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4 <br />different mating systems emerge. Of even greater interest, the <br />same species can show alternative mating systems under different <br />conditions. This, of course, is theory, and one needs to test <br />theory. Some desert springs are especially suited for such <br />testing. Cuatro Cienegas, in northern Mexico, is a splendid <br />example. <br />Many springs occur in the geological basin that holds Cuatro <br />Cienegas. Typically, each spring forms a pool with a stream <br />outlet that may connect to another pool or flow into an <br />evaporative pan. The conditions among the springs vary greatly. <br />They range from large to tiny, from very warm, e.g.. 346 C., to <br />relatively cool, and from productive to nonproductive. The <br />waters are delightfully clear and accessibly shallow. The <br />reproductive season is almost year-round in the warmer springs. <br />And the people in the local community of Cuatro Cienegas are <br />receptive and helpful. <br />What makes the springs there further remarkable is the <br />diversity of the fish fauna, 16 species in all. And the species <br />composition of the springs varies greatly. The community <br />structure is itself an important factor. It leads to measurable <br />differences in the trophic structures in different springs <br />because the feeding habits of the fish species range from <br />detrital feeding and herbivory through snail eating and <br />piscivory. <br />The scope of modes of reproduction among the 16 species of <br />fishes is a fair representation of the scope seen among all <br />fishes. I will run through them quickly, from simplest to most <br />advanced and make a few comments about the social systems where