March 30

Parental Investment Theory and Female Choice

Parental Investment Theory (Trivers 1972): - The fundamental principle states that the sex making the greater investment in offspring production should be the choosier sex regarding mate selection. - Investment includes the energetic costs of gamete production, gestation, and rearing. - Generally, females pay significantly higher costs for reproduction and possess a lower reproductive potential over their lifetime compared to males. - Example: The Mandarin duck exhibits distinct sexual roles based on these investment differences.
Sexual Dimorphism and Appearance: - Physical appearance often differs significantly between sexes. - Males frequently possess weapons (e.g., horns) or gaudy ornaments (bright, showy, or extravagant decorations). - Behavioral differences are common: males may engage in fighting, loud singing, or performing strange antics to secure access to females.

Ornamentation and the Darwinian Puzzle

Darwin's Dilemma: - Ornaments such as the elaborate plumage of Birds-of-Paradise did not initially seem to fit the theory of Natural Selection. - These traits are not essential for survival and are often energetically expensive to produce and maintain. - These traits make the possessor more prone to predators. - The core question was why these traits appear almost exclusively in males.
Evidence from Long-tailed Widowbirds (Euplectes progne): - Research shows that the mating success of male long-tailed widowbirds is directly dependent on the length of their tails. - This suggests that while the tail may be a survival handicap, it is a reproductive necessity.

Direct Benefits and Nuptial Gifts

Courting Gifts: - In certain species, males provide females with direct benefits to induce mating. - Nuptial gifts include food, assistance in rearing young, or access to high-quality territories with nesting sites.
Specific Examples: - Nursery web spiders: The female carries a spermatophore gifted by the male. - Penguins: Females may be won over by gifts like pebbles (referencing the BBC Earth source).
The "Ultimate Gift" (Sexual Cannibalism): - In some spider and mantid species, the male is consumed by the female after mating. - Redback spider (Australian black widow): Male suicide reduces the likelihood that the female will mate with another male, increasing the suicidal male's chances of passing on his genes. - Giant Asian Mantis: Engages in sexual cannibalism. - Evolutionary Logic: If the chance of a male finding another mate is extremely low, the evolutionary cost of suicide is low provided it ensures successful fertilization.

Indirect Benefits and "Good Genes" Hypotheses

The Bowerbird Case Study: - There are $20$ species found in New Guinea and Australia. - Males provide no parental care; all effort goes into display. - Males construct elaborate structures called "bowers" using sticks, grasses, stems, and human objects. - Early explorers mistakenly identified these bowers as human religious shrines. - In many cases, sperm is the only thing the female receives from the male, yet females remain highly selective.
Fisher's Hypothesis (Sexy Son Hypothesis): - Proposed by Fisher ( $1915$ , $1930$ ) and expanded by Weatherhead & Robertson ( $1979$ ). - Hypothesis: Females choose physically attractive males so that they will produce attractive sons. These "sexy sons" will be preferred by the next generation of choosy females, thereby increasing the mother's total number of grandchildren. - Selection for attractiveness is heritable. - Example: Stalk-eyed flies. Female preference for redder males is correlated with the redness of the father and the preference of the daughter.
Vigour and Performance Intensity: - Mate selection can occur in species where males lack traditional ornamentation. - Darwin identified "vigour" as the performance intensity of energetic activities. - Example: Golden-collared Manakin (Manacus vitellinus). Females prefer males that perform display moves at higher speeds. Differences of just $tens\,of\,milliseconds$ significant impact female preference. - Physiological Trade-offs: Increased heart rate and metabolic investment. While testosterone improves muscle speed for "sexy" displays, it simultaneously reduces muscle endurance (Tobiansky et al. 2020).
The Handicap Model (Zahavi 1975) - "Honest Signaling": - Fitness is signaled through handicaps (costly behaviors or morphologies). - A signal is reliable only if it is so costly that an individual with lower fitness could not afford to produce it. - The peacock's tail sends the message: "I have survived despite this massive handicap, therefore I possess superior genetic quality."

The Hamilton-Zuk Hypothesis

Parasite Resistance and Sexual Displays: - Suggested by Hamilton and Zuk ( $1982$ ). - Sexual displays are reliable indicators of genetic resistance to disease and parasites. - Females act as "diagnostic veterinarians," selecting males genetically equipped to survive the current local parasites. - This leads to a perpetual evolutionary arms race between hosts and parasites, meaning "good genes" are constantly changing.
Evidence from Three-Spined Sticklebacks: - Males develop red coloration, build nests, and perform a "zig-zag dance." - Experiment (Milinski and Bakker, 1990): Females prefer redder males. - Redness correlates with physical condition ( $greater\,mass\,per\,unit\,length$ ) and a lack of parasites. Intensity of the red color declines when the male is infected.
Evidence from European Green Lizards: - Variation in the brightness of the male's throat and belly color is associated with blood parasite infection levels. - Brightness is measured as the reflection of light in visible wavelengths from $320$ to $700\,nm$ . - Females choose brighter males to ensure offspring benefit from parasite-resistant genes.

Mating Systems and Ecological Factors

Sex Role Reversal: - In some species, females compete for males who provide the bulk of parental care. - Red Phalarope: Females are larger, more colorful, and aggressive. Males incubate eggs while females seek other mates. - Pipefish: Males select the largest, most ornamented females. Males incubate eggs in a special pouch. - Sea horse: Males possess a brood pouch for incubation.
Factors Influencing Systems: - Number and location of mates, mate quality, food availability, and the presence of predators or competitors.
Definitions of Mating Systems: - Monogamy: One male and one female pair for one or more seasons. Rare in most groups, but both parents often care for young. Examples: Bald eagles, swans, penguins, black vultures, voles. - Polygyny: One male mates with multiple females. This is the most common system in vertebrates ( $90\,\%$ ). Males may defend females directly or control resources like food or nesting sites. Females usually provide all care. Examples: Elephant seals, gorillas, deer. - Polyandry: One female mates with multiple males. Rare. Females defend males or resources, and males typically provide parental care. Example: Shorebirds. - Promiscuity: Both males and females mate with multiple partners within a single season.
Ecological Potential for Polygyny: - Monogamy often occurs when it is difficult for a male to defend more than one female (Lukas and Clutton-Brock 2013). - Example: Brushtail possums are monogamous when food/nest sites are widely separated but become polygynous when resources are clumped.

Foraging Behavior and Economics

Optimal Foraging Theory: - Basic Principle: Animals strive to maximize the energy acquired per unit of feeding time. - Natural selection acts on foraging efficiency to increase overall fitness.
Economics of Prey Choice (Eurasian Oystercatcher): - Meire and Ervynck ( $1986$ ) found birds select intermediate-sized bivalves. - Small bivalves: Not enough energy to offset the cost of finding and opening them. - Large bivalves: Too difficult and time-consuming to open. - Intermediate bivalves: Provide the highest energy gain for the effort, even if they are less abundant.
Shore Crabs and Mussels: - Crabs prefer mussel sizes that offer the highest rate of energy return based on "handling time." - Energy yield per unit of breaking time is the key metric.

Predation Risk and Foraging Decisions

The Wolf Effect in Yellowstone (Creel et al. 2005): - Predation risk from wolves altered the foraging patterns of elk (Cervus elaphus). - Elk prefer grasslands for foraging, but when wolves are present, they move into conifer forests for protection, despite the forests having less food.
Perceived Risk and Non-lethal Effects: - Song sparrows exposed to recordings of predators (Zanette et al. 2011) fed their young fewer times per hour compared to those exposed to non-predator recordings. - This demonstrates that the fear of predators alone can impact reproductive success.

Antipredator Behaviors and Group Living

Individual Defenses: - Slug caterpillars: Covered in stinging hairs to remain unpalatable. - Australian sea lions: Utilize unihemispheric slow-wave sleep (sleeping with one eye open and half the brain alert). - Peacock butterfly: Displays eyespots to startle predators. - Springbok: Performs "stotting" or "pronking" (stiff-legged jumping) to discourage pursuit. - Eastern hognose snake: Feigns death (playing dead) and emits a decay-like odor to deter predators that avoid carrion.
Benefits of Group Living: - Higher reproductive success, shared feeding, and shared care of young. - Reduced predation risk through the "many eyes" effect (vigilance) and collective defense. - Example: Musk oxen form a defensive circle, creating a formidable target. - Example: Wood Pigeons and Goshawks (Kenward 1978). The success rate of goshawk attacks drops from roughly $80\,\%-100\,\%$ against lone pigeons to only $6.2\,\%$ when attacking flocks of more than $50$ birds. In larger groups, predators are detected much sooner.