NSCI 1403 Lecture Notes - Lecture 5: Clasper, Mating System, External Fertilization
9 Jul 2016
School
Department
Course
Professor
Sexual Selection and the Evolution of Animal Mating
Systems 4/20/2016 7:00:00 PM
Darwin’s concept of sexual selection:
• Many species show sexual dimorphism
• Males in sexual dimorphic species often have conspicuous traits
such as bright coloration, long horns or anthers, and complex
courtship behavior
• These traits evolved because they improve the chance of mating
success
• Intrasexual selection(Male-male competition): males have to
actively fight to mate with the female. Males with desirable traits
would win the fight to mate with the female
• Intersexual selection (female choice)
• Body size subject to sexual selection
o Larger males may have the advantage in obtaining territories
and/or have access to more females
Alternative male mating tactics
• If you are not largest or most brightly colored you may have little
or no chance of mating
• Are there alternative mating strategies that give these lames some
chance of success?
o In frogs:
▪ External fertilization
▪ Amplexus: male clasps onto the females back
▪ Vocal signals
▪ Seasonal breeding, sometimes temporally concentrated
• How can a small male mate if the large males are present?
o The “satellite male” strategy:
▪ Hang out in the vicinity of the large male, knowing that
females will be attracted to his calling and do not call
▪ Intercept the female and try to mate with her
Mating system in horseshoe crabs has interesting parallels
• External fertilization
• Amplexus
find more resources at oneclass.com
find more resources at oneclass.com
• Mating peaks coincide with the full moon or new moon phases
• Satellite males are not attached to the female while she is mating
with another male
Components of male behavior
• Once the males are attached to the female they are very difficult to
detach. Other males almost never succeed at pushing off the male
from the female
• Are large males more likely to form mated pairs than small males?
Or alternatively is pairing random with respect to size?
• Do satellite males succeed in mating?
• What determines whether a male will be a mated male or satellite
male?
Size distribution:
• Males are typically smaller than females
• Conclusion: male body size is not related to mating tactic (mated
vs. satellite)
o This is probably enough because each clasper span is wide
enough to attach to any female in the population
o Even the smallest male have claspers large enough to attach
to the biggest female in the population
• The out come is simple a random collision between any male and
any female. “Random Collision” Model
o There are more average sized males in the population than
there are very small and very large males
o What is expected to be found therefore is average male
coupled with an average female
o What isn’t likely is small male pairing with large female
o Rarest combination: very large male pairing up with really
small female
Do satellite males succeed in mating?
• Given that they cant dislodge the mating male from the female,
why do they hang around?
• Are the eggs only fertilized by the attached male?
• Obtain dna samples from the attached males and satellite males
and if the male who is attached fertilized all of the eggs all the dna
of the offspring should trace back to him
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Sexual selection and the evolution of animal mating. Males with desirable traits would win the fight to mate with the female: intersexual selection (female choice, body size subject to sexual selection, larger males may have the advantage in obtaining territories and/or have access to more females. Mating system in horseshoe crabs has interesting parallels: external fertilization, amplexus, mating peaks coincide with the full moon or new moon phases, satellite males are not attached to the female while she is mating with another male. Components of male behavior: once the males are attached to the female they are very difficult to detach. Getting there first is priority: good vision enables you to see the females as they approach the shore, crabs with two alternative mating tactics (mating, satellite) largely determine the overall condition of their eyes and claspers. Birds have to have early experience in learning song, in order to learn the correct songs.
