My research interests center around two core and related questions: 1.) How do (abiotic) environments determine whether and where animals persist?, and 2.) How do animals respond, at cellular to organismal to population levels, to environmental challenges? In tackling these questions, I have worked with diverse animals that live in diverse environments: from tropical frogs (Dillon and Fiano, 2000) and orchid bees (Dillon and Dudley, 2004) to subtropical hummingbirds (Chai et al., 1999) to north temperate agricultural pests (Dillon et al., 2007) and pollinators (Reade et al., 2019) to Rocky Mountain stoneflies (Hotaling et al., 2020), Sierra Nevada fruit flies (Dillon and Frazier, 2006) and Himalayan bumble bees (Dillon and Dudley, 2014). Regardless of the organism, I combine detailed characterization of the environments they live in with physiological experiments to measure function in those environments and often with modeling to broaden findings to other systems.
Characterizing the environments organisms experience requires us to go “beyond the mean” (Dillon and Woods, 2016; Sheldon and Dillon, 2016) – variation in temperature in space and time is the reality, not an abstraction, but ecologists have long had an inordinate fondness for means. With colleagues around the world, I have been working to address this gap. We have documented striking global changes in diurnal and seasonal temperature variation that are likely to have major ecological consequences (Wang and Dillon, 2014), that effects of climate warming are likely more pronounced in the tropics despite the smaller magnitude of temperature change there (Dillon et al., 2010), and that microclimates may buffer or magnify effects of climate change dependent on size and behavior of the organisms that use them (Woods et al., 2015). I’ve further developed approaches to measure and manipulate temperature variation facilitating both cross-scale comparisons of climate signals and experiments that can elucidate which elements of those signals matter for organisms (Dillon et al., 2016). Ongoing work in my lab includes characterization of winter (particularly, ground) temperatures from local loggers and publicly available sensor networks, as well as using biophysical modeling approaches to predict ground temperatures anywhere. My postdoc, Travis Rusch, and PhD students Craig Garzella and Sarah Waybright are collaborating on these projects. Most recently, I am working on a perspective piece with several colleagues in which we develop a framework for incorporating extended phenotypes (the ways that organisms change their environments) into estimates of local climates, a step that is likely critical for better predicting climate change impacts.
A better understanding of the actual temperatures organisms experience is critical, but we also need a mechanistic understanding of how and why those temperatures affect organisms and how organisms may respond from short (acclimation; Dillon et al., 2007) to medium (plasticity; Gunderson et al., 2017) to long (adaptation; Dillon and Lozier, 2019) timescales. With students, I’ve developed new approaches for measuring organismal responses to thermal extremes (Oyen et al., 2016; Oyen and Dillon, 2018). These approaches allow us to uncover the cellular-level processes underlying responses to thermal stress (Hotaling et al., 2020; Pimsler et al., 2020), how the responses may have evolved across the landscape (Jackson et al., 2018, 2020), and to incorporate these findings into models of organism responses to climate change (Dillon et al., 2010; Sheldon and Dillon, 2016; Shah et al., 2020). We are particularly excited about our ongoing work aiming to understand cold tolerance from molecules to distributions (NSF EF-1921562). This collaboration with Franco Basile (University of Wyoming), Jeff Lozier and Jenna Fierst (University of Alabama), and Jamie Strange (Ohio State University) is allowing us to measure how changes in cellular constituents (metabolomics, lipidomics), adaptation of protein function (DNA), and changes in expression (RNA) determine shifts in cold tolerance across time (from minutes to seasons) and space (across habitats to among populations).
I have long been fascinated by animal flight (Chai et al., 1999; Dillon and Dudley, 2004), particularly in the context of high elevations where thin air should challenge lift generation and oxygen delivery (Dillon et al., 2006; Dillon and Frazier, 2006). I’ve traveled to some of the highest mountains in the world to study bumble bees, which miraculously do quite well in these extreme environments. In fact, we’ve discovered that these insect athletes have such large reserves of aerodynamic power and exceptional capacities for delivering oxygen to tissues that they can fly over the top of Everest (Dillon and Dudley, 2014)! Using high-speed videography, an exceptional undergraduate in my lab has also found what we believe is the first evidence for local adaptation in flight kinematics (Parsons and Dillon, in prep) – bees reared from high-altitude queens have strikingly different wing movements than their low altitude counterparts. However, these remarkable flight feats are energetically costly; with colleague Stacey Combes (University of California, Davis), I have measured the energetic cost of flight with loads. Contrary to dogma going back 60+ years proclaiming that kinematics are fixed, we discovered that individual bumble bees can actually modulate flight kinematics and that they do so to minimize energetic costs in response to changing flight conditions (Combes et al., 2020)–this is a truly shocking finding that, in my opinion, is going to open entire new fields of inquiry. Finally, we’ve used TEM, microCT, and respirometry to study how the characteristics of the tracheal respiratory system facilitate remarkable oxygen delivery capacity in insects (Vogt and Dillon, 2013; Shaha et al., 2013; Vogt et al., 2014). Most recently, we are finalizing a manuscript on flight physiology as it relates to temperature regulation. Using high-speed video, infrared thermography, and biophysical modeling, we have been able to demonstrate the cooling effect of flapping wings – bumble bees that might otherwise overheat are cooled by self-generated wind (Petranek and Dillon, in prep). Ongoing work will evaluate seasonal changes in flight physiology, particularly of queen bumble bees in relation to diapause (NSF OIS-1826834) – it is unclear how they reactivate flight muscles for spring foraging and nest initiation after 6-9 months of inactivity; and their ability to do so likely determines colony success in spring and summer.
A new focus in my lab extends our view beyond the growing season that dominates scientific attention to the often long period of quiescence that many (primarily temperate) organisms rely on to survive through severe winter conditions. This large, multi-institution, collaborative project (NSF OIS-1826834) has funded a new CliMet facility (BS402) which houses state of the art respirometry and thermography equipment and environmental chambers such that we can track bumble bees through diapause conditions to better understand this critical life history stage. We have determined temperature conditions that trigger diapause, how photoperiod changes influence the diapause response (MS student Claire Campion), how metabolic rates change pre-, during, and post-diapause (MS student Shayne Dodge), and tradeoffs between flight muscle and ovary development that alter the diapause and post-diapause response (lab scientist Megan Dillon). In collaboration with Franco Basile (University of Wyoming) and Dan Rule (University of Wyoming) and colleagues at NDSU, we are also characterizing metabolomic, lipidomic, and transcription changes in queen body compartments to probe the cellular level changes leading to the diapause transition. This flurry of work has led to 9 presentations accepted to 2 international meetings in November and January and 7 manuscripts in prep. And these experiments have opened up a whole new set of questions which will keep us very busy for the next few years.