Sperm competition

Gametes evolved to different sizes. Females by definition are the sex that produces larger gametes. Once they've deposited their smaller gametes, males are in the advantageous position to limit energetic input (e.g., leave). The females are then left with the decision to raise offspring or not, and this decision has important energetic implications. But males also have one evolutionary uncertainty to overcome: the certainty of paternity. If males release millions of sperm and can father multiple litters within a single reproductive season (by breeding with several females), the certainty of paternity is seldom assured, and the uncertainty of paternity may well be the greatest challenge that mammalian males face when it comes to reproduction. Not surprisingly, a myriad of adaptations has evolved to overcome this uncertainty—from mate guarding and mate defense to strategies inside the reproductive tract such as sperm competition. Also, if there are fights to prevent other males from mating, these males will fight to overcome barriers put in place by previous males. One could argue that when males fight, females ultimately win because whichever male succeeds probably has better genes or the kind of genes

A Dall's sheep ram (Ovis dalli) in the November rut season in a urine-testing pose, testing the female's urine for signs of estrus. (Photo by © Hugh Rose/Visuals Unlimited, Inc. Reproduced by permission.)
Florida manatee mother nursing her calf in Crystal River, Florida, USA. Photo by Animals Animals ©Franklin J. Viola. Reproduced by permission.)

copulation. The longer the sperm stays inside the female, the better the odds of fertilization by maximizing the amount of sperm and hence number of sperm cells active in the female tract. Interestingly, the tip of the penis (the glans) is used by males to remove sperm plugs deposited by other males.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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