Male-male Competition in Species With External Fertilization

I didn't have time to talk about male-male competition in species with external fertilization in class, but I have decided that this is an interesting enough topic to deserve its own post (and the subject is addressed in the Expected Learning Outcomes for Sexual Selection).

Most of the animals that we are used to thinking about have internal fertilization which means that the male mates with the female and then deposits sperm into the body of the female.  Other species, including many marine and aquatic species have external fertilization.  For example, in most fishes females deposit eggs either in a nest or in the open water column and then males swim by and deposit sperm in the water which will find the eggs and fertilize them.

Because it is possible for many males to attempt to fertilize the same eggs, in many cases the male that fertilizes the most eggs is simply the one that produces the most sperm. Thus, male-male competition in many species of fishes has led to the production of large amounts of sperm. In some cases the testes can make up almost half of the body mass of a male fish.

The Amazing Sex Life of Fish!!

Fish have some of the craziest and most interesting sex lives of all of the species on earth!  One of the interesting aspects of the sex lives of some of the coral reef fishes is that they change sex during their lives. Usually, individuals will start their lives as females and then change sex to males when they are older and larger (sequential hermaphroditism).

In some species, such as the blue-head wrasse, males defend territories that can contain several several females that he mates with.  In some populations there are fish with an interesting strategy.  There are fish that look and act just like females, but are actually males (these fish are known as "sneakers" - now the ridiculous photo at the top of the post should make sense).  The territorial male accepts the sneakers in his territory because he thinks that they are females.  When a female lays eggs in the territory the sneaker males sneak in, deposit a cloud of sperm, and fertilize some eggs! Pretty cool!!