of small sperm, so the minimum investment by a female is higher. The consequence is:

• the sex that invests more (usually the female) is limited by resources, and is choosy;
• the sex that invests less (usually the male) is limited by access to mates, and competes for them;
• when the roles are reversed, as in pipefish and seahorses where males brood the eggs, the females compete and display.

Parental care varies greatly. Many fish and insects give none: they release eggs and leave. Birds provide extensive care, often by both parents. Mammals provide pregnancy and milk, which is care by females, though in some species the males help, such as wolves and some monkeys. Male care is more likely when the male is sure of paternity, as with internal fertilization and mate guarding, or when the care greatly improves the survival of young.

How does sexual selection work?

Darwin proposed sexual selection for traits that increase mating success rather than survival. It has two forms:

• Intrasexual selection (competition within a sex): males fight or compete for access to females. It selects for large size, weapons such as antlers and horns, and strength. Elephant seal males are up to about three times heavier than females.
• Intersexual selection (mate choice): females choose males with certain traits, such as bright feathers, a long tail or a complex song. Females may gain “good genes” for the offspring, healthy mates or better resources. The peacock’s tail is an example. Peahens favor males with more eyespots in some studies. Andersson showed that experimentally lengthening the tails of male long-tailed widowbirds increased their mating success.

Showy traits are costly, and are therefore thought to be honest signals of quality, according to the handicap principle of Amotz Zahavi. Sexual selection can lead to sexual dimorphism, the differences in appearance between the sexes.

What is courtship?

Courtship is the series of behaviors that precede mating. It has several functions: to identify the species and sex of the partner (preventing mistakes and hybridization), to assess the quality of the partner, to synchronize the readiness of partners, and to reduce aggression. Courtship often uses fixed action patterns in a sequence in which each step releases the next one.

• Three-spined stickleback: the male develops a red belly and builds a nest, and performs a zigzag dance in front of a female with a swollen belly. She follows him to the nest, and he points to the entrance.
• Great crested grebes: pairs perform synchronized head-shaking, and the “weed dance” in which they rise breast to breast with weed in their bills.
• Nuptial gifts: in some insects and birds, the male offers food, such as a captured fly in dance flies, or a fish in terns. This shows his ability to provide.
• Bowerbirds: males construct and decorate a bower, and females choose according to its quality.
• Songs and calls: in frogs, birds, crickets and whales.

Mating systems compared

Worked example: reproductive success of males and females

In a population of elephant seals, a beach has 10 breeding females and 10 breeding males, so the sex ratio is 1:1. Suppose that each female produces 1 pup in the season. Because the dominant males hold harems, the 10 females are fertilized as follows: one dominant male fathers 6 pups, a second male fathers 3, a third male fathers 1, and the other 7 males father none.

1. Total pups: 6 + 3 + 1 = 10, which equals the 10 females × 1 pup.
2. Mean success of males: 10 pups ÷ 10 males = 1.0 pup per male.
3. Mean success of females: 10 pups ÷ 10 females = 1.0 pup per female. The means are always equal when the sex ratio is 1:1, because every pup has one father and one mother.
4. Variation: females all have 1 pup (range 1 to 1), while males range from 0 to 6. Seven of ten males, or 70 percent, have no offspring.
5. Interpretation: the large variation in male success is the reason why selection favors traits that help males compete, such as large body size, and why a male can gain more by mating with many females than a female can by mating with many males.

This is the logic of Bateman’s principle: in many species the reproductive success of males varies more than that of females, and increases more steeply with the number of mates. Bateman’s experiments with fruit flies in 1948 suggested this, though later reanalysis has questioned details of his methods, and the principle does not hold in all species.

How was mating behavior studied?

Darwin described sexual selection in 1871 in The Descent of Man, and Selection in Relation to Sex. Angus Bateman published his fruit fly experiments in 1948. Robert Trivers proposed the theory of parental investment in 1972. Amotz Zahavi put forward the handicap principle in 1975. Since the 1990s, DNA fingerprinting has revealed extra-pair paternity in many birds, changing the view of monogamy.

Which mistakes are common?

Mistake 1: Assuming that monogamous pairs are always faithful

Social monogamy is common, but genetic monogamy is not. Extra-pair young are found in many species of birds.

Mistake 2: Thinking that females are always the choosy sex

The choosy sex is the one that invests more in offspring. In seahorses and jacanas, the roles are reversed.

Mistake 3: Believing that sexual selection helps survival

Traits such as large antlers or long tails increase mating success, but may decrease survival.

Mistake 4: Confusing the sex ratio with the variance in success

The mean reproductive success of males and females is the same when the sex ratio is 1:1. The difference is in the variation.