2022 Centris pallida head widths

Historical data suggest that human climate and landscape modifications
have caused many bee species to decline in body size. Larger-bodied bee
species with narrow phenological and dietary breadth are most prone to
declines in body size over time, potentially due to temperature increases
during development or declines in resource allocation during development.
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
behaviors could threaten the prevalence of certain behavioral phenotypes
long-term. C. pallida (Hymenoptera: Apidae) are large-bodied solitary bees
with narrow phenological and dietary breadth found in the Sonoran Desert.
Males use alternative reproductive tactics (ARTs) and are dimorphic in
both morphology and behavior. Large-morph males are fixed on a patrolling
and fighting-based mate location strategy, while small-morph males
primarily use a sight-based hovering strategy. C. pallida male body size
has been studied since the 1970s in the same population, allowing for
comparison of population-level male morphological data associated with
unique behaviors. We collected data on male body size in 2022 and combined
it with published records to analyze body size trends from 1974-2022. We
find a persistent decline in the mean head width of patrolling males, as
well as distributional shifts towards smaller body sizes in the
populations of males found foraging and hovering. However, mating males
did not experience a decline in mean body size over time. We discuss
several hypotheses related the decline in average C. pallida male head
width. Finally, we advocate for C. pallida as a particularly good study
system for understanding the stability of ART systems with size-linked
behavioral phenotypes.