Food protein-carbohydrate ratio affects the trade-off between reproduction and lifespan in the wing-polymorphic cricket Gryllus firmus

Reproduction is considered to be a costly activity that negatively affects an animal’s lifespan.  However, trade-offs between lifespan and reproductive output are likely to be modulated by an animal’s ability to acquire and utilize resources, especially key nutrients like protein (p) and digestible carbohydrates (c).  Here, we examine whether the balance and amounts of p and c affect a trade-off between lifespan and reproduction in wing-polymorphic crickets.  We gave 137 newly molted adult females one of 13 diets containing different relative and total amounts of p and c, and measured consumption, total reproductive output (egg production), and adult lifespan.  Crickets adjusted feeding in response to food dilution and p-c ratio, consuming the greatest amount on the most dilute, p-biased food.  Total egg production was a function of both p-c ratio and density, and was maximized on the very protein-biased diet with intermediate total nutrient density.  Meanwhile, cricket lifespan was close to maximal across all five diets with intermediate total density, but was longest on the very carbohydrate-biased diet.  These results show that food p-c balance and density both determine whether a direct trade-off will occur between lifespan and reproduction.