Sociality, exotic ectoparasites, and fitness in the plural breeding rodent Octodon degus

Social animals are susceptible to high infection levels by contact-transmitted parasites due to increased conspecific interaction. Exotic parasites are known to have adverse consequences on native hosts. We examined the relationship between social group size and exotic ectoparasite loads, and adult infection levels with per capita fitness and offspring survival in the plural breeding rodent Octodon degus in central Chile. Degus at our site were almost entirely infected by two exotic ectoparasites: the fleas Leptopsylla segnis and Xenopsylla cheopis. Neither group size nor number of females per group predicted the abundance of either exotic flea species. The per capita number of pups (per capita fitness) that emerged from burrow systems used by known social groups was negatively correlated with abundance of L. segnis but not X. cheopis. On adults, X. cheopis abundance was three times greater than L. segnis but was not significantly correlated with per capita fitness. In females, L. segnis abundance was negatively correlated with peak body mass during pregnancy. Adult ectoparasite load was not correlated with offspring survival. Based on these results, we hypothesize that high infection levels of L. segnis result in decreased reproductive fitness of adult female degus but are not a cost of sociality because parasite loads are not predicted by social group size. Further work is needed to experimentally test this hypothesis and to determine if L. segnis serves as a vector for a deleterious pathogen. Lastly, the lack of native ectoparasites may explain why a previous study at our site determined that behavioral adaptations needed to cope with high ectoparasite burdens (e.g., grooming) are not extensive in degus; they simply have not had the coevolutionary time needed for selection of these behaviors.     

The influence of group size on natal dispersal in the communally rearing and semifossorial rodent, Octodon degus

In social or group living species, members of groups are expected to be affected differentially by competition through the effect of group size (i.e., the “social competition hypothesis”). This hypothesis predicts an increase in the probability of dispersal with increasing size of social groups. At a more mechanistic level and based on the known effects of competition on stress hormone levels, a positive relationship between group size and glucocorticoids of juveniles should be observed. We used a demographic approach to test these predictions on a natural population of the communally rearing and semifossorial rodent—Octodon degus. Burrow systems provide degus with places to rear offspring and to evade stressful thermal conditions and predators. Thus, we predicted dispersal to increase with increasing number of degus per main burrow system used, a measure of habitat saturation in degus. The probability of dispersal increased with increasing number of degus per main burrow system used. Mean fecal metabolites of cortisol in offspring increased, yet not statistically significantly, with the number of juveniles in groups. These results were consistent with a scenario in which competition drives natal dispersal in juveniles in social degus. In particular, competition would be the consequence of high degu abundance in relation to the abundance of burrow systems available at the time of offspring emergence.     

Ecological correlates of social organization in a communally breeding bird,the acorn woodpecker,Melanerpes formicivorus

Summary Acorn woodpeckers (Melanerpes formicivorus) were studied during the summers of 1976 and 1977 in the Chiricahua Mountains of Arizona to determine the ecological correlates of variations in unit size, storage behavior, and home range. Unit size varied from two to five adults, acorn storage capacity varied from 0 to 5,000 individual holes per unit, and home range size varied from 5.2 to 51.2 ha.There was a consistent but nonsignificant trend for units inhabiting pine-oak woodland to be larger and to maintain more food storage holes than units in oak-juniper or oak woodland. Pine-oak woodland had the highest oak species number, density, and basal area of any community in the study area, and showed the least annual variation in percent of oaks producing acorns during the study. However, pineoak woodland also had the lowest acorn productivity per hectare.More than four times as many oak trees produced acorns in 1977 than in 1976. Following the poor 1976 crop, unit home range size increased so that the mean number of oaks contained per home range size increased more than three and a half times in 1977. The number of woodpecker units containing nonbreeding adults decreased from 90% in 1976 to 20% in 1977. These data suggest that the size and reliability of acorn crops control the composition of acorn woodpecker social units.A comparison of acorn woodpecker population parameters in the Chiricahua Mountains and at the Hastings Reservation, California, was made, using data of MacRoberts and MacRoberts (1976). The California population was significantly more dense and maintained more storage trees and more individual storage holes per unit, but there was no evidence that more California units contained nonbreeding adults than did Chiricahua units.A model is presented that emphasizes the importance of acorn crop reliability as the major determinant of acorn woodpecker social organization (Fig. 2). Annual crop fluctuations affect the winter survival and dispersion of woodpecker units, and the density of the population in the following spring. Bad acorn years will result in decreased saturation of woodpecker nesting habitat by established groups. The model predicts a corresponding decrease in the frequency of units with helpers, as young adults take advantage of opportunities to breed on their own. Longer-term crop reliability determines the benefitcost ratio for establishing and maintaining food storage facilities. The observed geographic variation in storage behavior between California and Arizona populations is suggested to reflect differences in the long-term return on investment in expensive storage facilities.     

Long-term drivers of persistence and colonization dynamics in spatially structured amphibian populations

Many organisms live in networks of local populations connected by dispersing individuals, called spatially structured populations (SSPs), where the long-term persistence of the entire network is determined by the balance between 2 processes acting at the scale of local populations: extinction and colonization. When multiple threats act on an SSP, a comparison of the different factors determining local extinctions and colonizations is essential to plan sound conservation actions. We assessed the drivers of long-term population dynamics of multiple amphibian species at the regional scale. We used dynamic occupancy models within a Bayesian framework to identify the factors determining persistence and colonization of local populations. Because connectivity among patches is fundamental to SSPs dynamics, we considered 2 measures of connectivity acting on each focal patch: incidence of the focal species and incidence of invasive crayfish. We used meta-analysis to summarize the effect of different drivers at the community level. Persistence and colonization of local populations were jointly determined by factors acting at different scales. Persistence probability was positively related to the area and the permanence of wetlands, whereas it was negatively related to occurrence of fish. Colonization probability was highest in semipermanent wetlands and in sites with a high incidence of the focal species in nearby sites, whereas it showed a negative relationship with the incidence of invasive crayfish in the landscape. By analyzing long-term data on amphibian population dynamics, we found a strong effect of some classic features commonly used in SSP studies, such as patch area and focal species incidence. The presence of an invasive non-native species at the landscape scale emerged as one of the strongest drivers of colonization dynamics, suggesting that studies on SSPs should consider different connectivity measures more frequently, such as the incidence of predators, especially when dealing with biological invasions.     

The dynamics of brood desertion among communally breeding females in the treehopper, Publilia concava

In species that exhibit extended parental care, females sometimes lay eggs communally in order to redistribute the costs of offspring care. Communal egg-laying often involves redundancy in female effort, such that the number of females contributing to reproduction is more than is needed to provide adequate parental care. As a result, a subset of females will often abandon the communal brood, with the time of departure ranging from immediate abandonment after egg-laying (brood parasitism) to delayed abandonment with prolonged care (cooperative breeding). In this paper I approach the parental care dynamics of female-female broods as a desertion game similar to that of mate desertion in species with bi-parental care. I describe a field study of the insect Publilia concava (Hemiptera: Membracidae), a species that exhibits communal oviposition and a full range of egg guarding. This species exhibits full redundancy in female care, with no difference in survival between singly and doubly guarded broods. I find that double guarding is extremely rare in the population, with most communal broods having only one female guard. While this guard was usually the female that initiated the brood, these same females were more likely to abandon when secondary females arrived and when secondary females exhibited longer guarding durations. Paradoxically, the secondary females usually abandoned the broods they visited, resulting in up to 50% of broods with double abandonment. These unguarded broods suffered a 50% reduction in hatching success, reflecting an important risk for primary females that abandon egg masses to secondary females. Overall, P. concava exhibits desertion dynamics similar to mate desertion in vertebrates and it is likely that the theoretical work in this area will be useful for future work that addresses the allocation of parental care among communal breeders.     

Sociality reduces individual direct fitness in a communally breeding rodent,the colonial tuco-tuco (<Emphasis Type="Italic">Ctenomys sociabilis</Emphasis>)

In many social vertebrates, remaining in the natal group leads to at least short-term reductions in the direct fitness of philopatric animals. Among communally breeding rodents, the direct fitness costs of philopatry appear to increase as the frequency of successful natal dispersal decreases, suggesting a functional link between constraints on natal dispersal and the reproductive consequences of sociality. To explore this relationship empirically, I documented patterns of direct fitness among female colonial tuco-tucos (Ctenomys sociabilis), which are group-living subterranean rodents from southwestern Argentina. Demographic data suggest that successful natal dispersal is rare in this species, leading to the prediction that natal philopatry in C. sociabilis is associated with significant reductions in individual direct fitness. Using data obtained during 1996–2001, I compared the direct fitness of females that dispersed from their natal group and bred alone as yearlings to that of females that lived and bred in their natal group as yearlings. Philopatric yearlings reared significantly fewer young to weaning than did disperser (lone) yearlings. Although neither survival to a second breeding season nor the estimated lifetime number of pups reared to weaning differed between dispersal strategies, the annual direct fitness of group-living females was 23–40% less than expected, suggesting that philopatric animals experienced a substantial direct fitness cost by remaining in their natal group. These data yield important insights into the adaptive bases for group living in C. sociabilis and suggest that constraints on natal dispersal are an important factor favoring group living in this species.Communicated by J. Wilkinson     

Longevity,calling effort,and metabolic rate in two populations of cricket

Intraspecific variation in a resting metabolic rate (RMR) is likely to be an important determinant of energetic-resource use and may influence the resources subsequently available for allocation to traits not directly associated with somatic maintenance. The influence of RMR on resource availability could be especially important for condition-dependent sexual traits, such as cricket calls, that are themselves energetically costly to produce. RMR may also be associated with longevity, either negatively because individuals with a high RMR burn resources faster and die young, or positively as individuals with high RMR are more able to accrue resources to fuel survival. Additionally, the associations between RMR and other characters may vary across populations if differential selection or drift shapes these traits. Here we tested for differences in RMR, body mass, calling effort, and longevity in two populations of cricket Gryllodes sigillatus and then evaluated the potential influence of RMR on calling and longevity. We find that RMR, calling effort, and longevity varied across populations, but mass did not. Controlling for population and mass, RMR was not significantly associated with calling effort, but was negatively associated with longevity. These findings suggest that male crickets that live fast die young.     

Ecological energetics of Nerita (Archaeogastropoda,Neritacea) populations on Barbados,West Indies

Population energy budgets estimated on the assumption of steady state conditions for Nerita tessellata Gmelin, N. versicolor Gmelin, and N. peloronta L. on Barbados, W. Indies, are presented. Large differences in population structure, and hence energetics, occurred at different localities along the beach. Relatively high proportions (81 to 88%) of the assimilated energy were lost via metabolism. Assimilation efficiencies ranged from 39 to 43%, net growth efficiencies from 5 to 13%, and ecological efficiencies from 3 to 7%. For each species, production (P), energy flow (A) and total energy consumption (C) were expressed as functions of animal size, in order to facilitate gross estimations of the energy components for other populations for which data on size-frequency and density are available. Respiration studies of all three species in the laboratory failed to detect differences between respiration rates in air or under seawater.     

Ecological and economic considerations on fishing and rearing of Tapes phillipinarum in the lagoon of Venice

A population dynamic model for Tapes philippinarum has been developed, using experimental data for the estimation of mortality, and literature information for recruitment. The population dynamic model has been coupled to a eco-physiological model of T. philippinarum previously developed, in order to simulate the evolution of individual size and number of individuals in each age class.The resulting age-size class model has been used to analyse the implication of different scenarios of fishing/harvesting of the bivalve in the lagoons of the Northern Adriatic Sea, where fishery and aquaculture represent important economic activities.Ten years long simulations have been performed, in which initial density, harvesting efficiency, minimum harvested size, were varied. Comparisons between the different strategies are made in term of total yields and bio-economic income. The model gives suggestions on the optimal fishing effort, in case of fishery, and on optimal seeding size and seeding moment, in case of aquaculture.A discussion of model results provides indications on harvesting policies which are appropriate from ecological-economical point of view. The final result is that economically more profitable strategies coincide with ecologically more conservative policies.     

Reproductive cycles in two populations of the Pacific sand dollar Dendraster excentricus

East Pacific swimming crabs are poorly known ecologically. We trawled samples along 4 estuarine-continental shelf transects to provide information on the population structure and species composition of portunids along environmental stress gradients. Portunids comprised almost 99% of the benthic crabs. Callinectes toxotes dominates the freshest estuarine areas, but is replaced by C. arcuatus in estuary mouth and shallow-shelf samples. Deeper shelf samples are dominated by Euphylax robustus and Portunus sper. Great numbers of E. dovii, a pelagic species, were taken on the shelf; their size distribution implies intraspecific competition. Evidence is given that differences in salinity tolerance account for differential landward limits, but competition, predation by fishes, increased fouling and decreased available food exclude eurytopic species from physicochemically equable shelf waters.     

Kleptoparasites: a twofold cost of group living for the colonial spider, Metepeira incrassata (Araneae, Araneidae)

Several species of kleptoparasitic and araneophagic spiders (Araneae: Family Theridiidae, Subfamily Argyrodinae) are found in colonial webs of the orb-weaving spider Metepeira incrassata (Araneae, Araneidae) from Mexico, where they steal food and/or prey upon their spider hosts. Census data from natural M. incrassata colonies reveal that the incidence of these species increases with colony size. This pattern may reflect the presence of several other orb-weaving spiders, each with their own kleptoparasitic species, invading larger M. incrassata colonies. As the number of these associated spiders increases, so does the density and number of Argyrodinae species in M. incrassata colonies, suggesting that associated spiders might reduce their own kleptoparasite load by building their webs within M. incrassata colonies. This represents a twofold cost to M. incrassata, as a field enclosure experiment revealed that a primarily kleptoparasitic species (Argyrodes elevatus) may reduce prey available to their hosts, but a kleptoparasitic/araneophagic species (Neospintharus concisus) inflicts high mortality upon M. incrassata. However, the cost of kleptoparasitism and predation by these species may be offset in part for M. incrassata individuals in large colonies by certain defensive mechanisms inherent in groups, i.e., “attack-abatement” and “selfish herd” effects. We conclude that increased occurrence of kleptoparasitic and/or predatory Argyrodinae spiders is a consequence of colonial web building and is an important potential cost of group living for colonial web-building spiders.     

Anti-predator benefits of group living in white-nosed coatis (Nasua narica)

Predation is often considered an important factor in the evolution of sociality among animals. We studied mortality patterns and grouping behavior of white-nosed coatis (Nasua narica) at sites in southern Arizona, USA, and western Jalisco, México. Coatis were monitored by radio-tracking and recaptures for more than 3 years at each site. In both populations, predation by large felids, including jaguars (Panthera onca) and pumas (Puma concolor), accounted for more than 50% of mortality of adult coatis. Predation rates were significantly higher on solo coatis than on coatis in groups at both study sites. Predation rates were negatively related to group size in Arizona. However, mortality of juveniles, as measured by the ratios of juveniles to adult females, was not related to the number of adult females per group in Arizona or Jalisco. Coatis exhibit a suite of behaviors, including foraging with the juveniles in the center of the group, sharing vigilance, alarm calling, mobbing and attacking predators, and a highly synchronous birth season, that are all consistent with the hypothesis that predation has played an important role in shaping coati social behavior.     

Reproductive cycle,development and recruitment of two geographically separated populations of the dendrochirote holothurian Aslia lefevrei

The boreo-mediterranean holothurian Aslia lefevrei (Barrois) occupies cryptic habitats from low-water mark to ca. 20 m. During the present work specimens were collected from the west coast of Ireland (February 1979 to August 1980), where A. lefevreiis a prominent element in two discrete epibenthic assemblages, and from the north-west of Spain (September 1981 to August 1982), where this holothurian is densely aggregated in association with raft cultivation of the mussel Mytilus edulis. Its reproductive cycle, spawning process, embryological development, settlement, and early growth rates have been documented by means of laboratory and field studies. Depending on geographic location, spawning in the wild occurs between January and May. Development to the gastrula takes some 3 to 4 d. Uniformly ciliated dipleurula/auricularia hatch 12 to 24 h after gastrulation. The pentactula stage is attained approximately 7 d after fertilization, following which the larva settles. Newly settled individuals utilize the five primary tentacles for both feeding and locomotion. With the advent of small numbers of podia in the ambulacral regions, two of the primary tentacles prolong their locomotory and anchorage capabilities while the remaining tentacles develop adult characteristics and function solely in feeding. Growth was followed for an estimated 30 mo period using length-frequency analyses. In the first year of life, A. lefevrei attain an average length of 3.4 cm; individuals on the sea floor can attain up to 18 cm, indicating a possible life expectancy of 10 yr or more.Contribution No. 301 from the School of Marine Sciences, University College Galway, Ireland     

Social and ecological influences on activity budgets of vervet monkeys,and their implications for group living

Summary Activity patterns were documented over a 20-month period in six groups of vervet monkeys (Cercopithecus aethiops) in Amboseli National Park, Kenya. Group size varied both among groups and within groups through time. The home ranges of two groups were located in Acacia xanthophloea habitat whereas those of four adjacent groups were located in A. tortilis habitat. Repeated measures analysis of variance, three-way analysis of variance, and meta-analysis of group size effects were carried out on feeding, moving, resting, scanning, allo-grooming, and other behavior. Time spent feeding varied seasonally, peaking during periods when A. tortilis seeds were eaten. However, time spent feeding did not vary within or among groups, or between habitats, males and females, or dominants and subordinates. Males spend more time scanning and less time allo-grooming than females, and high-ranking individuals of both sexes spent more time scanning than low-ranking individuals. Among females, subordinates spent more time moving, whereas among males, dominants spent more time moving. Groups living in A. xanthophloea habitat spent more time moving and allo-grooming than groups in A. tortilis habitat but this may have been confounded by the fact that these two groups were on average larger than the others. Significant differences in activity budgets among groups demonstrate that activity patterns observed in one group are not always readily generalized to other groups even when they come from the same population. Group size analyses examined potential costs and benefits of group living. Increased intragroup competition, measured by time spent moving and feeding, was only weakly evident in larger groups. Individuals in larger groups allo-groomed more than individuals in smaller groups. Self-grooming also increased in larger groups. Individuals in larger groups spent less time scanning than individuals in smaller groups, but there was more scanning per group in larger groups than in smaller groups. Correspondence to: L.A. Isbell     

Frequency of multiple paternity varies between two populations of brown smoothhound shark,Mustelus henlei

Multiple paternity was recently observed in a population of the brown smoothhound shark, Mustelus henlei, from Las Barrancas, Baja California Sur, Mexico, with litters demonstrating the greatest percentage of multiple paternity for any shark species (93 % of litters and an average number of sires = 2.3). To determine whether this frequency is consistent elsewhere in the species’ range, 4 polymorphic microsatellite loci were used to determine the frequency of multiple paternity in 18 litters of M. henlei from Santa Catalina Island, CA, sampled in 2004, 2008, and 2012. Multiple paternity varied among sampling years with 2004 demonstrating multiple sires for 40 % of sampled litters (n = 10) with an average of 1.4 sires per litter and 2008/2012 demonstrating a total lack of multiply sired litters (n = 8). Although multiple paternity was detected in this study, the range of frequencies observed is lower than that observed in the Mexican population. Based on these findings, investigators should take location into consideration when assessing the existence of multiple paternity in future studies of elasmobranch species.     

Genetic structure of hybrid mussel populations in the west of Ireland: two hypotheses revisited

In Ireland, mussels on exposed rocky shores constitute an interbreeding mixture of two forms of mussels, the blue mussel, Mytilus edulis, and the Mediterranean mussel, M. galloprovincialis. Results from an Irish study in the 1980s, using partially diagnostic allozyme markers, indicated that mussels higher up the shore were more galloprovincialis-like than those lower down. In this study we set out to test two hypotheses: (a) recruits arriving on the shore are composed of genetically distinct cohorts that settle preferentially at different levels on the shore, and maintain genetic distinctiveness into adulthood; (b) recruits are genetically homogeneous, but once settled they diverge genetically over time, due to within-habitat site specific-selection. The diagnostic Me 15/16 DNA marker was used to analyse the genetic composition of newly-settled spat recruiting to artificial substrates, which were placed at two-week intervals from May–October 2002, on the mid- and low shore areas of two exposed sites in Galway Bay. Adult mussels were also collected on each sampling date. Results did not support the preferential settlement hypothesis, i.e., the genetic composition of primary settlers (≤ 500 μm) was similar between tidal heights and shores. Neither was there evidence of post settlement selective mortality, as adults were genetically similar to settling spat. In spat and adults the frequency of the M. galloprovincialis allele was high (0.56–0.80), due to high frequencies of M. galloprovincialis (> 37%) and hybrid (> 33%) genotypes, and correspondingly low frequencies of the M. edulis genotype (< 11%). Adult mussels from a nearby sheltered estuarine site, while significantly different to exposed shore mussels, still had low frequencies of the M. edulis genotype (< 17%), indicating no apparent advantage for the genotype in this environment. There are indications that the genetic composition of mussels may be changing on the Atlantic coasts of Ireland.     

Intracellular digestion in two sublittoral populations of Ostrea edulis (Lamellibranchia)

The rhythmicity of intracellular digestion was examined in two sublittoral populations of Ostrea edulis L. On the West coast of Ireland. During 12 h cycles at each station, 20 oysters were collected each hour, grouped as sub-samples of 5, facing each of the 4 cardinal points of the compass. A segment of digestive diverticula from each oyster was examined histologically and classified according to the digestive phases of the tubules. Oysters at both stations exhibited fluctuations in digestive activity which were not correlated with tidal ebb and flow nor with orientation to tidal currents. A relationship between variations in suspended particulate matter concentration in the water body and digestion is proposed. It is suggested that increases in the levels of particulate matter, by stimulating feeding, cause a significant increase in the proportion of absorptive-phase tubules 4 to 6 h later.