You decide to study two species of birds, both of which form monogamous pairs (one male and one female). In species 1, you find that the eggs in a pair's nest are in fact almost always the offspring of that pair. In species 2, you are surprised to find that many of the eggs in a nest were actually fathered by males of neighboring pairs. Apparently, mating outside of monogamous pairings is widespread in species 2. Given this information, what would be the logical prediction to make before comparing the testes size of males of the two species?
A) Testes of species 1 are larger than testes of species 2.
B) Testes of species 2 are larger than testes of species 1.
C) The testes size of species 1 will increase throughout its lifespan, eventually becoming larger than that of species 2.
D) There is no relationship between this observation and the size of testes.

Question

Quizplus · Accepted Answer

The fact that there is a high level of mating outside of monogamous pairings in species 2 suggests that there is likely to be sperm competition. In species with high levels of sperm competition, males tend to have larger testes in order to increase their chances of reproduction. Therefore, it is reasonable to predict that the males of species 2 have larger testes than those of species 1.

You Decide to Study Two Species of Birds, Both of Which