13
11/14/2012 1 Darwin: “Sexual size dimorphism is common.” male female rainforest spider female male elephant seals Many mammals males bigger than females Many spiders males smaller than females Sexual differences include differences in morphology, behavior and physiology. Morphology Behavior Physiology differences in hormones, brain structure, and gonads. Drawing of tufted coquette from “The Descent of Man and Selection in Relation to Sex” by Charles Darwin sexual selection results from “…advantages that certain individuals have over others of the same sex and species, in exclusive relation to reproduction…" Darwin attributed sexual differences to sexual selection. Quote from “Descent of Man” by C. Darwin Sexual selection is a special form of natural selection. Involves two mechanisms: A. Mate competition B. Mate choice Mate competition members of one sex compete with members of the same sex for access to mates bull elephant males male marine iguanas

14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

  • Upload
    ngonga

  • View
    215

  • Download
    0

Embed Size (px)

Citation preview

Page 1: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

1

Darwin: “Sexual size dimorphism is common.”

male

female

rainforest spider

female

male

elephant seals

Many mammalsmales bigger than females

Many spidersmales smaller than females

Sexual differences include differences in morphology, behavior and physiology.

Morphology

Behavior

Physiology differences in hormones, brain structure, and gonads.

Drawing of tufted coquette from “The Descent of Man and Selection in Relation to Sex” by Charles Darwin

sexual selection

results from “…advantages that certain individuals have over others of the same sex and species, in exclusive relation to reproduction…"

Darwin attributed sexual differences to sexual selection.

Quote from “Descent of Man�” by C. Darwin

Sexual selection is a special form of natural selection.

Involves two mechanisms:

A. Mate competition

B. Mate choice

Mate competition

members of one sex compete with members of the same sex for access to mates

bull elephant males

male marine iguanas

Page 2: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

2

Mate choice

members of one sex choose mates among members of the oppositesex

Mate Competition!

1. male-male contests

2. sperm competition

3. mate guarding

4. infanticide

5. alternative mating strategies

Types of Mate Competition

1. Male-Male Contests

Sometimes physical combat

Sometimes ritualized display

Contests typically reflect assessment of size or strength.

%

copulations

Mountain gorillas live in groups

Some groups have multiple males.

Male dominance established through aggression,

Which determines male mating success.

Dominant

male In elephant seals too, fighting establishes dominance status,

which determines male mating success.

Page 3: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

3

Among seals and related marine mammal species,

sexual size dimorphism is correlated with # females monopolized by males.

Video!!

http://dsc.discovery.com/tv/life/episodes.html

Page 4: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

4

2. Sperm Competition

= male-male competition at the level of sperm.

Common wherever females mate with more than one male.

Example. ‘Sperm wars’ in Drosophila

In many animal species, the last male to mate with a female fertilizes the next eggs out.

This pattern is known as last male precedence.

Last in, first out. Why?

In Drosophila, the mechanism was evaluated using males with genetically-engineered sperm.

Drosophilasperm which express a fluorescing protein (green tails)

and normal sperm (grey tails).

Photo courtesy of Kelly Dyer

Procedure: mate female first to male with one kind of sperm, then to male with other kind of sperm.

Possible outcomes:

1. Displacement of first sperm from storage sites.

2. Stratification in storage sites with first sperm buried deep in storage sites.

Page 5: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

5

stratification displacement

Colored areas are sites of sperm storage

(Male #1 in green; Male #2 in red)

Results: Last male precedence due to:

a. displacement of first male’s sperm from female storage organs.

BUT ALSO

b. incapacitation of first male’s sperm!

Luc Viatour

Copulation duration in dung fly, Scatophaga stercoraria

Female is ‘patch’

Male’s ‘decision variable’ is copulation duration

‘Currency of fitness’ is % eggs fertilized

Males defend dung patties as territories

Females lay eggs and larvae develop in patties

Optimal Behavior Under Sperm Competition

% Eggs fertilized

Copulation duration in dung flies

Time in copula (min)Search & guard

time ( 156. 5 min)

predicted

observed

Tradeoff between sperm transferred to one female and sperm available for next female

Alonso-Pimentel and Papaj (1996)

Increasing ratio of males to females should increase search time for next female…

… which should increase duration of copulation with current female.

We tested prediction in our lab using walnut flies.

C. Hedgcock

Effect of Sex Ratio on Mating Duration

Local Sex Ratio (male:female)

0

200

400

600

800

1000

n = 2

n = 31

n = 25

n = 16 n = 7

>1 >2 >31<1

Copulation Duration (sec)

Page 6: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

6

a. Increased sperm production-- some primate species

b. Retaliatory copulation

-- bighorn sheep

c. Copulatory plugs

-- butterflies

d. Traumatic insemination

-- bedbugs

e. Sperm trains-- murine rodents

f. Sperm removal devices-- dragonflies & damselflies

Strategies for Sperm Competition

a. Increased sperm production-- some primate species

b. Retaliatory copulation

-- bighorn sheep

c. Copulatory plugs

-- butterflies

d. Traumatic insemination

-- bedbugs

e. Sperm trains-- murine rodents

f. Sperm removal devices-- dragonflies & damselflies

Strategies for Sperm Competition

Sperm competition expected to be more intense when females mate multiply.

In primates, frequency of mating varies:

monogamous: males and females mate just once

polygynous: males mate multiply

multi-male/multi-female: both males and females mate multiply

Sperm competition expected to be most intense in this last mating system. Testes size for species with multi-male/multi-female ma-

ting system tend to fall above the line fit to all the data.

-0.07

-0.05

-0.03

-0.01

0.01

0.03

0.05

0.07

0 3 6 9 12

Number of Males

Testes Size

(controlled for Body Size)

Male flies held in more male-biased sex ratios grow larger testes!

Laura Conner and D. Papaj, unpubl.

a. Increased sperm production-- some primate species

b. Retaliatory copulation

-- bighorn sheep

c. Copulatory plugs

-- butterflies

d. Traumatic insemination

-- bedbugs

e. Sperm trains-- murine rodents

f. Sperm removal devices-- dragonflies & damselflies

Strategies for Sperm Competition

Page 7: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

7

Murine rodents have hooked sperm. Sperm hook together in female tract.

“Sperm trains” move faster than individual sperm.

Sperm are more hooked in species with larger testes, indicative of stronger sperm competition.

Fig. 7.24

Sperm morphology is highly variable!

Sperm removal in damselflies

Mating is weird!!!

Males have evolved:

• claspers to clutch female!

• a new penis!

• with a scrub brush!!

Female places genitalia over new penis.

Male scrubs old sperm from female’s sperm storage organ and deposits sperm.

Ischnura damselfly sperm removal devices

Page 8: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

8

a. post-copulatory mate guarding

Examples. Damselflies and many birds.

Males copulate and then guard until eggs laid.

3. Mate Guarding

b. pre-copulatory mate guarding

Example. Gammarus amphipods

Male guards immature female until she molts and is receptive.

3. Mate Guarding (cont’d)

4. Infanticide

A male lion kills cubs of rival when it takes over a pride.

WHY?

1. Fewer resources to someone else’s cub2. Females come into estrus sooner.

Example. Satellite males in frogs

Males hang out in the vicinity of a holder of a high-quality territory,

and mate with females as they move to the territory.

5. Alternative Mating Strategies

Example. Johnny-come-lately behavior in horseshoe crabs

Males crowd around a paired couple as they move onto the beach,

and release sperm in the vicinity of the couple.

Video

Page 9: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

9

Example. Alliance formation in savanna baboons.

Low-ranking males form coalitions and wrest females from a high-ranking male.

Kenneth M. Gale, Bugwood.org

Kenneth M. Gale, Bugwood.org

Example. Alliance formation in bottle-nosed dolphins

Coalitions of males:

1. herd females, preferring those in estrus

2. defend females from other males

3. wrest females from other coalitions

Tursiops species in Shark

Bay, Western Australia

BUT ARE COALITIONS MADE UP OF

LOW-RANKING MALES?

WE DON’T KNOW…

Example. 3 strategies in bluegill sunfish

Territorial males

Sneaker males

Female mimics

territorial male sneaker male female mimic

Example. Female mimicry in bluegill sunfish

Males similar in size and coloration to females.

Mimicry allows male to stay close to another’s territory,

and release sperm

when territory-holder

is pairing with a female.

territorial male

female mimic

Males come in three genetically distinct forms.

Example. Female mimicry in marine isopods

sneaker male

At equilibrium frequencies, all morphs have the same fitness.

rufous male Territorial Resident

white male Satellite

dark male Marginal male

Three alternative strategies in ruffs

Page 10: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

10

1. Territorial Residents defend territories from Marginal Males and other TR’s.

2. Marginal Males are floaters that may become TR’s.

3. Satellites form ‘couples’ with TR’s, are tolerated on their territories, and may steal copulations from the TR.

Are these genetically different?

Independents and Satellites are genetically different (= genetic polymorphism)

But MM’s and TR’s can change into each other (= developmental polymorphism)

Satellites Independents:- Territorial Residents- Marginal Males

The rock-paper-scissors game in side-blotched lizards

orange blue yellow

Ultra-dominant (v. aggressive)

Guard mates Mimic female & sneak matings

There are 3 male morphs in side-blotched lizards.

Differences are genetically-based.

Ultra-dominant orange beats mate-guarding blue;

Mate-guarding blue beats yellow-throated sneaker;

Yellow sneaker beats orange.

Net result: No one strategy beats all. Thus, all morphs persist!

Rock blunts scissors, paper covers rock, and scissors cuts paper…

orange blue yellow

Ultra-dominant (v. aggressive)

Large territories

Several females

Guard mates

Small territory

One female

Mimic female & sneak matings

No territory

Page 11: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

11

http://www.youtube.com/watch?v=YkSdLk3M5ew

http://bio.research.ucsc.edu/~barrylab/classes/animal_behavio

r/video_only.html

Do the sexes differ with respect to mate competition and mate choice?

Yes!

Yes!!

YES!!!

Generally speaking,

Males compete more intensely for mates than do females.

and

Females are choosier about their mates than are males.

Why?

Answer: Anisogamy

Anisogamy is the occurrence of gametes of different sizes

Males make many small gametes, termed sperm.

Females make few, large gametes, termed eggs.

Hamster egg and sperm

Why two gamete sizes?

Selection should favor:

1. gametes effective at fertilizing other gametes.

2. gametes, once fertilized, better at developing to adulthood.

Can the same type of gamete satisfy both #1 and #2?

NO! There is a TRADEOFF!!

A gamete that is good at #1 is probably not good at #2, and vice-versa.

So… gametes end up being

a. large and full of nutrients, OR

b. small and highly mobile

disruptive selection

small large small large small large

gamete size

Page 12: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

12

Anisogamy leads to three predictions.

Prediction #1: male fitness will be limited by number of mates,

whereas female fitness will not be limited by number of mates.

Bateman’s principlemale fitness improves with # mates, female fitness does not.

1 2 30

50

100

Fitness

No. of Mates

Evidence: In 1940’s, Bateman confirmed this prediction, using Drosophila fruit flies.

males

females

Prediction #2. Because females invest more into each gamete, they should invest more in offspring care.

Pattern: Females generally do invest more in parental care.

Prediction #3. Given greater investment in gametes and in offspring care,

females should be choosier about who they mate with,

and males should compete more for mates than do females.

Patterns: We generally see both patterns.

Nice story… is it true?

If true, exceptions should prove the rule:

Where males invest more in parental care, then males should be choosy and females should compete for mates.

In other words… role reversal on all counts!

Role Reversal in Mormon crickets

During mating, males transfer sperm plus a nutritious packet of protein,

This spermatophylax may be 25% of male’s weight.

Large male investment generates role reversal. Females compete for males.

Page 13: 14 mate competition '12 WEB - University of Arizona ... pdfs/14_mate... · Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more

11/14/2012

13

jacana

� Female jacana mates & lays eggs, female leave, male tends nest.

� Female finds new mate & lays eggs in new nest, then leaves. Etc.

� Female defends territoriescontaining the nests from other females.

� Females try to steal males by killing eggs of males in other territories (= ovicide).

Role reversal in jacanas

Last bit is evidence of females competing for males.

Also in rheas and red

phalaropes (VIDEO)

Role Reversal in Pipefish

Male brood pouch Male ‘giving birth’

Father tending to his young

Male pipefish do the caring…

In several species of pipefish, females have ornaments and males exercise mate choice.

Also a reversal of Bateman’s principle:

Female fitness increases with #’s of mates; male fitness changes little with #’s of mates.

Summary

Males compete more intensely for mates than do females.

and

Females are choosier about their mates than are males.

However… some scientists believe this view is incorrect.