Sexual Selection on Males Female Choice.docx

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Biomedical Engineering
BMEN 515
William Huddleston

Sexual Selection on Males: Female Choice  There is a great variety of species in which male reproductive success is limited by opportunities to mate but in which males are unable to monopolize either females themselves or any resource vital to females o In such species, males advertise for mates o Females typically inspect advertisements of several males before they choose a mate o Sexual selection by mate choice leads to the evolution of elaborate courtship displays o Beyond allowing females to identify a mate of the right species, male courtship displays were thought to function primarily in overcoming a general female reluctance to mate; once ready to mate, a female would accept any male at hand  When males cannot monopolize access to females, they often compete by advertising for mates o Basically when they can’t own them, they got to buy them Female Choice in Red-Collared Widowbirds  Read example on Female red-collared widow birds on Pg. 416-417. o Females find males with long tails sexy Female Choice in Gray Tree Frogs  Read example on gray tree frogs and their mating calls on Pg. 417-419. o Female gray tree frogs prefer males with longer calls than males with shorter calls. Choosy Females May Get Better Genes for Their Offspring  The displays given by males are indicators of genetic quality  Five aspects of offspring performance related to fitness: o Larval growth rate – faster is better o Time to metamorphosis – shorter is better o Mass at metamorphosis – bigger is better o Larval survival o Post-metamorphic growth – faster is better Choosy Females May Benefit Directly through the Acquisition of Resources  In many species the males provide food, parental care, or some other resource that is beneficial to the female and her young  Hanging flies live in the woods of eastern North America, where they hunt for insects  After a male catches an insect, he hangs from a twig and releases a pheromone to attract females  When a female approaches, the male presents his prey; if she accepts it, the pair copulates while she eats  The larger the prey, the longer it takes her to eat it, and the longer the pair copulates o The longer the pair copulates, the more sperm the female accepts from the male o If she finishes her meal in less than 20 minutes, the female breaks off the copulation and flies away looking for another male and another meal o The males giving her large gifts benefits her in two ways:  Provides her with more nutrients, allowing her to lay more eggs  Saves her from the need to hunt for herself o If the female is still eating after accepting all the sperm she can, the male grabs his gift back and flies off to look for a second female to share it with Choosy Females May Have Preexisting Sensory Biases  The pre-existing bias, or sensory exploitation, hypothesis holds that female preferences evolve first and that male mating displays follow  Read example on Water Mites on Pg. 423. o Males that tremble should get more of his spermatophores picked up by females than would a hypothetical male that does not tremble o A male that trembles would enjoy higher mating success o Therefore trembling is evolved by sexual selection  Sometimes choosy females may simply be responding to courting males as though the males were prey Other Explanations on Female Choice  It is also possible that female preferences are essentially arbitrary; one version of this idea is the sexy-son hypothesis o According to the sexy-son hypothesis, once a particular male advertisement display is favoured by a majority of females, selection on fema
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