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1. Morphology, Ecology, and Biogeography of Independent Origins of Cleaning Behavior Around the World

   https://doi.org/10.1093/icb/icz030  

   Baliga, Vikram B. ; Mehta, Rita S. ( May 2019 , Integrative and Comparative Biology)

   Members of an ecological guild may be anticipated to show morphological convergence, as similar functional demands exert similar selective pressures on phenotypes. Nature is rife with examples, however, where such taxa instead exhibit ‘incomplete’ convergence or even divergence. Incorporating factors such as character displacement by other guild members or variation in ecological specialization itself may therefore be necessary to gain a more complete understanding of what constrains or promotes diversity. Cleaning, a behavior in which species remove and consume ectoparasites from “clientele,” has been shown to exhibit variation in specialization and has evolved in a variety of marine habitats around the globe. To determine the extent to which specialization in this tropic strategy has affected phenotypic evolution, we examined the evolution of cleaning behavior in five marine fish families: Labridae, Gobiidae, Pomacanthidae, Pomacentridae, and Embiotocidae. We used a comparative framework to determine patterns of convergence and divergence in body shape and size across non-cleaning and cleaning members within these five clades. Highly specialized obligate cleaning, found in the Indo-Pacific and the Caribbean, evolved in the Labridae and Gobiidae at strikingly similar times. In these two regions, obligate cleaning evolves early, shows convergence on an elongate body shape, and ismore »restricted to species of small body size. Facultative cleaning, shown either throughout ontogeny or predominately in the juvenile phase, exhibits a much more varied phenotype, especially in geographic regions where obligate cleaning occurs. Collectively, our results are consistent with varying extents of an ecological specialization constraining or spurring morphological evolution in recurrent ways across regions.

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2. Scaling patterns of body plans differ among squirrel ecotypes

   https://doi.org/10.7717/peerj.14800  

   Linden, Tate J. ; Burtner, Abigail E. ; Rickman, Johannah ; McFeely, Annika ; Santana, Sharlene E. ; Law, Chris J. ( January 2023 , PeerJ)

   Body size is often hypothesized to facilitate or constrain morphological diversity in the cranial, appendicular, and axial skeletons. However, how overall body shape scales with body size ( i.e. , body shape allometry) and whether these scaling patterns differ between ecological groups remains poorly investigated. Here, we test whether and how the relationships between body shape, body size, and limb lengths differ among species with different locomotor specializations, and describe the underlying morphological components that contribute to body shape evolution among squirrel (Sciuridae) ecotypes. We quantified the body size and shape of 87 squirrel species from osteological specimens held at museum collections. Using phylogenetic comparative methods, we first found that body shape and its underlying morphological components scale allometrically with body size, but these allometric patterns differ among squirrel ecotypes: chipmunks and gliding squirrels exhibited more elongate bodies with increasing body sizes whereas ground squirrels exhibited more robust bodies with increasing body size. Second, we found that only ground squirrels exhibit a relationship between forelimb length and body shape, where more elongate species exhibit relatively shorter forelimbs. Third, we found that the relative length of the ribs and elongation or shortening of the thoracic region contributes the most to bodymore »shape evolution across squirrels. Overall, our work contributes to the growing understanding of mammalian body shape evolution and how it is influenced by body size and locomotor ecology, in this case from robust subterranean to gracile gliding squirrels.« less
3. Sex-specific plasticity and the nutritional geometry of insulin-signaling gene expression in Drosophila melanogaster

   https://doi.org/10.1186/s13227-021-00175-0  

   McDonald, Jeanne M. ; Nabili, Pegah ; Thorsen, Lily ; Jeon, Sohee ; Shingleton, Alexander W. ( December 2021 , EvoDevo)

   Abstract Background Sexual-size dimorphism (SSD) is replete among animals, but while the selective pressures that drive the evolution of SSD have been well studied, the developmental mechanisms upon which these pressures act are poorly understood. Ours and others’ research has shown that SSD in D. melanogaster reflects elevated levels of nutritional plasticity in females versus males, such that SSD increases with dietary intake and body size, a phenomenon called sex-specific plasticity (SSP). Additional data indicate that while body size in both sexes responds to variation in protein level, only female body size is sensitive to variation in carbohydrate level. Here, we explore whether these difference in sensitivity at the morphological level are reflected by differences in how the insulin/IGF-signaling (IIS) and TOR-signaling pathways respond to changes in carbohydrates and proteins in females versus males, using a nutritional geometry approach. Results The IIS-regulated transcripts of 4E-BP and InR most strongly correlated with body size in females and males, respectively, but neither responded to carbohydrate level and so could not explain the sex-specific response to body size to dietary carbohydrate. Transcripts regulated by TOR-signaling did, however, respond to dietary carbohydrate in a sex-specific manner. In females, expression of dILP5 positively correlated withmore »body size, while expression of dILP2,3 and 8, was elevated on diets with a low concentration of both carbohydrate and protein. In contrast, we detected lower levels of dILP2 and 5 protein in the brains of females fed on low concentration diets. We could not detect any effect of diet on dILP expression in males. Conclusion Although females and males show sex-specific transcriptional responses to changes in protein and carbohydrate, the patterns of expression do not support a simple model of the regulation of body-size SSP by either insulin- or TOR-signaling. The data also indicate a complex relationship between carbohydrate and protein level, dILP expression and dILP peptide levels in the brain. In general, diet quality and sex both affect the transcriptional response to changes in diet quantity, and so should be considered in future studies that explore the effect of nutrition on body size.« less
4. Evolutionary allometry and ecological correlates of fang length evolution in vipers

   https://doi.org/10.1098/rspb.2022.1132  

   Holding, Matthew L. ; Trevine, Vivian C. ; Zinenko, Oleksandr ; Strickland, Jason L. ; Rautsaw, Rhett M. ; Mason, Andrew J. ; Hogan, Michael P. ; Parkinson, Christopher L. ; Grazziotin, Felipe G. ; Santana, Sharlene E. ; et al ( September 2022 , Proceedings of the Royal Society B: Biological Sciences)

   Traits for prey acquisition form the phenotypic interface of predator–prey interactions. In venomous predators, morphological variation in venom delivery apparatus like fangs and stingers may be optimized for dispatching prey. Here, we determine how a single dimension of venom injection systems evolves in response to variation in the size, climatic conditions and dietary ecology of viperid snakes. We measured fang length in more than 1900 museum specimens representing 199 viper species (55% of recognized species). We find both phylogenetic signal and within-clade variation in relative fang length across vipers suggesting both general taxonomic trends and potential adaptive divergence in fang length. We recover positive evolutionary allometry and little static allometry in fang length. Proportionally longer fangs have evolved in larger species, which may facilitate venom injection in more voluminous prey. Finally, we leverage climatic and diet data to assess the global correlates of fang length. We find that models of fang length evolution are improved through the inclusion of both temperature and diet, particularly the extent to which diets are mammal-heavy diets. These findings demonstrate how adaptive variation can emerge among components of complex prey capture systems.
5. Bat Dentitions: A Model System for Studies at the Interface of Development, Biomechanics, and Evolution

   https://doi.org/10.1093/icb/icac042  

   Santana, Sharlene E. ; Grossnickle, David M. ; Sadier, Alexa ; Patterson, Edward ; Sears, Karen E. ( May 2022 , Integrative And Comparative Biology)

   The evolution of complex dentitions in mammals was a major innovation that facilitated the expansion into new dietary niches, which imposed selection for tight form–function relationships. Teeth allow mammals to ingest and process food items by applying forces produced by a third-class lever system composed by the jaw adductors, the cranium, and the mandible. Physical laws determine changes in jaw adductor (biting) forces at different bite point locations along the mandible (outlever), thus, individual teeth are expected to experience different mechanical regimes during feeding. If the mammal dentition exhibits functional adaptations to mandible feeding biomechanics, then teeth are expected to have evolved to develop mechanically advantageous sizes, shapes, and positions. Here, we present bats as a model system to test this hypothesis and, more generally, for integrative studies of mammal dental diversity. We combine a field-collected dataset of bite forces along the tooth row with data on dental and mandible morphology across 30 bat species. We (1) describe, for the first time, bite force trends along the tooth row of bats; (2) use phylogenetic comparative methods to investigate relationships among bite force patterns, tooth, and mandible morphology; and (3) hypothesize how these biting mechanics patterns may relate to themore »developmental processes controlling tooth formation. We find that bite force variation along the tooth row is consistent with predictions from lever mechanics models, with most species having the greatest bite force at the first lower molar. The cross-sectional shape of the mandible body is strongly associated with the position of maximum bite force along the tooth row, likely reflecting mandibular adaptations to varying stress patterns among species. Further, dental dietary adaptations seem to be related to bite force variation along molariform teeth, with insectivorous species exhibiting greater bite force more anteriorly, narrower teeth and mandibles, and frugivores/omnivores showing greater bite force more posteriorly, wider teeth and mandibles. As these craniodental traits are linked through development, dietary specialization appears to have shaped intrinsic mechanisms controlling traits relevant to feeding performance.

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