Chapter 9. Chapter 9 Graphic Content

A female with one mate produces 60 offspring. This is the same number as for a male mating with one female. The two numbers are the same because they are measuring the same thing: one male mating with one female. Whatever the number of offspring produced, it is recorded in the left-most bar in each graph.

A female with two mates produces 50 offspring. It is not clear why females with two mates would produce fewer offspring than females with one mate. At least two explanations are possible. First, because these numbers are measured for different females—that is, a female’s reproductive output can be measured when she has one mate or two mates, but not both—this may just be a consequence of natural variation among females. Maybe those in the two-mate group just happened to have fewer offspring, unrelated to the number of mates they had. Second, perhaps the presence of two males created stress, which somehow reduced either the female’s or the males’ fertility. For additional data, it would be useful to have error bars to determine why the number of offspring is lower for females with two mates. These would give us a sense of how much variation there is in reproductive output among females given access to two males (and among females given access to just one male). If there is much variation within those two groups, then the lower number seen here may just be an artifact of the individual females measured. In this case, if greater numbers of birds were used in the study, the average reproductive output would be the same for females given access to one mate or two mates.

Females with four mates would most likely still have about 60 offspring, while males with four mates would have about 150 offspring. These numbers assume the same trends shown in the graphs.

It appears that females are not limited by access to mates. Rather, they seem to be limited by their own production of eggs. Males’ reproductive output, however, does seem to be constrained by the number of mates they have. If a male has more mates, he has greater reproductive output.

To make comparisons between the data in the two graphs, it is essential that they are measuring the same thing, using the same scales. Otherwise, it would be like comparing apples and oranges.