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

Aaah.. the better to find you

Indian Moon Moth

Up to 7 miles away….

Elaborate Sensory Structures

Wedge-shaped Beetle

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