Sons and daughters represent different costs to the mother. Generally, in the animal kingdom, sons are more "expensive" for a mother to produce, as they tend to be larger and more colorful. The mother provides resources (such as glucose, various vitamins, minerals, and hormones) for her young, both when she makes the egg and when she raises her young. However, these resources are allocated differently between sons and daughters, as they have different requirements. Think about peacocks, for example. A peahen (female) is quite small and her feathers are a dull brown. In comparison, a peacock (male) is bigger, very brightly colored, and has long, elaborate tail feathers. These long, brightly colored tail feathers allow a male to attract a mate. It takes a lot of energy to make these feathers, which is why sons are considered "expensive." The energy to grow elaborate feathers comes from the male himself, but his mother gives him extra hormones and other resources to keep him healthy and help ensure that he can grow a long, colorful tail. This initial maternal investment helps her young to survive.
Survival also depends on the animal's environment.

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