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

Alexander J. Austin; James D. J. Gilbert

Diet also influenced the survival of larvae, with those held on the more dilute diets 274 suffering greater mortality (parametric survival regression, χ 2 =72.7, df=6, p<0.0001, Fig. 4) . Despite not converging upon an intake target in nutritional space, larvae were 296 nevertheless not consuming the diets at random, instead seeming to align on a P:C ratio of about 297 1:1.8 (Fig. 6a) . For both carbohydrate and protein we saw differences in consumption from what 298 would have been expected for each larva based on random consumption, and this effect was 299 dependent on the specific set of diet choices (carbohydrate: linear mixed models with ID as a 300 random effect and "diet combination" and "random or observed" as predictor variables; dropping 301 the interaction, χ 2 =42.76, df=3, p<0.0001; protein: dropping the interaction, χ 2 =16.91, df=3, 302 p<0.001, Fig. 7) . 303 Moreover, when visualised as the amounts of P and C consumed during each 48h 304 treatment period (Fig. 7a, b) , it is clear that larvae were achieving a degree of homeostasis in C 305 consumption compared to what would be expected under random consumption of each diet 306 choice (Fig. 7c, Fig. 7d ). In contrast, their consumption of P aligned very closely with what 307 would be expected under random consumption (Fig. 7) . The copyright holder for this preprint (which was not peer-reviewed) is the author/funder.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 bicornis may therefore prioritise carbohydrate intake in order to provide adequate lipid stores to 340 meet energy demands of their developing eggs (Beenakkers et al., 1985) . Nevertheless, (Bosch 341 and Vicens, 2005) found little correlation between body size in O.cornuta and fecundity, which 342 was instead more related to longevity and provisioning rate. It is also worth noting that although 343 we found that high carbohydrate increased cocoon weight we did not identify a fitness peak at 344 which cocoon weight was maximised. Further work could employ a wider range of diet ratios in 345 order to locate this. Although high-carbohydrate consumption increased survival to pupation, among those larvae 355 that ended up eating low quantities of carbohydrate, protein consumption mediated survival, with 356 those on higher protein diets (higher P:C) experiencing greater survival (Fig. 5) . This pattern Interestingly, when allowed to self-select their diets, O.bicornis larvae did not cluster in 380 nutritional space but were instead spread out along a P:C ratio of 1:1.8 (Fig. 6a) , approximately 381 according to overall diet concentration. Notwithstanding the variable amounts actually eaten, 382 assuming at least that this 1:1.8 ratio reflects the ratio of the larvae's true intake target, this (Fig. 2b) , they were clearly unable to compensate for the 396 reduced nutrient concentration, consuming less of each macronutrient than larvae fed more 397 concentrated diets (Fig. 2a) . Despite the fact that total food consumption was similar across 398 treatments in the choice phase (Fig. 6b) , larvae on less concentrated diets may have been unable 399 to reach the same point in nutritional space due to consumption rate limitations. Raubenheimer, 2012b). Larval bees feed on pollen, which is amongst the most protein-rich of 425 plant tissues (Mattson, 1980) , and, as such, carbohydrate may be the more limiting nutrient for 426 larvae. It is likely that starch, the standard form of digestible carbohydrate storage in