To hold or not to hold? The effects of prey type and size on the predatory strategy of a venomous snake

Patterns of morphological variation and ecological correlates in the skull of vipers (Serpentes: Viperidae)

The skull of vipers is a highly kinetic anatomical structure involved in envenomating and consuming of prey. Morphological knowledge about the viperid skull is based on studies on some groups of species, but information on its variation within the whole family and its functional morphology is still scarce. In this study, we aimed to explore variation in skull morphology among species of the three subfamilies of Viperidae, and test whether that variation correlates with macrohabitat and diet. We performed quantitative analyses of the viperid skull based on broad taxonomic sampling and two methodological approaches: linear and geometric morphometrics. The results of both approaches showed that much of the variation lies in differences of shape and relative size of the premaxilla, the nasals, the frontals, and the parietals. The results indicated that phylogeny and size influence the shape of the skull, but we also found evidence of morphological differentiation between arboreal and terrestrial species and in species with mammal specialist diet. Our findings imply that, besides evolutionary allometry and phylogenetic signal, demands of particular diets coupled with use of certain habitats have in part shaped morphological evolution of the viperid skull.

Armed to the teeth: The underestimated diversity in tooth shape in snakes and its relation to feeding behavior and diet

The structure, composition, and shape of teeth have been related to dietary specialization in many vertebrate species, but comparative studies on snakes' teeth are lacking. Yet, snakes have diverse dietary habits that may impact the shape of their teeth. We hypothesize that prey properties, such as hardness and shape, as well as feeding behavior, such as aquatic or arboreal predation, or holding vigorous prey, impose constraints on the evolution of tooth shape in snakes. We compared the morphology of the dentary teeth of 63 species that cover the phylogenetic and dietary diversity of snakes, using 3D geometric morphometrics and linear measurements. Our results show that prey hardness, foraging substrate, and the main feeding mechanical challenge are important drivers of tooth shape, size, and curvature. Overall, long, slender, curved teeth with a thin layer of hard tissue are observed in species that need to maintain a grip on their prey. Short, stout, less curved teeth are associated with species that undergo high or repeated loads. Our study demonstrates the diversity of tooth morphology in snakes and the need to investigate its underlying functional implications to better understand the evolution of teeth in vertebrates.

Evolutionary allometry and ecological correlates of fang length evolution in vipers

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.

The Effects of Temperature on the Kinematics of Rattlesnake Predatory Strikes in Both Captive and Field Environments

The outcomes of predator-prey interactions between endotherms and ectotherms can be heavily influenced by environmental temperature, owing to the difference in how body temperature affects locomotor performance. However, as elastic energy storage mechanisms can allow ectotherms to maintain high levels of performance at cooler body temperatures, detailed analyses of kinematics are necessary to fully understand how changes in temperature might alter endotherm-ectotherm predator-prey interactions. Viperid snakes are widely distributed ectothermic mesopredators that interact with endotherms both as predator and prey. Although there are numerous studies on the kinematics of viper strikes, surprisingly few have analyzed how this rapid movement is affected by temperature. Here we studied the effects of temperature on the predatory strike performance of rattlesnakes (Crotalus spp.), abundant new world vipers, using both field and captive experimental contexts. We found that the effects of temperature on predatory strike performance are limited, with warmer snakes achieving slightly higher maximum strike acceleration, but similar maximum velocity. Our results suggest that, unlike defensive strikes to predators, rattlesnakes may not attempt to maximize strike speed when attacking prey, and thus the outcomes of predatory strikes may not be heavily influenced by changes in temperature.

Male energy reserves, mate-searching activities, and reproductive success: alternative resource use strategies in a presumed capital breeder

Sexual selection studies often focus on morphological traits that are important only in the later stages of mate acquisition. Comparatively little is known about traits that lead to mate acquisition, such as mate-searching activities. We experimentally manipulated body condition (i.e., the energy reserves) in male puff adders (Bitis arietans) prior to the mating season using supplemental feeding in the field, and used radio-telemetry and DNA paternity analyses to characterize the relationships between male energy reserves, mating activities, and reproductive success. We found that male mobility is a strongly sexually selected trait because males that travelled further in search of females have higher female encounter rates and reproductive success. However, supplemental feeding did not have a significant effect on mating activities or reproductive success, because control snakes compensated by foraging more often during the mating season. The time invested in foraging by control snakes did not come at the costs of decreased mating activities or opportunity compared to fed snakes, because the latter spent the spare time resting. Our experimental field research directly demonstrates the link between male mobility and reproductive success, identifying the ultimate mechanism leading to the evolution of prolonged male mate-searching activities in snakes, and indicates that male puff adders, presumed capital breeders, adjust their breeding tactics according to resource availability.

What makes a fang? Phylogenetic and ecological controls on tooth evolution in rear-fanged snakes

BACKGROUND

Fangs are a putative key innovation that revolutionized prey capture and feeding in snakes, and - along with their associated venom phenotypes - have made snakes perhaps the most medically-significant vertebrate animals. Three snake clades are known for their forward-positioned fangs, and these clades (Elapidae, Viperidae, and Atractaspidinae) contain the majority of snakes that are traditionally considered venomous. However, many other snakes are "rear-fanged": they possess potentially venom-delivering teeth situated at the rear end of the upper jaw. Quantification of fang phenotypes - and especially those of rear-fanged species - has proved challenging or impossible owing to the small size and relative rarity of many such snakes. Consequently, it has been difficult to understand the evolutionary history of both venom and prey-capture strategies across extant snakes. We quantified variation in the dentition of 145 colubriform ("advanced") snake species using microCT scanning and compared dental characters with ecological data on species' diet and prey capture method(s) to understand broader patterns in snake fang evolution.

RESULTS

Dental traits such as maxilla length, tooth number, and fang size show strong phylogenetic signal across Colubriformes. We find extreme heterogeneity and evolutionary lability in the rear-fanged phenotype in colubrid (colubrine, dipsadine, and natricine lineages) and lamprophiid snakes, in contrast to relative uniformity in the front fanged phenotypes of other groups (vipers and, to a lesser extent, elapids). Fang size and position are correlated with venom-use in vipers, elapids, and colubrid snakes, with the latter group shifting fangs anteriorly by shortening the entire maxillary bone. We find that maxilla length and tooth number may also be correlated with the evolution of dietary specialization. Finally, an ancestral state reconstruction suggests that fang loss is a recurring phenomenon in colubrid snakes, likely accompanied by shifts in diet and prey capture mode.

CONCLUSIONS

Our study provides a framework for quantifying the complex morphologies associated with venom use in snakes. Our results suggest that fang phenotypes, and particularly the rear-fanged phenotype, in snakes are both diverse and labile, facilitating a wide range of ecological strategies and contributing to spectacular radiations of these organisms in tropical and subtropical biomes worldwide.

Has snake fang evolution lost its bite? New insights from a structural mechanics viewpoint

Venomous snakes-the pinnacle of snake evolution-are characterized by their possession of venom-conducting fangs ranging from grooved phenotypes characterizing multiple lineages of rear-fanged taxa to tubular phenotypes present in elapids, viperids and atractaspidines. Despite extensive research, controversy still exists on the selective pressures involved in fang phenotype diversification. Here, we test the hypothesis that larger fangs and consequently a shift to an anterior position in the maxilla evolved to compensate for the costs of structural changes, i.e. higher stress upon impact in tubular fangs compared to grooved fangs. Direct voxel-based stress simulations conducted on high-resolution µCT scans, analysed within a phylogenetic framework, showed no differences in stress distribution between the three fang phenotypes, despite differences in (relative) fang length. These findings suggest that additional compensatory mechanisms are responsible for the biomechanical optimization and that fang length might instead be related to differential striking behaviour strategies.