Evolution test 3

monogamy

a mating system where a male and a female mate exclusively with one another during a given breeding season

serial monogamy

forming exclusive pairs for one season before finding new partners in the next

Oldfield mouse

genetic analysis reveals that long-term monogamy is the strict rule: over 90% of the time, the male residing in a burrow is the true genetic father of the litters born there

Fitness Consequences and Preferred Mates

Choosing a high-quality partner provides immediate reproductive gains. In laboratory experiments, significantly more offspring were born to Oldfield mouse pairs consisting of a male and his preferred female compared to pairs forced together with non-preferred females.

The Mate-Assistance Hypothesis

Monogamy often evolves when offspring survival requires two parents. In harsh environments where food is scarce or predators are abundant, a male achieves higher evolutionary fitness by bringing food to the nest and defending offspring than by searching for additional mates

prairie voles

courting males and females approach one another, mate, and rapidly build a durable pair bond. Once bonded, they exhibit extreme selectivity, turning intensely aggressive toward unfamiliar individuals of the opposite sex.

nucleus accumbens

dopamine signaling here

D2 Receptors

When dopamine activates ____ during initial courtship and mating, it facilitates the formation of the pair bond

D1 Receptors

Once the pair bond is established, ____ activity increases. Activation of D1 receptors actually inhibits new pair bonding and triggers selective aggression against intruder males or females

Polygyny

occurs when a single male mates with multiple females during a breeding season, while females mate with only one male

Female Defense Polygyny

When females are short-lived, have low fecundity, mate shortly after adulthood, and naturally cluster together, males directly monopolize and defend the female groups. In the Australian wasp genus Epsilon, males mature early and search out unopened brood cells containing emerging virgin females. Once a cluster is uncovered, the dominant male aggressively repels rival males and mates with each female as she emerges.

Lekking (Arena Mating)

When females are widespread and resources cannot be defended, males set up and defend tiny, temporary display territories clustered together in an arena called a lek. Females visit these leks solely to inspect males and choose a mate

sexy-son hypothesis

proposes that females select males displaying prime physical indicators or pheromones because those traits will be inherited by their male offspring, making their sons irresistible to the next generation of females

sandflies

females consistently choose males whose male offspring subsequently dominate leks in the next generation

peacocks

females prefer leks with the highest concentration of males

males within their lek are frequently genetic kin

polyandry

occurs when females mate with multiple males during a breeding season

can be simultaneous or sequential

simultaneous

maintaining multiple mates in the same timeframe

sequential

forming short-term exclusive bonds one after another

Wattled jacanas

males incubate eggs and provide all parental care, while larger females aggressively defend territories encompassing multiple male nests

polyandry benefits to females

sperm replenishment, direct material benefits (nutrients, protection), genetic diversity, and convenience (avoiding male harassment)

patrilines

In social insects like ants and bees, a single queen mates with multiple worker males, creating multiple ___ within the nest

colonies with higher ___ are vastly more successful at establishing new nests and maintaining healthy, diverse microbiomes

female dispersion tracks

resources (food, shelter)

male dispersion tracks

females

Polygyny Threshold Model (PTM)

predicts that a female will choose to become the second mate of a male on a high-quality, resource-rich territory if her expected reproductive success there equals or exceeds what she could achieve as the sole mate on a poor-quality territory

promiscuous system

When polyandry and polygyny coexist in the same population

severe sexual conflict

Male Preference Hierarchy

Polygyny (sole access to multiple females) > Monogamy > Polygynandry (shared access to multiple females) > Polyandry (shared access to one female)

Female Preference Hierarchy

Polyandry / Polygynandry > Monogamy / Polygyny

sperm competition

the direct physical and physiological race between sperm from different males to fertilize available egg

Copulation Duration and Sperm Displacement

In dungflies, the last male to mate displaces previously stored sperm. The proportion of eggs fertilized by the last male increases with copulation length. However, because displacement operates with diminishing returns over time, males face a trade-off between maximizing displacement and leaving to search for new mates

Velocity versus Endurance Trade-offs

In broadcast spawners like sea urchins, fast-moving sperm require up to 100 times fewer cells to fertilize eggs compared to slow sperm. However, energetic constraints create a strict negative correlation between velocity and lifespan: fast sperm burn through their energy rapidly and die quickl

Ejaculate Plasticity

Males adjust sperm count based on the perceived risk of extrapair copulations (EPCs). When human or animal couples spend significant time apart, males ejaculate substantially higher sperm numbers upon reunion compared to when they spent continuous time together

kamikaze sperm hypothesis

proposes that some sperm morphs evolve specialized structures to physically block or destroy rival sperm rather than fertilize eggs. Conversely, in the desert ant (Cataglyphis savignyi), sperm actively aggregate into cooperative bundles to navigate the queen's storage organ effectively

cryptic mate choice

females exert post-copulatory control by preferentially storing, using, or discarding sperm from specific males—ensuring that physical mating does not automatically guarantee paternity

anisogamy

the asymmetry in gamete size between sexes

Females

produce few, large, resource-heavy gametes (eggs)

Males

produce vast quantities of small, energetic gametes (sperm)

reproductive success is limited primarily by how many females he can fertilize

Bateman’s principle

1. Females should be the choosier sex because making a poor mating choice wastes an enormous portion of their reproductive capacity

2. Greater choosiness by females leads to higher variance in male reproductive success

Intrasexual selection

Direct competition among members of one sex (typically males) for access to the opposite sex. Think of red deer stags clashing antlers or male horses battling for territory

Intersexual selection

Mate choice, where individuals of one sex (typically females) actively select their partners based on specific traits, colors, or displays

preoptic area (POA) and ventromedial hypothalamus (Vm

brain regions tied to reproductive priming

Watching a preferred male win

Triggered intense gene expression tied to reproductive priming

Watching a preferred male lose

triggered high gene expression in the lateral septum (LS), an area associated with anxiety-like responses, effectively shutting down her mating readiness

Scorpionfly Nuptial Gifts

Male scorpionflies (Hylobittacus apicalis) present females with captured prey (flies, beetles, or aphids). Females evaluate the gift size before mating

Honest indicators and parasite resistance

To avoid being fooled by male "cheaters”

Traits so costly to produce that only high-quality males can afford them

Pronghorn Antelopes

Female pronghorns undertake long searches to mate with males capable of defending large harems. Long-term studies show that offspring sired by these attractive harem-defenders have significantly higher survival rates

Major Histocompatibility Complex (MHC)

Animals often prefer mates with dissimilar immune system genes (MHC) to grant their offspring diverse disease defense mechanisms. Female sticklebacks use olfactory cues to detect MHC peptide ligands (amino acid strings), preferring males that optimize their offspring's total MHC allele diversity.

Runaway Sexual Selection

Proposed by Sir Ronald Fisher, this model relies on a genetic correlation between two distinct loci:

1. A locus coding for female preference.

2. A locus coding for a male trait

If some females randomly develop a genetic preference for a male trait (e.g., long eye stalks in stalk-eyed flies), they mate with those males. Their sons inherit the exaggerated male trait, while their daughters inherit the preference for that trait. This sets off a self-reinforcing genetic feedback loop where both the trait and the preference "run away" to extreme lengths across generations.

Sensory Bias (Sensory Exploitation) Model

This model focuses on how a preference originates. It proposes that a male trait arises to exploit a neurobiological bias that females already possess for non-mating reasons (such as foraging or predator avoidance).

sexual imprinting

learning the physical features of their parents and using them as a template for future mates

dendritic spine density decreases

Once the male encounters a female and completes sexual imprinting

stabilizing the learned mate template and locking in his preference

Mate-choice copying

occurs when a female's likelihood of choosing a specific male increases after she observes another female mating with him (Y - X > 0).

Lekking Grouse

In black grouse (Tetrao tetrix) and sage grouse (Centrocercus urophasianus), males gather on communal display arenas called leks. When researchers placed stuffed dummy female models on randomly chosen male territories, wild females eagerly mated with those specific males. The unanimity of female choice increases dynamically as more females observe others mating

The genetics of copying

OT "knockout" mice learn normally in basic tasks, but completely lose the ability to copy mate choices, proving that social learning in mating has dedicated genetic and endocrine foundations

Oxytocin (OT)

A neurohormone governing social recognition

Song learning in cowbirds

Cross-fostering juvenile brown-headed cowbirds from South Dakota with adults from Indiana showed that birds adopt the song styles and mating preferences of their cultural tutors, demonstrating how cultural transmission maintains population-specific mating traditions

Roaring contests

Males exchange roars. Roaring rate directly reflects physical condition and muscle stamina

Parallel walks

If roaring rates are matched, stags walk side-by-side to assess body size

Clashing antlers

Physical fighting occurs only if two stags are evenly matched in size and roaring ability

strong positive correlation

correlation between harem size and sexual size dimorphism (males being larger than females)

Parental males

Large, territorial males that build nests, attract females, and spend days fanning and defending eggs

Sneaker males

Small, agile males that hide in nearby vegetation. When a parental male and female spawn, the sneaker dashes in, releases a cloud of sperm in seconds, and flees

Satellite males

Mid-sized males that mimic female color patterns and behavior. They hover near a spawning pair, tricking the parental male into courting both "females" at once while the satellite fertilizes the eggs

spermatophores

sperm packets on delicate stalks in the environment without direct female contact

identical by descent

copies of a gene inherited from a shared recent common ancestor

coefficient of relatedness

“r”

The probability that two individuals share an allele identical by descent

Inclusive fitness

the reproductive success of genetic relatives facilitated by an individual's help

Hamilton’s Rule

predicts that an altruistic trait will spread in a population whenever r x b > c

Offspring rule

using surviving offspring as the biological currency

evolutionary theory of family

kinship theory with ecological constraints theory and reproductive skew theory