The geometry of dependence: solitary bee larvae prioritize carbohydrate over protein in parentally provided pollen

Alexander J. Austin; James D. J. Gilbert

Bees are critical not only to global ecological stability but also to humans' food security, 44 as major pollinators for 90% of the world's food crops (Klein et al., 2018 (Klein et al., , 2007 Potts et al., 45 2016) and many wildflowers (Ollerton et al., 2011) . Many wild and domesticated bees have seen 46 marked declines in recent decades, with both significant range contractions and extinctions 47 maintain themselves (Simpson and Raubenheimer, 2012a) . Understanding how animals cope in a 66 changing nutritional environment requires us to understand not only how animals gather the 67 correct balance of nutrients they need, but also how they adjust their foraging when resources are 68 imbalanced (Simpson and Raubenheimer, 2012b) . 69 In bees, the larval stage is where almost all growth occurs (Nijhout et al., 2014) as well as distinctly different diet to adults, adults primarily feeding on nectar (although see Cane, 2016) 77 and larvae feeding almost solely on pollen (Muth et al., 2016) . 78 The Geometric Framework for Nutrition (GF) allows us to investigate foraging decisions 79 made by animals in multi-dimensional "nutrient space" (Simpson and Raubenheimer, 2012b, 80 1993). The GF can be used to determine an organism's intake target (the optimal amount and 81 balance of macronutrients that an animal should aim to consume in order to achieve maximal 82 fitness (Simpson and Raubenheimer, 1993) as well as how that target is achieved. Additionally, 83 we can use the GF to investigate the rule of compromise -that is, the rules governing 84 consumption that an animal uses when it is unable to reach its intake target with the nutritional 85 options available (e.g. Lee et al., 2004b; Simpson and Raubenheimer, 2001) . 86 The GF has provided profound insights into broad topics from ant agriculture (Shik et al., 87 2016) to human obesity (Simpson and Raubenheimer, 2005) . While the GF has been used to 88 . CC-BY-NC-ND 4.0 International license is made available under a The copyright holder for this preprint (which was not peer-reviewed) is the author/funder. It . https://doi.org/10.1101/397802 doi: bioRxiv preprint investigate the nutrition of some highly social hymenopterans ( larval nutrition in social species, principally because it is rarely possible to accurately track 93 nutrition within a colony -food brought in by workers is often shared and/or modified within the 94 nest, and is then continually fed to the larvae (Field, 2005) . This means that nutritional insights 95 from GF studies into the parent-offspring relationship are currently limited. 96 In solitary bees, by contrast, typically each reproductive female provisions each of her 97 offspring individually with a single, independent "pollen ball" before sealing the cell and 98 leaving. This pollen ball contains all the resources that the larva will need to grow to adulthood. Scriber and Slansky, 1981) in order to determine 122 their rules of compromise and the diet composition that maximised fitness. In a second "choice" 123 phase, we then provided larvae with targeted choices between sets of two imbalanced diets that 124 differed in their P:C ratios to determine whether larvae consistently aim for an intake target. The copyright holder for this preprint (which was not peer-reviewed) is the author/funder. It . https://doi.org/10.1101/397802 doi: bioRxiv preprint S1a,b). Completed nests, signified by a mud plug at the entrance, were then brought into the 157 laboratory. Early trials