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

2. 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

3. 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

4. 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

5. 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?

6. 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?

7. Gulf pipefish ♀ ♂

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

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

10. 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.

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

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

13. Polygyny
Lek polygyny
Example: Marine iguanas
Photo: Martin Wikelski

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

15. 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

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

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

18. 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

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

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

21. 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

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

23. Social vs. genetic monogamy
Video: blue-footed boobie

24. 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