Evolution of prolonged development: a life table analysis for periodical cicadas

Area of Science:

• Evolutionary Biology
• Ecology
• Entomology

Background:

• Conventional evolutionary theory suggests natural selection favors early reproduction.
• The prolonged developmental periods (13 and 17 years) of periodical cicadas present a challenge to this theory.
• Previous hypotheses suggested increased fecundity or reduced mortality risk with longer development.

Purpose of the Study:

• To test if 17-year periodical cicadas exhibit greater fecundity than 13-year cicadas.
• To estimate the survivorship curve for periodical cicadas.
• To evaluate the demographic hypothesis explaining prolonged cicada development.

Main Methods:

• A cohort of 17-year cicadas was monitored from 1979 to 1996.
• Survivorship was assessed by tracking mortality patterns over the developmental period.
• Ovary weight, as a proxy for fecundity, was compared between 17-year and 13-year cicada adults.

Main Results:

• Most cicada mortality occurred within the first two years of development.
• Adult densities remained relatively stable across monitored sites.
• Seventeen-year cicadas possessed heavier ovaries, indicating higher fecundity, particularly Magicicada cassini (1.80x) compared to Magicicada decim (1.16x).

Conclusions:

• Increased fecundity associated with longer development adequately explains prolonged life cycles in periodical cicadas, especially for M. cassini.
• For M. decim, low mortality rates balanced the modest fecundity advantage of longer development.
• The demographic hypothesis provides a sufficient explanation, though reduced predation and selection for larger body size may also contribute.