Regulation of nutrient intake in nectar-feeding birds: insights from the geometric framework

Abstract

A nectar diet is simple in composition and easily digested, but may vary greatly in its proportions of sugar and water. Here we apply the geometric framework, a modelling approach for investigating how animals balance nutrient needs in multidimensional and dynamic nutritional environments, to captive whitebellied sunbirds (Cinnyris talatala). We address the question of how these small birds (~8 g) prioritize sugar and water intake, and how dietary salt content interacts with sugar and water intake. Sunbirds kept at 20˚C and provided with moderate to high sucrose concentrations (≥1 M), together with supplementary water, converge on an intake target of 2.79 g day -1 of sucrose and 7.72 g day -1 of water: equivalent to 0.85 M sucrose. When the birds are given more dilute sucrose concentrations, they defend their sugar intake by overingesting water, up to a ceiling of 47 g day -1 . Sugar intake thus gets priority over water intake, but the birds have a finite capacity to over-ingest water to gain the target level of sugar.Regulation appears to be less precise when birds are given a choice between two sucrose solutions than when they choose between a sugar solution and supplementary water. Intake targets vary in response to internal and external factors, and sunbirds increase their sugar intake in response to increased activity and cold, irrespective of nectar concentration. They also compensate for interruptions in foraging activity, whether overnight or during the day.Interactive effects became evident when sodium was included as a third nutrient: on very dilute nectar (≤0.1 M), where sunbirds lose body mass, the addition of sodium to the diet helps to achieve the carbohydrate intake target, while raising the ceiling on water intake. This analysis provides a new perspective on nectarivory, while adding to the comparative database on nutrient regulation and emphasising water as a nutrient.

Differences in larval nutritional requirements and female oviposition preference reflect the order of fruit colonization of Zaprionus indianus and Drosophila simulans

“…This approach allows us to decrease the nutritional complexity of foods down to manageable sizes, while introducing sufficient complexity to allow the exploration of interactions between macronutrients (Raubenheimer and Simpson, 1997;Simpson and Raubenheimer, 1999). Nutritional geometry has been used to explore the response of life history traits and behavioral strategies to the macronutrients in a broad range of animals (Kohler et al, 2012;Simpson et al, 2015;Rothman et al, 2014).…”

Drinking problems on a ‘simple’ diet: physiological convergence in nectar-feeding birds

“…For example, the prioritisation of sugar intake over water intake has recently been confirmed by use of the geometric framework to examine nutrient intake regulation in whitebellied sunbirds (Köhler et al, 2012); when maintained in captivity at 20°C, these birds (body mass 9 g) reach a daily intake target of 2.77 g of sucrose despite a 10-fold range of sugar concentrations [from 0.25 to 2.5 mol l −1 sucrose (Nicolson and Fleming, 2003a)]. How these animals maintain such a constant energy and osmotic balance despite this remarkable range of diets has stimulated research across the globe.…”

Balancing heat, water and nutrients under environmental change: a thermodynamic niche framework

“…This red line therefore represents a ‘rule of compromise’ (Raubenheimer & Simpson ) whereby a non‐negative water balance is achieved and sufficient dry matter intake to pay for all metabolic processes (growth, maintenance, differentiation and reproduction), assuming dry food is balanced in nutrient composition. This rule of compromise assumes (infinitely) high costs to exceeding dry food needs and that there are no costs to over‐ingesting water (but see Köhler, Raubenheimer & Nicolson ); with different costs for ingesting excess water or dry food, the slope and curvature of the line will change (Simpson et al . ).…”