Effects of size at metamorphosis on stonefly fecundity, longevity, and reproductive success |
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Authors: | Brad W Taylor Chester R Anderson Barbara L Peckarsky |
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Institution: | (1) Department of Entomology, Cornell University, Ithaca, NY 14853, USA, US;(2) Rocky Mountain Biological Laboratory, P. O. Box 519, Crested Butte, CO 81224, USA, US |
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Abstract: | Many organisms with complex life cycles show considerable variation in size and timing at metamorphosis. Adult males of Megarcyssignata (Plecoptera: Perlodidae) are significantly smaller than females and emerge before females (protandry) from two western Colorado
streams. During summer 1992 stoneflies from a trout stream emerged earlier in the season and at larger sizes than those from
a colder fishless stream, and size at metamorphosis did not change over the emergence period in either stream. We performed
two experiments to determine whether variation in size at metamorphosis affected the fecundity, reproductive success and longevity
of individuals of this stonefly species and if total lifetime fecundity was affected by the number of matings. In the first
experiment, total lifetime fecundity (eggs oviposited) was determined for adult females held in small plastic cages in the
field. Males were removed after one copulation, or pairs were left together for life and allowed to multiply mate. Most copulations
occurred in the first few days of the experiment. Females in treatments allowing multiple matings had significantly lower
total lifetime fecundity and shorter adult longevity than females that only mated once. Multiple matings also reduced longevity
of males. Fecundity increased significantly with female body mass at emergence, but only for females that mated once. While
multiple matings eliminated the fecundity advantage of large female body size, number of matings did not affect the significant
positive relationship between body mass at metamorphosis and longevity of males or females. In a second experiment designed
to determine if body mass at emergence affected male mating success, we placed one large and one small male Megarcys in an observation arena containing one female and recorded which male obtained the first mating. The large and the small
male had equal probabilities of copulating with the female. Copulations usually lasted all night, and the unmated male made
frequent, but unsuccessful attempts to take over the copulating female. Our data suggest that selection pressures determining
body size at metamorphosis may operate independently on males and females, resulting in evolution of sexual size dimorphism,
protandry, and mating early in the adult stage. We emphasize the importance of interpreting the fitness consequences of larval
growth and development on the timing of and size at metamorphosis in the context of the complete life cycle.
Received: 1 July 1997 / Accepted: 12 November 1997 |
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Keywords: | Drumming Protandry Sexual size dimorphism Size at metamorphosis Total lifetime fecundity |
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