DOI: 10.5061/dryad.kd51c5b9n
title: The presence of a guard vicariously drives split sex ratios in a facultatively social bee

description: Split sex ratios provide broad insights into how reproductive strategies
 evolve and historically have special relevance to the evolution of
 eusociality. Yet almost no attention has been directed to situations where
 split sex ratios may potentially decrease the payoffs for worker-like
 behaviour, increasing selective thresholds for eusociality. We examined
 sex ratios in a facultatively social colletid bee, Amphylaeus morosus. Sex
 ratios in this bee vary strongly with the presence of a nest guard and in
 a pattern that does not conform to assumptions of previous models in which
 split sex ratios facilitate altruism. While the production of daughters
 was constant across social and solitary nests, mothers produced more brood
 when a non-reproductive guard was present, but these extra brood were all
 male. This leads to split sex ratios, vicariously driven by guards that
 are unable to manipulate sex ratios in their favour. Importantly, if
 guarding becomes more common in a population this would lead to an excess
 of males and lower the genetic value of these extra males to guards,
 effectively putting a brake on selection for worker-like behaviour.
description: Nests of <em>Amphyaleus</em>
 <em>morosus</em> (n = 298) were collected from the Dandenong
 Ranges, Victoria, Australia throughout the reproductive season. Nests were
 sampled across five consecutive years (2017-2021; equating to four
 reproductive seasons) and seven separate collections.
 To assess patterns of investment sex ratios in colonies of
 <em>A. morosus</em>, brood sex, wet weight, and brood cell
 position were recorded. Pupae were weighed on a Thermoline precision
 balance to ± 0.1 mg. The numerical sex ratio (NSR) was calculated as the
 number of male brood divided by the total number of brood that reached
 pupation to a point that they could be reliably sexed (NSR = Σmale brood/
 Σmale brood + female brood). Investment sex ratio (ISR) was calculated as
 a product of the numerical sex ratio and the pupal weight ratio was
 calculated from mean brood sex pupal weight and was used to test whether
 the observed numerical sex ratio deviated from the investment a mother
 allocates to each sex. Brood that had died before nests were opened were
 not included in pupal weight measurements but were used for numerical sex
 ratio calculations. To examine sex allocation patterns across cell
 positions in the nest, cell position was coded so that ‘cell 1’
 corresponded to the first cell provisioned (furthest from the nest
 entrance), following Hearn et al. (2022). For some analyses, brood that
 reached adulthood were pooled across all nests and the pupal sex was
 treated as a binomial response variable (female = 0, male = 1). For all
 analyses, we define solitary nests as nests containing one or no adult
 female at the time of nests collection and social nests as colonies
 containing two adult females or genome-inferred social nests (see Hearn et
 al. (2022)), but recognise that social nesting is likely to be more common
 than our nest census data suggests (SI Appendix).   All
 statistical analyses were performed in SPSS version 27.0 (IBM, Armonk, NY)
 and R version 4.0.4 (R Core Team 2018). Tests of normality and
 homoscedasticity were assessed using a Shapiro–Wilk test and Levene’s
 test. A Chi-square goodness of fit test was used to determine if observed
 numerical sex ratio significantly differed from an expected null
 hypothesis of equal investment. Where necessary we used arcsine
 transformed values of sex ratio and the corresponding confidence
 intervals.