Reproductive isolating mechanisms between two sympatric sibling species of sea snakes

Mechanisms that maintain species isolation within sympatric congeners have attracted analysis in many kinds of organisms, but not in snakes. We studied two sibling species of amphibious sea snakes (Laticauda colubrina and L. frontalis) on the island of Efate, in the Pacific Ocean republic of Vanuatu. The two taxa are almost identical morphologically, except that L. colubrina grows much larger than L. frontalis. No natural hybrids have been reported, and geographic distributions of the two taxa suggest the possibility of sympatric speciation. Our fieldwork shows that the two taxa are often syntopic and overlap in breeding seasons. Behavioral studies in outdoor arenas show that the separation between these two taxa is maintained by species-specific cues that control male courtship. Males of both species courted conspecific females but not heterospecific females. The proximate mechanism driving this separation involves chemical cues. Adult females of both taxa possess distinctive lipids in the skin. Males directed courtship behavior (chin-pressing) to hexane-extracted samples of lipids from conspecific but not heterospecific females. Males of the dwarf species (L frontalis) were more selective courters than were those of the larger taxon (L. colubrina), perhaps because a preference for courting larger females means that L. colubrina males would be unlikely to court L. frontalis-sized (i.e., small) females even in the absence of pheromonal barriers.     

Three sympatric species of Neotoma: dietary specialization and coexistence

Summary Three sympatric species of Neotoma occur in the southern Great Basin Desert in northern Arizona. Observations and experiments from 1980–1984 focused on diet and den selection to determine to what extent woodrats partition available food and shelter. Analyses included microscopic inspection of feces from live-trapped animals, forage moisture content, and seasonal habitat utilization. Each species of woodrat was found to selectively forage on a different genus of the three evergreens on the study site: N. albigula was the only species to eat appreciable amounts of Yucca, while N. devia, specialized on Ephedra epidermis, and N. stephensi on Juniperus. Observations in the laboratory showed a linear dominance hierarchy where the larger species dominated smaller ones, i.e., N. albigula>N. stephensi>N. devia. To determine if such a hierarchy existed in the field, the behaviorally dominant species (N. albigula and N. stephensi) were continually removed (from a 25 ha experimental plot) over a 12-month period leaving only the subordinate species (N. devia) in the area. In these experiments, 40% of the dominant-species dens became occupied by 20 of the subordinate-species on the removal plot, whereas there were no interspecific den site (n=39) changes among species on the control plot. Removal of the two dominant Neotoma spp resulted in an increase of N. devia from a pre-removal high of 16 to a post-removal population of 26 individuals. These data suggest that while these woodrats may not compete for food, the subordinate species compete with the dominant species for den sites, prime dens being sequestered by the behaviorally dominant species.     

Trophic specialization and morphological divergence between two sympatric species in Lake Catemaco,Mexico

The association of morphological divergence with ecological segregation among closely related species could be considered as a signal of divergent selection in ecological speciation processes. Environmental signals such as diet can trigger phenotypic evolution, making polymorphic species valuable systems for studying the evolution of trophic‐related traits. The main goal of this study was to analyze the association between morphological differences in trophic‐related traits and ecological divergence in two sympatric species, Astyanax aeneus and A. caballeroi, inhabiting Lake Catemaco, Mexico. The trophic differences of a total of 70 individuals (35 A. aeneus and 35 A. caballeroi) were examined using stable isotopes and gut content analysis; a subset of the sample was used to characterize six trophic and six ecomorphological variables. In our results, we recovered significant differences between both species in the values of stable isotopes, with higher values of δ¹⁵N for A. caballeroi than for A. aeneus. Gut content results were consistent with the stable isotope data, with a higher proportion of invertebrates in A. caballeroi (a consumption of invertebrates ten times higher than that of A. aeneus, which in turn consumed three times more vegetal material than A. caballeroi). Finally, we found significant relationship between ecomorphology and stable isotopes (r = .24, p < .01), hence, head length, preorbital length, eye diameter, and δ¹⁵N were all positively correlated; these characteristics correspond to A. caballeroi. While longer gut and gill rakers, deeper bodies, and vegetal material consumption were positively correlated and corresponded to A. aeneus. Our results are consistent with the hypothesis that morphological divergence in trophic‐related traits could be associated with niche partitioning, allowing the coexistence of closely related species and reducing interspecific competition.     

The thermoregulatory strategy of two sympatric colubrid snakes affects their demography

Population dynamics of terrestrial vertebrates are affected by climatic fluctuations, notably in ectotherms. An understanding of the interaction between physiology and demographic processes is necessary to predict the impacts of climate change. Reptiles are particularly sensitive to temperature, but only a few studies have explored the relationship between thermoregulatory strategy and demography in these animals. Using 12 years of mark-recapture data on two sympatric colubrid snakes (Hierophis viridiflavus and Zamenis longissimus), we tested whether demographic parameters are influenced by contrasted thermoregulatory strategies. The thermophilic and conspicuous species (H. viridiflavus) grew faster than the thermoconforming and secretive species (Z. longissimus), and this difference was most pronounced in open habitats, suggesting that the metabolic benefits associated with high thermal preferences depend on environmental factors at small spatial scales. Survival varied annually in both species, but was not lower in H. viridiflavus despite a higher degree of exposure. In Z. longissimus, survival was negatively affected by low temperatures during the active season, possibly underlying an exposure trade-off.     

Thermal benefits of artificial shelters in snakes: A radiotelemetric study of two sympatric colubrids

1.

In temperate climates, reptiles face constraining thermal conditions, and thus tradeoff predator avoidance against thermoregulatory requirements.     

The cephalic vascular anatomy of three species of sea snakes

The primary and secondary elements of the cephalic vascular system in some sea snakes are similar to those of the generalized ophidian pattern. The three species examined in this study revealed only minor variations in vascular morphology; these variations appear to be correlated with myological differences among the three species. For example, in Hydrophis melanocephalus it appears that the depressor mandibulae artery is displaced by the cranially expanded insertion of the semi-spinalis and spinalis muscles. A preliminary hypothesis is put forth that explains the apparent constancy of the cephalic vascular system of ophidians in terms of possible constraints due to cranial kinesis.     

Thermal strategies and energetics in two sympatric colubrid snakes with contrasted exposure

The thermoregulatory strategy of reptiles should be optimal if ecological costs (predation risk and time devoted to thermoregulation) are minimized while physiological benefits (performance efficiency and energy gain) are maximized. However, depending on the exact shape of the cost and benefit curves, different thermoregulatory optima may exist, even between sympatric species. We studied thermoregulation in two coexisting colubrid snakes, the European whipsnake (Hierophis viridiflavus, Lacépède 1789) and the Aesculapian snake (Zamenis longissimus, Laurenti 1768) that diverge markedly in their exposure, but otherwise share major ecological and morphological traits. The exposed species (H. viridiflavus) selected higher body temperatures (~30°C) than the secretive species (Z. longissimus, ~25°C) both in a laboratory thermal gradient and in the field. Moreover, this difference in body temperature was maintained under thermophilic physiological states such as digestion and molting. Physiological and locomotory performances were optimized at higher temperatures in H. viridiflavus compared to Z. longissimus, as predicted by the thermal coadaptation hypothesis. Metabolic and energetic measurements indicated that energy requirements are at least twice higher in H. viridiflavus than in Z. longissimus. The contrasted sets of coadapted traits between H. viridiflavus and Z. longissimus appear to be adaptive correlates of their exposure strategies.     

Genetic monandry in 6 viviparous species of true sea snakes

Using a suite of 10 highly variable microsatellite loci, we conducted genetic paternity analyses for 76 embryos in the broods of 12 pregnant females representing 6 viviparous species of true sea snakes (Hydrophis clade) in the family Elapidae. To our surprise, we uncovered no evidence for multiple paternity within any of the clutches despite the fact that the genetic markers showed high intraspecific heterozygosities and as many as 20 conspecific alleles per locus. This outcome stands in sharp contrast to the rather high (but also variable) frequency of multiple paternity previously reported in many other reptilian species. However, because our study appears to be the first assessment of genetic parentage for any sea snake species (and indeed for any member of the elapid clade), we can only speculate as to whether this apparent monandry by females is a broader phylogenetic characteristic of elapid snakes or whether it might relate somehow to the sea snakes' peculiar lifestyle that uniquely combines viviparity with a marine existence.     

Cross-incompatibility between two sympatric polyploid Solanum species

Summary Preliminary results from a large number of reciprocal crosses between the closely related sympatric species S. gourlayi Hawkes (2n=4x=48) and S. oplocense Hawkes (2n=6x=72) indicated that they are difficult to hybridize. Pollen-pistil incompatibility barriers were detected via fluorescent microscopy. The cross incompatibility reaction occurred at three sites in 6x×4x crosses; on the stigma, in the first one-third of the style, and in the first two-thirds of the style. In the reciprocal 4x×6x crosses the incompatibility reaction invariably occurred in the ovary. Backcrosses of interspecific pentaploid hybrids (that were occasionally formed) to both parental populations were fully compatible, partially compatible, and fully incompatible with three sites of cross-incompatibility reaction similar to those observed in 6x×4x crosses, respectively. Both polyploid species were found to be selfcompatible, whereas their F1 hybrids were found to be self-incompatible. An hypothesis based on interactions of dominant cross-incompatibility (CI) genes in pistils and dominant specific complementary genes in pollen grains is postulated to explain these observations. The cross-incompatibility system that appears to be operating in nature between 4x S. gourlayi and 6x S. oplocense provides a way for gene exchange between sympatric populations without threatening the identity of either species.     

Temperature‐induced multi‐species cohort effects in sympatric snakes

In reptiles, reproductive maturity is often determined by size rather than age. Consequently, growth early in life may influence population dynamics through effects on generation time and survival to reproduction. Because reproductive phenology and pre‐ and post‐natal growth are temperature dependent, environmental conditions may induce multi‐species cohort effects on body size in sympatric reptiles. I present evidence of this using 10 years of neonatal size data for three sympatric viviparous snakes, Dekay''s Brown snakes (Storeria dekayi), Red‐bellied Snakes (S. occipitomaculata), and Common Garter snakes (Thamnophis sirtalis). End‐of‐season neonatal size varied in parallel across species such that snout–vent length was 36%–61% greater and mass was 65%–223% greater in years when gestating females could achieve higher April–May (vs. June–July or August–September) operative temperatures. Thus, temperature had a larger impact during follicular enlargement and ovulation than during gestation or post‐natal growth. Multi‐species cohort effects like these may affect population dynamics and the magnitude of these effects may increase with climate change.     

Right-handed snakes: convergent evolution of asymmetry for functional specialization

External asymmetry found in diverse animals bears critical functions to fulfil ecological requirements. Some snail-eating arthropods exhibit directional asymmetry in their feeding apparatus for foraging efficiency because dextral (clockwise) species are overwhelmingly predominant in snails. Here, we show convergence of directional asymmetry in the dentition of snail-eating vertebrates. We found that snakes in the subfamily Pareatinae, except for non-snail-eating specialists, have more teeth on the right mandible than the left. In feeding experiments, a snail-eating specialist Pareas iwasakii completed extracting a dextral soft body faster with fewer mandible retractions than a sinistral body. The snakes failed in holding and dropped sinistral snails more often owing to behavioural asymmetry when striking. Our results demonstrate that symmetry break in dentition is a key innovation that has opened a unique ecological niche for snake predators.     

Biosystematics of two sympatric species ofEucharis (Amaryllidaceae)

Eucharis candida andE. formosa are two often sympatric species of bulbous geophytes restricted to neotropical rain forest understory. The species are most common in eastern Ecuador, and are the only two east Ecuadorean species of the genus found north of the Pastaza valley. Data from phenetic, karyotypic, and preliminary isozyme electrophoretic analyses of both species are represented. The species are distinguishable phenetically and karyologically, but isozyme-based relationships are more complex. Phenetic resolution of the isozyme phenotypes supports recognition of two species in Ecuador. A Peruvian isolate ofE. formosa, though not morphologically distinct, shows both allozyme and chromosomal divergence from Ecuadorean populations. Cladistic relationships based on overall allozyme data do not support species distinction, but a novel electrophoretic phenotype for glutathione reductase is shared only by individuals ofE. candida. An apparent geographic component within the monophyletic groups resolved in the cladogram suggests that some degree of gene flow between these two species has been maintained without the complete loss of morpholgoical species identity. This may have been mediated either by artificial population structures due to a probable long history of cultivation, or via Pleistocene refugia effects. Both species may have originated in eastern Ecuador from a common ancestral population which has since radiated outward, perhaps several times.     

北京地区同域分布的普通和黑头种群密度比较

2007年4月至7月,2007年12月至2008年1月,分别在北京市门头沟区小龙门森林公园、百花山自然保护区以及延庆县松山自然保护区,对同域分布的普通(Sitta europaea)和黑头(Sitta villosa)两种同域分布的类种群密度进行了调查。普通在小龙门森林公园、百花山自然保护区和松山自然保护区的种群密度分别是40.92、96.67只/km2和16.67只/km2。而黑头在上述3个调查区域的种群密度分别是2.03、16.67只/km2和23.33只/km2。普通的总体平均密度(51.4只/km2)高于黑头(20.68只/km2)。两种类分布的海拔和林型存在着差异。普通在海拔高度1070—1350 m的分布比较多。在海拔段1070—1250 m区间,普通的数量随海拔高度的增加呈递增的趋势。而黑头在海拔600—800 m分布较多,随着海拔高度的增加,种群数量呈递减趋势。普通主要分布在阔叶林和针阔混交林中,而黑头主要分布在针叶林中。     

Expression pattern of three-finger toxin and phospholipase A2 genes in the venom glands of two sea snakes, Lapemis curtus and Acalyptophis peronii : comparison of evolution of these toxins in land snakes, sea kraits and sea snakes

Background  

Snake venom composition varies widely both among closely related species and within the same species, based on ecological variables. In terrestrial snakes, such variation has been proposed to be due to snakes' diet. Land snakes target various prey species including insects (arthropods), lizards (reptiles), frogs and toads (amphibians), birds (aves), and rodents (mammals), whereas sea snakes target a single vertebrate class (fishes) and often specialize on specific types of fish. It is therefore interesting to examine the evolution of toxins in sea snake venoms compared to that of land snakes.     

Contrasting life-history traits of two sympatric smooth-hound species: implication for vulnerability

In this study, life-history traits (maximum and average size, size at maturity and fecundity) of two congeneric smooth-hounds, Mustelus mustelus and Mustelus punctulatus, which share a geographical distribution and experience a similar fishing exploitation, were estimated and compared between species. The results indicated a lower maximum and average size, a lower size at maturity and a higher fecundity in M. punctulatus compared with those in M. mustelus. Considering that these two species co-occur in the same areas and are caught by the same fishing gears, the results indicate a higher vulnerability to exploitation of M. mustelus compared with that of M. punctulatus.     

Foliar carbon isotope discrimination in Larix species and sympatric evergreen conifers: a global comparison

Larches (Larix spp.), deciduous conifers, occur in the northern hemisphere in cold-temperate and boreal climates – an environment normally thought to favor evergreen tree species. We compare foliar carbon isotope discrimination (Δ), instantaneous water use efficiency, total foliar nitrogen concentration, and specific leaf area (for a subset of sites) between Larix spp. and co-occurring evergreen conifers at 20 sites throughout the natural range of larches. Except for Larix occidentalis in the xeric Intermountain West, USA, Δ is significantly (P < 0.05) greater for larches than co-occurring evergreen conifers at 77% of the sites, suggesting that larches use water less efficiently. At elevations greater than 3000 m, the Δ of Larix spp. and co-occurring conifers converge, suggesting that water is not the limiting resource. Foliar nitrogen concentration and specific leaf area are two ecophysiological characteristics that are positively correlated with high photosynthetic capacity. Foliar nitrogen concentration is significantly greater for larches than evergreen conifers at 88% of the sites and specific leaf area is approximately three times greater for larches than co-occurring conifers. Future studies should examine the potential effect that global warming may have on the distribution of larch forests because the water use efficiency of larches is commonly less than co-occurring evergreen conifers and the boreal and high-latitude environments are likely to experience the greatest climate warming. Received: 23 May 1997 / Accepted: 28 October 1997     

Pollen competition as a reproductive isolating mechanism between two sympatric Hibiscus species

Differences in pollen tube growth rates (certation) between heterospecific (foreign) and conspecific pollen may strongly influence whether hybrid offspring are produced after mixed pollen loads are delivered to a stigma. For both members of a sympatric species pair, Hibiscus moscheutos and H. laevis, pollination by pure loads of foreign pollen resulted in fruit set that was not significantly different from conspecific pollination, indicating that pure loads of foreign pollen could readily result in hybrid offspring. However, the number of seeds per fruit from pure foreign pollinations was significantly less than that of pure conspecific pollination. Simultaneous mixed pollination resulted in a proportion of hybrid seeds (detected by an electrophoretic marker enzyme) that was significantly lower than expected based upon the capacity of foreign pollen to effect fertilization when applied in pure pollinations. After these 50/50% pollen mixtures were applied to stigmas, 8.0 and 7.4% hybrids were produced when H. moscheutos and H. laevis were the ovule parents, respectively. For these Hibiscus species, pollen competition appears to function as a barrier to hybridization that is of moderate intensity compared with similar barriers occurring between other recently studied sympatric species pairs.     

Relationship between spatial working memory performance and diet specialization in two sympatric nectar bats

Behavioural ecologists increasingly recognise spatial memory as one the most influential cognitive traits involved in evolutionary processes. In particular, spatial working memory (SWM), i.e. the ability of animals to store temporarily useful information for current foraging tasks, determines the foraging efficiency of individuals. As a consequence, SWM also has the potential to influence competitive abilities and to affect patterns of sympatric occurrence among closely related species. The present study aims at comparing the efficiency of SWM between generalist (Glossophaga soricina) and specialist (Leptonycteris yerbabuenae) nectarivorous bats at flowering patches. The two species differ in diet--the generalist diet including seasonally fruits and insects with nectar and pollen while the specialist diet is dominated by nectar and pollen yearlong--and in some morphological traits--the specialist being heavier and with proportionally longer rostrum than the generalist. These bats are found sympatrically within part of their range in the Neotropics. We habituated captive individuals to feed on artificial flower patches and we used infrared video recordings to monitor their ability to remember and avoid the spatial location of flowers they emptied in previous visits in the course of 15-min foraging sequences. Experiments revealed that both species rely on SWM as their foraging success attained significantly greater values than random expectations. However, the nectar specialist L. yerbabuenae was significantly more efficient at extracting nectar (+28% in foraging success), and sustained longer foraging bouts (+27% in length of efficient foraging sequences) than the generalist G. soricina. These contrasting SWM performances are discussed in relation to diet specialization and other life history traits.