Mating Systems & Sexual Selection

Advanced Prerequisites: Mechanisms of Behavior IBO USABO ethology

Overview

Sexual selection (Darwin’s second major selective mechanism, distinct from natural selection for survival) is selection arising specifically from variance in mating success rather than variance in survival — and it is the single mechanism that explains the most exam-tested oddities in the animal kingdom: elaborate ornaments that reduce survival odds, males that fight to the point of injury, and mating systems ranging from strict monogamy to promiscuity within closely related species. This page builds directly on the honest-signaling framework from Animal Communication (many sexually selected traits are honest signals of quality) and connects forward to Kin Selection, Altruism & Eusociality, since parental investment asymmetry — introduced here — is also the starting point for understanding parent-offspring conflict.

Key Concepts

Intersexual and Intrasexual Selection

Sexual selection operates through two distinguishable routes, and exam questions consistently require identifying which route a described trait results from:

Intersexual selection (mate choice) — one sex (typically female, see parental investment below) chooses mates based on a trait, so that trait increases in frequency because it makes its bearer more chosen, not more survival-fit. The peacock’s train (see the handicap principle in Animal Communication) is the standard example, favored because peahens preferentially mate with more elaborately trained males.

Comparative fitness-trade-off figure contrasting an “additive model” (parent-offspring begging conflict) against a “multiplicative model” of mate choice (illustrated with a peacock and peahen), each showing benefit, trade-off, and resulting fitness curves against a shared resource axis. Source: Springer/BMC Biology

Intrasexual selection (mate competition) — members of one sex (typically male) compete directly with each other for access to mates, so that traits favoring competitive success (large body size, weapons, aggression) increase in frequency through winning contests rather than through being chosen. Elephant seal male bulls, which fight for control of a harem and show extreme size dimorphism relative to females as a direct result, are the standard example.

Illustration of adult male, adult female, and pup elephant seals to scale, showing the male’s much greater size and distinctive proboscis compared to the female. Source: ScienceDirect Topics (northern elephant seal)

Both routes can act on the same species simultaneously and even on the same trait — red deer stag antlers are used both for direct male-male combat (intrasexual) and are separately assessed by females as a quality signal during mate choice (intersexual).

Parental Investment Theory

Parental investment theory (Robert Trivers) explains why mate choice and competition are so consistently asymmetric between the sexes: the sex that invests more in each individual offspring (metabolically costly gametes, gestation, lactation, direct offspring care) becomes the limiting resource for the other sex’s reproductive success, and consequently becomes the more selective, choosier sex, while the lower-investing sex faces stronger selection to compete for access to the higher-investing sex. In most mammals, female obligate investment (gestation + lactation) is far higher than male minimum investment (a single mating), predicting — and generally matching — female choosiness and male-male competition; species with unusually high relative male investment (see sex-role-reversed systems below) predict, and show, the reverse pattern.

Mating System Diversity

Mating systems are classified by how many mates each sex has across a breeding season, and the classification connects directly back to parental investment asymmetry:

System	Pattern	Typical driver	Example
Monogamy	One male, one female	Offspring require investment from both parents to survive	Many songbirds (often with some extra-pair mating despite social monogamy)
Polygyny	One male, multiple females	High potential male reproductive payoff, defensible females or resources	Elephant seals, red deer harems
Polyandry	One female, multiple males	Sex-role reversal — male investment exceeds female’s	Phalaropes (female is larger, more brightly colored, competes for males; male alone incubates)
Promiscuity	Multiple mates, no pair bond, either sex	Little or no benefit to pairing; offspring survival doesn’t depend on biparental care	Chimpanzees

Mating system diagram using blue (male) and pink (female) icons and arrows: monogamy (1:1), polygyny (1 male:multiple females), polyandry (multiple males:1 female), polygynandry (multiple:multiple, with parental investment), and promiscuity (multiple:multiple, without parental investment). Source: Springer, “Reference Work Entry”

Sex-role reversal in phalaropes and pipefish/seahorses (in the latter, males brood fertilized eggs in a specialized pouch, making male investment per offspring exceed female’s) is a heavily tested confirmation of parental investment theory precisely because it demonstrates the investment asymmetry, not male/female identity per se, is the causal variable — when investment reverses, so does the pattern of choosiness and competition.

Three photographs of red phalaropes in breeding plumage: (1) a typical male, (2-3) females — females are the more brightly colored, more strikingly patterned sex in this species, the reverse of the usual vertebrate pattern. Source: ResearchGate

Sperm Competition

Sperm competition occurs whenever a female mates with multiple males within a single reproductive cycle, so that sperm from different males compete directly to fertilize the same set of eggs — this creates selection on males for traits that increase paternity share independent of pre-copulatory mate choice or combat. Documented adaptations include: increased relative testis size and sperm production in species/populations with higher female mating promiscuity (confirmed comparatively across primates — chimpanzees, which are highly promiscuous, have much larger relative testis size than gorillas, which are polygynous with low female mating rates, despite gorillas being far larger overall); mate guarding (a male remaining with a female after mating to prevent her from remating before fertilization); and copulatory plugs (a physical barrier left in the female reproductive tract after mating, documented in some rodents and insects, that delays or blocks subsequent males’ sperm from reaching the eggs).

Source: International Biology Olympiad 2023, Theory 2 (official exam) — submitted as a substitute for the originally-requested testis-size chart, and a better fit for the IBO/USABO syllabus this guide targets: a real released exam question testing the exact testis-mass/body-mass/mating-system inference logic described in the text.

Official IBO 2023 answer key and reasoning for Q.35, citing Harcourt, Harvey, Larson & Short (1981) “Testis weight, body weight and breeding system in primates,” Nature 293 — the same foundational comparative-primatology paper underlying the chimpanzee-vs-gorilla testis-size claim in the text. Source: International Biology Olympiad 2023, Theory 2 (official answer key)

Comparative Structures

Concept	Selective route	Sex typically favored to be choosy/competitive	Example
Peacock train	Intersexual	Females choose	Peafowl
Elephant seal harem combat	Intrasexual	Males compete	Elephant seals
Red deer antlers	Both simultaneously	Both	Red deer stags
Phalarope sex-role reversal	Intersexual, reversed	Males choose (females compete)	Phalaropes
Relative testis size	Post-copulatory (sperm competition)	N/A — operates after mating	Chimpanzee vs. gorilla testis size

Common Exam Questions

“Distinguish intersexual from intrasexual selection, and classify each of the following as one or the other: female peahens preferring elaborately trained males; male elephant seals fighting for harem control.”
“Explain, using parental investment theory, why males typically compete for mates while females are typically the choosier sex in mammals, and predict what pattern would be expected in a species with sex-role reversal.”
“Phalarope males alone incubate eggs while females are larger, more colorful, and compete for mates. Explain how this observation supports parental investment theory rather than contradicting it.”
“Chimpanzees have much larger relative testis size than gorillas despite being smaller-bodied overall. Explain this using sperm competition and the two species’ differing mating systems.”
“Define a copulatory plug and explain what selective pressure favors its evolution.”
“Explain why monogamy is more common in species where offspring require significant biparental care to survive to independence.”

Visual Reference

Interactive

Parental investment asymmetry predictor (HTML/JS, no new library) — user sets relative male and female investment levels on a slider and the tool predicts which sex should be more competitive vs. more choosy, letting the user “discover” the sex-role-reversal prediction by pushing male investment above female investment.

Mating system classifier (drag-and-drop, HTML/JS) — named species scenarios (with brief mating-behavior descriptions) sorted into monogamy/polygyny/polyandry/promiscuity bins, reinforcing the classification against real examples rather than abstract definitions.

Static (placed inline in Key Concepts above, next to the concept each one illustrates, rather than collected here.)

Practice Problems

A bird species shows extra-pair mating (individuals paired socially but also mating outside the pair) despite being classified as socially monogamous. Explain why genetic paternity testing was necessary to reveal this, and what it implies about the limits of classifying mating systems by observed pair-bonding alone.
Using parental investment theory, predict the expected relative testis size (large or small, relative to body size) in a species where females typically mate with only one male per reproductive cycle, and explain your reasoning.
Red deer antlers are used in direct physical combat between males and are also assessed by females during mate choice. Explain why this trait cannot be attributed to intrasexual selection or intersexual selection alone.
A seahorse species has males that carry and gestate fertilized eggs in a brood pouch. Predict which sex should be more selective about mates and which should compete more intensely for mating opportunities, and justify your prediction.
Explain why mate guarding behavior (a male remaining near a female after mating) is a predicted response to the risk of sperm competition, and describe what a researcher would expect to observe if female mating rate in a population were experimentally reduced.