Date of Award:

5-2026

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Committee Chair(s)

Karen Kapheim

Committee

Karen Kapheim

Committee

Susannah French

Committee

Zachariah Gompert

Committee

Ralph Meyer

Committee

Theresa Pitts-Singer

Abstract

When faced with stressors such as pathogen infection, organisms must invest energy in defense or risk death. Because resources are limited, mounting an immune response often comes at the expense of other costly activities such as reproduction. The costs of immunity are shaped by factors including behavior, nutrition, age, and evolutionary history.

Social insects (e.g., ants, bees, and wasps) offer a powerful system for studying these trade-offs because queens in these colonies appear to escape them—living long lives while reproducing prolifically. Bees, which include species that range from those which nest solitarily to those with complex social systems and many strategies in between, are ideal for testing how social evolution influences immune investment. However, most of the research has focused on honey bees, leaving major gaps in our understanding of solitary species and species that can nest both with and without social behavior (i.e. facultatively social species). My dissertation addresses this gap by examining how immune responses and their costs are shaped by social evolution and life history traits across bees.

In my first chapter, I tested how worker help and reproduction influence queen lifespan in the facultatively social sweat bee, Megalopta genalis, where foundresses can live solitarily or as queens with worker daughters. Queens, although longer-lived and more reproductive, did not differ from solitary foundresses in immune response. However, larger queens showed stronger immune responses, suggesting that better developmental nutrition and worker help may contribute to their extended lifespan without influencing the immune response.

In my second chapter, I tested how social evolution and life history traits shape immune investment. Competing hypotheses predict that social bees either enhance their immunity due to pathogen exposure or reduce it through evolved social hygiene behaviors. I found vi support for the social group hypothesis: social bees generally had higher immune responses than small solitary bees, but large-bodied solitary species exceeded all others, indicating that both body size and sociality shape immune investment.

Finally, in the solitary alkali bee, Nomia melanderi, early-life immune challenge did not affect survival or reproductive development but altered gene expression linked to aging and reproduction, revealing hidden physiological costs.

Together, this work expands our understanding of how immunity interacts with social behavior and physiology to shape life history trade-offs in bees.

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