Centris pallida(Hymenoptera: Apidae) male body size decreases across five decades

Abstract

Historical data suggest that many bee species have declined in body size. Larger-bodied bees with narrow phenological and dietary breadth are most prone to declines in body size over time. This may be especially true in solitary, desert-adapted species that are vulnerable to climate change such as Centris pallida (Hymenoptera: Apidae). In addition, body size changes in species with size-linked behaviours could threaten the prevalence of certain behavioural phenotypes long-term. C. pallida solitary bees are found in the Sonoran Desert. Males use alternative reproductive tactics (ARTs) and are dimorphic in both morphology and behaviour. C. pallida male body size has been studied since the 1970s in the same population. The authors collected body size data in 2022 and combined it with published records from 1974-2022. The authors find a persistent decline in the mean head width of patrolling males, and shifts towards smaller body sizes in the populations of males found foraging and hovering. Both morphs declined in average body size, and the proportion of large-morph males in the population decreased by 8%.Mating males did not decline in mean body size over the last five decades. The authors discuss hypotheses related to the decline in C. pallida male head width. Finally, the authors advocate for C. pallida as an excellent study system for understanding the stability of ARTs with size-linked behavioural phenotypes.

Impact of body size on critical thermal maxima in female solitary desert bees

“…Historical data suggest there has been a recent decline in insect body sizes, possibly due to climate change (Sheridan & Bickford, 2011;Wonglersak et al, 2021; but see Baar et al, 2016). Some bees show evidence of this pattern (Barrett & Johnson, 2022;Herrera et al, 2022). If smaller body size is associated with reduced CT max , climate-change-driven decreases in mean body size and increases in extreme temperatures can magnify climate threats to insects.…”

“…Centris pallida is a solitary bee species that lives in the Sonoran Desert of the United States and Mexico. Females show great variation in body size (from 4.33 to 5.95 mm in head width within the same population; Barrett & Johnson, 2022). Importantly, females regularly experience high temperatures, with nesting sites reaching ground temperatures in excess of 60 C (Barrett, 2022).…”

Individual reflectance of solar radiation confers a thermoregulatory benefit to dimorphic males bees (Centris pallida) using distinct microclimates

“…In male Colias butterflies, intraspecific increases in radiative heat gain due to melanization allowed for longer flight periods at cooler air temperatures, increasing access to mating opportunities [48,49]. The mean body size of the C. pallida male population increases throughout the morning (both for males foraging, and males engaged in mating-relevant behaviors [50]), suggesting smaller males may use their darker coloration to increase radiative heat gain and their access to mates during cooler early morning periods when large males are not yet as prevalent in the population.…”

Informing policy and practice on insect pollinator declines: Tensions between conservation and animal welfare

“…Insect pollinators are in peril: anthropogenic climate and habitat changes have caused abundance, diversity, and body size declines, as well as range, phenology, and ecological relationship shifts (Cane et al, 2006;Bartomeus et al, 2011;Kuhlmann et al, 2012;Burkle et al, 2013;Barrett and Johnson, 2022;Turley et al, 2022). In 10.…”