Sexual divergence in diets and morphology in Fijian sea snakes Laticauda colubrina (Laticaudinae)

In the Fiji Islands, female yellow‐lipped sea kraits (Laticauda colubrina) grow much larger than males, and have longer and wider heads than do conspecific males of the same body length. This morphological divergence is accompanied by (and may be adaptive to) a marked sex divergence in dietary habits. Adult female sea kraits feed primarily on large conger eels, and take only a single prey item per foraging bout. In contrast, adult males feed upon smaller moray eels, and frequently take multiple prey items. Prey size increases with snake body size in both males and females, but the sexes follow different trajectories in this respect. Female sea kraits consume larger eels relative to predator head size and body length than do males. Thus, the larger relative head size of female sea kraits is interpreted as an adaptation to consuming larger prey items. Our results are similar to those of previous studies on American water snakes (natricines) and Australian file snakes (acrochordids), indicating that similar patterns of sex divergence in dietary habits and feeding structures have evolved convergently in at least three separate lineages of aquatic snakes.     

Specializations of the Body Form and Food Habits of Snakes

Viperid snakes have stouter bodies, larger heads, and longerjaws than snakes in other families; there are no major differencesbetween the two subfamilies of vipers in these features. A suiteof morphological characters that facilitates swallowing largeprey finds its greatest expression among vipers, but certainelapid and colubrid snakes have converged upon the same bodyform. The number of jaw movements required to swallow prey islinearly related to the size of a prey item when shape is heldconstant. Very small and very large prey are not disproportionatelydifficult for a snake to ingest. Vipers swallow their prey withfewer jaw movements than do colubrids or boids and can swallowprey that is nearly three times larger in relation to theirown size. Proteolytic venom assists in digestion of prey, andmelanin deposits shield the venom glands from light that woulddegrade the venom stores. Ancillary effects of the morphologicalfeatures of vipers, plus the ability to ingest a very largequantity of food in one meal, should produce quantitative andqualitative differences in the ecology and behavior of vipersand other snakes.     

Do lizards and snakes really differ in their ability to take large prey? A study of relative prey mass and feeding tactics in lizards

Adaptations of snakes to overpower and ingest relatively large prey have attracted considerable research, whereas lizards generally are regarded as unable to subdue or ingest such large prey items. Our data challenge this assumption. On morphological grounds, most lizards lack the highly kinetic skulls that facilitate prey ingestion in macrostomate snakes, but (1) are capable of reducing large items into ingestible-sized pieces, and (2) have much larger heads relative to body length than do snakes. Thus, maximum ingestible prey size might be as high in some lizards as in snakes. Also, the willingness of lizards to tackle very large prey items may have been underestimated. Captive hatchling scincid lizards (Bassiana duperreyi) offered crickets of a range of relative prey masses (RPMs) attacked (and sometimes consumed parts of) crickets as large as or larger than their own body mass. RPM affected foraging responses: larger crickets were less likely to be attacked (especially on the abdomen), more likely to be avoided, and less likely to provide significant nutritional benefit to the predator. Nonetheless, lizards successfully attacked and consumed most crickets ≤35% of the predator’s own body mass, representing RPM as high as for most prey taken by snakes. Thus, although lizards lack the impressive cranial kinesis or prey-subduction adaptations of snakes, at least some lizards are capable of overpowering and ingesting prey items as large as those consumed by snakes of similar body sizes.     

The influence of prey-scent stimuli on predatory behavior of the North American copperhead Agkistrodon contortrix (Serpentes: Viperidae)

Viperid snakes strike, envenomate, and release mammalian preyto prevent being harmed by the prey; snakes must then trackprey in the process of strike-induced chemosensory searching.Because rattlesnakes prefer to track and consume envenomatedprey, it would seem that the scent of envenomated tissue iskey to the tracking process. After striking rodents, rattlesnakesalso retain a specific chemical search image of prey items.I examined this behavioral pattern in copperheads (Agkistrodoncontortrix) from three US populations with documented dietarybiases toward mammals (Kansas), lepidopteran larvae (Texas),and amphibians (Louisiana), respectively. Experiments were conductedto assess whether copperheads form a specific search image ofnon-envenomated mouse, hornworm, and frog prey items. Additionalexperiments tested the relative importance of envenomated tissueto prey scent. Results indicate that copperheads do not forma specific search image of prey items. Preference for non-envenomatedprey items is in the order mouse > hornworm > frog forall three populations; therefore, the innate behavioral preferencefor types of prey does not match the dietary biases noted inthe literature. Envenomated mice and hornworms were preferredto all nonenvenomated prey items, but most trials involvingenvenomated frogs did not suggest envenomated prey preference.Overall, these results suggest that when the snakes search forprey, envenomated tissue stimuli are more important to snakesthan scents arising from the prey itself. Searching and consumptionbehaviors seem to be independent, suggesting that strike-inducedchemosensory searching and consumption are more complicatedbehavioral processes than previously recognized.     

Interactions between locomotion,feeding, and bodily elongation during the evolution of snakes

Information from lizard lineages that have evolved a highly elongate (snake‐like) body form may clarify the selective forces important in the early evolution of snakes. Lizards have evolved bodily elongation via two distinct routes: as an adaptation to burrowing underground or to rapid locomotion above ground. These two routes involve diametrically opposite modifications to the body plan. Burrowing lizards have elongate trunks, small heads, short tails, and relatively constant body widths, whereas surface‐active taxa typically have shorter trunks, wider heads, longer tails, and more variable body widths. Snakes resemble burrowing rather than surface‐active (or aquatic) lizards in these respects, suggesting that snakes evolved from burrowing lizards. The trunk elongation of burrowing lizards increases the volume of the alimentary tract, so that an ability to ingest large meals (albeit consisting of small individual prey items) was present in the earliest snakes. Subsequent shifts to ingestion of wide‐bodied prey came later, after selection dismantled other gape‐constraining morphological attributes, some of which may also have arisen as adaptations to burrowing through hard soil (e.g. relatively small heads, rigid skulls). Adaptations of snake skulls to facilitate ingestion of large prey have evolved to compensate for the reduction of relative head size accompanying bodily elongation; relative to predator body mass, maximum sizes of prey taken by snakes may not be much larger than those of many lizards. This adaptive scenario suggests novel functional links between traits, and a series of testable predictions about the relationships between squamate morphology, habitat, and trophic ecology. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 293–304.     

Effects of season, sex and body size on the feeding ecology of turtle-headed sea snakes (Emydocephalus annulatus) on IndoPacific inshore coral reefs

In terrestrial snakes, many cases of intraspecific shifts in dietary habits as a function of predator sex and body size are driven by gape limitation and hence are most common in species that feed on relatively large prey and exhibit a wide body-size range. Our data on sea snakes reveal an alternative mechanism for intraspecific niche partitioning, based on sex-specific seasonal anorexia induced by reproductive activities. Turtle-headed sea snakes (Emydocephalus annulatus) on coral reefs in the New Caledonian Lagoon feed entirely on the eggs of demersal-spawning fishes. DNA sequence data (cytochrome b gene) on eggs that we palpated from stomachs of 37 snakes showed that despite this ontogenetic stage specialization, the prey comes from a taxonomically diverse array of species including damselfish (41 % of samples, at least 5 species), blennies (41 %, 4 species) and gobies (19 %, 5 species). The composition of snake diets shifted seasonally (with damselfish dominating in winter but not summer), presumably reflecting seasonality of fish reproduction. That seasonal shift affects male and female snakes differently, because reproduction is incompatible with foraging. Adult female sea snakes ceased feeding when they became heavily distended with developing embryos in late summer, and males ceased feeding while they were mate searching in winter. The sex divergence in foraging habits may be amplified by sexual size dimorphism; females grow larger than males, and larger snakes (of both sexes) feed more on damselfish (which often lay their eggs in exposed sites) than on blennies and gobies (whose eggs are hidden within narrow crevices). Specific features of reproductive biology of coral reef fish (seasonality and nest type) have generated intraspecific niche partitioning in these sea snakes, by mechanisms different from those that apply to terrestrial snakes.     

The energetic advantages of slug specialization in garter snakes (genus Thamnophis)

We tested the hypothesis that dietary specialization by foraging garter snakes is accompanied by increased assimilation efficiency on specialist prey items. Our comparison included two closely related garter snake species considered to be slug specialists (Thamnophis ordinoides and Thamnophis elegans terrestris), one fish specialist (Thamnophis couchii), and one diet generalist (Thamnophis elegans elegans). Our results suggest that slug specialists have an energetic advantage over non-slug-eating snakes when both eat slugs. Slug specialists T. ordinoides and T. e. terrestris both have higher assimilation and net assimilation efficiencies when eating slugs than do generalists T. e. elegans and T. couchii. The slug specialists did not experience decreased efficiency when eating fish. Therefore, there was no apparent digestive trade-off for the slug specialists when eating other prey.     

Does body size influence thermal biology and diet of a python (Morelia spilota imbricata)?

Current theory predicts that larger‐bodied snakes not only consume larger prey (compared with smaller individuals), but may also have a different range of prey available to them due to their thermal biology. It has been argued that smaller individuals, with lower thermal inertia (i.e. faster cooling rates at nightfall when air temperature falls and basking opportunities are limited), may be thermally restricted to foraging and hunting during the day on diurnally active prey, and have reduced capacity to hunt crepuscular and nocturnal prey species. This predictive theory was investigated by way of dietary analysis, assessment of thermal biology and thermoregulation behaviour in an ambush forager, the south‐west carpet python (Morelia spilota imbricata, Pythonidae). Eighty‐seven scats were collected from 34 individual pythons over a 3‐year radiotelemetry monitoring study. As predicted by gape size limitation, larger pythons took larger prey; however, 65% of prey items of small pythons were represented by nocturnally active, small mammals, a larger proportion than present in larger snakes. Several measures of thermal biology (absolute body temperature, thermal differential of body temperature to air temperature, maximum hourly heating and cooling rates) were not strongly affected by python body mass. Additionally, body temperature was only influenced by the behavioural choice of microhabitat selection and was not affected by python body size or position, suggesting that these behavioural choices do not allow smaller pythons to vastly increase their temporal foraging window. By coupling dietary analysis, measures of body temperature and behavioural observations of free‐ranging animals, we conclude that, contrary to theoretical predictions, a small body size does not thermally restrict the temporal window for ambush foraging in M. s. imbricata. An ontogenetic or size‐determined switch from ambush feeding to actively foraging on slower prey would account for the differences in prey taken by these animals. The concept of altered foraging behaviour warrants further investigation in this species.     

Feeding Strategies in Marine Snakes: An Analysis of Evolutionary, Morphological, Behavioral and Ecological Relationships

Analysis of 1,063 stomach contents from 39 species of sea snakesindicates that about one-third of the shallow, warm, marine,Indo-Australian fish families are preyed upon by sea snakes.Families of eels and gobies are taken by the greatest numbersof snake species. Most species of sea snakes feed on fish familieswhose members are relatively sedentary, dwelling along the bottom,within burrows or reef crevices. With one exception, a fishegg-eating specialization found uniquely in the Aipysurus-Emydocephaluslineage, the dietary habits of sea snakes cannot be categorizedaccording to the snakes' three phylogenetic lineages. Eels,mullet-like, rabbitfish-like and goby-like fish forms are takenby all three lineages. Two or three snake species are generalists,and numerous ones specialize on eels, goby-like fish or catfish.There are differences among sea snake species in the relationshipbetween snake neck girth and the maximum diameter of the prey;in the relationships of both snake gape measurements and fanglength, to the type of prey taken; and in the relationship ofsnake shape and body proportions to the prey selected. Severalmodes of feeding have been observed among sea snakes: feedingin nooks and crannies in the bottom or in reefs, cruising nearthe bottom, and feeding in drift lines. Analysis of percentdigestion of stomach contents and projections backward to thetimes of prey capture provides evidence for feeding periodicity.The greatest amount of diet overlap is for two species of seasnakes which do not both occur at the same locality. Where speciesdo co-occur, diet overlap index values are lower. The numbersof species present as well as their relative abundances varyamong localities as does the relative importance of generalists,eel-eaters, egg-eaters and other specialized feeders.     

Why do Juvenile Chinese Pit‐Vipers (Gloydius shedaoensis) Select Arboreal Ambush Sites?

Ontogenetic shifts in habitat use are widespread, especially in ectothermic taxa in which juveniles may be an order of magnitude smaller than large adult conspecifics. The factors that generate such habitat shifts are generally obscure, but we studied an unusual system that allowed us to compare consequences of habitat selection between adults and juveniles. Pit‐vipers (Gloydius shedaoensis) on a small island in north‐eastern China feed almost entirely on seasonally migrating birds. During the spring bird‐migration period, individual snakes consistently re‐used either arboreal or terrestrial ambush sites. Snakes in trees were smaller (and more philopatric) than snakes on the ground. This ontogenetic shift in habitat use may reflect the difficulty of capturing birds on the ground, especially by small snakes. In laboratory trials, large (adult) pit‐vipers struck faster, further and more accurately than did small (juvenile) snakes. In experiments with free‐ranging snakes, the proportion of strikes hitting the bird was lower for juveniles than for adults, and lower for terrestrial snakes than for arboreal snakes. Additionally, adult snakes generally seized the bird by the head whereas juveniles frequently struck the body or wings (and thus, obtained a less secure grip). Arboreal ambush sites may facilitate prey capture not only because they give access to smaller birds but also because they render the bird's location more predictable and, hence, enable the snake to position itself optimally prior to the prey's arrival. Because juvenile pit‐vipers are less capable strikers, and are small relative to available prey items, they may benefit from the greater ease of prey capture from branches. Thus, the ontogenetic shift in habitat selection within this species may be because of ontogenetic shifts in the vipers’ ability to capture and ingest large, mobile prey.     

Thermal correlates of foraging-site selection by Chinese pit-vipers (Gloydius shedaoensis,Viperidae)

• 1.Do thermal factors influence foraging-site selection by ectothermic predators? Snake species that obtain their prey from ambush must remain immobile for long periods, precluding overt behavioural thermoregulation; and some “ambush” snakes use thermal cues to detect endothermic prey. Plausibly, alternative ambush sites might differ either in equilibrial body temperatures available to snakes, or in the thermal “background” against which prey items must be detected.
• 2.We examined this topic with field data on pit-vipers (Gloydius shedaoensis) on a small island in northeastern China. Adult snakes feed only on migrating passerine birds. The snakes ambush birds both from arboreal perches (branches of small trees) and from the ground.
• 3.Arboreal versus terrestrial ambush sites differed both in operative temperatures and thermal “backgrounds” available to the snakes. Operative temperatures inside copper models were lower in trees than on the ground (because of wind), and snakes in arboreal ambush sites were cooler than those in terrestrial sites. Thermal backgrounds from arboreal perches were cooler (and thus, provided more contrast against prey items) than did backgrounds available from terrestrial ambush-sites.
• 4.Thermal factors thus modify the suitability of alternative ambush locations for these pit-vipers, but with a trade-off: a snake in a tree can “see” its prey more clearly, but may not be warm enough (and hence, able to strike fast enough) to capture it. Further work is required to determine whether or not snakes actually use such thermal differences as criteria for the selection of ambush sites.

     

Snake phylogeny based on osteology,soft anatomy and ecology

Relationships between the major lineages of snakes are assessed based on a phylogenetic analysis of the most extensive phenotypic data set to date (212 osteological, 48 soft anatomical, and three ecological characters). The marine, limbed Cretaceous snakes Pachyrhachis and Haasiophis emerge as the most primitive snakes: characters proposed to unite them with advanced snakes (macrostomatans) are based on unlikely interpretations of contentious elements or are highly variable within snakes. Other basal snakes include madtsoiids and Dinilysia--both large, presumably non-burrowing forms. The inferred relationships within extant snakes are broadly similar to currently accepted views, with scolecophidians (blindsnakes) being the most basal living forms, followed by anilioids (pipesnakes), booids and booid-like groups, acrochordids (filesnakes), and finally colubroids. Important new conclusions include strong support for the monophyly of large constricting snakes (erycines, boines. pythonines), and moderate support for the non-monophyly of the trophidophiids' (dwarf boas). These phylogenetic results are obtained whether varanoid lizards, or amphisbaenians and dibamids, are assumed to be the nearest relatives (outgroups) of snakes, and whether multistate characters are treated as ordered or unordered. Identification of large marine forms, and large surface-active terrestrial forms, as the most primitive snakes contradicts with the widespread view that snakes arose via minute, burrowing ancestors. Furthermore, these basal fossil snakes all have long flexible jaw elements adapted for ingesting large prey ('macrostomy'), suggesting that large gape was primitive for snakes and secondarily reduced in the most basal living foms (scolecophidians and anilioids) in connection with burrowing. This challenges the widespread view that snake evolution has involved progressive, directional elaboration of the jaw apparatus to feed on larger prey.     

Do juvenile gape-limited predators compensate for their small size when feeding?

When juvenile and adult animals occur syntopically, juveniles are at a distinct performance disadvantage due to their absolutely small size. Yet, optimal foraging theory predicts that juvenile predators should feed efficiently in order to compete with adults for food, and to minimize their exposure to predators. Previous authors have suggested that one way for juvenile animals to accomplish these ecological tasks is by increasing their overall feeding performance relative to adults (compensation hypothesis). Nonetheless, only a handful of studies have tested whether juvenile animals have increased feeding performance (e.g. decreased ingestion and/or handling times relative to body size) compared with adults. We tested this hypothesis by examining the ontogeny of head dimensions and feeding performance (ingestion time and number of mandibular protractions) on fish prey for broad-banded water snakes Nerodia fasciata . Individuals were fed fish scaled in a 1:1 ratio to their head width. All head dimensions scaled with significant negative allometry versus body size, and thus smaller snakes had relatively larger heads for their body size compared with larger snakes. By contrast, most head variables (except head volume) exhibited positive allometry versus head length, demonstrating that larger snakes had larger head dimensions relative to head size compared with smaller snakes. In the performance trials, smaller snakes had worse feeding performances when feeding on similarly sized fish prey (relative to their head width) compared with larger snakes. Therefore, these data show that smaller water snakes do not compensate for their size through increased feeding performance.     

Effects of body size,diet, and transience on the demography of the arboreal boid snake Corallus grenadensis on Carriacou (Grenada Grenadines,West Indies)

Remarkably little is known about the demography of snakes in the family Boidae. This lack of information may be attributed, in part, to low population densities on the Neotropical mainland, rendering capture-recapture methods impractical for many species. Conversely, islands support fewer species but snake densities can be much higher. Corallus grenadensis is an arboreal boid endemic to the Grenada Bank and, relative to mainland boids, can be amazingly abundant. As young, its diet is comprised largely of native Anolis lizards, a ubiquitous and abundant food source; it then undergoes an ontogenetic shift in diet to a less abundant resource, rodents. From 2015 to 2019, we marked 254 C. grenadensis and used capture–recapture models to estimate abundance, capture probabilities, survival, and the proportion of transients. We hypothesized that the transient effect would increase with body size (snout–vent length [SVL]), prompted by their ontogenetic shift in diet. Capture probabilities increased with sampling effort and decreased with increasing SVL. Abundance ranged from 96 to 141 individuals and annual resident survival was 0.71, 95% confidence interval (CI) = 0.54–0.83. The proportion of transients increased with increasing SVL, with the estimate being distinguishable from zero starting at ~810 mm SVL, coinciding with the size at which their dietary shift from ectothermic to endothermic prey begins. Ontogenetic dietary shifts are widespread in snakes and occur in at least 11 of 17 species of West Indian boids. Thus, the prominence of transients in our study may be indicative of its demographic and ecological importance among other snake species.     

Arboreal ambush site selection by pit-vipers Gloydius shedaoensis

For a sit-and-wait predator, the choice of ambush site may be a crucial determinant of foraging success. During fieldwork on a small island in northeastern China, we explored the availability and use of arboreal ambush sites (tree branches) selected by Shedao pit-vipers, Gloydius shedaoensis. The snakes were highly selective at a variety of spatial scales. For example, they displayed strong biases in terms of which tree species were used, which individual trees within each species were used and which branches were used within a tree. Snakes disproportionately used trees that were on the edge rather than the interior of thickets, and branches that faced out towards the clearing rather than back towards the thicket. Branches at an angle slightly above horizontal were preferred. The snakes used branches visited at high rates by potential prey, that provided effective camouflage, and with thermal and visual backgrounds (cool, bright) that contrasted strongly with avian prey items (hot, dark). The snakes used perches close to the ground (the area of greatest bird activity) despite suboptimal visual and thermal backgrounds. Use of thicker branches by larger snakes, and by snakes containing recently ingested prey items, may contribute to effective camouflage. Thermoregulation did not appear to influence foraging site selection.     

Is dietary niche breadth linked to morphology and performance in Sandveld lizards Nucras (Sauria: Lacertidae)?

The functional characteristics of prey items (such as hardness and evasiveness) have been linked with cranial morphology and performance in vertebrates. In lizards particularly, species with more robust crania generally feed on harder prey items and possess a greater bite force, whereas those that prey on evasive prey typically have longer snouts. However, the link between dietary niche breadth, morphology, and performance has not been explicitly investigated in lizards. The southern African genus Nucras was used to investigate this link because the species exhibit differing niche breadth values and dietary compositions. A phylogeny for the genus was established using mitochondrial and nuclear markers, and morphological clusters were identified. Dietary data of five Nucras species, as reported previously, were used in correlation analyses between cranial shape (quantified using geometric morphometrics) and dietary niche breadth, and the proportion of hard prey taken and bite force capacity. Dietary niche breadth and the proportion of hard prey eaten were significantly related to cranial shape, although not once phylogeny was accounted for using a phylogenetic generalized least squares regression. The proportion of evasive prey eaten was a significant predictor of forelimb length when phylogeny was taken into account. We conclude that, in Nucras, the percentage of evasive prey taken co‐evolves with forelimb morphology, and dietary niche breadth co‐evolves with cranial shape. However, although head width is correlated with the proportion of hard prey eaten, this appears to be the result of shared ancestry rather than adaptive evolution. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 674–688.     

Rapid and repeated origin of insular gigantism and dwarfism in Australian tiger snakes

It is a well-known phenomenon that islands can support populations of gigantic or dwarf forms of mainland conspecifics, but the variety of explanatory hypotheses for this phenomenon have been difficult to disentangle. The highly venomous Australian tiger snakes (genus Notechis) represent a well-known and extreme example of insular body size variation. They are of special interest because there are multiple populations of dwarfs and giants and the age of the islands and thus the age of the tiger snake populations are known from detailed sea level studies. Most are 5000-7000 years old and all are less than 10,000 years old. Here we discriminate between two competing hypotheses with a molecular phylogeography dataset comprising approximately 4800 bp of mtDNA and demonstrate that populations of island dwarfs and giants have evolved five times independently. In each case the closest relatives of the giant or dwarf populations are mainland tiger snakes, and in four of the five cases, the closest relatives are also the most geographically proximate mainland tiger snakes. Moreover, these body size shifts have evolved extremely rapidly and this is reflected in the genetic divergence between island body size variants and mainland snakes. Within south eastern Australia, where populations of island giants, populations of island dwarfs, and mainland tiger snakes all occur, the maximum genetic divergence is only 0.38%. Dwarf tiger snakes are restricted to prey items that are much smaller than the prey items of mainland tiger snakes and giant tiger snakes are restricted to seasonally available prey items that are up three times larger than the prey items of mainland tiger snakes. We support the hypotheses that these body size shifts are due to strong selection imposed by the size of available prey items, rather than shared evolutionary history, and our results are consistent with the notion that adaptive plasticity also has played an important role in body size shifts. We suggest that plasticity displayed early on in the occupation of these new islands provided the flexibility necessary as the island's available prey items became more depauperate, but once the size range of available prey items was reduced, strong natural selection followed by genetic assimilation worked to optimize snake body size. The rate of body size divergence in haldanes is similar for dwarfs (h(g) = 0.0010) and giants (h(g) = 0.0020-0.0025) and is in line with other studies of rapid evolution. Our data provide strong evidence for rapid and repeated morphological divergence in the wild due to similar selective pressures acting in different directions.     

Morphological integration and adaptation in the snake feeding system: a comparative phylogenetic study

A long-standing hypothesis for the adaptive radiation of macrostomatan snakes is that their enlarged gape--compared to both lizards and basal snakes--enables them to consume "large" prey. At first glance, this hypothesis seems plausible, or even likely, given the wealth of studies showing a tight match between maximum consumed prey mass and head size in snakes. However, this hypothesis has never been tested within a comparative framework. We address this issue here by testing this hypothesis in 12 monophyletic clades of macrostomatan snakes using recently published phylogenies, published maximum consumed prey mass data and morphological measurements taken from a large sample of museum specimens. Our nonphylogenetically corrected analysis shows that head width--independent of body size--is significantly related to mean maximum consumed prey mass among these clades, and this relationship becomes even more significant when phylogeny is taken into account. Therefore, these data do support the hypothesis that head shape is adapted to prey size in snakes. Additionally, we calculated a phylogenetically corrected morphological variance-covariance matrix to examine the role of morphological integration during head shape evolution in snakes. This matrix shows that head width strongly covaries with both jaw length and out-lever length of the lower jaw. As a result, selection on head width will likely be associated with concomitant changes in jaw length and lower jaw out-lever length in snakes.     

Apparent decoupling of prey recognition ability with prey availability in an insular snake population

Numerous studies on the feeding behavior of snakes have reported the consistency of tongue-flick responses with their natural diets. For representatives of widely distributed, dietary generalist species from particular localities, we can expect that their tongue-flick responses to potential prey unavailable in their original habitats have been reduced whereas those to prey common in the habitats have been enhanced. To test this hypothesis, intraspecific variation in tongue-flick responses to prey chemicals was examined using ingestively naive snakes (Elaphe quadrivirgata) from dietarily different populations: populations from the main Japanese island, where the snakes' diet predominantly consists of sympatric frogs, and from Mikura-jima Island, where no frogs occur and the snakes thus chiefly prey on lizards. We presented chemical stimuli from six items including those from their natural and potential prey (fish, frog, lizard, mouse, water, and cologne) to newborn snakes. Significant effects of stimuli on the tongue-flick responses were detected. On the other hand, effects of population and interaction between stimuli and population were not significant, and individual comparisons revealed no significant interlocality differences in responses to either frog or lizard chemicals. Thus, our hypothesis was not supported. However, in the Mikura-jima sample, significantly fewer snakes responded to frog chemicals than in the main island sample. The significance of the inconsistency between prey recognition ability and prey availability in the Mikura-jima population are discussed. Received: October 17, 2000 / Accepted: December 14, 2000     

Feeding preferences in 2 disjunct populations of tiger snakes, Notechis scutatus (Elapidae)

Variations at both the genetic and phenotypic levels play animportant role in responses to food and food-related stimuli.Knowledge of such variations is crucial to understanding howpopulations adapt to changing environments. We investigatedthe dietary preferences of 2 tiger snake populations and comparedthe responses of diet-naive animals (laboratory-born neonates),diet-controlled animals (laboratory-reared juveniles), and naturaldiet–experienced animals (wild-caught adults) to visualand chemical cues from 6 prey types (mouse, skink, silver gull,chicken, shearwater, and frog). The mainland population inhabitsa swamp, feeds mostly on frogs, and suffers heavy predation.The second population inhabits a small nearby offshore islandwith no standing water (no frogs); feeds mostly on skinks, mice,and, as adults, silver gull chicks; and suffers no known predation.Although different prey are eaten in the 2 populations, adultwild-caught snakes from both populations showed a significantpreference for 3 types of prey (frog, mouse, and chick), irrespectiveof their natural diet. Neonates responded to all prey cues morethan they did to control stimuli in both populations. However,the island neonates showed significantly higher interest insilver gull chick stimuli (the main prey of the island adultsnakes) than did their mainland conspecifics. Laboratory-bredjuveniles displayed behavioral plasticity by significantly increasingtheir response to mice after being fed baby mice for 7 months.We conclude that genetic-based differences in food-related cuesare important in tiger snakes but that they are also capableof behavioral plasticity. Island adult and neonate snakes exhibitedresponses to prey types no longer consumed naturally (frog),suggesting that behavioral characters may have been retainedfor long periods under relaxed selection. Island neonates showeda strong interest in a novel prey item (silver gull). This resultcomplements previous work describing how island snakes havedeveloped the ability to swallow larger prey than usual, aswell as seemingly developing a taste for them.