Sexual selection & mating systems Marine Vertebrates: Lecture 16 Sexual selection & mating systems

Why do males usually compete, while females choose? Hypothesis 1: “Eggs are expensive, sperm is cheap!” More energy invested per egg than per sperm Females: limited by egg production Males: limited by number of mates

Is sperm really cheap? It often takes a large number of sperm to fertilize a single egg, due to Hostile environment within female Sperm competition among males

Why do males usually compete, while females choose? Hypoth. 2 (R. Trivers): Individual with the most total parental investment chooses. Often the female (i.e. mammals) Sometimes the male… Photo: pbs.org

Sexual selection Characteristics/behaviors are selected for that maximize the chances of acquiring mates. What are some of these characteristics? Which gender is likely to have the most obvious sexually-selected characteristics?

Sexual selection and polyandry in pipefishes and sea horses Why pipefish and seahorses? Question Is there a relationship between the type of mating system and degree of sexual selection? Species Gulf pipefish: Polyandry (no polygyny) Dusky pipefish: Polygynandrous Broad-nosed pipefish: Polygynandrous W. Australian seahorse: Monogamous Expected results?

Gulf pipefish ♀ ♂

Photo: Institute of Biology of the Southern Seas, Ukraine Broad-nose pipefish ♀ ♂ Photo: Institute of Biology of the Southern Seas, Ukraine

Hippocampus spp. Pair (Same genus as W. Australian seahorse Note similar size and coloration of pair. www.divegallery.com

Mating systems Polygyny: One male mates with several females Why considered the male “default”? Monogamy: One male mates with one female Social vs. genetic monogamy Polyandry: One female mates with several males Polygynandry: Male mates with >1 female, female mates with >1 male.

Photo: www.driftersister.com Polygyny Female defense polygyny Example: Northern elephant seals Photo: www.driftersister.com Photo: www.wetasschronicles.com

Male-male competition and sexual dimorphism (seals) NOTE: Each point represents a species AL11150.jpg

Polygyny Lek polygyny Example: Marine iguanas Photo: Martin Wikelski

Polygyny: Cetacea Sperm whales Highly skewed sex ratios on mating grounds Sexual dimorphism & battle scars Females: synchronous estrus Adaptive value?

Polygyny: Cetacea Humpback whales Skewed sex ratios Asynchronous ovulation Asynchronous arrival on mating grounds Paternity studies Many males with no offspring No male with >3

Polygyny? Orcas Matrilineal groups Males associate with mother’s group Mate outside groups (based on genetic studies using “skin darts” for sampling) www.pacificwhaler.com

Alternative male strategies Plainfin midshipmen Dominant male features and behavior Sneaker male features and behavior Photo: Cornell University

Alternative male strategies Salmon Dominant males Top two in photo Jacks (smaller, less time at sea) Bottom photo by Ian Fleming yusukekoseki.michikusa.jp/research.htm

California Sheephead and sex reversal California sheephead female Photo: UC Santa Cruz California sheephead male Photo: V. Sanchez

Other sex reversers: bluehead wrasse Bluehead wrasse, intial phase Bluehead wrasse, terminal phase male Photo: P. Humann

Monogamy: alternate hypotheses Mate assistance: it takes two parents to raise the offspring Example: Adelie penguins Both parents needed for chick survival Photo: Karen Haberman

Monogamy: alternate hypotheses Danger “theory” Leaving  increases chance of dying if predation rates are high. Example: Blue-spotted jawfish www.reefcorner.com

Social vs. genetic monogamy Video: blue-footed boobie www.amazingjourneys.net

Polyandry/Polygynandry Potential advantages of multiple matings by females? Females reproductive success limited by mates rather than gametes (rare) Example: polyandrous pipefish Higher rates of conception (i.e. % of eggs fertilized) Good genes hypothesis: Insures the highest quality mate/offspring Genetic variability among offspring Material benefits