Gravisensation and modulation of gravitactic responses by other sensory cues in the monarch butterfly (Danaus plexippus).

Using the monarch butterfly (Danaus plexippus), we studied how animals can use cues from multiple sensory modalities for deriving directional information from their environment to display oriented movement. Our work focused on determining how monarchs use gravity as a cue for oriented movement and determined how cues from other sensory modalities, cues that by themselves also produce oriented movement (visual and magnetic directional cues), might modulate gravisensation. In two tests of gravisensation (movement in a vertical tube; righting behavior), we found that monarchs display negative gravitaxis only (movement opposite to the direction of gravity). Negative gravitaxis can be modulated by either visual (light) or magnetic field cues (inclination angle) that provide directional information. The modulation of gravity-mediated responses, however, depends on the relationship between cues when presented during trials, such as when cues are in accord or in conflict. For example, when light cues that elicit positive phototaxis conflicted with negative gravitaxis (light from below the monarch), monarch gravisensation was unaffected by directional light cues. We also found that the antennae play a role in gravity-mediated movement (righting), as, with antennae removed, monarch movement behavior was no longer the same as when the antennae were intact. Our results demonstrate that monarchs can use and integrate multiple, multimodal cues for oriented movement, but that the use of such cues can be hierarchical (that is, one cue dominant for movement), and the hierarchy of cues, and the responses towards them when found together, depends on the physical relationships between cues during movement.

The cost of movement: assessing energy expenditure in a long-distant ectothermic migrant under climate change.

Migration is an energetically taxing phenomenon as animals move across vast, heterogeneous landscapes where the cost of transport is impacted by permissible ambient conditions. In this study, we assessed the energetic demands of long-distance migration in a multigenerational ectothermic migrant, the monarch butterfly (Danaus plexippus). We tested the hypotheses that temperature-dependent physiological processes reduce energy reserves faster during migration than previously estimated, and that increasing climatic temperatures resulting from the climate crisis will intensify baseline daily energy expenditure. First, we reared monarchs under laboratory conditions to assess energy and mass conversion from fifth instar to adult stages, as a baseline for migratory adult mass and ontogenetic shifts in metabolic rate from larvae to adult. Then, using historical tag-recapture data, we estimated the movement propensity and migratory pace of autumn migrants using computer simulations and subsequently calculated energy expenditure. Finally, we estimated the energy use of monarchs based on these tag-recapture data and used this information to estimate daily energy expenditure over a 57 year period. We found support for our two hypotheses, noting that incorporating standard metabolic rate into estimates of migratory energy expenditure shows higher energy demand and that daily energy expenditure has been gradually increasing over time since 1961. Our study shows the deleterious energetic consequences under current climate change trajectories and highlights the importance of incorporating energetic estimates for understanding migration by small, ectothermic migrants.

Lack of evidence for a fine-scale magnetic map sense for fall migratory Eastern North American monarch butterflies (Danaus plexippus).

How first-time animal migrants find specific destinations remains an intriguing ecological question. Migratory marine species use geomagnetic map cues acquired as juveniles to aide long-distance migration, but less is known for long-distance migrants in other taxa. We test the hypothesis that naïve Eastern North American fall migratory monarch butterflies (Danaus plexippus), a species that possesses a magnetic sense, locate their overwintering sites in Central Mexico using inherited geomagnetic map cues. We examined whether overwintering locations and the abundance of monarchs changed with the natural shift of Earth's magnetic field from 2004 to 2018. We found that migratory monarchs continued to overwinter at established sites in similar abundance despite significant shifts in the geomagnetic field, which is inconsistent with monarchs using fine-scale geomagnetic map cues to find overwintering sites. It is more likely that monarchs use geomagnetic cues to assess migratory direction rather than location and use other cues to locate overwintering sites.

Oriented migratory flight at night: Consequences of nighttime light pollution for monarch butterflies.

We show that light trespass-a form of nighttime light pollution (NLP)-elicits normal daytime clock-mediated migratory behavior in fall monarch butterflies during their night-cycle. In controlled indoor flight simulator studies isolating the role of NLP on the expression of oriented migratory flight using a time-compensated sun compass,a full-spectrum light source consistent with lights used outdoors at night by the public,triggered proper fall directional flight at night in monarchs. Monarchs remained quiescent when initially placed in the flight simulator in the dark, but flight was immediately triggered when our light source was turned on. This nighttime behavior was identical to that seen in outdoor free-flying fall conspecifics during the day. The light source provided directional cues equivalent to those provided by the sun and could either phase-advance or phase-delay monarchs. Our study highlights the negative consequences of NLP on diurnal animals, especially those that rely on clock-mediated behavior.

Cardiovascular contributions and energetic costs of thermoregulation in ectothermic vertebrates.

Ectothermic vertebrates use a suite of physiological and behavioral mechanisms to thermoregulate, which result in various thermoregulatory strategies from thermoconformity to thermoregulation. Here, we present a novel synthesis of theoretical and empirical methods to determine cardiovascular contributions to heat transfer in free-living ectothermic vertebrates. We start by identifying the fundamental components of heat transfer and the cardiovascular mechanisms for physiological modulation of heat exchange, and then integrate these components into a single, integrative framework: the cardiovascular heat exchange framework (CHEF). We demonstrate that this framework can identify details of the thermoregulatory strategy in two turtle species, most notably the preponderance of instances where turtles use physiological mechanisms to avoid overheating, suggesting vulnerability to climate change. As modulated physiological contributions to heat flow incur a greater energy demand than relying on unmodulated passive heat transfer, we then asked whether we could characterize the energetic costs of thermoregulation. We measured field metabolic rate (FMR) in free-living turtles and used the CHEF to determine FMR while actively or passively thermoregulating. Comparing an individual's actual FMR to the rate calculated assuming absence of thermoregulation revealed that painted turtles, a partial thermoregulator, elevate their daily energy expenditure (DEE) by about 25%, while box turtles, a thermoconformer, have a DEE that is nearly unchanged as a result of thermoregulation. This integrative framework builds a new paradigm that provides a mechanism to explain correlations between energy demand and thermoregulatory strategy, quantifies the energetic costs of thermoregulation, and identifies the role of cardiovascular contributions to thermoregulation in free-living animals.

A behavioral assay to test sensory-cue-guided oriented flight in monarch butterflies under controlled conditions.

Many animals use sensory cues to guide movement. Testing animals under conditions in which cues can be isolated and manipulated is key for understanding the function of cues. Here, we present a protocol to assess the flight of migratory monarch butterflies (Danaus plexippus). We describe procedures to optimize and conduct trials, especially under indoor conditions. This protocol facilitates testing monarchs in various experimental conditions including during their subjective night when they are not normally flying. For complete details on the use and execution of this protocol, please refer to Parlin et al. (2022).1.

Dimorphic cocoons of the robin moth, Hyalophora cecropia, reflect the existence of two distinct architectural syndromes.

The architectural design of animal structures forms part of an individual's extended phenotype that can be subjected to strong selection pressures. We examined cocoon architectural dimorphism in robin moths (Hyalophora cecropia), which construct multilayered silk-woven cocoons that possess either a 'baggy' or 'compact' morphology. These dimorphic cocoons reflect extended phenotypes that can enable survival during a critical developmental period (pupal stage to adult emergence), with cocoons occurring either sympatrically or as monomorphic groups across different climatic regions in North America. We hypothesized that cocoon dimorphism is related to the cocoon's role as a mediating barrier for moisture. We predicted that the macro- and micro-architectural differences between the cocoon morphs would be consistent with this function. We compared the cocoon morphs in terms of their orientation when spun under natural field conditions, examined how these orientations affected cocoon water absorption under simulated rain trials, and performed material surface tests to compare the hydrophobicity of cocoons. We found that compact cocoons had traits that increased water resistance, as they had significantly greater hydrophobicity than baggy cocoons, because they absorbed less water and released water vapor faster. In contrast, the increased water absorptiveness of baggy cocoons can allow for greater moisture retention, a function related to the prevention of desiccation. Our study suggests that cocoon dimorphism in robin moths reflects distinct architectural syndromes, in which cocoons are spun to optimize either water resistance or retention. These different functions are consistent with strategies that act to respond to uncertain external environmental conditions that an individual might encounter during development.

A laboratory-based study examining the properties of silk fabric to evaluate its potential as a protective barrier for personal protective equipment and as a functional material for face coverings during the COVID-19 pandemic.

The worldwide shortage of single-use N95 respirators and surgical masks due to the COVID-19 pandemic has forced many health care personnel to use their existing equipment for as long as possible. In many cases, workers cover respirators with available masks in an attempt to extend their effectiveness against the virus. Due to low mask supplies, many people instead are using face coverings improvised from common fabrics. Our goal was to determine what fabrics would be most effective in both practices. Under laboratory conditions, we examined the hydrophobicity of fabrics (cotton, polyester, silk), as measured by their resistance to the penetration of small and aerosolized water droplets, an important transmission avenue for the virus causing COVID-19. We also examined the breathability of these fabrics and their ability to maintain hydrophobicity despite undergoing repeated cleaning. Laboratory-based tests were conducted when fabrics were fashioned as an overlaying barrier for respirators and when constructed as face coverings. When used as material in these two situations, silk was more effective at impeding the penetration and absorption of droplets due to its greater hydrophobicity relative to other tested fabrics. We found that silk face coverings repelled droplets in spray tests as well as disposable single-use surgical masks, and silk face coverings have the added advantage over masks such that they can be sterilized for immediate reuse. We show that silk is a hydrophobic barrier to droplets, can be more breathable than other fabrics that trap humidity, and are re-useable via cleaning. We suggest that silk can serve as an effective material for making hydrophobic barriers that protect respirators, and silk can now be tested under clinical conditions to verify its efficacy for this function. Although respirators are still the most appropriate form of protection, silk face coverings possess properties that make them capable of repelling droplets.

Activity and movement of free-living box turtles are largely independent of ambient and thermal conditions.

BACKGROUND: Ectotherms are assumed to be strongly influenced by the surrounding ambient and environmental conditions for daily activity and movement. As such, ecological and physiological factors contribute to stimuli influencing navigation, extent of movement, and therefore habitat use. Our study focused on the intensity of activity (from acceleration data) and extent of movement (from GPS and thread trailing data) of Eastern box turtles (Terrapene carolina carolina) in a fragmented landscape near their northern population limit. First, we quantified the thermal performance curve of box turtles using activity as a measure of performance. Second, we investigated ecological factors that could influence activity and movement and characterized the movement as extensive (exploration) and intensive (foraging). RESULTS: In contrast to previous lab work investigating effects of temperature on activity, we found no relationship between box turtle activity and temperature in the field. Furthermore, box turtle activity was consistent over a wide range of temperatures. Cluster analysis categorized movement recorded with GPS more as intensive than as extensive, while thread trailing had more movement categorized as extensive than intensive. Box turtle activity was higher during the morning hours and began to decrease as the day progressed. Based on the microclimate conditions tested, we found that box turtle movement was influenced by precipitation and time of day, and activity was most influenced by absolute humidity, ambient temperature, cloud cover, and time of day. CONCLUSIONS: Our model ectotherm in this study, the Eastern box turtle, had activity patterns characteristic of a thermal generalist. Sampling resolution altered the characterization of movement as intensive or extensive movement, possibly altering interpretation. More information on the resolution needed to definitively identify foraging and exploratory behavior in turtles is needed. Activity and movement were nearly independent of environmental conditions, which supports the overall interpretation that turtle performance is that of a broad environmental generalist. Future studies of movement of other turtle and reptile species are needed to determine the generality of these findings.

Thermoregulatory performance and habitat selection of the eastern box turtle (Terrapene carolina carolina).

Environmental conditions may affect individual physiological processes that influence short-term performance and ultimately growth, survival and reproduction. As such, habitats selected by animals must provide suitable and adequate resources. Ectothermic species are highly dependent on climatic conditions and ambient temperatures that dictate body temperature regulation and in turn physiological processes. We investigated the thermoregulatory performance, habitat selection, and movements of an ectothermic vertebrate, the Eastern box turtle (Terrapene carolina carolina) to assess the importance of thermoregulatory physiology in habitat selection. We evaluated the relationship between habitat selection and thermoregulatory performance in Southwest Ohio over two active seasons from May until October. We found that T. carolina selected shaded habitats, including evergreen and deciduous forests, as well as herbaceous grasslands, conformed to the ambient temperatures throughout the active season, although these habitats had temperatures below those expected based on thermal optima of box turtles. Further, we found that movement was not correlated with internal body temperature. Our study shows that thermal conditions are not paramount in habitat selection of box turtles, but that cooler temperatures do not have an effect on the extent of their locomotion.