Academic journal article The American Midland Naturalist

Reproduction Decreases Life Span in the Giant Waterbug (Belostoma Flumineum)

Academic journal article The American Midland Naturalist

Reproduction Decreases Life Span in the Giant Waterbug (Belostoma Flumineum)

Article excerpt

ABSTRACT.-Senescence is the result of age-specific trade-offs among life history traits. Energetic trade-offs among various components of an organism's energy budget can also affect an individual's life span. An increase in reproductive effort, for instance, can result in less energy available for maintenance of body tissues resulting in a shorter life span. We investigated the effects of reproduction on longevity of giant waterbugs (Belostoma flumineum). Female giant waterbugs oviposit eggs onto the backs of males which then provide all post-copulatory parental care. The number of reproductions was manipulated in both males and females, and male waterbugs were divided into groups that provided parental care or did not This allowed us to determine the relative costs of mating versus parental care. Both male and female waterbugs maintained as virgins outlived those that bred. Parental care incurred a greater cost in male waterbugs than the act of mating, but the number of reproductions had no effect on life span suggesting that a single reproductive event is as costly as many. In both males and females the age at first reproduction was significantly positively correlated with age at death.

INTRODUCTION

Senescence, the process by which organisms age and ultimately die, is generally considered to be a product of the decreasing ability of natural selection to remove detrimental genes expressed after an individual has reproduced (Comfort, 1954; Medawar, 1955, 1957; Rose, 1991). Since life span has a direct impact on the lifetime fitness of an organism, it is generally accepted that it will be correlated with other fitness traits. Typically, the suite of fitness traits an organism possesses cannot be maximized concurrently, necessarily resulting in trade-offs among various life history characteristics (Steams, 1992). Life span, for example, has been shown to be positively correlated with age at first reproduction (Clarke and Maynard Smith, 1961; Wattiaux, 1968; Sokal, 1970; Mertz, 1975; Law, 1979; Rose and Charlesworth, 1980,1981; Luckinbill et al., 1984; Partridge and Barton, 1993; Zwaan et al., 1995; Sgro and Partridge, 1999; Sgro et al., 2000). Individuals that reproduce early die sooner than individuals that delay reproduction.

Life span is also shown to be negatively affected by increased rates of reproduction (Murdoch, 1966; Loschiavo, 1968; Tinkle et al., 1970; Ricklefs, 1977; Haukioja and Hakala, 1978). This has generally been explained as either a result of increased extrinsic mortality (predation, disease) due to compromising reproductive behaviors or of competing energy requirements. When extrinsic mortality is experimentally removed in laboratory situations, the effect of energetic trade-offs in decreasing life span can be addressed. An organism's energy budget can be divided into four components to include growth, maintenance, reproduction and storage (Gadgil and Bossert, 1970; Congdon et al., 1982). Since available energy is finite in many, if not all, cases, when more energy is allocated to one area there is necessarily less available for other compartments. Individuals that invest more energy into reproduction, therefore, have less available energy to maintain their cells and tissues, which can result in a shorter life span. While this relationship has been shown in a variety of organisms, most studies have focused on females (Snell and King, 1977; Law, 1979; Partridge and Farquhar, 1981; Nur, 1984; Clutton-Brock et al., 1988; Fowler and Partridge, 1989; Service, 1989; Prowse and Partridge, 1997; for exceptions see: Partridge and Farquhar, 1981; Service, 1989; Prowse and Partridge, 1997). Females tend to have higher levels of parental investment per offspring than males (Trivers, 1972), typically making them a better choice for investigating energetic trade-offs between reproduction and life span. One would predict, however, that increased rates of reproduction would decrease life span in both sexes of organisms in which significant amounts of parental investment is supplied by males. …

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