Life History Strategies

Pacific salmon

Red kangaroo

Mayfly

Bamboo

Life History: refers to any aspect of the developmental pattern and mode of reproduction of an organism.

We can consider five fundamental aspects of life history: Size Metamorphosis Diapause Senescence Reproductive Patterns

The side-blotched lizard, Uta stansburiana, shows significant differences in life history characteristics across its range. It is found from Mexico in the south to Washington state in the north. Lizards in the part of the range are larger, lay more eggs and have a longer reproductive period than those in the north. They also may experience more predation.

Guppies are native to streams in Venezuela and Trinidad. Their life histories have been studied for many years by John Endler and David Reznick.

In some pools, males display prominent secondary sexual characteristics: Bright colors Prominent tail fins In other pools, the male guppes are drab in coloration. The reason? ….. predators.

Guppies (Poecilia reticulata) occupy pools separated by waterfalls. In pools where predators, such as the pike cichlid (Crenicichla altra), are present, males are drab. Where predators are absent, male guppies are brightly colored and attract females.

After several generations, guppies raised in low- and high-predation environments evolve different features. As measured by the number of bright, conspicuous spots, males become more brightly colored (low predation) or drab (high predation).

In this case, fitness results from a trade-off between the advantageous and disadvantageous consequences of bright coloration. The way in which things play out depends on the level of predation to which the guppies are subjected.

Other life history traits can be correlated to predation as well.

Laboratory studies demonstrated that the differences in life history traits were heritable, resulting from differences in fitness between guppies demonstrating different traits in different conditions. LIFE HISTORY STRATEGY A behavior or set of behaviors used by an individual to deal with an important life-history problem (for example finding a mate, rearing young, obtaining food, etc.). As with other definitions, the human term “strategy” that implies conscious thought is used as a shorthand; no conscious planning is required, even though it might appear that the behaviors are rational and planned in the human sense. The use of the word "strategy" is simply a shorthand that expresses the appearance of the result of some behaviors. It is generally assumed that in most species strategies are largely innate, are produced by the usual genetic and developmental mechanisms, and are acted on by natural selection. However, strategies can also be learned, even in relatively simple animals.

Time and Energy Budgets Both energy and time are limited for organisms. Both must be allocated among competing demands. As a result, organisms must make trade-offs in budgeting time and energy. These “decisions” help to define an organism’s life history “strategy”. An example can be seen in the rock pipit

The rock pipit adjusts the amount of time that it devotes to different activities in response to environmental conditions. In harsh winters, more time must be spent foraging. This time must come at the expense of other activities.

Human liver fluke

Schistosomiasis, life cycle In blood vessels of the human gut, adult worms mate and release eggs that reach the interior of the gut and are shed with the feces. Larvae hatch in water and enter their second host, a snail, in a form known as the mother sporocyst. Eventually the larvae within leave the snail and enter the water. The larvae pierce the skin of humans walking bare-foot in the water, usually tending agricultural fields. These larvae mature into adults again becoming lodged in the blood vessels of the gut to complete the cycle. Schistosomiasis, life cycle

Swifts are polymorphic with regard to the number of eggs they lay – some lay two eggs and others lay three eggs. Why don’t all swifts lay three eggs? Wouldn’t this lead to greater fitness? David Lack found that “three-egg” swifts produced more offspring in mild years, but were less successful than those laying two eggs in harsh years. Why? In harsh years, there is not enough food to provide for three young birds. All of the offspring suffer, and the overall production of young is lower.

Life history strategies evolve under different environmental demands Life history strategies evolve under different environmental demands. This can be diagrammatically represented with alternative energy budget allocations. The size of the arrow represents the size of the energy investment. (a) Free of beetle attack, beans allocate more to toxins and growth than to reproduction. (b) Under beetle attack, beans evolved a strategy of increased reproduction, overwhelming beetles with a large output of seeds, but at the expense of toxin production and vegetative growth.

Unstable environment, density independent K Stable environment, density dependent interactions small size of organism large size of organism energy used to make each individual is low energy used to make each individual is high many offspring are produced few offspring are produced early maturity late maturity, often after a prolonged period of parental care short life expectancy long life expectancy each individual reproduces only once individuals can reproduce more than once in their lifetime type III survivorship pattern in which most of the individuals die within a short time but a few live much longer type I or II survivorship pattern in which most individuals live to near the maximum life span