Post on 10-Feb-2016
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Short & SweetMale Intrasexual
Competition
Intrasexual Competition• Selection on mating behavior, either
through competition among members of one sex (usually males) for access to members of the other sex
• Males tend to invest more of their reproductive effort into mating effort than parenting effort– Finding, attracting, Evaluating potential mates
Males Short & Sweet• Male- male competition tends to be
more confrontational than in females• Male fitness is more variable
Recall Bateman’s Principle• Bateman 1948• The sex which invests the
most in producing offspring becomes a limiting resource over which the other sex will compete– Male strategy– Female strategy– Males should be eager,
females…“The female, with the rarest exceptions, is less eager than the male…she is coy,And may often be seen endeavoring for a long time to escape.”
Bateman’s Experiment • Mated drosophila• Male fitness depends
on maximizing mating frequency
• Promiscuous males have 3-4X more offspring
• Females less than 2X as many offspring when mated with multiple males
• Win big, lose big
Bateman’s Principle• Implications• Males are typically facing
– Stronger selection forces– More Intrasexual competition– Greater expression of secondary sexual
characteristics – Win big, lose big
Bateman’s Principle• Implications• Females are typically facing
– Energy is the limiting resource for achieving fitness
– Females should be choosier who they mate with• Wasted gametes costlier• Typically invest more in cases of parental
care
Males Win Big, Lose Big • Ave offspring
produced by males & females similar
• Males exhibit more variance in number of offspring produced
• Male potential reproductive output is higher than females
The potential for a big “payoff” results in risky behavior
Recall Trivers…• Expanded Bateman’s principle to extend beyond
gamete scope to include all aspects of reproductive effort– Gestation, feeding, protection
• The sex that invests less should adopt a strategy that maximizes copulatory events– Win big, lose big strategy applies to all aspects of
reproductive effort• The sex that invests more should adopt a strategy
that maximizes choice..– Hold back reproduction event until the best male is
identified
Males Are Riskier
Males Are Riskier
Males Are Riskier
Males Are Riskier
http://www.lifeinthefastlane.ca/why-women-live-longer-than-men/humor-humour
Males Are Riskier• http://www.youtube.com/watch?v=m
CudcVno1gQ
Male Survivorship Low Relative to Females
• Risky behavior in males leads to lower survivorship relative to females
• Risky behavior leads to– More exposure to
• Predators• Disease• Increased aggression & violent behavior
– Increased energetic cost– High levels of testosterone
Sahara Gerbil
Soay Rams
Low Male Survivorship
Short Lifespan Influences Reproductive Effort
• Male strategies influenced by future reproductive potential
• Probability of dying before next reproduction is high
• Since females are the limiting factor, males typically invest more in mating effort– Males in competition for limiting resource
• Male can only “Win Big” if he outcompetes other males– Males willing to stress, starve & die for
reproduction
Sex Can Mean Death for a Male!
The things boys have to do to get a date…..
Male-Male Competition• Andersson, 1994 “Sexual Selection”• Male reproductive strategy dependent on
female strategy• 5 fundamental areas of male-male
competition – Scrambles– Endurance Rivalry– Contests– Sperm competition– Mate choice (Female choice)
Finding Potential Partners• Scrambles• In some species the females remain in one
location– Maintain a territory– Females are dispersed in the environment
according to resource availability• The male maximizes his ability to find a
mate by maximizing his search area & his ability to search
Scramble Competition• Male fitness increased by adaptations that
allow him to find mates• Results in adaptations that increase
– Sensory systems & structures– Mobility– Increased spatial ability
• Early search & Swift location of mates– Early reproduction
Moths as Scramblers• Male moths & butterflies have more
elaborate antennae than females• Sensory structures• Receptive female will pick a safe location
& emit a chemical pheromone (chemical attractant)
• Male who first reaches her has greater probability of fertilizing her eggs
Indian Moon Moth
Up to 7 miles away….
Elaborate Sensory Structures
Golden Saltmarsh Mosquito
My What Big Eyes You Have
Many fly families males have larger eyes to facilitate mate location
Newts As Scramblers• Red spotted newts• Tail fin size correlated with
fertilization frequency• Adult newt populations average 2
males for each female– Steep intrasexual competition
• Males with bigger fins can get to females more efficiently than his competitors
Red Spotted Newt
Swim Little Buddy…
Sahara Gerbil• Increased spatial acuity & locomotor
physiology
Male Mammals Have Bigger Hippocampus???
Scramble Competition Leads to early Maturation in Males
• Males of short lived species exhibit early maturation
• The earlier a male matures the less males are around for competition– Selection pressure for males to mature early
• Males that mature later encounter fewer non-inseminated females
Fig Wasp• Female lays eggs in fig• Male wasps develop & hatch earlier than
females• Emerge earlier than females• Allows scramble competition for emerging
virgin females• Some species males do not wait for
females to emerge– Males chew into fig & mate with females before
they hatch
Endurance Rivalry• Sexual selection favors traits that improve
endurance of males• Male remains reproductively active during
a large part of the season– Mating success correlated with length of time
spent at breeding sites• Allows male to increase the number of
females he mates with• Consequences can decrease survival
Endurance Rivalry• Favors adaptations that allow male
to persist• Larger body size• Stamina
Antechinus sp• Insectivorous
marsupial• Big bang reproduction• Remain at mating site
without feeding until they die– Stomach ulcers,
disease, parasites– http://
www.youtube.com/watch?v=zv7b-KPg9hY
Endurance Rivalry• Lekking• During mating season males of lekking
species gather into small clusters of territories (lek)
• Males each defend a territory within the lek via competitive mating display
• Meet daily before & during mating season display & spar
• Superior males earn prime spots
Sage Grouse Lek
Sage grouse
• http://www.youtube.com/watch?v=m0M8pZnNlnI
Contests Competition• Combat Contests
– Traits that improve success in fights• Large size• Strength• Weaponry• Agility
• Strength signals– Favors Threat signals
• Alternate tactics– Alternate mating tactics of inferior competitors,
avoiding contests with superior rivals
Combat Contests• Males engage in direct combat or
other physical contests• Natural selection favors Adaptations
such as– Weapons– Strength – Large size– Aggression
Kangaroo Boxing
• http://www.youtube.com/watch?v=yl7g4GR7kzg
Moose Fighting
• http://www.youtube.com/watch?v=jLnoPkF_6_w
Dendrobates auratus
• http://www.youtube.com/watch?v=_8lylgXCEfg&feature=related
Rhinoceros Beetle
• http://www.youtube.com/watch?v=vWWHHe9rCl0
Elephant seal• Male bulls 20 ft long, 6600 lbs
– Males 5X larger than the female• Proboscis amplifies sound• Less than 1 in 10 successfully maintain
breeding territory– Ano Nuevo
• 115 males• 5 high ranking performed 123 of 144 observed copulations
– 4% of males produced 85% of offspring• Selection favors large aggressive males• Breeding males mate 1-2 yrs• http://www.youtube.com/watch?
v=EisUYeKTtS8&feature=related
Combat Consequences
High cost is tolerated because payoff is BIG
Sex on the beach• http://www.youtube.com/watch?
v=LhRco2_OlMY• 1 in 1000 cows suffocated• Females will seek sex from smaller
males