Sexual Selection & the Evolution of Animal Mating Systems
Darwin’s concept of sexual selection
•Many species show sexual dimorphism.
•Males in sexually dimorphic species often have conspicuous traits such as bright coloration, long
horns or antlers, and complex courtship behaviors.
•These traits evolved because they improve the chance of mating success.
•Intrasexual selection (male-male competition)
•Intersexual selection (female choice)
•Another body trait subject to sexual selection is body size.
•Larger males may have an advantage in obtaining territories and/or have access to more females
Alternative male mating tactics
•If you are not the biggest or most colorful male, you may have little or no chance of mating with a
•Are there alternative mating strategies that give these males some chance of success?
•Frog mating behavior is well studied for these concepts
•amplexus (Once a male frog has found a mate, he needs to take up a position in which he can
fertilize the female's eggs)
•vocal signals (convey info about body size; deeper voice is larger and thus more attractive)
•seasonal breeding, sometimes temporarily concentrated
•How can a small male mate if larger males are present?
•The satellite male strategy:
•hang out in vicinity of of large male knowing the females will be attracted to their calling; small
•intercept the female and tries to mate with her
•The mating systems in horseshoe crabs has parallels to frogs
•external fertilization, amplexus, mating peaks concede with full moon or new moon phases
• Male claspers are used for amplexus; males hold on to female; very difﬁcult to separate clasped male
•sexual size dimorphism in horseshoe crabs; males typically smaller but theres a wide range
•male body size is not related to mating tactic (mated vs satellite)
•This is probably because any male’s clasper span is wide enough to clasp to any female
•Random Collision Model used in experiment
•Paternity analysis using DNA Markers
•If there are no other males present, the attached male has sperm for all eggs
•when present, satellite males release sperm and some achieve fertilization
•being a satellite is an alternative male mating tactic for those males that do not attach to a female