ANIMALS HAVE MANY MATING STRATEGIES

by Whit Gibbons

June 27, 2010

Years ago, on an early summer evening, I watched a big salamander creep through a swamp. The animal was a greater siren, one of the largest salamanders in the world. The one I watched that night was more than two feet long. I stood on the bank for several minutes, following its movements with my flashlight.

It may have been looking for food, but my hope was that it would encounter another greater siren of the opposite sex. Because despite the immense size of this native salamander of the Southeast, few people see it in the wild, and ecologists know next to nothing about its breeding habits. I was hoping to be the first to see sirens mate.

The study of animal mating systems and social structure is an intriguing area of ecological research. Mating among animals is not a random, haphazard process. Within each species, mating strategies and patterns--often highly complex and intricate--have been molded throughout the species' evolutionary history. Each year our knowledge across the spectrum of species increases and patterns begin to take shape.

Some species, like Canada geese, are relatively monogamous, often remaining with the same mate for life. Male wild donkeys may keep a harem of females and physically prevent younger males from mating with them. The timing of mating also varies among species. Most frogs breed at night. Most birds breed during the day. Zebras breed year round. Garter snakes mate in the spring. Timber rattlers look for mates in the fall.

Ecologists collect information about when, where and how mating occurs. These ecological voyeurs try to determine the social structure of animal populations and unravel the complexities and relationships among individuals. Each new discovery can increase our understanding of the relationships between the ecology, evolution and breeding pattern of the species.

A tropical frog found in Guyana, South America, provides evidence that an individual animal's actions may be best for propagation of that individual's genes but not best for producing the most offspring. The females, if given a choice, will select male mating partners that are about 80 percent of their own length. Experiments revealed that this female-to-male size ratio produced the highest fertilization of a female's eggs. When the male was larger than the female, it fertilized significantly fewer eggs because it was not in proper position for the sperm to reach all the eggs. Males smaller than the optimal size did not have an adequate sperm supply to fertilize all the eggs. So to maximize egg fertilization and have the highest reproductive success, a female needs a mate about 80 percent her size.

The males, however, have their own agenda, and a larger male can physically displace a smaller one chosen by the female. Thus, a larger one may end up mating with her. These bullies of the frog world fertilize a lower percentage of the female's eggs. But they produce some of their own offspring and pass on their own genes, which would not have happened if they had not mated.

Bullfrogs practice a satellite mating strategy in which smaller but opportunistic males sometimes successfully mate. When a male bullfrog gives a mating call to attract the female, a smaller male may remain quiet but alert as she approaches. When a female passes by on her way to check out the calling male, a small, silent, satellite male may intercept her and mate. Such competition between males can reduce the reproductive success of some females while ensuring the propagation of a particular male's genes.

Every animal has a mating story. Yet in spite of our having discovered enough about breeding systems to identify patterns and classify them into categories, basic information about individual behaviors and strategies remains to be discovered for most species. Perhaps mating patterns exist of which we are still unaware. One day, when someone adds the siren breeding story to the list, we may discover that one of our native wetland species has a fascinating mating strategy of its own.

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