An individual's absolute fitness depends on the timing, quality, and quantity of reproduction. This life history incorporates fundamental components of reproductive output and survival, including size at birth and maturity, rate of growth, age at maturity, size and number of offspring, and senescence. In general, these life-history traits arise from an individual's schedule of investment in growth, maintenance, and reproduction. Because all organisms must cope with limited resources, investment in one function tends to reduce resources available for other functions, creating trade-offs between life-history traits. For example, allocation to current reproduction often reduces an individual's survival and/or future reproductive efforts. Such trade-offs create correlations between life-history traits. Thus, in some species, individuals reach sexual maturity at a young age, produce many small offspring, and live short lives. Individuals of other species mature late, produce few large offspring, and live long lives (Holmes and Sherry 1997: Read and Harvey 1989; Roff 1992; Stearns 1992).