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1. Eye‐body allometry across biphasic ontogeny in anuran amphibians

   https://doi.org/doi.org/10.1007/s10682-021-10102-3  

   Shrimpton, S. J.; Streicher, J. W.; Gower, D. J.; Bell, R. C.; Fujita, M. K.; Schott, R. K.; Thomas, K. N. (January 2021, Elements in evolutionary economics)

   Animals with biphasic lifecycles often inhabit different visual environments across ontogeny. Many frogs and toads (Amphibia: Anura) have free-living aquatic larvae (tadpoles) that metamorphose into adults that inhabit a range of aquatic and terrestrial environments. Ecological differences influence eye size across species, but these relationships have not yet been explored across life stages in an ontogenetic allometric context. We examined eye-body size scaling in a species with aquatic larvae and terrestrial adults, the common frog Rana temporaria, using a well-sampled developmental series. We found a shift in ontogenetic allometric trajectory near metamorphosis indicating prioritized growth in tadpole eyes. To explore the effects of different tadpole and adult ecologies on eye-body scaling, we expanded our taxonomic sampling to include developmental series of eleven additional anuran species. Intraspecific eye-body scaling was variable among species, with 8/12 species exhibiting a significant change in allometric slope between tadpoles and adults. Traits categorizing both tadpole ecology (microhabitat, eye position, mouth position) and adult ecology (habitat, activity pattern) across species had significant effects on allometric slopes among tadpoles, but only tadpole eye position had a significant effect among adults. Our study suggests that relative eye growth in the preliminary stages of biphasic anuran ontogenies is somewhat decoupled and may be shaped by both immediate ecological need (i.e. tadpole visual requirements) and what will be advantageous during later adult stages. 

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2. Compensation for a costly ornament depends on the development of flight performance in stalk-eyed flies

   https://doi.org/10.3389/fetho.2023.1242198  

   Vance, Jason T.; Pehl, Kayla; Acakpo, Comonla J.; Swallow, John G. (October 2023, Frontiers in Ethology)

   Burghardt, G.M. (Ed.)

   Several species of stalk-eyed flies exhibit exaggerated sexual dimorphism where females favor males with longer eyespans. Longer eyespan increases a fly’s moment of inertia, and may, therefore, impact flight behavior and fitness, specifically maneuverability and predator evasion. However, these putative costs may be ameliorated by co-selection for compensatory traits, as flies with longer eyespans tend to have larger thoraces and wings, which allows them to perform turns similar to flies with shorter eyespans. Furthermore, the capacity to compensate for a potentially costly ornament may not be fixed across the life-history of the adult stage, as stalk-eyed flies achieve sexual maturity at 3-4 weeks of age, accompanied by significant growth of reproductive tissues and organs. Thus, growth of the abdomen and body mass over time may impose constraints on flight performance that may affect whether an adult reaches the age of reproductive viability. The purpose of this study was to investigate the flight performance of stalk-eyed flies and its relationship to body morphology and development. The flight performance of 1-to-30 day oldTeleopsis dalmanni(n=124) andDiasemopsis meigenii(n=83) were assessed by presenting normoxic, variable-density mixtures of heliox (O₂, N₂and He) in 10% increments ranging from air to pure heliox; the least-dense gas allowing flight represented maximal performance. Flight kinematics were analyzed using high-speed (5930fps) videography. Immediately following flight assessment, flies were euthanized, photographed, dissected and weighed. In both species, total body mass, thorax and abdominal mass increased across age. Wing kinematics and maximal flight capacity were associated with thorax mass, and increased with age as flies became heavier. Although flies with longer eyespans were indeed heavier, they had larger wings and thoraces; however, maximal flight capacity and kinematics were generally independent of eyespan. Thus, bearing long eye-stalks did not impair flight performance, nor did the increase in mass attributable to reproductive maturation. Instead, variation in flight performance appears associated with the development of the flight motor, and improved ratio of thorax-to-total mass, across age. 

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3. Seasonal plasticity in morphology and metabolism differs between migratory North American and resident Costa Rican monarch butterflies

   https://doi.org/10.1002/ece3.9796  

   Tenger‐Trolander, Ayşe; Julick, Cole R.; Lu, Wei; Green, Delbert André; Montooth, Kristi L.; Kronforst, Marcus R. (February 2023, Ecology and Evolution)

   Abstract Environmental heterogeneity in temperate latitudes is expected to maintain seasonally plastic life‐history strategies that include the tuning of morphologies and metabolism that support overwintering. For species that have expanded their ranges into tropical latitudes, it is unclear the extent to which the capacity for plasticity will be maintained or will erode with disuse. The migratory generations of the North American (NA) monarch butterflyDanaus plexippuslead distinctly different lives from their summer generation NA parents and their tropical descendants living in Costa Rica (CR). NA migratory monarchs postpone reproduction, travel thousands of kilometers south to overwinter in Mexico, and subsist on little food for months. Whether recently dispersed populations of monarchs such as those in Costa Rica, which are no longer subject to selection imposed by migration, retain ancestral seasonal plasticity is unclear. To investigate the differences in seasonal plasticity, we reared the NA and CR monarchs in summer and autumn in Illinois, USA, and measured the seasonal reaction norms for aspects of morphology and metabolism related to flight. NA monarchs were seasonally plastic in forewing and thorax size, increasing wing area and thorax to body mass ratio in autumn. While CR monarchs increased thorax mass in autumn, they did not increase the area of the forewing. NA monarchs maintained similar resting and maximal flight metabolic rates across seasons. However, CR monarchs had elevated metabolic rates in autumn. Our findings suggest that the recent expansion of monarchs into habitats that support year‐round breeding may be accompanied by (1) the loss of some aspects of morphological plasticity as well as (2) the underlying physiological mechanisms that maintain metabolic homeostasis in the face of temperature heterogeneity. 

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4. Experimental studies suggest differences in the distribution of thorax elasticity between insects with synchronous and asynchronous musculature

   https://doi.org/10.1098/rsif.2023.0029  

   Casey, Cailin; Heveran, Chelsea; Jankauski, Mark (April 2023, Journal of The Royal Society Interface)

   Insects have developed diverse flight actuation mechanisms, including synchronous and asynchronous musculature. Indirect actuation, used by insects with both synchronous and asynchronous musculature, transforms thorax exoskeletal deformation into wing rotation. Though thorax deformation is often attributed exclusively to muscle tension, the inertial and aerodynamic forces generated by the flapping wings may also contribute. In this study, a tethered flight experiment was used to simultaneously measure thorax deformation and the inertial/aerodynamic forces acting on the thorax generated by the flapping wing. Compared to insects with synchronous musculature, insects with asynchronous muscle deformed their thorax 60% less relative to their thorax diameter and their wings generated 2.8 times greater forces relative to their body weight. In a second experiment, dorsalventral thorax stiffness was measured across species. Accounting for weight and size, the asynchronous thorax was on average 3.8 times stiffer than the synchronous thorax in the dorsalventral direction. Differences in thorax stiffness and forces acting at the wing hinge led us to hypothesize about differing roles of series and parallel elasticity in the thoraxes of insects with synchronous and asynchronous musculature. Specifically, wing hinge elasticity may contribute more to wing motion in insects with asynchronous musculature than in those with synchronous musculature. 

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5. Divergence in Body Mass, Wing Loading, and Population Structure Reveals Species-Specific and Potentially Adaptive Trait Variation Across Elevations in Montane Bumble Bees

   https://doi.org/10.1093/isd/ixab012  

   Lozier, Jeffrey D; Parsons, Zachary M; Rachoki, Lois; Jackson, Jason M; Pimsler, Meaghan L; Oyen, Kennan J; Strange, James; Dillon, Michael E (September 2021, Insect Systematics and Diversity)

   Hines, Heather (Ed.)

   Abstract Biogeographic clines in morphology along environmental gradients can illuminate forces influencing trait evolution within and between species. Latitude has long been studied as a driver of morphological clines, with a focus on body size and temperature. However, counteracting environmental pressures may impose constraints on body size. In montane landscapes, declines in air density with elevation can negatively impact flight performance in volant species, which may contribute to selection for reduced body mass despite declining temperatures. We examine morphology in two bumble bee (Hymenoptera: Apidae: Bombus Latreille) species, Bombus vancouverensis Cresson and Bombus vosnesenskii Radoszkowski, across mountainous regions of California, Oregon, and Washington, United States. We incorporate population genomic data to investigate the relationship between genomic ancestry and morphological divergence. We find that B. vancouverensis, which tends to be more specialized for high elevations, exhibits stronger spatial-environmental variation, being smaller in the southern and higher elevation parts of its range and having reduced wing loading (mass relative to wing area) at high elevations. Bombus vosnesenskii, which is more of an elevational generalist, has substantial trait variation, but spatial-environmental correlations are weak. Population structure is stronger in the smaller B. vancouverensis, and we find a significant association between elevation and wing loading after accounting for genetic structure, suggesting the possibility of local adaptation for this flight performance trait. Our findings suggest that some conflicting results for body size trends may stem from distinct environmental pressures that impact different aspects of bumble bee ecology, and that different species show different morphological clines in the same region. 

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