The theory of r- and ^-selection illustrated that as life-history traits evolve, trade-offs have taken place among growth, maintenance, and reproduction. Another way of approaching life-history evolution is to examine the allocation of energy devoted to reproduction over a life span. This is known as reproductive effort. Energetic trade-offs are found in all aspects of a life history. When the parent organism devotes energy to reproduction it has less energy to devote to growth and maintenance. The energy devoted to reproduction can be partitioned in various ways: large versus small offspring, small versus large litter size, the amount of energy devoted to parental care, etc.
Cole (1954) described the dichotomy of semelparous versus iteroparous reproduction. In semelparity, reproduction is channeled into one major reproductive effort. Most insects, many invertebrates, fish, and many plants (annuals, biennials, and some bamboos) have this life cycle. A semelparous life history in a disturbed habitat offers no mystery. More interesting are organisms which are long-lived, yet have semelparous reproduction. For example, mayflies often spend several years as larvae, and periodical cicadas live 13 or 17 years below ground as juveniles, but the adult phases are only a few hours or days in mayflies and a few weeks in periodical cicadas. Hawaiian silverswords (Argyroxiphium spp.) live 7 to 30 years before flowering once and dying. Some bamboo species delay flowering for 100-120 years before producing a massive seed crop and dying. Janzen (1976) and others have proposed that this life history allows escape of highly vulnerable juvenile stages through predator satiation.
Iteroparity is common among most vertebrates and perennial plants. Iteroparous species, nevertheless, are extremely diverse in their life histories: (i) short versus long pre-reproductive periods; (ii) annual versus periodical reproduction; (iii) small versus large amount of reproductive effort; and (iv) many small offspring versus a few large offspring.
Since energy devoted to reproduction is not available for growth and maintenance, reproduction itself has a "cost" in terms of increased mortality or decreased growth of the adult organism. Thus, an individual that reproduces in a given year often has reduced survivorship and/or may reproduce at a lower rate in the near future. In addition, there is usually a limit to the number of offspring that an adult or pair of adults can successfully produce without causing harm to themselves, their survivorship, their future reproduction, or the survivorship of this year's brood.
Therefore, an organism may have greater evolutionary fitness over the long term if it postpones reproduction or limits the allocation of energy to current reproduction. This will be true if the energetic allocation to growth and maintenance (as opposed to reproduction) produces a sufficient gain in future reproduction to compensate for losses in the present. According to this argument, an organism may have greater evolutionary fitness over the long term if it postpones (or limits the allocation of energy to) reproduction in the current year.
Field studies documenting the effect of reproduction on growth, survival, and future reproduction are not abundant. Primack and Hall (1990) showed that reproduction in pink lady's slipper orchids (Cypripedium acaule) is limited by bee pollination, but seed production could be greatly increased through hand pollination. As seed production increased, the cost of reproduction took effect in the third and fourth years. Handpollinated plants had lower growth and flowering rates than controls. For example, an average-sized hand-pollinated plant lost 10-13% of leaf area and had a 5-16% lower flowering rate as compared to control plants. In red deer (American elk) (Cervus elaphus) (Clutton-Brock 1984, Clutton-Brock et al. 1982, 1989) and in lizards (Tinkle 1969) it has been shown that great reproductive effort leads to declining fecundity and reduced survivorship. In American bison (Bison bison) sons suckle longer than daughters (up to 15 months). Cows that have produced sons breed later and are more likely to be barren in the next year as compared with cows that had females in the previous year (Wolff 1988).
Experimental field studies are often done with birds, in which the clutch size can be manipulated. Reid (1987) studied the optimal brood size of the glaucous-winged gull (Larus glaucescens) in which the natural range of chicks per nest is 1-3. Reid added and subtracted eggs, producing broods of from one to seven chicks per nest. He found that when more than three chicks were present in the nest, adult survivorship declined significantly.
A study by Beissinger (1990) on snail kites (Rostrhamus sociabilis) at two field sites is also instructive. Brood size was manipulated with similar results. The normal brood range is 1-3 eggs. With only one egg in a nest, since one parent can raise one young successfully, both females and males often deserted the nest and tried to start another one. Desertion rates were almost 100% with one egg per nest, with females deserting twice as often as males. With two young per nest, desertion rate was about 50%. With three young in the nest there was no desertion. All broods with one or two young were 100% successful at fledging. However, the kites had difficulty raising three young and were unable to raise four young. Desertion is an adaptive response that allows the kites to adjust their parental investment to the number of chicks present in a nest.
In the Venezuela field site, Beissinger found that no parents raised four young, and only 40% of nests with three young raised them all. At control nests no broods of three young occurred naturally (205 nests). In Florida, however, 22% of the control nests fledged three young. The investigators found that total food delivery rates increased up to three young per nest, but there was no further increase with four per nest. Thus, natural selection closely controls the clutch size and the amount of parental effort per nest in snail kites.
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