In animals without paternal care, the number of offspring sired by a male increases as the number of females he mates with increases. This fact has driven the evolution of multiple matings in the males of many species. It is less obvious why females mate multiple times, because it would seem that the number of offspring that a female has would be limited by her resources and not by the number of her mates, as long as she has at least one mate. To look for advantages of multiple mating, a study of the Gunnison’s prairie dog followed females to find out how many times they mated (Hoogland 1998). They then followed the same females to discover whether they gave birth later. The results are compiled in the following table:

Did the number of times that a female mated affect her probability of giving birth?

a. Calculate expected frequencies for a contingency test.

b. Examine the expected frequencies. Do they meet the assumptions of the  contingency test? If not, what steps could you take to meet the assumptions and make a test?

c. An appropriate test shows that the number of mates of the female prairie dogs is associated with giving birth. Does this mean that mating with more males increases the probability of giving birth? Can you think of an alternative explanation?

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Number of times female mated Number of times female mated Number who gave birthNumber who gave birth Number who didn't give birthNumber who didn't give birth 11 8181 22 8585 33 44 55 6161 1717 55 66 88 00 00 00