Specific dynamic action and postprandial thermophily in juvenile northern water snakes,Nerodia sipedon

1. We measured oxygen consumption of seven juvenile Northern water snakes, Nerodia sipedon, at 3, 19, 41, and 89 h after feeding. 2. Oxygen consumption 19 h after eating was significantly higher than at 3, 41, and 89 h after feeding and the values at 3 h post-feeding were significantly higher than those at 41 and 89 h. 3. We monitored substrate temperature preference in nine juvenile N. sipedon at 2000, 0000, 0800, 1200, and 1600 h for 3 days. 4. After feeding and at night snakes selected warmer substrate temperatures.     

Genetic analysis of the mating system and opportunity for sexual selection in northern water snakes (Nerodia sipedon)

We used data collected over 3 years at two study sites to quantifythe rates and consequences of multiple paternity and to determinethe opportunity for selection on male and female northern watersnakes (Nerodia sipedon). We sampled litters from 45 femalesthat gave birth to 811 offspring. Using eight microsatelliteDNA loci (probability of exclusion of nonparental males >0.99), we assigned paternity to 93% of neonates from one studypopulation and 69% of neonates from the other population. Observationsof participation in mating aggregations predicted individual reproductive success poorly for two reasons. First, males regularlycourted nonreproductive females. Second, more than half ofall sexually mature males obtained no reproductive successeach year, despite the fact that many of them participatedin mating aggregations. The number of sires per litter ranged from one to five, with 58% of all litters sired by more thanone male. Multiple paternity increased with female size, apparentlyboth because bigger females mated with more males and becausethe larger litters of big females provide paternity opportunitiesto more males. Multiple paternity was also more prevalent inyears with shorter mating seasons. We detected no advantage to multiple paternity in reducing either the number of unfertilizedovules or stillborn young. Despite the majority of males siringno young each year, some males fathered young with as manyas three different females in one year. Male reproductive successincreased by more than 10 offspring for each additional mate,whereas female success increased by fewer than 2 offspring foreach additional mate. The opportunity for sexual selectionwas more than five times higher in males than females.     

Microgeographic population genetic structure in the northern water snake, Nerodia sipedon sipedon detected using microsatellite DNA loci

We describe the isolation and genetic characterization of eight microsatellite DNA loci from the northern water snake, Nerodia sipedon sipedon and use these loci to analyse levels of genetic differentiation between local (< 2 km apart) populations of these snakes in Ontario. These loci are variable, with expected heterozygosities ranging from 0.28 to 0.91, and can correctly exclude nonsires in parentage analyses with a high probability (0.998). Population analyses reveal significant deviation from expected heterozygosity levels for one population, probably a result of a null allele(s) at a single locus and small but significant levels of genetic differentiation among all three populations. This demonstrates that microgeographic genetic structure exists in this species, possibly due to limited dispersal.     

Histological aspects of natural eustrongyloid infections of the northern water snake, Nerodia sipedon

Encapsulated as well as unencapsulated fourth stage larvae of Eustrongylides sp. were found in four northern water snakes, Nerodia sipedon, from Virginia. Unencapsulated nematodes occurred within muscle, subcutaneous tissue and in the coelom. Encapsulated nematodes occurred in three forms: recently encapsulated, degenerating, or sclerotic; they were found on the mesentery, in muscle and subcutaneous tissue. Encapsulation was associated with muscular hypotrophy, fibroplasia and eosinophilia.     

Dynamic changes in body form during swimming in the water snake Nerodia sipedon

Video records of swimming water snakes show that during moderate to rapid swimming, the rear half to two-thirds of the trunk is compressed laterally, approaching the body form of some sea snakes. Body form of swimming snakes differed significantly from their shape when resting on a flat surface or when anesthetized and suspended in water. The extent of lateral flattening is positively correlated with swimming speed, a relationship generally supported by tests of trunk models in a flow tank. In Nerodia, the ability to temporarily flatten the trunk depends on kinetic costovertebral joints, a large compressible body cavity, and the absence of ventral skeletal support - features found in most snakes. Histological studies and manipulations of partially dissected preserved specimens showed that the resting angle of the ribs is maintained by localized elastic hypertrophy of the costovertebral capsular ligament. Trunk form during swimming in Nerodia is proposed to arise from anteromedial movement of the distal rib powered by deep muscles acting in concert with those proposed to generate undulation of the vertebral column.     

Sexual size dimorphism and male combat in snakes

Summary This paper reviews published literature on snakes to test the hypothesis that large male size, relative to female size, evolves because of the advantage it confers in male combat. Analysis of the data reveals a high correlation between the occurrence of male combat, and sexual dimorphism in which the male is the larger sex. This correlation holds (i) within the total sample of snake species (n=224), (ii) within the family Colubridae (n=134), and (iii) in a comparison between the eight families of snakes for which data are available. These results strongly support the hypothesis that large male size is an adaptation to intrasexual competition. The analysis also shows that females are larger than males in about 66% of snake species, that male combat is known in only about 15% of species, and that both sexual size dimorphism and the incidence of male combat tend to be distributed along taxonomic lines.     

Intraspecific variation in ejaculate traits of the northern watersnake (Nerodia sipedon)

Sperm competition occurs when the sperm of more than one male compete to fertilize the eggs of a female. In reptiles, sperm competition is particularly prevalent and is an important agent of sexual selection in males. Spermatogenesis in reptiles can be energetically expensive, suggesting that there may be costs to producing high-quality ejaculates. The northern watersnake Nerodia sipedon has a mating system characterized by aggregations in which a single female mates with multiple males, resulting in high levels of multiple paternities. Under these circumstances sperm competition is likely important, and selection should favour sperm and ejaculate traits that enhance a male's reproductive success. In this study, we examined intraspecific variation in ejaculate quality (sperm length, motility, sperm density, spermactocrit) in male watersnakes and determined whether ejaculate traits varied with body size and condition, using both size-corrected mass and haematocrit as indices of condition. We found large variation among males in all these traits, except for sperm length. Although there was significant variation in sperm length among males, the majority of variation in sperm length occurred within rather than among individuals. Males with high haematocrit had sperm that were less variable with respect to length, and large males produced ejaculates that were less concentrated with respect to sperm than small males. The lack of condition dependence of most ejaculate traits is consistent with previous studies that indicate that male reproductive effort in this species is generally not energy limited, perhaps because of opportunistic foraging during the mating season.     

Reduction in locomotor ability as a cost of reproduction in gravid snakes

Summary Recent studies suggest that lower survival among gravid squamate reptiles may be partially the result of decreased locomotor ability during gestation. In this study, we compared the speed and endurance of female garter snakes (Thamnophis marcianus), before, during, and after pregnancy. Gravid snakes had significantly lower locomotor performance than did non-gravid females, and performance varied among stages of gestation, reaching a minimum 0–6 weeks prior to parturition. Both number of offspring and relative clutch mass were inversely correlated with locomotor performance; as females increased these traits, locomotor ability decreased. If reduced locomotor performance results in greater risk of predation and/or lowered foraging ability, then natural selection (operating via differential mortality or feeding rates of gravid females) may result in important constraints on both clutch size and relative clutch mass in squamates.     

Sexual selection and adult male orangutan long calls

Long calls given by adult male orangutans (Pongo pygmaeus) have been hypothesized to mediate interindividual spacing among male orangutans and to attract female orangutans, to high-ranking mates over long distances. To test these two hypotheses I conducted observations of calling behaviour and a series of experimental playbacks of recorded long calls. The findings were consistent with the male-spacing hypothesis but did not support the mate-attraction hypothesis. Experimental playbacks demonstrate that long calls regulate spacing between males through an approach-avoidance system based on dominance relationships: the highest-ranking adult male approaches calls, while lower-ranking, males avoid calls. The results of playbacks also reveal that sexually active females do not move toward long calls, and therefore do not indicate that long calls function in attracting mates. This latter finding suggests that female mate choice may not have been an important selective factor during the evolution of long calls.     

Standard and digestive metabolism in the banded water snake, Nerodia fasciata fasciata

Estimating energy costs by respirometry is fundamental to many studies of the ecology, behavior and evolution of reptiles. However, traditional respirometry procedures seldom incorporate objective techniques for removal of outliers from estimates of metabolic parameters. We demonstrate how computer-automated respirometry equipment, which records many respiratory measurements over short intervals, can be coupled with mathematical procedures to produce robust estimates of pre- and post-prandial metabolism in banded water snakes (Nerodia fasciata fasciata). Standard metabolic rate of N. f. fasciata was estimated to be 1.21 ml O2/h (mass = 30.21 +/- 0.74 g) at 25 degrees C. After ingestion of a fish equaling 20% of their body mass, snakes exhibited a fivefold increase in metabolic rate with peak O2 consumption rate (VO2) reaching 6.5 ml O2/h. Total cost of digestion was 5.44 kJ, equivalent to approximately 21% of the energy in the meal. Repeated measurements of metabolism in the same individuals revealed that our methods yielded similar results, even when individuals exhibited different patterns of VO2 variation between respiratory trials. Our results underscore the importance of obtaining many VO2 measurements, coupled with objective removal of outlier values from estimates of metabolic rate, especially when metabolic values are to be interpreted in a comparative context.     

Resource allocation to growth,reproduction and survival in Gladiolus : The cost of male function

Theoretical models of life-history evolution assume trade-offs between present and future reproduction and/or survival. Models of the evolution of sex assume trade-offs between male function and female function. Generally, experiments designed to evaluate the cost of reproduction on other functions tend to ignore male function. The present work on Gladiolus takes into account simultaneously the different primary functions of the plant and separates sexual reproduction into one male component (pollen production) and one female component (seed production). The study of environmental (within-clone), between-clone and genetic correlations using strains of Gladiolus and principal component analysis show that trade-offs exist between male function, female function and survival, including both characters of plant vigour, perennation (corm production) and vegetative propagation (cormel production). Phenotypic correlations, using different species and species-hybrids, have been obtained which confirm these results. In particular, these results underline the importance of the impact of pollen production on the other functions.     

Sexual dimorphism and cost of reproduction in the dioecious shrub Lindera benzoin (Lauraceae)>

We examined sexual dimorphism in reproductive allocation and its effects on growth and subsequent reproduction in a natural population of the dioecious woody shrub Lindera benzoin (L.) Blume. In addition to comparing natural patterns of growth and reproduction in a marked population of 251 females and 87 males, we experimentally examined the effect of reduced reproduction on future growth and reproduction in female plants and examined sexual dimorphism in carbon/nutrient balance. Our results suggest that females of L. benzoin bear greater reproductive costs in terms of both biomass and minerals. These costs were measurable in terms of current biomass and allocation of nitrogen to reproduction, as well as subsequent growth, reproduction, and tissue carbon/nutrient balance. Based upon the results of a fruit-thinning experiment and path analysis, fruit production in 1991 appeared to have direct negative effects on fruit production in 1992, an effect that was not necessarily mediated through effects on plant growth. We discuss our results in the context of other intrinsic and extrinsic factors that can influence growth, reproductive costs, and mortality in this species.     

Good species despite massive hybridization: genetic research on the contact zone between the watersnakes Nerodia sipedon and N. fasciata in the Carolinas, USA

Genomic markers generated with the amplified fragment length polymorphism method revealed extensive, panmictic-like hybridization along the narrow contact zone between the water snakes Nerodia sipedon and Nerodia fasciata in the Carolinas, USA. However, asymmetric distributions of diagnostic markers between both species and low frequencies of backcrossed hybrids with a high value of interspecific mixture infer selection against certain genotypes. This is consistent with a pronounced genetic and morphological preponderance of N. fasciata characters in the hybrid zone. Despite massive hybridization within the contact zone, the existence of nearly fixed genetic markers and the potential inferiority of certain hybrid genotypes support the species status of the two taxa and corroborate known, but nondiagnostic differences in morphology and ecology. This study stretches the applicability of species concepts to cases, where the genetic compatibility between two closely related species is very high, yet, they still evolve and persist as independent entities.     

Sexual reproduction of Daphnia in a deep temperate reservoir: the phenology and genetics of male formation

Long-term persistence of a Daphnia population is allowed by the production of dormant stages, produced mostly through sexual reproduction. We investigated the spatio-temporal dynamics of male production in a spring population of Daphnia galeata in the reservoir, and compared the genetic structure of three groups within this population: male-, female-producing females, and adult males. With a fine resolution sampling design and the use of highly variable microsatellite markers we revealed that: (1) the spring period of male offspring production was delimited in time with minimum interannual variation to about 3 weeks, and in space to the upper 5-m water layer; (2) there were no remarkable changes in the clonal composition of male-producing females within the period of male production; (3) overall certain clones exhibited a higher tendency to produce male offspring and therefore the clonal structure of male-producing lineages was significantly different from that of female-producing lineages; and (4) the clonal structure of male-producing females was not significantly different from that of adult males occurring later in the reservoir. This suggests that males were not subjected to any significant selective forces till maturity and the male-producing females confer a long-term fitness advantage over female-producing females.     

Measuring the cost of reproduction

Gravid and barren Daphnia pulex were exposed to a variety of predators in laboratory aquaria. Small fish (guppies, sticklebacks and shiner fry) consistently preferred the gravid females, establishing the existence of a behavioural cost of reproduction. However, no such cost was associated with predation by more efficient visual predators (sunfish) or by nonvisual predators (hydras), and the excess of gravid females eaten by backswimmers was found to be attributable to their distribution in the water column. Moreover, the cost associated with predation by small fish was observed only when the Daphnia were presented against a light background, and was abolished when a dark background was substituted. In a further series of experiments with guppies we attempted to show that each egg added to the brood caused a decrease in survival; in two such experiments survival rate was related to body size but not to fecundity, while in a third the effect of body size did not appear, and a negative correlation between survival and fecundity could be demonstrated. Although these experiments unambiguously demonstrate a cost of reproduction they also illustrate the elusiveness of the phenomenom and emphasize the need to develop theories which specify the type and magnitude of costs generated by different ways of life.