THE TRIVERS-WILLARD HYPOTHESIS
"In species with a long period of parental investement after birth of young, one might expect biases in parental behavior toward offspring of different sex, according to the parental condition; parents in better condition would be expected to show a bias toward male offspring."
Males produce more offspring than females:
Mother's Health Hypothesis
The Red Deer
Crafoord Prize in Biosciences 2007
http://www.boston.com/news/globe/ Trivers himself
Adams, J, 1989. Natural selection of parental ability to vary resource partitioning among offspring? Oikos 56: 135-137.
Lee, D.E. and Sydeman, W.J. 2009. North Pacific climate mediates offspring sex ratio in northern elephant seals. Journal of Mammalogy 90: 1-8.
Dugdale, H.L., Macdonald, D.W., Newman, C. 2003.Offspring sex ratio variation in the European badger, Meles meles. Ecology 84: 40-45.