You decide to study two species of birds, both of which form monogamous pairs (one male and one female). In species 1, the eggs
in a pair's nest are almost always the offspring of that pair. In species 2, 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. What would be the logical prediction to make before comparing the testes size of males of the two species?
B-Testes of species 2 are larger than testes of species 1.
Explanation:
Relatively bigger tests are associated with female and/or male promiscuity across a wide range of invertebrate and vertebrate taxa.
Short 1979; Harcourt et al. 1981;
Two general theories to explain the interspecific difference in test size were proposed: 1) sperm competition (Parker 1970) and 2) sperm depletion (Short 1981). Data available suggest that sperm competition in the evolution of the size of avian testes may be a more important selective force than sperm depletion. As a result, species-specific test size data were used in comparative studies, particularly of birds, as a convenient and supposedly reliable index of sperm competition levels which further leads to bird promiscuity.
