lifespan. r-selected organisms usually: mature rapidly and have an early age of first reproduction have a relatively short lifespan have a large number of offspring at a time, and few reproductive events, or are semelparous have a high mortality rate and a low offspring survival rate have minimal parental care/investment K-selected organisms usually: mature more slowly and have a later age of first reproduction have a longer lifespan have few offspring at a time and more reproductive events spread out over a longer span of time have a low mortality rate and a high offspring survival rate have high parental investment Variation Variation is a major part of what LHT studies, because every organism has its own life history strategy. Differences between strategies can be minimal or great. For example, one organism may have a single offspring while another may have hundreds. Some species may live for only a few hours, and some may live for decades. Some may reproduce dozens of times throughout their lifespan, and others may only reproduce one or twice. Trade-offs An essential component of studying life history strategies is identifying the trade-offs that take place for any given organism. Energy use in life history strategies is regulated by thermodynamics and the conservation of energy, and the "inherent scarcity of resources", so not all traits or tasks can be invested in at the same time. Thus, organisms must choose between tasks, such as growth, reproduction, and survival, prioritizing some and not others. For example, there is a trade-off between maximizing body size and maximizing lifespan, and between maximizing offspring size and maximizing offspring number. This is also sometimes seen as a choice between quantity and quality of offspring. These choices are the trade-offs that life history theory studies. One significant trade off is between somatic effort (towards growth and maintenance of the body) and reproductive effort (towards producing offspring). Since an organism can't put energy towards doing these simultaneously, many organisms have a period where energy is put just toward growth, followed by a period where energy is focused on reproduction, creating a separation of the two in the life cycle. Thus, the end of the period of growth marks the beginning of the period of reproduction. Another fundamental trade-off associated with reproduction is between mating effort and parenting effort. If an organism is focused on raising its offspring, it cannot devote that energy to pursuing