How Did the Spider Cross the River? Behavioral Adaptations for River-Bridging Webs in Caerostris darwini (Araneae: Araneidae)

Interspecific coevolution is well described, but we know significantly less about how multiple traits coevolve within a species, particularly between behavioral traits and biomechanical properties of animals' "extended phenotypes". In orb weaving spiders, coevolution of spider behavior with ecological and physical traits of their webs is expected. Darwin's bark spider (Caerostris darwini) bridges large water bodies, building the largest known orb webs utilizing the toughest known silk. Here, we examine C. darwini web building behaviors to establish how bridge lines are formed over water. We also test the prediction that this spider's unique web ecology and architecture coevolved with new web building behaviors.We observed C. darwini in its natural habitat and filmed web building. We observed 90 web building events, and compared web building behaviors to other species of orb web spiders.Caerostris darwini uses a unique set of behaviors, some unknown in other spiders, to construct its enormous webs. First, the spiders release unusually large amounts of bridging silk into the air, which is then carried downwind, across the water body, establishing bridge lines. Second, the spiders perform almost no web site exploration. Third, they construct the orb capture area below the initial bridge line. In contrast to all known orb-weavers, the web hub is therefore not part of the initial bridge line but is instead built de novo. Fourth, the orb contains two types of radial threads, with those in the upper half of the web doubled. These unique behaviors result in a giant, yet rather simplified web. Our results continue to build evidence for the coevolution of behavioral (web building), ecological (web microhabitat) and biomaterial (silk biomechanics) traits that combined allow C. darwini to occupy a unique niche among spiders

Digging the optimum pit: Antlions, spirals and spontaneous stratification

Most animal traps are constructed from self-secreted silk, so antlions are rare among trap builders because they use only materials found in the environment. We show how antlions exploit the properties of the substrate to produce very effective structures in the minimum amount of time. Our modelling demonstrates how antlions: (i) exploit self-stratification in granular media differentially to expose deleterious large grains at the bottom of the construction trench where they can be ejected preferentially, and (ii) minimize completion time by spiral rather than central digging. Both phenomena are confirmed by our experiments. Spiral digging saves time because it enables the antlion to eject material initially from the periphery of the pit where it is less likely to topple back into the centre. As a result, antlions can produce their pits—lined almost exclusively with small slippery grains to maximize powerful avalanches and hence prey capture—much more quickly than if they simply dig at the pit’s centre. Our demonstration, for the first time to our knowledge, of an animal using self-stratification in granular media exemplifies the sophistication of extended phenotypes even if they are only formed from material found in the animal’s environment

Low Dose Isoflurane Exerts Opposing Effects on Neuronal Network Excitability in Neocortex and Hippocampus

The anesthetic excitement phase occurring during induction of anesthesia with volatile anesthetics is a well-known phenomenon in clinical practice. However, the physiological mechanisms underlying anesthetic-induced excitation are still unclear. Here we provide evidence from in vitro experiments performed on rat brain slices that the general anesthetic isoflurane at a concentration of about 0.1 mM can enhance neuronal network excitability in the hippocampus, while simultaneously reducing it in the neocortex. In contrast, isoflurane tissue concentrations above 0.3 mM expectedly caused a pronounced reduction in both brain regions. Neuronal network excitability was assessed by combining simultaneous multisite stimulation via a multielectrode array with recording intrinsic optical signals as a measure of neuronal population activity

Male pygmy hippopotamus influence offspring sex ratio

Pre-determining fetal sex is against the random and equal opportunity that both conceptus sexes have by nature. Yet, under a wide variety of circumstances, populations shift their birth sex ratio from the expected unity. Here we show, using fluorescence in situ hybridization, that in a population of pygmy hippopotamus (Choeropsis liberiensis) with 42.5% male offspring, males bias the ratio of X- and Y-chromosome-bearing spermatozoa in their ejaculates, resulting in a 0.4337±0.0094 (mean±s.d.) proportion of Y-chromosome-bearing spermatozoa. Three alternative hypotheses for the shifted population sex ratio were compared: female counteract male, female indifferent, or male and female in agreement. We conclude that there appears little or no antagonistic sexual conflict, unexpected by prevailing theories. Our results indicate that males possess a mechanism to adjust the ratio of X- and Y-chromosome-bearing spermatozoa in the ejaculate, thereby substantially expanding currently known male options in sexual conflict

Influence of fire prevention management strategies on the diversity of butterfly fauna in the eastern Pyrenees

Fire prevention management is becoming a necessity in many Mediterranean locations to regulate fire of natural or human origin. However, very few studies have determined the real effects of the strategies adopted on local fauna. Butterflies are sensitive to local changes and they can thus serve as indicators of environmental changes. Three different types of fire prevention management approaches in three different localities in the Eastern Pyrenees (France) were performed and the butterfly community composition was investigated. We show that of the 80 species of butterflies observed, 36 % can be considered as biological markers. An original objective treatment of data using hierarchical distance analysis combined with a neural network analysis (Self-Organizing Maps) was applied in this study. Our conclusions are that the overall number of species is maintained independently of the fire prevention type but that some important changes are observed among butterfly communities, with a clear reduction of the numbers of endemic/specialized species in favour of generalist ones for the two most drastic fire prevention management approaches studied here. The influence of such approaches is discussed on the basis of the conservation of Mediterranean species of Lepidoptera

Ultraviolet reflectance of spiders and their webs

To determine the reflectance of spider webs and spiders under ultraviolet (UV) light, spiders and their webs were photographed under normal (white) light and under UV light. It was found that all silks in araneoid webs reflect slightly more UV light than white light; i.e., they had a positive UV-brightness. However, the often cited, particularly high UV-brightness of stabilimenta could not be confirmed. Spiders differed in their UV-brightness, with most spiders reflecting less UV light than white light. Based on the knowledge of the visual system of insects and invertebrates it is suggested that the main function of stabilimenta is predator defense. However, drawing a final conclusion requires more knowledge on the way potential predators and prey perceive spiders, spider webs and stabilimenta

Inbreeding and outbreeding in African rhinoceros species

Effective breeding strategies in ex situ conservation require an optimal balance between inbreeding and outbreeding, as both can lead to a decrease in population fitness. Thus the optimization of breeding strategies to maintain genetic diversity entails a profound knowledge of the actual situation (including conservation units). This study examines the consequences of inbreeding and outbreeding in captive populations of two threatened species, the white and the black rhinoceros (Ceratotherium simum and Diceros bicornis), based on data from recent studbooks (2004). We also assessed the conservation units of the black rhinoceros, whose classification into subspecies remains a matter of discussion. Theory predicts that juvenile mortality increases with increasing degree of inbreeding. We calculated inbreeding coefficients and examined possible correlations with juvenile mortality rates. To assess the conservation units of the black rhinoceros, we compared juvenile mortality rates of outbred animals with those of non-outbred animals and additionally performed a geographical distance analysis. With both approaches we aimed to draw breeding borderlines between putative conservation units to preserve the genetic diversity. Our assessment of the current inbreeding situation based on records in international studbooks was severely limited by small sample sizes in both the white and the black rhinoceros. For the same reason we could not evaluate the conservation units in the black rhinoceros. Nonetheless, we conclude that inbreeding and outbreeding must be followed closely in both species, as their consequences can be severe

Laboratory methods for maintaining and studying web-building spiders

Web-building spiders are an important model system to address questions in a variety of biological fields. They are attractive because of their intriguing biology and because they can be fairly easily collected and maintained in the laboratory. However, the only published instructions for working with web-building spiders are somewhat Outdated and not easily accessible. This paper aims to provide an up-to-date guide on how to best collect, keep and study web-building spiders. In particular, it describes how to obtain spiders by capturing them or by raising them from cocoons, how to keep and feed spiders in the laboratory and how to encourage them to build webs. Finally it describes how to document and analyze web building and web structure

Effects of experimental small-scale grassland fragmentation on the extent of grazing damage in Trifolium repens seedlings

We examined the extent of grazing damage to seedlings of white clover (Trifolium repens) in experimentally fragmented calcareous grasslands. The experimental set-up consisted of 12 blocks distributed over three sites. Each block contained one large (4.5x4.5 m), one medium (1.5x1.5 m) and two small (0.5x0.5 m) fragments and corresponding control plots (Zschokke et al 2000). Fragmentation was maintained by frequently mowing the area between the fragments. A fence around the study sites excluded large herbivores. Seedlings of T. repens planted in petri dishes were exposed to invertebrate herbivores in fragments and control plots for 2-3 days both in summer and autumn. In summer - unlike autumn - grazing intensity expressed as leaf biomass loss per petri dish and day was 43% lower in fragments than in control plots. Grazing intensity in summer was positively correlated with relative gastropod density (number of individuals from 12 species), but not with relative grasshopper density (number of individuals from 15 species). In autumn, grazing intensity was correlated neither with gastropod nor with grasshopper density. It is suggested that a combination of factors can explain the seasonal difference in fragmentation effect on grazing intensity. Different herbivore species are active at different seasons and/or under different environmental conditions. Furthermore, different herbivore species react differently to the type of grassland fragmentation chosen in the present study. We conclude that fragmentation effects on grazing depend on which of the potential herbivore species react to fragmentation and which of them are active during the investigation period

Homology, behaviour and spider webs : web construction behaviour of Linyphia hortensis and L-triangularis (Araneae : Linyphiidae) and its evolutionary significance

Linyphiidae is the second largest family of spiders. Using Linyphia hortensis and L. triangularis, we describe linyphiid sheet-web construction behaviour. Orb-web construction behaviour is reviewed and compared with that of nonorb-weaving orbicularians. Phylogenetic comparisons and the biogenetic law are applied to deduce behavioural homology. Linyphia webs were constructed gradually and in segments over a period of many days and had a long lifespan. Two construction behaviours, supporting structure and sticky thread (ST) (within the sheet) were observed. ST construction behaviour in linyphiids is considered homologous to sticky spiral construction in orb-weavers. Overall web construction conformed to the pattern of alternate construction of sticky and nonsticky parts as observed in theridiids. Linyphiids had no problem in switching between structure construction and ST construction even during a single behavioural bout. Both web construction behaviours in linyphiids were nonstereotypic, which is unusual in orbicularians. This might be due to the loss of control mechanisms at genetic level, probably by macro mutation. Lack of stereotypic behaviour might have played a substantial role in the origin of the diverse web forms seen in nonorb-weaving orbicularians. This hypothesis is consistent with patterns observed in the orbicularian phylogeny