High-Protein Diets Shorten Female but Not Male Lifespans and Have Minimal Effects on Egg Production in the Migratory Locust.

AbstractIn many species, reproduction and lifespan are maximized at different dietary protein-to-carbohydrate (p∶c) ratios. Protein-biased diets tend to favor higher reproduction, while carbohydrate-biased diets tend to favor longer lifespans, but the mechanisms responsible for the apparent survival costs of reproduction remain unclear. Prior studies of this topic for females have primarily measured egg production, which could be an inadequate measure of fitness, and have not assessed the rate and timing of reproduction across lifespan, which can provide insights into fitness costs and benefits. To test how variation in dietary p∶c ratio effects are integrated across whole lifespans, we reared migratory locusts (Locusta migratoria) from fifth instar through adulthood on one of three artificial isocaloric diets varying in p∶c ratios (14p∶28c, 21p∶21c, 35p∶7c). We then put pairs of locusts in cages and each week measured male and female survival, the number of eggs and egg pods laid, and hatchlings per reproductive pair throughout the lifespan. Carbohydrate-biased diets increased female lifespans, but macronutrient content had no effect on male lifespans. Locusts eating a carbohydrate-biased diet incurred the cost of a 1-wk delay in the onset of egg laying; however, macronutrient balance did not affect the total number of hatchlings produced over their entire lifespans. Locusts consuming protein-biased diets laid more eggs per pod but fewer total egg pods, and their eggs were lighter than eggs laid by locusts on other diets. Earlier reproduction did not appear to fully explain the reduced longevity of locusts on high-protein diets, as locusts feeding on a balanced diet had similar reproductive schedules but had longer lifespans. In summary, our data show that reproductively active females are more susceptible to the negative effects of high protein on longevity than males and that macronutrient balance affects some nuances of egg production but does not affect overall hatchling success in lab populations. Macronutrient balance may have a greater impact on reproductive success in field populations, depending on predation pressures (reproducing earlier may be paramount), food availability (hatchlings from larger eggs may be more robust to starvation), or other factors.