Lake Erie water snakes revisited: Morph- and age-specific variation in relative crypsis

Summary Populations of water snakes (Nerodia sipedon insularum) on islands in western Lake Erie are variable in colour pattern, consisting of unbanded, intermediate, and banded morphs. In contrast, mainland populations (N. s. sipedon) consist solely of banded morphs. Previous investigators hypothesized that natural selection favoured unbanded morphs on exposed island shorelines and banded morphs in overgrown mainland habitats and that gene flow from mainland populations was responsible for the persistence of banded morphs on islands. To clarify the potential role of natural selection, I quantified relative crypsis among morphs and age classes of water snakes by comparing the size of patches making up their colour patterns with the size of patches in island and mainland backgrounds. This analysis reveals that if unbanded morphs are more cryptic than intermediate and banded morphs on islands, it is only in the young-of-the-year age class. For older snakes on islands and for all snakes on the mainland, unbanded morphs are consistently less cryptic than intermediate and banded morphs. Given these results, the net direction of selection in island populations should depend on the intensity of predation on different age classes of snakes. Overall, selection may favour unbanded morphs (e.g. if predation occurs primarily on young-of-the-year), intermediate and banded morphs (e.g. if predation occurs primarily on older snakes), or be weak or absent (e.g. certain combinations of predation on young-of-the-year and older snakes). Using estimates of relative crypsis to guide reanalysis of morph frequency data, I find support for the hypothesis that unbanded morphs are favoured by natural selection in island populations.     

COLOR-PATTERN VARIATION IN LAKE ERIE WATER SNAKES: THE ROLE OF GENE FLOW

In an effort to clarify the evolutionary processes influencing color-pattern variation in Lake Erie island water snake (Nerodia sipedon) populations, rates of gene flow among island and mainland populations were estimated from patterns of allozymic variation detected using electrophoresis. Rates of gene flow were high with Nm, the number of migrants per generation, averaging 25.5 among island sites, 9.2 between the Ontario mainland and the islands, and 3.6 between the Ohio mainland and the islands. Based on estimates of current population size from mark-recapture work and of past population size extrapolated from the extent of shoreline habitat, values of m between island and mainland populations ranged from 0.0008–0.01. Synthesis of estimates of the rate of gene flow with information on inheritance of color pattern, the strength of natural selection, and population history supports the hypothesis that color-pattern variation in island populations results from a balance between gene flow and natural selection. However, depending on the mode of inheritance of color pattern, stochastic processes such as drift may have been important in the initial stages of differentiation between island and mainland populations.     

COLOR PATTERN POLYMORPHISM IN THE LAKE ERIE WATER SNAKE,NERODIA SIPEDON INSULARUM

Populations of the water snake, Nerodia sipedon, on islands in western Lake Erie are polymorphic for color pattern. These populations include banded, intermediate, and unbanded morphs while surrounding mainland populations consist solely of the banded morph. The hypothesis that this polymorphism is maintained by strong selection and migration pressures is widely accepted. Unbanded morphs are apparently more cryptic along island shorelines while banded morphs are more cryptic on the mainland. Migration of banded morphs from the mainland explains their persistence in island populations. Data collected in a capture-mark-recapture program on six islands provide no evidence of differential selection among morphs; morph frequencies do not differ among age classes, between once-captured and multiply-captured snakes, or between scarred and unscarred snakes. Furthermore, herring gulls, the most common snake predators in the island area, appear to detect banded and unbanded model snakes with equal ease. High site fidelity of water snakes and the distribution of morphs among islands suggest that migration from the mainland is not common. However, islands close to each other are similar in morph frequency, and water snakes have colonized islands elsewhere in the Great Lakes, indicating that some migration does occur. Recently, the frequency of banded morphs has increased in island populations while adult population sizes have declined. This increase in banded morphs is interpreted as reflecting an increased impact of migration from the mainland into these reduced populations. One scenario for the evolution and maintenance of this polymorphism is that selection was important in establishing unbanded morphs in island populations as they became isolated from the mainland. As populations declined to their present size, the impact of migration from the mainland increased and is now swamping the effect of selection. Further declines in island population size may result in fixation of the banded morph.     

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.     

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.     

Gene flow and melanism in Lake Erie garter snake populations

Melanistic garter snakes ( Thamnophis sirtalis ) are unusually common near Lake Erie, apparently because selection for thermoregulatory ability in cool lake-shore habitats (which favours melanistic morphs) outweighs selection for crypsis (which favours striped morphs). However, morph frequencies are highly variable among sites, suggesting that random genetic drift also influences colour pattern. In an effort to better understand the evolutionary processes influencing garter snake colour patterns, we estimated Fx and Nm (the number of migrants per generation) among island and mainland populations from patterns of allozymic variation detected using electrophoresis. Estimates of Nm were high, ranging from 2.7 to 37.6 between pairs of study sites and making it unlikely that differences in morph frequencies among sites were solely the result of random genetic drift. Furthermore, differences in F _st estimates between colour pattern (a one-locus two-allele trait) and allozyme loci suggest that colour pattern alleles are not in Hardy-Weinberg equilibrium, most likely as a result of natural selection. Comparison of allozymic data from Lake Erie with those from more distant sites suggests that gene flow occurs over long distances in T. sirtalis.     

Opposing fitness consequences of colour pattern in male and female snakes

Local populations of the adder, Vipera berus, are polymorphic for dorsal colour pattern, containing both melanistic (black) and zig-zag patterned individuals. Colour patterns in snakes influence crypsis and thermoregulatory capacity and therefore may be subjected to natural selection. To find an explanation for the maintenance of this polymorphism I examined temporal and spatial variation in morph frequency, and tested for differential selection among morphs using data from a six year capture-mark-recapture study. The data derive from six groups of islands in the Baltic Sea off the Swedish east coast, two mainland localities near the coast, and one inland locality. Morph frequency did not change over time within a population but varied among populations: melanistic individuals were not found at the inland locality, but comprised from 17 to 62% of the coastal and island populations. Adders frequently moved between islands within a group, but the tendency to disperse was independent of morph. These results suggest that the polymorphism is stable and maintained by a deterministic process. Scar frequency was twice as high among melanistic as among zig-zag snakes, and melanistic individuals were easier to capture, indicating that predation may be higher on the melanistic morph. Colour morphs did not differ in body size, but analysis of recapture data shows evidence for differential survival among morphs. Zig-zag males survived better than melanistic males, but the relative survival rates of morphs were reversed in females. This difference was consistent through time and may be due to sexual differences in behaviour, with melanism increasing predation intensity when associated with male but not with female behaviour. Opposing fitness consequences of colour pattern in the two sexes may help maintain colour polymorphism within populations of Vipera berus.     

Body-size evolution on islands: are adult size variations in tiger snakes a nonadaptive consequence of selection on birth size?

Mean adult size has been used as the traditional measure of body size to explain trends of insular gigantism and dwarfism in a wide array of taxa. However, patterns of variation in body size at birth have received surprisingly little attention, leaving open the possibility that adult body-size differences are nonadaptive consequences of selection acting on neonate body size. Here I used an empirical and correlative approach to test this hypothesis in a mosaic of 12 island and mainland snake populations in Australia. Data collected on 597 adult and 1,084 neonate tiger snakes showed that (1) both adult and neonate mean body sizes varied strongly across populations; (2) prey diversity and size convincingly explained birth-size variations: birth size-notably, gape size-correlated with prey size; (3) neonate snout-vent length was significantly correlated with neonate gape size; and (4) neonate snout-vent length was significantly correlated with adult snout-vent length. Postnatal growth rates recorded under common-garden conditions differed across populations and were correlated with mean prey size. These data collectively suggest that (1) prey size is the main driver for the evolution of body size at birth in gape-limited predators, (2) adult size variations may reflect selective forces acting on earlier life stages, and (3) adult size variations may also reflect resource availability during ontogeny (notably, prey diversity).     

Selection underlies phenotypic divergence in the insular Azores woodpigeon

The study of phenotypic evolution in island birds following colonization is a classic topic in island biogeography. However, few studies explicitly test for the role of selection in shaping trait evolution in these taxa. Here, we studied the Azores woodpigeon (Columba palumbus azorica) to investigate differences between island and mainland populations, between females and males, and interactions between geographical origin and sex, by using spectrophotometry to quantify plumage colour and linear measurements to examine external and skeletal morphology. We further tested if selection explains the observed patterns by comparing phenotypic differentiation to genome‐wide neutral differentiation. Our findings are consistent with several predictions of morphological evolution in island birds, namely differences in bill, flight and leg morphology and coloration differences between island and mainland birds. Interestingly, some plumage and morphological traits that differ between females and males respond differently according to geographical origin. Sexual dimorphism in colour saturation is more pronounced in the mainland, but this is driven by selection on female plumage coloration. Differences in flight morphology between females and males are also more pronounced in the mainland, possibly to accommodate contrasting pressures between migration and flight displays. Overall, our results suggest that phenotypic differentiation between mainland and island populations leading to divergent sexual dimorphism patterns can arise from selection acting on both females and males on traits that are likely under the influence of natural and sexual selection.     

Island colonisation and the evolutionary rates of body size in insular neonate snakes

Island colonisation by animal populations is often associated with dramatic shifts in body size. However, little is known about the rates at which these evolutionary shifts occur, under what precise selective pressures and the putative role played by adaptive plasticity on driving such changes. Isolation time played a significant role in the evolution of body size in island Tiger snake populations, where adaptive phenotypic plasticity followed by genetic assimilation fine-tuned neonate body and head size (hence swallowing performance) to prey size. Here I show that in long isolated islands (>6000 years old) and mainland populations, neonate body mass and snout-vent length are tightly correlated with the average prey body mass available at each site. Regression line equations were used to calculate body size values to match prey size in four recently isolated populations of Tiger snakes. Rates of evolution in body mass and snout-vent length, calculated for seven island snake populations, were significantly correlated with isolation time. Finally, rates of evolution in body mass per generation were significantly correlated with levels of plasticity in head growth rates. This study shows that body size evolution occurs at a faster pace in recently isolated populations and suggests that the level of adaptive plasticity for swallowing abilities may correlate with rates of body mass evolution. I hypothesise that, in the early stages of colonisation, adaptive plasticity and directional selection may combine and generate accelerated evolution towards an ‘optimal'' phenotype.     

Large mainland populations of South Island robins retain greater genetic diversity than offshore island refuges

For conservation purposes islands are considered safe refuges for many species, particularly in regions where introduced predators form a major threat to the native fauna, but island populations are also known to possess low levels of genetic diversity. The New Zealand archipelago provides an ideal system to compare genetic diversity of large mainland populations where introduced predators are common, to that of smaller offshore islands, which serve as predator-free refuges. We assessed microsatellite variation in South Island robins (Petroica australis australis), and compared large mainland, small mainland, natural island and translocated island populations. Large mainland populations exhibited more polymorphic loci and higher number of alleles than small mainland and natural island populations. Genetic variation did not differ between natural and translocated island populations, even though one of the translocated populations was established with five individuals. Hatching failure was recorded in a subset of the populations and found to be significantly higher in translocated populations than in a large mainland population. Significant population differentiation was largely based on heterogeneity in allele frequencies (including fixation of alleles), as few unique alleles were observed. This study shows that large mainland populations retain higher levels of genetic diversity than natural and translocated island populations. It highlights the importance of protecting these mainland populations and using them as a source for new translocations. In the future, these populations may become extremely valuable for species conservation if existing island populations become adversely affected by low levels of genetic variation and do not persist.     

Rapid prey‐induced shift in body size in an isolated snake population (Notechis scutatus,Elapidae)

Abstract Geographic divergence in phenotypic traits between long‐isolated populations likely has a genetic basis, but can phenotypic plasticity generate such divergence rapidly in the initial stages of isolation? Australian tiger snakes (Notechis scutatus, Elapidae) provide a classic model system for the evolution of body size: mean adult sizes are relatively invariant in mainland populations, but many offshore islands have dwarf or giant populations. Previous work has shown a genetic basis to this divergence in long‐isolated islands (>10 000 years), but what of the initial stages of this process? Human translocation of mainland snakes to Carnac Island 90 years ago gives us a unique opportunity to assess the proximate reasons for the giant size of Carnac Island animals compared with mainland conspecifics. Our data suggest a major role for phenotypic plasticity. Feeding trials on captive snakes from both island and mainland populations showed a strong link between food intake and growth rates, similar in the two populations. Snakes given abundant food grew much larger than we have ever recorded in the wild, demonstrating that observed mean body sizes are driven by food availability rather than genetic limits to growth. In combination with earlier work showing genetic divergence in growth rates in snakes from long‐isolated islands, our data suggest that geographical divergence in mean adult body sizes in this system initially is driven by a rapid shift due to phenotypic plasticity, with the divergence later canalized by a gradual accumulation of genetic differentiation.     

Oxygen consumption and the energetics of island-dwelling Florida cottonmouth snakes

We measured oxygen consumption (Vo(2)) to estimate standard metabolic rates (SMR) in cottonmouth snakes (Agkistrodon piscivorus conanti) from Seahorse Key and the adjacent peninsula of northern Florida. The island population is unusual because adult snakes feed on fish that are dropped by colonial nesting birds, and food resources are temporally limited relative to that of mainland populations. We found no differences in SMR between island and mainland snakes at any of four experimental temperatures (15 degrees -30 degrees C), suggesting that any adjustments to energy limitations involve other aspects of physiology or behavior. As with other viperid species, the SMR of cottonmouths is about one-half of that expected from interspecific allometric regressions previously reported for snakes generally. Allometric mass exponents of SMR averaged 0.76 and were not affected by temperature. We found that Vo(2) increased with temperature (Q(10) = 2.4-2.8) and was elevated 29% during scotophase compared with photophase. Neonates exhibited elevated Vo(2)compared with older juveniles of similar size, apparently due to assimilation of yolk that is present in the neonatal gut. In adult snakes, specific dynamic action (SDA) following feeding resulted in four- to eightfold increases in Vo(2), with magnitude and duration related positively to relative meal size. The total energy devoted to SDA increased with meal size and averaged 32.8%+/-4.4% of total ingested energy. We estimate that a nonreproductive 500-g adult cottonmouth at Seahorse Key uses 3,656 kJ of assimilated energy annually for maintenance and activity, which requires ingestion of approximately 1 kg of fish.     

Evolution of color variation in dragon lizards: quantitative tests of the role of crypsis and local adaptation

Abstract Many animal species display striking color differences with respect to geographic location, sex, and body region. Traditional adaptive explanations for such complex patterns invoke an interaction between selection for conspicuous signals and natural selection for crypsis. Although there is now a substantial body of evidence supporting the role of sexual selection for signaling functions, quantitative studies of crypsis remain comparatively rare. Here, we combine objective measures of coloration with information on predator visual sensitivities to study the role of crypsis in the evolution of color variation in an Australian lizard species complex (Ctenophorus decresii). We apply a model that allows us to quantify crypsis in terms of the visual contrast of the lizards against their natural backgrounds, as perceived by potential avian predators. We then use these quantitative estimates of crypsis to answer the following questions. Are there significant differences in crypsis conspicuousness among populations? Are there significant differences in crypsis conspicuousness between the sexes? Are body regions “exposed” to visual predators more cryptic than “hidden” body regions? Is there evidence for local adaptation with respect to crypsis against different substrates? In general, our results confirmed that there are real differences in crypsis conspicuousness both between populations and between sexes; that exposed body regions were significantly more cryptic than hidden ones, particularly in females; and that females, but not males, are more cryptic against their own local background than against the background of other populations. Body regions that varied most in contrast between the sexes and between populations were also most conspicuous and are emphasized by males during social and sexual signaling. However, results varied with respect to the aspect of coloration studied. Results based on chromatic contrast (“hue’ of color) provided better support for the crypsis hypothesis than did results based on achromatic contrast (“brightness’ of color). Taken together, these results support the view that crypsis plays a substantial role in the evolution of color variation and that color patterns represent a balance between the need for conspicuousness for signaling and the need for crypsis to avoid predation.     

Convergence in reduced body size,head size,and blood glucose in three island reptiles

Many oceanic islands harbor diverse species that differ markedly from their mainland relatives with respect to morphology, behavior, and physiology. A particularly common morphological change exhibited by a wide range of species on islands worldwide involves either a reduction in body size, termed island dwarfism, or an increase in body size, termed island gigantism. While numerous instances of dwarfism and gigantism have been well documented, documentation of other morphological changes on islands remains limited. Furthermore, we lack a basic understanding of the physiological mechanisms that underlie these changes, and whether they are convergent. A major hypothesis for the repeated evolution of dwarfism posits selection for smaller, more efficient body sizes in the context of low resource availability. Under this hypothesis, we would expect the physiological mechanisms known to be downregulated in model organisms exhibiting small body sizes due to dietary restriction or artificial selection would also be downregulated in wild species exhibiting dwarfism on islands. We measured body size, relative head size, and circulating blood glucose in three species of reptiles—two snakes and one lizard—in the California Channel Islands relative to mainland populations. Collating data from 6 years of study, we found that relative to mainland population the island populations had smaller body size (i.e., island dwarfism), smaller head sizes relative to body size, and lower levels of blood glucose, although with some variation by sex and year. These findings suggest that the island populations of these three species have independently evolved convergent physiological changes (lower glucose set point) corresponding to convergent changes in morphology that are consistent with a scenario of reduced resource availability and/or changes in prey size on the islands. This provides a powerful system to further investigate ecological, physiological, and genetic variables to elucidate the mechanisms underlying convergent changes in life history on islands.     

Selection in natural populations. IV. British housemice (Mus musculus)

In a number of populations of semi-commensal housemice in England and Wales, the older individuals are on the average larger and more variable in molar width than the younger individuals of the same populations. Several environmental variables have been shown to be unimportant with respect to these within-population differences, and it therefore seems likely that the differences in this age-invariant character are due to directional and destabilizing natural selection. The mean selection intensity is about 0.21 on the variance and about 0.09 on the mean. There is heterogeneity among populations, but not between sexes, in this selection; the amount of selection on at least the mean is related to the kind of food eaten. This selection is absent on an island off Wales, but there a discrete dental polymorphism present in 1957 was lost after a severe reduction in population size in 1959.     

CEPAEA NEMORALIS (L.) IN THE CHANNEL ISLANDS: ISLAND HISTORIES AND GENETIC VARIATION

Populations of C. nemoralis from the Channel Islands show strongbetween-island variation in their shell colour and banding polymorphism,despite the general similarity of the habitats sampled on eachisland. Morph-frequency distributions do not, in general, matchthose found in similar open habitats in England and Wales inregions where visual selection for crypsis is effective. Somechanges have occurred over the last 30 years, reflecting changingpatterns of land-use. A detailed comparison is made with C.nemoralis populations on the Isles of Scilly, where inter-islanddifferences are much less marked, but where morph-frequencydistributions are even more unlike those found in similar habitatson the British mainland. These patterns can be interpreted interms of rising Holocene sea-levels: the Scillies were probablycolonized once, when a single larger island existed, while manyof the Channel Islands have a history of separate isolationsfrom differing parts of the French mainland. Population historieshave an effect on present genetic make-up, even for loci onwhich selection can and does operate. (Received 20 January 1997; accepted 12 May 1997)     

Allelic diversity at the Mhc-DQA locus of woodmouse populations (Apodemus sylvaticus) present in the islands and mainland of the northern Mediterranean

Aim Polymorphism at neutral markers and at MHC loci in rodent populations living on islands is generally low. The main genetic factors that may contribute to a reduced level of genetic variability are genetic drift, reduced gene flow and founder events. We investigated the pattern of polymorphism at the second exon of the Mhc‐DQA gene in island populations of Apodemus sylvaticus and in their mainland counterparts to investigate the pattern of MHC polymorphism in a phylogeographical context and to assess the impact of insularity on diversity at this locus. Location Eight north Mediterranean populations of Apodemus sylvaticus were studied, including five island populations (Majorca, Minorca, Porquerolles, Port‐Cros and Sicily) and three mainland populations. Methods cDNA sequencing and nucleotide sequences analyses. Synonymous and non‐synonymous substitutions were examined at the PBR and non‐PBR sites. The DQA allelic distribution in populations was compared with the woodmouse phylogeography. Results This study presents novel DQA alleles. High polymorphism of the DQA locus is recorded in natural populations of A. sylvaticus with 13 alleles being widely distributed irrespective of the geographical origin and palaeoclimatic history of populations. The DQA locus does not show the expected pattern for non‐synonymous substitutions at the PBR sites. However, island populations show a weak loss of polymorphism in comparison with their mainland counterparts. Main conclusions The DQA locus in the woodmouse seems to be subject to weak selection and does not allow resolution of phylogeographical relationships among European woodmouse populations. The presence of at least three alleles in island populations and the maintenance of five alleles between the two European lineages over 1.5 Myr suggest that balancing selection may act within populations, and more precisely within island populations, to maintain genetic variability. This study shows that phylogeographical studies are a prerequisite for any genetic investigation of selected genes in natural populations.     

Adult-young interactions in island and mainland populations of the deermouse Peromyscus maniculatus

Summary The demographic and ecological characteristics of island populations of small mammals have received increasing attention in recent years, but few studies have compared the behavioral characteristics of island populations with those of mainland populations. Behavior is considered an important variable because it is believed by many to be a crucial factor affecting the population dynamics and demography of natural populations. In particular, among many species of rodents, the social behavior of adults towards juveniles is cited as an important factor influencing dispersal patterns and population regulation. The present study compares social interactions between adults and juveniles of island and mainland populations of the deermouse Peromyscus maniculatus, and attempts to relate differences in behavior to the demographic differences between the two populations. Adult mice were trapped on the mainland of British Columbia and on one of the Gulf Islands off the British Columbia coast, and allowed to breed in the laboratory. Male and female juveniles from both populations were then tested with their own parents and with unrelated male and female adults. The results demonstrate that island adults show almost no aggression towards either own or unrelated young. Mainland adults likewise show little aggression towards their own young, but a proportion of the population, consisting of both male and female adults, shows severe aggression towards unrelated juveniles of both sexes. These results suggest four major conclusions: 1) behavior may be the mechanism responsible for the demographic differences reported for these island and mainland populations; 2) female aggression may be a more important factor in deermouse population dynamics than has been previously recognized; 3) since parents show little aggression towards their own young, adult aggression may be a significant factor in juvenile mortality and emigration only after juveniles have initiated dispersal away from their natal sites; and 4) adult aggression controls the number of both male and female juveniles which are recruited into the population.     

Body Size Evolution in Insular Speckled Rattlesnakes (Viperidae: Crotalus mitchellii)

Background

Speckled rattlesnakes (Crotalus mitchellii) inhabit multiple islands off the coast of Baja California, Mexico. Two of the 14 known insular populations have been recognized as subspecies based primarily on body size divergence from putative mainland ancestral populations; however, a survey of body size variation from other islands occupied by these snakes has not been previously reported. We examined body size variation between island and mainland speckled rattlesnakes, and the relationship between body size and various island physical variables among 12 island populations. We also examined relative head size among giant, dwarfed, and mainland speckled rattlesnakes to determine whether allometric differences conformed to predictions of gape size (and indirectly body size) evolving in response to shifts in prey size.

Methodology/Principal Findings

Insular speckled rattlesnakes show considerable variation in body size when compared to mainland source subspecies. In addition to previously known instances of gigantism on Ángel de la Guarda and dwarfism on El Muerto, various degrees of body size decrease have occurred frequently in this taxon, with dwarfed rattlesnakes occurring mostly on small, recently isolated, land-bridge islands. Regression models using the Akaike information criterion (AIC) showed that mean SVL of insular populations was most strongly correlated with island area, suggesting the influence of selection for different body size optima for islands of different size. Allometric differences in head size of giant and dwarf rattlesnakes revealed patterns consistent with shifts to larger and smaller prey, respectively.

Conclusions/Significance

Our data provide the first example of a clear relationship between body size and island area in a squamate reptile species; among vertebrates this pattern has been previously documented in few insular mammals. This finding suggests that selection for body size is influenced by changes in community dynamics that are related to graded differences in area over what are otherwise similar bioclimatic conditions. We hypothesize that in this system shifts to larger prey, episodic saturation and depression of primary prey density, and predator release may have led to insular gigantism, and that shifts to smaller prey and increased reproductive efficiency in the presence of intense intraspecific competition may have led to insular dwarfism.