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

♀

♂

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.

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

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

Photo: Cornell University

•Plainfin midshipmen Dominant male

features and behavior

Sneaker male features and behavior

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, terminal phase male

Photo: P. Humann

Bluehead wrasse, intial phase

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