Social aggregation of the marine isopod Cirolana harfordi does not rely on the availability of light‐reducing shelters

Social aggregation under shelters can afford benefits to animals such as protection from predators. Many isopods and insects are negatively phototactic and this may help them gravitate towards shelter. Previous studies show that, when placed in an arena with two red shelters, specimens of the marine isopod Cirolana harfordi and the terrestrial isopod Porcelio scaber pick one of the two shelters at random and aggregate under it, demonstrating social aggregation under light‐reducing shelters. In the present study, an arena with two clear shelters was used to determine whether group sizes of 4, 8, 12 and 16 C. harfordi specimens display social aggregation when the shelters do not accommodate negative phototaxis. In all group sizes, C. harfordi specimens picked one of the shelters at random and significantly more animals aggregated under this shelter compared with the other. Cirolana harfordi also displayed aggregation in an arena with no shelters. Accordingly, C. harfordi specimens do not require shelters that reduce light to display social aggregation. The ability to locate suitable shelter under which there is no substantial reduction in light could benefit the animal in a natural environment comprising heavily shaded areas, as well as at night.     

Aggregation behavior of Harmonia axyridis under non‐wintering conditions

The invasive multicolored Asian ladybeetle, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), aggregates inside dwellings during winter to avoid cold weather. This adaptive behavior disturbs homeowners, because of the large numbers of individuals that aggregate, which induces allergic reactions. The migratory flight patterns of this species have been well documented, with individuals preferentially moving toward prominent and high color contrast elements. However, the factors involved in the selection of aggregation sites by this species have yet to be elucidated. Here, we evaluated the influence of (i) the density of individuals and (ii) the type of available shelters on decisions by H. axyridis to settle and aggregate under shelters. A dual choice bioassay conducted in the laboratory demonstrated the presence of mutual attraction to conspecifics. We also found that individuals preferentially settled under red covered shelters compared to transparent shelters, and that the type of shelter outweighed the effect of social interactions among conspecifics. Moreover, this experiment was performed under non‐wintering conditions, providing the first evidence that aggregative behavior in this species can also occur under those specific conditions.     

Aggregation in woodlice: social interaction and density effects

Terrestrial isopods are known to be sensitive to humidity, brightness or temperature. Until now, aggregation was assumed to depend on these sensitivities as a result of individual preferences. In this paper, we show that the social component is also important in the isopod aggregation phenomenon. In experimental arenas with two identical shelters up to nearly 90% of woodlice aggregated under shelters. This aggregation was quick as in 10 minutes most of the animals aggregated, irrespective of their density. Nonetheless, 10-15% of the animals walked around the arena, rarely forming very small and short-lasting aggregates outside shelters. Woodlice aggregated preferably under one of the shelters in 77% of experiments. Indeed, almost 80% of the animals out of 40, 60 or 80 animals in the arena aggregated under one shelter. In arenas with 100 individuals the aggregations were proportionally smaller (70%). Our results revealed that 70 animals was a maximum number of woodlice in an aggregate. We concluded that the location of aggregates is strongly governed by individual preferences but the dynamics of aggregation and collective choice are controlled by social interaction between congeners. The tested densities of the animals in the arena did not impact the aggregation patterns.     

Plant Size-dependent Escaping Behavior of Gregarious Nymphs of the Desert Locust, Schistocerca gregaria

The present study involves the tracking of marching bands of more than 300,000 gregarious nymphs of the desert locust, Schistocerca gregaria, to examine shelter plant preference and how species and size of shelter plants and nymphal group sizes jointly influence the escaping behavior of 4th- and 5th-instar gregarious nymphs. Field observations are conducted during daytime and night-time in the Sahara Desert in Mauritania. Three dominant plant species have been identified at the survey site: Hyoscyamus muticus, Panicum turgidum, and Nucularia perrini. The smallest mean plant size among the three plant species is H. muticus. Gregarious nymphs perch on all the three plant species irrespective of time, and form various sizes of groups ranging from <10 to >10,000 nymphs. Groups of gregarious locusts perching on the plants show either escaping or sheltering behavior in response to an approaching observer. Percentages of nymphal groups showing escaping behavior for H. muticus, P. turgidum and N. perrini are 96.4, 47.6 and 19.5 %, respectively. Defensive behavior is not affected by nymphal group size but by species and size of shelter plants. Nymphal groups tend to show escaping behavior when their perching plants are relatively small. No groups escape from their perching plants during night. These results might indicate that gregarious nymphs do not have a strong shelter plant preference and change their defensive behavior depending on species and size of sheltering plants and light conditions.     

Differences in competitive ability for the occupancy of shelters in triatomines

Triatomines (Hemiptera: Reduviidae: Triatominae) are nocturnal blood‐sucking insects. During daylight hours they remain in an akinetic state inside their shelters, whereas at dusk they become active and move outside. When they are outside their shelters during the photophase, triatomines are vulnerable to diurnal predators and the period just before dawn is critical to their survival. This work analyses the existence of competitive interactions involved in the occupancy of shelters by triatomines. Behavioural assays were performed in which nymphs of different stages, nutritional status or species were released in an experimental arena containing a space‐limited artificial shelter. The proportions of individuals occupying the shelter during the photophase were quantified to estimate the competitive abilities of each stage and species. Intraspecific comparisons showed higher levels of shelter occupancy for fourth over fifth instars and fed over unfed nymphs of Triatoma infestans. Interspecific comparisons showed higher rates of shelter occupancy for Triatoma sordida in comparison with T. infestans, and for T. infestans over Rhodnius prolixus. Arrival order was also relevant to determining shelter occupancy levels: early arrival was advantageous in comparison with later arrival. The study of intra‐ and interspecific competitive interactions for shelter occupancy provides relevant information about colonization and recolonization processes in the natural environments of triatomines.     

Shelter selection in the kissing bugs Triatoma infestans and Rhodnius prolixus: Intra- and inter-specific interactions and quality evaluation

Triatomines display most of their activities during the night. Before sunrise, they search, select and occupy adequate shelters to stay during the photophase, avoiding exposure to diurnal predators. In this work, we first explored the interactions between individuals of the same or different species in the shelter selection process of Triatoma infestans (Klug 1834) and Rhodnius prolixus (Stål 1859). When two groups of insects (either of different nutritional status, nymphal instar or species) were released together over an experimental arena containing two identical shelters, all nymphs were distributed randomly, suggesting the absence of intra- and inters-pecific interactions. Secondly, we analysed their preferences for particular features of shelters by releasing one group of insects (either T. infestans or R. prolixus) over an arena containing two different refuges. Nymphs exhibited preferences for darker shelters with a vertical orientation of its substrate and elevated from the ground, highlighting the importance of such features in a shelter selection context. We conclude that these species disregard the presence of other individuals but evaluate certain characteristics of the shelters to choose them. This information may contribute to understanding the colonization/recolonization dynamic processes of these Chagas disease vectors.     

Source and specificity of chemical cues mediating shelter preference of Caribbean spiny lobsters (Panulirus argus)

Caribbean spiny lobsters display a diversity of social behaviors, one of the most prevalent of which is gregarious diurnal sheltering. Previous research has demonstrated that shelter selection is chemically mediated, but the source of release and the identity of the aggregation signal are unknown. In this study, we investigated the source and specificity of the aggregation signal in Caribbean spiny lobsters, Panulirus argus. We developed a relatively rapid test of shelter choice in a 5000-l laboratory flume that simulated flow conditions in the spiny lobster's natural environment, and used it to examine the shelter preference of the animals in response to a variety of odorants. We found that both males and females associated preferentially with shelters emanating conspecific urine of either sex, but not with shelters emanating seawater, food odors, or the scent of a predatory octopus. These results demonstrate specificity in the cues mediating sheltering behavior and show that urine is at least one source of the aggregation signal.     

Species composition and life histories of shelter-building caterpillars on Salix miyabeana

We studied the species composition and life history patterns of shelter‐building microlepidoptera on the willow Salix miyabeana in Hokkaido, northern Japan. We identified 23 microlepidopteran species across seven families that constructed leaf shelters. Species in Tortricidae and Pyralidae comprised approximately 90% of the total number of sampled shelter‐building microlepidoptera that reached adult eclosion in the laboratory. Seasonal changes in the density of leaf shelters showed two peaks: early June and mid‐August. In June, caterpillars of Gypsonoma bifasciata, Gypsonoma ephoropa, Acleris issikii and Saliciphage acharis were the principal shelter builders, while in August shelters were constructed primarily by caterpillars of Nephopterix adelphella, A. issikii and S. acharis. Approximately 90% of leaf shelters were constructed on the top portions of shoots, suggesting that most shelter‐building caterpillars prefer to build leaf shelters here.     

Gregariousness and repellent defences in the survival of phytophagous insects

Group living has both costs and benefits for plant‐feeding insects, but defence against predators is the most widely acknowledged benefit. Gregarious folivores typically have warning coloration and elaborate anti‐predator defences. Do these defences protect these species from predation? To see if protection from predators generally results from gregariousness, I compared the shapes of published survivorship curves of externally feeding, gregarious and solitary Lepidoptera and Symphyta. Gregarious species are less likely than solitary species to die in the larval stages. However, solitary species that have anti‐predator defences do not have higher larval survival compared to gregarious species. This result, along with evidence from experimental manipulations of group size, suggests that repellent defences per se do not increase survival of gregarious larvae. Group behaviour is undoubtedly important in affecting the higher larval survival of gregarious species, but we currently cannot determine whether predator learning, dilution of risk, or rapid development contribute most to increasing survival.     

Ecological consequences of shelter sharing by leaf-tying caterpillars

A wide variety of insect herbivores construct and inhabit leaf shelters (ties, rolls, folds, and webs). Shelter construction can lead to a high rate of secondary occupation by other arthropods, including other species of constructors. The consequences for the inhabitants of secondarily occupying these shelters are currently unknown. In this study, we conducted field experiments to examine the fitness consequences (survival and attack by natural enemies) for caterpillars that (i) occupy a shelter with conspecifics vs. occur singly; and (ii) establish a new shelter vs. colonize a pre‐existing one. In addition, we conducted factorial laboratory experiments to test the hypothesis that caterpillars sharing shelters with conspecifics might have reduced construction costs (a potential benefit of shelter‐sharing or secondary occupation). Larvae of Psilocorsis quercicella Clemens (Lepidoptera: Oecophoridae) placed in white oak [Quercus alba L. (Fagaceae)] leaf ties alone or in groups of three had equal likelihood of survival from natural enemies. This same caterpillar species, however, had a higher disappearance rate when placed in pre‐existing leaf ties than when placed in newly formed ones, suggesting a potential cost of secondary colonization. A similar experiment with a closely related species [Psilocorsis cryptolechiella (Chambers)], however, failed to detect a cost of secondarily occupying shelters made on beech, Fagus grandifolia Ehrh. (Fagaceae). In the laboratory experiment, we found no evidence of shelter‐sharing benefits; rather larvae reared in shelters in groups of three had lower pupal mass (and thus lower potential fecundity) than larvae reared singly, suggesting a cost of shelter sharing. Moreover, groups of larvae forced to repeatedly construct new shelters tended to have reduced survival relative to the other treatment, suggesting that energetic constraints are more likely to reduce fitness when larvae cohabit shelters. Taken together, these results indicate that the common phenomenon of shelter sharing by leaf‐tying caterpillars has either neutral or negative effects for the occupants. The fact that these leaf‐tying caterpillars actually share shelters may simply reflect limited availability of oviposition sites.     

Causes and consequences of aggregation by neonatal tiger snakes (Notechis scutatus,Elapidae)

Although snakes traditionally have been regarded as asocial animals, recent studies have revealed complex interactions among neonatal snakes and their mothers. We noticed frequent aggregation by captive neonatal Australian elapids (tiger snakes, Notechis scutatus), and conducted simple experiments to clarify the proximate causation of, and potential consequences of, aggregative behaviour. Litters of neonates exhibited statistically significant aggregation (clustering) in empty containers, especially if the test area was subjected to rapid cooling. Aggregation was most pronounced inside shelter‐sites, and familiar shelters (i.e. containing scent cues from the litter) attracted snakes more than did novel (unscented) shelters. Snakes in larger aggregations cooled more slowly (reflecting their higher combined mass and thus, thermal inertia) and higher body temperatures facilitated neonatal locomotor performance, retreat‐site location and anti‐predator tactics. Plausibly, aggregation in neonatal tiger snakes (and other reptiles) functions to retard cooling rates, with the result that the young snakes are better able to evade or repel attacks by predators.     

Gregarious behaviour of evasive prey

Gregarious behavior of potential prey was explained by Hamilton (1971) on the basis of risk-sharing: The probability of being picked up by a predator is small when one makes part of a large aggregate of prey. This argument holds only if the predator chooses its victims at random. It is not the case for herds of evasive prey in the open, where prey's gregarious behavior, favorable for the fast group members, makes it easier for the predator to home in on the slowest ones. We show conditions under which gregarious behavior of the relatively fast prey individuals leaves slowest prey with no other choice but to join the group. Failing to do so would signal their vulnerability, making them a preferred target for the predator. Analysis of an n+1 player game of a predator and n unequal prey individuals clarifies conditions for fully gregarious, partially gregarious, or solitary behavior of the prey.     

Cues For Shelter Use in a Phytophagous Insect

Many insects spend a large proportion of their life inactive, often hiding in shelters. The presence of shelters may, therefore, influence where insects feed. This study examines stimuli affecting the use of shelters by adults of the pine weevil, Hylobius abietis (L.) (Coleoptera, Curculionidae). This species is an economically important forest pest in Europe since the adults feed on the stem bark of newly planted conifer seedlings. When there are hiding or burrowing places present in close proximity to a seedling, pine weevils may hide there and repeatedly return to feed on the same seedling. Experiments were conducted in a laboratory arena with above-ground or below-ground shelters and in the presence or absence of wind. Pine weevils were highly attracted to shelters both above and below ground. Weevils in shelters were often observed assuming a characteristic “resting” posture. Experiments with opaque and transparent shelters showed that visual stimuli are used for orientation towards shelters and also increase the probability of an individual remaining in a shelter. The presence of wind increased the weevils’ propensity to use shelters both above and below ground. The present study indicates that shelters have a major influence on the behavior of the pine weevil and possible implications of the results are discussed.     

Riparian Forest Restoration: Increasing Success by Reducing Plant Competition and Herbivory

The reestablishment of riparian forest is often viewed as “best management practice” for restoring stream ecosystems to a quasi‐natural state and preventing non‐point source contaminants from entering them. We experimentally assessed seedling survivorship and growth of Quercus palustris (pin oak), Q. rubra (red oak), Q. alba (white oak), Betula nigra (river birch), and Acer rubrum (red maple) in response to root‐stock type (bare root vs. containerized), herbivore protection (tree shelters), and weed control (herbicide, mowing, tree mats) over a 4‐year period at two riparian sites near the Chester River in Maryland, U.S.A. We started with tree‐stocking densities of 988/ha (400/ac) in the experimental plots and considered 50% survivorship (i.e., a density of 494/ha [200/ac] at crown closure) to be an “acceptable or minimum” target for riparian restoration. Results after four growing seasons show no significant difference in survivorship and growth between bare‐root and containerized seedlings when averaged across all species and treatments. Overall survivorship and growth was significantly higher for sheltered versus unsheltered seedlings (49% and 77.6 cm vs. 12.1% and 3.6 cm, respectively) when averaged across all species and weed control treatments. Each of the five test species exhibited significantly higher 4‐year growth with shelter protection when averaged across all other treatments, and all species but river birch had significantly higher survivorship in shelters during the period. Seedlings protected from weeds by herbicide exhibited significantly higher survivorship and growth than seedlings in all other weed‐control treatments when averaged across all species and shelter treatments. The highest 4‐year levels of survivorship/growth, when averaged across all species, was associated with seedlings protected by shelters and herbicide (88.8%/125.7cm) and by shelters and weed mats (57.5%/73.5 cm). Thus, only plots where seedlings were assisted by a combination of tree shelters and either herbicide or tree mats exhibited an “acceptable or minimum” rate of survivorship (i.e.,>50%) for riparian forest restoration in the region. Moreover, the combined growth and survivorship data suggest that crown closure over most small streams in need of restoration in the region can be achieved most rapidly (i.e., 15 years or less) by protecting seedlings with tree shelters and controlling competing vegetation with herbicides.     

Female German cockroaches join conspecific groups based on the incidence of auditory cues

Insects deciding whether to approach and join a group of conspecifics may utilize olfactory and auditory signals, or cues, from that group as indicators of its size or the suitability of its shelter. Here we show (1) that German cockroaches, Blattella germanica L. (Dictyoptera: Blattellidae), in groups wing‐fan; (2) that the incidence of wing‐fanning (WF) increases proportionately to group size; and (3) that gravid females utilize auditory cues associated with WF behaviour when they decide whether or not to enter a shelter. In binary choice arena bioassays, proportionately more gravid females avoided shelters associated with play‐back of high‐incidence male‐ or female‐produced WF sound, indicating a high‐density group of conspecifics, but sought shelters associated with play‐back of low‐incidence WF sound, indicating a low‐density group of conspecifics. These auditory cues seem to convey information on group size or density, avoid sensory fatigue in enclosed environments, and allow sonotactic orientation to the group's location.     

Does shelter enhance early seedling survival in dry environments? A test with eight Mediterranean species

Question: Do solid‐walled polyethylene tubes and mesh fabric tubes improve the short‐term survival of eight Mediterranean tree and shrub species often used in the restoration of arid environments? Location: We conducted two experimental plantations in degraded field sites in the province of Almería (SE Spain), under arid Mediterranean conditions. Methods: One‐year‐old seedlings of Ceratonia siliqua, Juniperus phoenicea, Olea europaea, Pinus halepensis, Pinus pinaster, Quercus coccifera, Quercus ilex and Tetraclinis articulata were planted either sheltered by one of the above shelter tubes, or by being left unsheltered. Survival was recorded the first growing season after planting, which was a very dry season. Results: Overall, seedling survival ranged from as little as 0% to 24%, and tree shelters consistently enhanced survival in Quercus species only, ranging from 16% in walled shelters to 8% in mesh shelters. Shelters failed to boost survival in the six remaining species. Conclusion: The results of this study suggest that both walled and mesh shelters were mostly ineffective at increasing seedling survival for the Mediterranean species used in this experiment; these species coincide with those used in restoration programs. The use of shelters in restoration programs conducted in arid environments should be reconsidered, while walled shelters might be advisable for Mediterranean Quercus species only. Further research is necessary to develop and assess improved types of shelters for arid environments.     

Field Observations of Agonism in the Crayfish, Orconectes rusticus: Shelter Use in a Natural Environment

Crayfish are aggressive animals that compete to acquire resources such as shelters, food, and mates. Shelters are a primary resource that crayfish use for protection from conspecifics and predators. Despite the importance of this resource, no field research has been performed that studies the acquisition and control of this resource. The present study examines shelter use in a natural habitat and the impact that shelter ownership has on the intensity and outcome of agonistic encounters. A stationary underwater camera was used to observe crayfish, Orconectes rusticus, shelter use and agonistic interactions in a natural lake environment. These shelters were formed naturally in iron outcroppings found on the limestone and detritus benthos. Crayfish activity and shelter use was found to be dependent upon a circadian cycle with most of the shelter use occurring during the morning to early afternoon (05:00–13:00 hours). Agonistic encounters in the presence of shelters resulted in short, low intensity interactions. Interestingly, fight outcomes were not significantly affected by shelter ownership, but were primarily determined by size differential of combatants. This outcome may be due to the prevalence of shelters within this habitat. Contrary to laboratory studies, these results indicate that shelter ownership may not be an important factor in determining resource‐holding potential in some habitats.     

No evidence of sickness behavior in immune‐challenged field crickets

Sickness behavior is a taxonomically widespread coordinated set of behavioral changes that increases shelter‐seeking while reducing levels of general activity, as well as food (anorexia) and water (adipsia) consumption, when fighting infection by pathogens and disease. The leading hypothesis explaining such sickness‐related shifts in behavior is the energy conservation hypothesis. This hypothesis argues that sick (i.e., immune‐challenged) animals reduce energetic expenditure in order have more energy to fuel an immune response, which in some vertebrates, also includes producing an energetically expensive physiological fever. We experimentally tested the hypothesis that an immune challenge with lipopolysaccharide (LPS) will cause Gryllus firmus field crickets to reduce their activity, increase shelter use and avoid foods that interfere with an immune response (i.e., fat) while preferring a diet that fuels an immune response (i.e., protein). We found little evidence of sickness behavior in Gryllus firmus as immune‐challenged individuals did not reduce their activity or increase their shelter‐seeking. Neither did we observe changes in feeding or drinking behavior nor a preference for protein or avoidance of lipids. Males tended to use shelters less than females but no other behaviors differed between the sexes. The lack of sickness behavior in our study might reflect the fact that invertebrates do not possess energetically expensive physiological fever as part of their immune response. Therefore, there is little reason to conserve energy via reduced activity or increased shelter use when immune‐challenged.     

Patterns of aggregation behaviour in six species of cockroach: comparing two experimental approaches

Cockroaches (Blattaria) are considered universally gregarious insects. In this study, we tested the hypothesis that even related species and/or males and females of the same species may exhibit different aggregation patterns. We compared aggregation behaviour in six cockroach species belonging to three groups: Eublaberus, Blaberus, and Gromphadorhina‐Princisia (all Blaberidae). For comparison, binary choice and free aggregation arena tests were used. The results revealed that blaberid clades clearly differed in their aggregation patterns. Females of Eublaberus species aggregated, whereas only a low aggregation tendency was recorded in conspecific males. Both sexes of Blaberus species aggregated, especially when tested in the arena. The Madagascar hissing cockroaches of the Gromphadorhina‐Princisia clade are not gregarious – the aggregation tendency was entirely absent in males of the two species studied and in females of Princisia vanwaerebeki van Herrewege, and was only weakly demonstrated in females of Gromphadorhina portentosa (Schaum). The observed patterns of aggregation are compared with previous reports concerning social behaviour of blaberid species.     

Understanding Animal Companion Surplus in the United States: Relinquishment of Nonadoptables to Animal Shelters for Euthanasia

Relinquishing a nonhuman animal to a shelter is a complex decision that, it is often believed, ultimately may represent a breakdown of the human-animal bond. The result of such a breakdown is an animal companion surplus in the United States, which is no better evidenced than by the statistics documenting the millions of animals euthanized at shelters every year. This research examined the companion animals who are relinquished by their owners to shelters for adoption and compared them specifically to those relinquished for euthanasia. The study also compared the owner characteristics of the animals in these two groups. Although the majority of dogs and cats relinquished are clearly adoptable, a sizable number of these pets presented to shelters for euthanasia have problems precluding their adoptability: old age, illness, and refractory behavior.