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.     

Sex-specific niche partitioning and sexual size dimorphism in Australian pythons (Morelia spilota imbricata)

Sexual dimorphism is usually interpreted in terms of reproductive adaptations, but the degree of sex divergence also may be affected by sex-based niche partitioning. In gape-limited animals like snakes, the degree of sexual dimorphism in body size (SSD) or relative head size can determine the size spectrum of ingestible prey for each sex. Our studies of one mainland and four insular Western Australian populations of carpet pythons ( Morelia spilota ) reveal remarkable geographical variation in SSD, associated with differences in prey resources available to the snakes. In all five populations, females grew larger than males and had larger heads relative to body length. However, the populations differed in mean body sizes and relative head sizes, as well as in the degree of sexual dimorphism in these traits. Adult males and females also diverged strongly in dietary composition: males consumed small prey (lizards, mice and small birds), while females took larger mammals such as possums and wallabies. Geographic differences in the availability of large mammalian prey were linked to differences in mean adult body sizes of females (the larger sex) and thus contributed to sex-based resource partitioning. For example, in one population adult male snakes ate mice and adult females ate wallabies; in another, birds and lizards were important prey types for both sexes. Thus, the high degree of geographical variation among python populations in sexually dimorphic aspects of body size and shape plausibly results from geographical variation in prey availability.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 113–125.     

Sexual differences in morphology and niche utilization in an aquatic snake,Acrochordus arafurae

Filesnakes (Acrochordus arafurae) are large (to 2 m), heavy-bodied snakes of tropical Australia. Sexual dimorphism is evident in adult body sizes, weight/length ratios, and body proportions (relative head and tail lengths). Dimorphism is present even in neonates. Two hypotheses for the evolution of such dimorphism are (1) sexual selection or (2) adaptation of the sexes to different ecological niches. The hypothesis of sexual selection is consistent with general trends of sexually dimorphic body sizes in snakes, and accurately predicts, for A. arafurae, that the larger sex (female) is the one in which reproductive success increases most strongly with increasing body size. However, the sexual dimorphism in relative head sizes is not explicable by sexual selection.The hypothesis of adaptation to sex-specific niches predicts differences in habitats and/or prey. I observed major differences between male and female A. arafurae in prey types, prey sizes and habitat utilization (shallow versus deep water). Hence, the sexual dimorphism in relative head sizes is attributed to ecological causes rather than sexual selection. Nonetheless, competition between the sexes need not be invoked as the selective advantage of this character divergence. It is more parsimonious to interpret these differences as independent adaptations of each sex to increase foraging success, given pre-existing sexually-selected differences in size, habitat or behavior. Data for three other aquatic snake species, from phylogenetically distant taxa, suggest that sexual dimorphism in food habits, foraging sites and feeding morphology, is widespread in snakes.     

Sexual dimorphism, reproductive biology, and food habits of two species of African filesnakes (Mehelya, Colubridae)

The ecology and general biology of African snakes remains virtually unstudied, even in highly distinctive species such as the filesnakes (genera Mehelya and Gonionotophis ). Our measurements and dissections of preserved specimens provided information on body sizes, sexual dimorphism in size and bodily proportions, clutch sizes, and food habits of two Mehelya species. In both M. capensis and M. nyassae , females attain sexual maturity at the same size as conspecific males, but grow to much larger sizes. Mehelya capensis displays extreme differences in body shape between males and females at the same body length: females have longer and wider heads, thicker bodies, and larger eyes (relative to both head length and head width) than do conspecific males. Dimorphism in body proportions is less marked in M. nyassae. Female reproductive cycles are seasonal in M. capensis , and clutch sizes are larger in this species than in its smaller congener (5-11 eggs in M. capensis , 2-6 eggs in M. nyassae ).
Contrary to popular wisdom, Mehelya are not specialized ophiophages. Mehelya nyassae feeds primarily upon lygosomatine skinks, including many fossorial taxa. Mehelya capensis has a broader diet, feeding on a wide variety of terrestrial lizards (especially agamids and gerrhosaurids) and snakes. Toads are also common prey items. The diversity of prey types taken by M. capensis suggests that these snakes may use ambush predation as well as active foraging. Mehelya is strongly convergent with Asian elapids of the genus Bungarus in its morphology (triangular body shape; powerful jaws; visible interstitial skin), behaviour (nocturnality; reluctance to bite when harassed), and diet (feeding on elongate reptiles, including snakes). Observations of preyhandling and ingestion by captive snakes are needed to clarify possible selective forces for the evolution of the unusual traits shared by these taxa.     

Morphology,Reproduction and Diet in Australian and Papuan Death Adders (Acanthophis,Elapidae)

Death adders (genus Acanthophis) differ from most other elapid snakes, and resemble many viperid snakes, in their thickset morphology and ambush foraging mode. Although these snakes are widely distributed through Australia and Papua New Guinea, their basic biology remains poorly known. We report morphological and ecological data based upon dissection of >750 museum specimens drawn from most of the range of the genus. Female death adders grow larger than conspecific males, to about the same extent in all taxa (20% in mean adult snout-vent length,  =  SVL). Most museum specimens were adult rather than juvenile animals, and adult males outnumbered females in all taxa except A. pyrrhus. Females have shorter tails (relative to SVL) than males, and longer narrower heads (relative to head length) in some but not all species. The southern A. antarcticus is wider-bodied (relative to SVL) than the other Australian species. Fecundity of these viviparous snakes was similar among taxa (mean litter sizes 8 to 14). Death adders encompass a broad range of ecological attributes, taking a wide variety of vertebrate prey, mostly lizards (55%), frogs and mammals (each 21%; based on 217 records). Dietary composition differed among species (e.g. frogs were more common in tropical than temperate-zone species), and shifted with snake body size (endotherms were taken by larger snakes) and sex (male death adders took more lizards than did females). Overall, death adders take a broader array of prey types, including active fast-moving taxa such as endotherms and large diurnal skinks, than do most other Australian elapids of similar body sizes. Ambush foraging is the key to capturing such elusive prey.     

The influence of sex and body size on food habits of a giant tropical snake, Python reticulatus

1. In many animal species, dietary habits shift with body size, and differ between the sexes. However, the intraspecific range of body sizes is usually low, making it difficult to quantify size-associated trophic shifts, or to determine the degree to which sex differences in diet are due to body-size differences. Large snakes are ideal for such a study, because they provide a vast range of body sizes within a single population.
2. More than 1000 Reticulated Pythons ( Python reticulatus ) from southern Sumatra were examined, with specimens from 1·5 to > 6 m in snout–vent length, and from 1 to 75 kg in mass. Females attained much larger body sizes than did conspecific males (maxima of 20 vs 75 kg, 5 vs 7 m), but had similar head lengths at the same body lengths.
3. Prey sizes, feeding frequencies and numbers of stomach parasites (ascarid nematodes) increased with body size in both sexes, and dietary composition changed ontogenetically. Small snakes fed mostly on rats, but shifted to larger mammalian taxa (e.g. pangolins, porcupines, monkeys, wild pigs, mouse deer) at 3–4-m body length.
4. Adult males and females showed strong ecological divergence. For some traits, this divergence was entirely caused by the strong allometry (combined with sexual size dimorphism), but in other cases (e.g. feeding frequency, dietary composition), the sexes followed different allometric trajectories. For example, females shifted from rats to larger mammals at a smaller body size than did conspecific males, and feeding frequencies increased more rapidly with body size in females than in males. These allometric divergences enhanced the degree of sex difference in trophic ecology induced by sexual size dimorphism.     

Sexual dimorphism of body and relative head sizes in neonatal common garter snakes

Morphological and behavioral differences between sexes are commonplace throughout the animal kingdom. Body size is one of the most obvious sex differences frequently found in snakes. However, the developmental origins of size differences in many species, including snakes, are not well known. We examined post-natal variation in sexual size dimorphism in garter snakes Thamnophis sirtalis . The weights, body and tail lengths, and head sizes of male and female neonates born to mothers collected from ecologically dissimilar habitats on Beaver Island, Lake Michigan were compared. Sexual size dimorphism was prominent. Overall, males had significantly longer bodies and tails than females. Females were significantly heavier and had larger heads than male snakes. Maternal site affected head but not body measurements, perhaps due to differences in prey availability. The body condition of maternal females predicted neonatal body length. Significant litter variation suggests heritable variation in morphological traits possibly correlated with feeding success and survival.     

Sexual dimorphism in the tusked frog, Adelotus brevis (Anura:Myobatrachidae): the roles of natural and sexual selection

At least two adaptive processes can lead to the evolution of sexual dimorphism: sexual selection (e.g. male-male combat) or natural selection (e.g. dietary divergence). We investigated the adaptive significance of a distinctive pattern of sexual dimorphism in a south-eastern Australian frog, Adelotus brevis. Male Adelotus grow larger than female conspecifics, have larger heads relative to body size, and have large paired projections (‘tusks’) in the lower jaw. All of these traits are rare among anurans. We quantified the degree of dimorphism in Adelotus, and gathered data on diets and mating systems of this species to evaluate the possible roles of sexual selection and dietary divergence in favoring die evolution of these sexually dimorphic traits. Analysis of prey items in alimentary tracts revealed significant sex differences in prey types. For example, females ate proportionally more arthropods and fewer molluscs than did males. However, this difference is likely to be a secondary consequence of habitat differences between the sexes (due in turn to their different reproductive roles) rather than a selective force for the evolution of sexual dimorphism. Calling males spend their time in moist habitats where pondsnails are abundant, whereas females are more often encountered in the drier arthropod-rich woodlands. A three-year behavioural ecology study on a field population revealed that reproductive males engage in agonistic interactions, with the sexually dimorphic tusks used to attack rivals. Larger body size contributed to male reproductive success. Small males were excluded from calling sites and, among the calling males, larger animals had higher reproductive success (numbers of matings) than did smaller individuals. Hence, the atypical pattern of sexual dimorphism in Adelotus brevis seems to have resulted from sexual selection for larger body size and tusk size in males, in the context of male-male agonistic behaviour, rather than natural selection for ecological divergence between the sexes.     

Determinants of dietary specialization: a comparison of two sympatric species of sea snakes

Why do some predator species specialize on only a single type of prey whereas others take a broad range? One critical determinant may be the ontogenetic range of body sizes of the predator compared to that of its prey. If any single prey taxon spans only part of the range of prey sizes ingestible by the predator, then the predator will be more likely to take multiple prey taxa. We exploit a model system that provides a robust opportunity to test this hypothesis. We studied two sympatric species of predatory sea snakes, similar in size and general ecology that feed on anguilliform fishes from different habitats in the Great Lagoon of New Caledonia. Eel species from soft‐bottom habitats must construct their own burrows, and thus tend to be more slender‐bodied and less variable in body size than eel species that inhabit variable‐sized crevices among hard coral. As a result, a laticaudine sea snake species (Laticauda saintgironsi) that feeds on hard‐coral‐dwelling eels relies primarily on a single prey species: juveniles take young eels whereas adults consume adult eels of the same species. In contrast, a laticaudine species (L. laticaudata) that forages on soft‐bottom eels switches its prey ontogenetically: juveniles take small eel species whereas adults consume large eel species. Thus, habitat‐imposed constraints on the range of body sizes within each prey taxon generate a striking difference in the degree of dietary specialization of two closely related, sympatric predator species.     

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.     

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.     

Allometry and selection in a novel predator–prey system: Australian snakes and the invading cane toad

Because many organismal traits vary with body size, interactions between species can be affected by the respective body sizes of the participants. We focus on a novel predator–prey system involving an introduced, highly toxic anuran (the cane toad, Bufo marinus ) and native Australian snakes. The chance of a snake dying after ingesting a toad depends on the size of the snake and the size of the toad, and ultimately reflects the effect of four allometries: (1) physiological tolerance (the rate that physiological tolerance to toad toxin changes with snake size); (2) swallowing ability (the rate that maximal ingestible toad size (i.e. snake head size) increases with snake body size); (3) prey size (the rate that prey size taken by snakes increases with snake head size) and (4) toad toxicity (the rate that toxicity increases with toad size). We measured these allometries, and combined them to estimate the rate at which a snake's resistance changes with toad toxicity. The parotoid glands (and thus, toxicity) of toads increased disproportionately with toad size (i.e. relative to body size, larger toads were more toxic) but simultaneously, head size relative to body size (and thus, maximal ingestible prey size relative to predator size) declined with increasing body size in snakes. Thus, these two allometries tended to cancel each other out. Physiological tolerance to toxins did not vary with snake body size. The end result was that across snake species, mean adult body size did not affect vulnerability. Within species, however, smaller predators were more vulnerable, because the intraspecific rate of decrease in relative head size of snakes was steeper than the rate of increase in toxicity of toads. Thus, toad invasion may cause disproportionate mortality of juvenile snakes, and adults of the sex with smaller mean adult body sizes.     

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.     

Does ‘gliding while gravid’ explain Rensch's rule in flying lizards?

Among species with sexual size dimorphism (SSD), taxa in which males are the larger sex have increasing SSD with increasing body size, whereas in taxa in which females are the larger sex, SSD decreases with body size: Rensch's rule. We show in flying lizards, a clade of mostly female‐larger species, that SSD increases with body size, a pattern similar to that in clades with male‐biased SSD or more evenly mixed SSD. The observed pattern in Draco appears due to SSD increasing with evolutionary changes in male body size; specifically divergence in body size among species that are in sympatric congeneric assemblages. We suggest that increasing body size, resulting in decreased gliding performance, reduces the relative gliding cost of gravidity in females, and switches sexual selection in males away from a small‐male, gliding advantage and toward selection on large size and fighting ability as seen in many other lizards. Thus, selection for large females is likely greater than selection for large males at the smaller end of the body size continuum, whereas this relationship reverses for species at the larger end of the continuum. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 270–282.     

蓝尾石龙子的头部两性异形和食性

通过测量头、体大小和胃检研究浙江泰顺产蓝尾石龙子 (Eumeceselegans)个体发育过程中两性异形和食性的变化。蓝尾石龙子成体个体大小和头部大小的两性差异显著 ,雄性大于雌性。不同发育阶段雌性头长与SVL的线性回归斜率无显著差异 ,头宽与SVL线性回归斜率的差异显著 ,成体和SVL <5 0mm幼体头宽随SVL的增长速率显著小于SVL为 5 0 - 6 9mm的幼体。雄性头部相对于SVL呈加速式异速生长。两性比较发现 :雌雄幼体头长和头宽随SVL的增长速率无显著差异 ,SVL <5 0mm幼体特定SVL的头长和头宽无显著的两性差异 ,但SVL为 5 0 - 6 9mm的雄性幼体头长和头宽大于SVL相同的雌性幼体 ;雄性成体头长和头宽随SVL的增长速率显著大于雌性。SVL <5 0mm的雌性幼体头部相对小于SVL为 5 0 - 6 9mm的同性幼体 ,性成熟雌体头部相对小于SVL为 5 0 - 6 9mm的同性幼体。雌性幼体、雄性幼体、雌性成体和雄性成体食物生态位宽度分别为 12 3、 12 5、 4 8和 14 4。雌雄幼体食物生态位重叠度最高 ,雌雄成体食物生态位重叠度次之 ,成体与幼体食物生态位重叠度较小。成体摄入食饵的大小 (用胃内完整食物长度的平均值表示 )和变化范围大于幼体。两性成、幼体摄入的食饵大小差异显著。两性个体摄入的食饵大小均与其SVL呈正相关 ,表明较大     

Sexual differences in head form and diet in a population of Mexican lance‐headed rattlesnakes,Crotalus polystictus

Sexual dimorphism of phenotypic traits associated with resource use is common in animals, and may result from niche divergence between sexes. Snakes have become widely used in studies of the ecological basis of sexual dimorphism because they are gape‐limited predators and their head morphology is likely to be a direct indicator of the size and shape of prey consumed. We examined sexual dimorphism of body size and head morphology, as well as sexual differences in diet, in a population of Mexican lance‐headed rattlesnakes, Crotalus polystictus, from the State of México, Mexico. The maximum snout–vent length of males was greater than that of females by 21%. Males had relatively larger heads, and differed from females in head shape after removing the effects of head size. In addition, male rattlesnakes showed positive allometry in head shape: head width was amplified, whereas snout length was truncated with increased head size. By contrast, our data did not provide clear evidence of allometry in head shape of females. Adults of both males and females ate predominately mice and voles; however, males also consumed a greater proportion of larger mammalian species, and fewer small prey species. The differences in diet correspond with dimorphism in head morphology, and provide evidence of intersexual niche divergence in the study population. However, because the sexes overlapped greatly in diet, we hypothesize that diet and head dimorphisms in C. polystictus are likely related to different selection pressures in each sex arising from pre‐existing body size differences rather than from character displacement for reducing intersexual competition. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 633–640.     

Climate-driven impacts of prey abundance on the population structure of a tropical aquatic predator

In the present study we explore how annual variation in climate (late wet-season rainfall) affects population demography in a gape-limited obligate piscivorous predator, the Arafura filesnake Acrochordus arafurae in the Australian tropics. These aquatic snakes display extreme sexual dimorphism, with body sizes and relative head sizes of females much larger than those of males. Two consecutive years with low rainfall during the late wet season reduced the abundance of small but not large sized fish. Although snake residual body mass (RBM, calculated from a general linear regression of ln-transformed mass to ln-SVL) decreased after the first year with low prey availability, it was not until the second year that reduced prey abundance caused a dramatic decline in filesnake survival, and hence in population numbers. Thus, our results suggest that most snakes survived the first year of reduced prey abundance, but a successive year with low prey availability proved fatal for many animals. However, the effects of prey scarcity on RBM and survival fell disproportionately on some size classes of snakes. Medium-sized animals (large males and intermediate-sized females) were affected more dramatically than were small or large snakes. We attribute the higher survival of small snakes to their lower energy needs compared to medium-sized individuals, and the higher survival of large snakes to the continued abundance of large prey (mainly large catfish). Two successive years with low abundance of smaller sized prey thus massively modified the size-structure of the filesnake population, virtually eliminating large males and intermediate-sized females. Our field data provide a clear demonstration of the ways in which stochastic variation in climatic conditions can have dramatic effects on predator population demography, mediated via effects on prey availability.     

Feeding ecology of North American gopher snakes (Pituophis catenifer, Colubridae)

Studies of food relations are important to our understanding of ecology at the individual, population and community levels. Detailed documentation of the diet of large‐bodied, widespread snakes allows us to assess size‐dependent and geographical variation in feeding preferences of gape‐limited predators. Furthermore, with knowledge of the food habits of sympatric taxa we can explore possible causes of interspecific differences in trophic niches. The feeding ecology of the North American gopher snake, Pituophis catenifer, was studied based on the stomach contents of more than 2600 preserved and free‐ranging specimens, and published and unpublished dietary records. Of 1066 items, mammals (797, 74.8%), birds (86, 8.1%), bird eggs (127, 11.9%), and lizards (35, 3.3%) were the most frequently eaten prey. Gopher snakes fed upon subterranean, nocturnal and diurnal prey. The serpents are primarily diurnal, but can also be active at night. Therefore, gopher snakes captured their victims by actively searching underground tunnel systems, retreat places and perching sites during the day, or by pursuing them or seizing them while they rested at night. Gopher snakes of all sizes preyed on mammals, but only individuals larger than 40 and 42 cm in snout–vent length took bird eggs and birds, respectively, possibly due to gape constraints in smaller serpents. Specimens that ate lizards were smaller than those that consumed mammals or birds. Gopher snakes raided nests regularly, as evidenced by the high frequency of nestling mammals and birds and avian eggs eaten. Most (332) P. catenifer contained single prey, but 95 animals contained 2–35 items. Of the 321 items for which direction of ingestion was determined, 284 (88.5%) were swallowed head‐first, 35 (10.9%) were ingested tail‐first, and two (0.6%) were taken sideways. Heavier gopher snakes took heavier prey, but heavier serpents ingested prey with smaller mass relative to snake mass, evidence that the lower limit of prey mass did not increase with snake mass. Specimens from the California Province and Arid Deserts (i.e. Mojave, Sonoran and Chihuahuan Deserts) took the largest proportion of lizards, whereas individuals from the Great Basin Desert consumed a higher percentage of mammals than serpents from other areas, and P. catenifer from the Great Plains ate a greater proportion of bird eggs. Differences in prey availability among biogeographical regions and unusual circumstances of particular gopher snake populations may account for these patterns. Gopher snakes have proportionally longer heads than broadly sympatric Rhinocheilus lecontei (long‐nosed snake), Charina bottae (rubber boa) and Lampropeltis zonata (California mountain kingsnake), which perhaps explains why, contrary to the case in P. catenifer, the smaller size classes of those three species do not eat mammals. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77 , 165–183.     

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.     

Built to bite? Differences in cranial morphology and bite performance between narrow‐ and broad‐headed European glass eels

The presence of two phenotypes in a single species is a widespread phenomenon, also observed in European eel (Anguilla anguilla). This dimorphism has been related to dietary differences in the subadult elver and yellow eel stages, with broad‐heads generally feeding on harder and/or larger‐bodied prey items than narrow‐heads. Nevertheless, both broad‐ and narrow‐headed phenotypes can already be found among glass eels, the stage preceding the elver eel stage. As these glass eels are considered nonfeeding, we investigate here to what degree the observed variation in head width is reflected in variation in the musculoskeletal feeding system, as well as whether this reflects the same variation observed in the older, dimorphic yellow eels. Additionally, we investigate whether musculoskeletal differences between broad‐ and narrow‐headed glass eels have implications on their feeding performance and could thus impact prey preference when eels start feeding. Therefore, we compared the cranial musculoskeletal system of five broad‐ and narrow‐headed glass eels using 3D‐reconstructions and simulated the glass eel's bite force using the data of the muscle reconstructions. We found that the variation in the musculoskeletal system of glass eels indeed reflects that of the yellow eels. Broader heads were related to larger jaw muscles, responsible for mouth closure. Accordingly, broad‐heads could generate higher bite forces than narrow‐headed glass eels. In addition, broader heads were associated with higher coronoid processes and shorter hyomandibulae, beneficial for dealing with higher mechanical loadings and consequently, harder prey. We, thus, show that head width variation in glass eels is related to musculoskeletal differences which, in turn, can affect feeding performance. As such, differences in prey preference can already take place the moment the eels start feeding, potentially leading to the dimorphism observed in the elver and yellow eel stage.