Resource assessment in hermit crabs: the worth of their own shell

Animals gather information about the quality of a resource throughits assessment and behave accordingly as a result of adaptivemotivational changes. In the hermit crab Pagurus longicarpus,we investigated whether an individual was affected in its motivationto acquire a new shell by the quality of the domicile shell(own resource value [ORV]), of the offered shell (external resourcevalue [ERV]), or of both and asked whether its motivation wasaltered by the information gathered during shell investigation.We analyzed the behavior of hermit crabs inhabiting shells ofdiffering qualities and compared their willingness to acquirean offered shell—optimal, smaller than optimal, or largerthan optimal—by measuring the latency to approach it,the number of shell investigation, and its total duration. Crabsin smaller shells (SSs) approached more quick and often theoffered shell, whereas crabs in larger shells investigated theoffered shell more thoroughly. The readiness of crabs to approachthe offered shell and the extent of its investigation were independentof the ERV but were exclusively affected by the ORV, whereasthe number and duration of shell investigation did not changewith time as investigation proceeded, except for crabs in SSs.These results suggest that P. longicarpus' motivation to acquirea new shell is exclusively influenced by the value of the shellit inhabits rather than by the quality of the shell it is offeredand that this species does not gather—or does not use—informationabout ERV during investigation.     

How resource quality differentially affects motivation and ability to fight in hermit crabs

Contesting animals typically gather information about the resource value and that information affects fight motivation. However, it is possible that particular resource characteristics alter the ability to fight independently of the motivation. Using hermit crabs, we investigate how the resource in terms of shell quality affects both motivation and ability to fight. These crabs fight for shells, but those shells have to be carried and may impose physiological costs that impede fight vigour. We find that the shell has different effects on motivation and ability. Potential attackers in very small shells were highly motivated to attack but, rather than having enhanced ability, unexpectedly quickly fatigued and subsequently were not more successful in the fights than were crabs in larger shells. We also examined whether defending crabs could gather information about the attacker''s shell from the vigour of the attack. Defending crabs gave up quickly when a potential gain had been assessed, indicating that such information had been gathered. However, there was no indication that this could be owing to the activity of the attacker and the information is probably gathered via visual assessment of the shell.     

Olfactory Discrimination of Individual Scents in the Subterranean Rodent Ctenomys talarum (tuco-tuco)

Biological odours that convey cues regarding individual identity can provide valuable information mediating many aspects of mammalian social relationships such us dominance hierarchies, group memberships, territorial and mating activities. The ability of the subterranean rodent, tuco‐tuco (Ctenomys talarum), to discriminate between soiled shavings, urine and faeces from different individuals was investigated using the habituation–discrimination paradigm. Discrimination by both males and females was tested using scents obtained from same‐ and opposite‐sex individuals. Each test subject was habituated for three consecutive days to odour samples from the same individual; on the fourth day a scent from a novel individual was provided. For all odour sources, animals spent significantly less time investigating the habituation scent over successive trials, indicating that animals perceived the stimulus as familiar. For all stimuli, test subjects spent more time investigating the novel odour, rather than the familiar one. Animals spent more time investigating soiled shavings than urine or faeces. Both males and females discriminated novel from familiar odours in shavings and urine regardless of the gender of the odour donors. In contrast, test animals discriminated between familiar and novel odours in feces only when the fecal donors were of different sex from subjects. Possible territorial and reproductive functions of individual scent discrimination are discussed.     

Shell decay rates of native and alien freshwater bivalves and implications for habitat engineering

1. Spent shells of bivalves can provide habitat for other organisms, as well as playing important roles in biogeochemical cycles. The amount of spent shell material that will accumulate at a site depends on rates of both shell production and decay, although the latter is rarely considered. 2. We measured the instantaneous decay rates of four species of freshwater bivalves across a range of sites in south‐eastern New York, and found that rates varied by more than 500‐fold across sites and species. 3. Differences in decay rates were related to water chemistry (Ca, pH, dissolved inorganic C), the presence of a current, and the size of the bivalve shell. 4. Combining these decay rates with estimates of shell production derived from the literature, we conclude that the Unionidae, Corbicula, and Dreissena are all capable of producing large accumulations (>10 kg dry mass m⁻²) of spent shells, while members of the Sphaeriidae probably rarely will produce such large accumulations. 5. Hence the replacement of native unionid bivalves by the alien Corbicula and Dreissena may have little effect on standing stocks of spent shells, unless the aliens invade sites where unionids are scarce or absent.     

Sizing‐up the Competition: Factors Modulating Male Display Behavior During Mate Competition

When competing for mates, males of many species use cues from their rivals to evaluate their chances of success. Signaling behavior is a vital component of male–male contests and courtship, and may inform males of a rival's quality or intentions. We used eastern fence lizards (Sceloporus undulatus) to investigate how the time a male spent signaling during mate competition is influenced by his quality, his rival's quality relative to his own, and the value of a contested female. Furthermore, we examined how a male's behavioral response to a competitor's signals would be mediated by his relative quality. We simulated natural encounters by allowing two males to compete over a single female in the laboratory. We measured the time males spent performing two types of displays (pushups and shudders) and categorized male behavioral responses to rival pushup and shudder displays. Time spent signaling was not related to a male's absolute quality (body and head size, condition, and badge sizes), or his quality relative to that of his rival, although males did spend more time performing pushups when competing over females in better condition. Male behavior was also influenced by his rival's signals, such that males of relatively lower quality than their opponents were more likely to aggressively respond to rival pushups and shudders. We discuss these results with respect to the evolution and function of signaling behavior in courtship and male–male contests.     

Postoccupancy evaluation of staff,visitors, and three species of animals in a zoo setting

Postoccupancy evaluation (POE) was used to assess newly constructed zoo exhibits from the perspective of three user groups: zoo staff, zoo visitors, and the animals. Staff represents a generally understudied user group in zoo-based POEs. We asked staff to rate the animals' space, the visitors' space, and the staff's space at previous and new exhibits. We also compared zoo visitors' ratings of the animals' behavior and environments, overall exhibit impressions, and the time visitors spent viewing previous and new exhibits. Lastly, we compared activity and space use of a Komodo dragon (Varanus komodoensis), two red pandas (Ailurus fulgens), and one rhinoceros (Rhinoceros unicornis) in their previous and new exhibits. Staff rated animal, visitor, and staff areas higher at the new exhibits compared to the previous exhibits. Visitors also rated several factors higher and spent more time at the new exhibits. The most naturalistic exhibit received the most favorable ratings in all categories and animal activity increased visitor stay time. We found that red pandas were less active in their new exhibit, and the Komodo dragon and rhino showed no difference in activity. The red pandas and the Komodo dragon used more available space in their new exhibits; however, we recommend using Electivity index to examine resource preferences for these species, whose enclosure use has been less studied than large mammals. We emphasize the importance of including staff in zoo-based POE, make other recommendations for future POE studies, and discuss various factors that could have influenced our results.     

The growth ofTracheleuglypha dentata (Rhizopoda: Testacea) in clonal cultures under different trophic conditions

Twenty-four clones ofTracheleuglypha dentata were cultivated under different trophic conditions. Half were cultivated with soil extract which induced low bacterial growth. The other 12 were cultivated in the same medium with the addition of Cerophyl to enhance bacterial growth. The dynamics of each clone was followed for about 3 months by counting both live animals and empty shells. The rate of decomposition of empty shells was negligible. The number of empty shells are a direct measure for mortality. Parameters measured were densities, intrinsic rate of growth, generation time, production, and mortality. The densities increased in most of the clones until about the third week; it then reached a plateau or decreased slowly until the end of the experiment. After this initial period, the live animals closed their pseudostome with a thin membrane and remained quiescent until fresh food was given. The level of the plateau was significantly six times higher in the well-fed than in the starved clones, a difference due to a higher rate of growth induced by a shorter generation time during the first 10 days. This high growth produced giant shells, teratological forms created by the fusion of several shells. Mortality appeared at about the 20th day. Because it was higher in the well-fed clones than in the starved clones, it brought the densities of both clones to the same level after 3 months. The first empty shells were produced by either precocious mortality or abortion, as they did not contain reserve plates necessary for a new shell prior to reproduction. The mortality of adults came later and was probably linked with the age of the culture. The starved cells had an unusual morphology which disappeared when the medium was renewed.     

Vacancy Chains Provide Aggregate Benefits To Coenobita clypeatus Hermit Crabs

Vacancy chain theory describes a unique mechanism for the sequential distribution of animal resources across multiple individuals. This theory applies to any resources, such as shelters or nest sites, that are discrete, reusable, and limited in use to single individuals or groups at one time. Hermit crabs rely on gastropod shells for shelter, and a single vacant shell can initiate a chain of sequential shell switches that distributes new resources across many individuals. Using the terrestrial hermit crab Coenobita clypeatus , we examined the previously untested theoretical prediction that this process will yield trickle-down resource benefits to vacancy chain participants (aggregate benefits). In laboratory experiments, we measured improvements in shell quality when a single vacant shell was provided to groups of eight crabs. We found that crabs participating in vacancy chains (averaging 3.2 individuals) gained significant reductions in their shell crowding. In addition, vacancy chains terminated early when experimental groups included a single crab occupying a damaged shell, because damaged vacancies always remained unoccupied. Hermit crabs in damaged shells were more likely to win resource contests for high quality shells against size-matched hermit crabs in crowded shells. Finally, field additions of many new shells to an island population of C. clypeatus hermit crabs reduced average shell crowding for crabs of all sizes, possibly from propagation of benefits through vacancy chains. These results provide empirical support for the theoretical prediction that vacancy chains should provide benefits distributed across many vacancy chain participants. Since shelter-based vacancy chains likely occur in other animals, additional studies of vacancy chain processes should provide new insights into resource acquisition behaviors in diverse animal groups.     

The effects of gastropod shell availability and habitat characteristics on shell utilization by the intertidal hermit crab Pagurus longicarpus Say

The shell utilization patterns of two intertidal populations of the hermit crab, Pagurus longicarpus Say, were studied. The populations differed with respect to the physical characteristics of their habitats and the availability of empty gastropod shells. The first population was in an estuary connected to Narragansett Bay. This population had few, if any. empty shells available in the area. The second population was in a rock-cobble area directly on Block Island Sound. There were large numbers of empty gastropod shells available for this population. The consistently high gastropod mortality on this shore appears to be caused by the animals being washed up on the beach, where they die due to desiccation.Data from samples collected from June to November of both 1974 and 1975 showed that there were distinct differences in the shell utilization patterns of the two populations. A greater proportion of the animals collected from the estuary population inhabited damaged shells and/or shells with symbionts compared to the individuals from the population with large numbers of empty shells available.The results of shell selection experiments indicated that individuals from both populations inhabited less than preferred shells. The population with large numbers of empty shells available was qualitatively shell-limited because of the effect of physical factors on the morphology of the gastropod species of that area. In addition, the shell selection experiments showed that individuals from the two areas had significantly different shell preferences. This result may be explained by the effects of the environment on the growth rates of the hermit crabs and/or as an indication that shell preferences may vary in response to the physical factors of a given habitat.     

Same resource,different benefits: hermit crab shell structure advantages owners,but not intruders in agonistic interactions

Resources may confer advantages by enhancing their owners’ fighting ability (resource holding potential; RHP). While the resource-correlated RHP hypothesis has been recognized as a determinant of agonistic success in different taxa, this has mostly been based on assessment of either the intruder or the owner, but only rarely in both contestants. We tested whether the internal structure of shells affects hermit crabs’ RHP, both as owners defending the shell against eviction and as intruders attempting to gain access to an occupied shell. We conducted contests (n?=?60) to compare the success in shell eviction by intruders in intact shells vs. in shells with the columella artificially reduced, and the success of shell retention by owners in intact shells vs. shells with the columella reduced. The internal configuration of the shell showed different resource-correlated RHP effects depending on the individual’s role in the fight. The presence of a columella in the intruder’s shell did not affect the likelihood that they would evict their opponents. However, owners resisted more evictions in shells with intact columella than those in shells with reduced columella. Our results demonstrate that the same resource can offer different RHP advantages to owners and intruders during an agonistic interaction.

     

Caedichnus,a New Ichnogenus Representing Predatory Attack on the Gastropod Shell Aperture

Predatory traces, in which the tracemaker has damaged the prey animal's skeleton to kill and consume it, have a deep fossil history and have received much scientific attention. Several types of predatory traces have been assigned to ichnotaxa, but one of the most studied predatory traces, the wedge-shaped excision produced as a result of attacks mainly by crustaceans on the apertures of gastropod shells, has yet to be described as an ichnotaxon. We propose the ichnogenus Caedichnus to describe the shell damage produced by aperture peeling behavior. Caedichnus is produced by predators that are unable to crush their prey's shells outright. Depending on the predator's peeling ability and the prey's withdrawal depth within the shell, the trace can extend through several whorls of the shell. Aperture peel attacks may fail, allowing such damage to be repaired by surviving gastropods. Thus, the types of attacks that produce Caedichnus may exert selective pressure on prey to evolve better-defended shells (in the case of gastropods) or to inhabit better-defended shells (in the case of hermit crabs). The identification of these trace fossils will enhance our understanding of how predation influences the morphological, and even behavioral, evolution of prey organisms.     

The effects of shell size and coil orientation on reproduction in female hermit crabs, Clibanarius vittatus

We investigated the effects of shell coil orientation and shell size on reproduction in field populations of the hermit crab, Clibanarius vittatus. Females were collected in the intertidal in Beaufort, NC. Shell parameters were measured and size (cephalothorax length) and reproductive status were determined for 70 females occupying Busycon shells. Crabs were categorized as berried (eggs on the pleopods), mature ovaries, or non-reproductive (no eggs). For berried females, the number of eggs was recorded. By offering a separate group of females access to empty shells, it was possible to calculate optimal shell size and the deficit in shell size for field-collected animals.Females that were berried were in shells closer to the optimal shell size than females with mature ovaries, both for shell weight and shell volume. And females with mature ovaries were in shells that were closer to the optimal size than females that were non-reproductive. For both categories of females without eggs on the pleopods, the majority of females were in shells that were too big (in weight and internal volume). While the percentage of berried females did not differ between dextral (Busycon carica) and sinistral (Busycon sinistrum) shells, the non-reproductive females had a much smaller deficit in volume in sinistral shells compared to dextral shells. For berried females, there was no relationship between the magnitude of their shell deficit and the number of eggs carried. Our results suggest that reproduction is inhibited when females occupy shells sufficiently greater than the optimal shell size.     

Resistance to shell breaking in two intertidal snails

The ability of shells to withstand shell breaking forces has been examined in two intertidal prosobranchs, Nucella lapillus and Littorina littorea , using four methods: measuring shell strength on a compressive testing machine, measuring the shell to body mass ratio, measuring the shell thickness and measuring the ability of crabs to break shells in aquarium experiments. Nucella lapillus consistently showed a relationship between shell vulnerability and environmental variables: the shells were easier to break at sites where rock and boulder movement was the least. Although some between-site differences were found in L. littorea shells, these were less than in N. lapillus and did not relate to environment variables: the shells were easier to break at sites where exposure to wave action was the least. Although some between-site differences were found in L. littorea shells , these were less than in N. lapillus and did not relate to environmental factors. However, both species appear to grow into a size refuge in which they are secure from predation by shore crabs at the sites where these crabs are commonest.     

Are the preference and selection patterns of hermit crabs for gastropod shells species- or site-specific?

Experimental analyses of hermit crabs and their preferences for shells are essential to understand the intrinsic relationship of the crabs' dependence on shells, and may be useful to explain their shell use pattern in nature. The aim of this study was to evaluate the effect of crab species and site on the pattern of shell use, selection, and preference in the south-western Atlantic hermit crabs Pagurus brevidactylus and Pagurus criniticornis, comparing sympatric and allopatric populations. Differently from the traditional approach to evaluate shell preference by simply determining the shell selection pattern (i.e., the number of shells of each type selected), preference was defined (according to [Liszka, D., Underwood, A.J., 1990. An experimental design to determine preferences for gastropod shells by a hermit-crab. J. Exp. Mar. Biol. Ecol., 137(1), 47–62]) by the comparison of the number of crabs changing for a particular shell type when three options were given (Cerithium atratum, Morula nodulosa, and Tegula viridula) with the number of crabs changing for this same type when only this type was offered. The effect of crab species was tested at Cabelo Gordo Beach, where P. brevidactylus was found occupying shells of C. atratum, M. nodulosa, and T. viridula in similar frequencies, whereas P. criniticornis occupied predominantly shells of C. atratum. In laboratory experiments the selection patterns of the two hermit-crab species for these three gastropods were different, with P. criniticornis selecting mainly shells of C. atratum, and P. brevidactylus selecting more shells of M. nodulosa. The shell preference was also dependent on crab species, with P. criniticornis showing a clear preference for shells of C. atratum, whereas P. brevidactylus did not show a preference for any of the tested shells. The effect of site was tested for the two species comparing data from Cabelo Gordo to Preta (P. brevidactylus) and Araçá beaches (P. criniticornis). The pattern of shell use, selection, and preference was demonstrated to be dependent on site only for P. brevidactylus. The results also showed that the shell use pattern of P. criniticornis can be explained by its preference at both sites, whereas for P. brevidactylus it occurred only at Cabelo Gordo, where the absence of preference was correlated with the similar use of the three gastropod species studied. Finally, the results showed that the shell selection pattern cannot be considered as a measure of shell preference, since it overestimates crab selectivity.     

Behavioral budgets of captive sea otter mother–pup pairs during pup development

Instantaneous sampling of eight behavior categories for a captive mother sea otter (Enhydra lutris) and her four pups was conducted continuously during the first three months following each pup's birth. Initially, pup-grooming and rest occupied the largest proportion of the mother's time. Despite a steady decline in the amount of time the mother spent pup-grooming, the mother continued to groom the pup a substantial amount of time three months after the pup's birth. The proportion of time the mother spent resting, self-grooming, or swimming did not change significantly over the study period, suggesting that these behaviors are important to the mother's self-maintenance. The newborn pups' behaviors were initially limited to rest, active, and nursing, with large proportions of time spent resting and nursing. A significant increase in self-grooming occurred regularly by week 2, followed by increases in swimming (week 3), diving (week 6), and feeding (week 7). Three months after birth, the pups' behavior budgets were similar to their mother, with the exception that pups self-groomed substantially less than their mother self-groomed. The lower proportion of pup self-grooming may be due to the continued close proximity to their mother in captivity, which afforded extended association, and allowed the mother to continue to perform this complex behavior for the pup. © 1993 Wiley-Liss, Inc.     

The effect of experience on the selective behaviour of song thrushes feeding on artificial populations ofCepaea (held)

Song thrushes fed on populations of bread-stuffedCepaea shells. The results indicate that experience influences the selective behaviour of thrushes for both shell colour and shell size. Such behaviour could lead to frequency dependent selection in natural populations. Within the ranges of size and colour offered, thrushes learned from a colour rather than a size experience when both were varied simultaneously.     

Sexual,habitat‐constrained and parasite‐induced dimorphism in the shell of a freshwater mussel (Anodonta anatina,Unionidae)

Intraspecific trends in freshwater mussel (unionoid) shells that are consistently associated with differences in the mussels' sex and/or parasitic infestation can potentially be used to reconstruct sex ratios or parasitic levels of modern and ancient unionoid populations. In contrast to morphological patterns within mammal species, such dimorphic trends within unionoid species are, however, poorly understood. This study investigates, for the first time, to what extent sex, trematode infection and indirect habitat effects determine shell morphology in the freshwater mussel Anodonta anatina. Three of the five study populations displayed significant sexual shell width dimorphism. Here, shells of females were significantly wider than males, probably as a result of altered shell growth to accommodate marsupial gills. In two of these populations, female shells were additionally significantly thinner than those of males, which could be a result of resource depletion by offspring production. Two other A. anatina populations showed no significant dimorphic patterns, and our results indicate that this interpopulational difference in the degree of sexual dimorphism may reflect the overarching effect of habitat on morphology. Thus, populations in the most favourable habitats exhibit faster growth rates, attain larger maximum sizes and produce more offspring, which results in more swollen gills and consequently more inflated shells of gravid females compared to less fecund populations. None of the populations showed any evidence for sexual dimorphism in overall size, growth rate, sagittal shape and density of shells. In addition to sexual dimorphisms, infestation by bucephalid trematode parasites (Rhipidocotyle sp.) significantly altered sagittal and lateral shell shape of A. anatina in one of the populations, with infected specimens growing wider and more elongated. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Sex differences in diet and foraging behavior in white-faced capuchins (Cebus capucinus)

I examined sex differences in diet and foraging behavior in two groups of white-faced capuchin monkeys (Cebus capucinus)in a tropical dry forest at Santa Rosa National Park, Costa Rica. I tested three hypotheses: sex differences in diet and foraging behavior are best explained by (1) sexual dimorphism, (2) the energy demands of pregnancy and lactation for females;and (3) avoidance of competition between the sexes. Sexual dimorphism offered the best explanation of sex differences in the diet and foraging behavior of C. capucinus,accurately predicting that males do more strenuous foraging activity, make less use of small foraging supports, and spend more time on or near the ground. Females spent more time foraging than males did but probably obtained a lower protein yield per unit foraging time. Females exploited more small and embedded invertebrates, while males ate more large invertebrates and vertebrate prey. Pregnant and lactating females spent more time resting and less time foraging than other females did, increasing their foraging return by focusing on foods requiring little handling. There was little evidence of competition avoidance between the sexes.     

Egg shell quality,clutch size and hatching success of the great tit (Parus major) and the pied flycatcher (Ficedula hypoleuca) in an air pollution gradient

Egg shell thickness, egg volume, clutch size and hatching success of Parus major and Ficedula hypoleuca were studied at 14 study sites around a copper smelter complex in Harjavalta, south-west Finland, in 1991–1993. In 1991–1992 unhatched eggs were collected to measure shell quality. F. hypoleuca was more susceptible to pollutants than P. major, the response of which was weaker in all aspects studied. Egg shells of F. hypoleuca were about 17% thinner and eggs were about 8% smaller in volume near the factory than at a distance of 10 km. The clutch size of F. hypoleuca was significantly smaller and hatching success markedly lower at a study site next to the factory complex than at all other sites. In P. major, variation in shell thickness and egg volume was not significantly related to the distance from the pollution source. Clutch size and hatching success of P. major did not significantly differ among study sites, although the trend in hatching success was in the same direction as in F. hypoleuca. Clutches of both species contained less shell material and both species had more nests without eggs near the factory than further away. The surface structure of the eggshells was studied by scanning electron microscope. Especially in F. hypoleuca, the egg shell surface was more rough and porous near the factory. The roles of Ca and heavy metals in shell thinning are discussed.