“The geometry of nutrient-space based life-history trade-offs: Sex-specific effects of macronutrient intake on the trade-off between encapsulation ability and reproductive effort in decorated crickets”
James Rapkin, Kim Jensen, C. Ruth Archer, Clarissa M. House, Scott K. Sakaluk, Enrique del Castillo, and John Hunt (Apr 2018)
An analytical approach to measure nutrient based life-history trade-offs
Sex-specific trade-offs between immune function and reproduction are largely driven by the divergent protein and carbohydrate requirements to maximize these traits in the sexes
Life-history theory assumes that trade-offs result from traits competing for limited resources, with the resource most commonly studied being the quantity and/or quality of diet. Recent studies have started using artificial diets that vary both the ratio of protein to carbohydrate and the overall nutritional value of the diet to create a multi-dimensional nutritional space, using a method called the geometric framework of nutrition. These studies have suggested that life-history trade-offs are often regulated by the intake of specific nutrients; however, a formal approach to identify and quantify the strength of such trade-offs is lacking.
In their article appearing in The American Naturalist, Rapkin et al. present an analytical approach for quantifying nutrient based life-history trade-offs using the geometric framework. They suggest that trade-offs occur whenever life-history traits of interest are maximized in different regions of nutritional space and that the strength of any trade-offs can be quantified using both the angle and the distance between nutritional optima.
Rapkin et al. then test their proposed analytical approach, examining the effect of protein and carbohydrate intake on the trade-off between reproduction and aspects of immune function in male and female decorated crickets (Gryllodes sigillatus). They show that female encapsulation ability and egg laying increased with the intake of both protein and carbohydrate, whereas male encapsulation ability increased with protein intake but male calling effort increased with carbohydrate intake. The trade-offs between traits were therefore shown to be larger in males than females with significant negative correlations between the traits in males, non-overlapping regions of nutritional optima, and larger estimates of the angle and distance between these nutritional optima. A greater consideration of specific nutrient effects on life-history trade-offs will, therefore, be important for future studies.
Life-history theory assumes that traits compete for limited resources resulting in trade-offs. The most commonly manipulated resource in empirical studies is the quantity or quality of diet. Recent studies using the Geometric Framework for nutrition, however, suggest that trade-offs are often regulated by the intake of specific nutrients but a formal approach to identify and quantify the strength of such trade-offs is lacking. We posit that trade-offs occur whenever life-history traits are maximized in different regions of nutrient space, as evidenced by non-overlapping 95% confidence regions of the global maximum for each trait, and large angles (θ) between linear nutritional vectors and Euclidean distances (d) between global maxima. We then examined the effects of protein and carbohydrate intake on the trade-off between reproduction and aspects of immune function in male and female Gryllodes sigillatus. Female encapsulation ability and egg production increased with the intake of both nutrients, whereas male encapsulation ability increased with protein intake but calling effort increased with carbohydrate intake. The trade-offs between traits was therefore larger in males than females, as demonstrated by significant negative correlations between the traits in males, non-overlapping 95% confidence regions and larger estimates of θ and d. Under dietary choice, the sexes had similar regulated intakes but neither optimally regulated nutrient intake for maximal trait expression. We highlight that greater consideration of specific nutrient intake is needed when examining nutrient-space based trade-offs.