The ability of a male to mate multiply is likely to be an important contributor to female fertility, as matings are concentrated in the periods shortly before dusk and after dawn when single males mate with several females [ 16].
Sperm depletion and exhaustion, due to males mating multiply, may be costly to females as well [ 9].
But when the mother mates multiply, matrigenes are more related than patrigenes to the siblings affected, and matrigenes are therefore predicted to be less selfish.
At least under some circumstances, mating multiply appears to be harmful for females [9] [12] both as a result of behavioural interactions and as a side effect of postcopulatory sexual selection [13] [15].
Although laboratory mating trials are able to detect genetic differences in intrinsic female preferences [27], this might not reflect relative male mating success in nature where females are constantly harassed by males [28], [29] and are mating multiply [30], [31].
From a classical perspective, males are expected to increase their fitness by mating multiply with many females, while females are assumed to require only one or a few matings to maximise their fertility [3]; [8].
However if multiple mating is advantageous for females, as is increasingly recognized across many taxa, then it is time to examine mating decisions from the perspective that both female and male fitness can be maximized by mating multiply.
If, for example, offspring from multiply mated females are larger at birth or grow faster (two fitness correlates) than offspring from singly mated mothers, then multiply mated females could attain greater fitness benefits for the same number of offspring produced.
The differences in offspring from singly and multiply mated females are "astonishing," says evolutionary and behavioral biologist Anne Houde of Lake Forest College in Illinois.
Alternatively, fitness benefits for males are often very strong, implying a strong selection pressure for mating multiply, with possible consequences for mating rates in both sexes (Halliday and Arnold 1987; Uller and Olsson 2008; W. Forstmeier, personal communication).
