Upload
ngonga
View
215
Download
0
Embed Size (px)
Citation preview
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
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.
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
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.
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)
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
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
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
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
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
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
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.
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.