Glauco Machado

Using δ13C stable isotopes to quantify individual-level diet variation

Individual-level diet variation can be easily quantified by gut-content analysis. However, because gut contents are a ‘snapshot’ of individuals’ feeding habits, such cross-sectional data can be subject to sampling error and lead one to overestimate levels of diet variation. In contrast, stable isotopes reflect an individual’s long-term diet, so isotope variation among individuals can be interpreted as diet variation. Nevertheless, population isotope variances alone cannot be directly compared among populations, because they depend on both the level of diet variation and the variance of prey isotope ratios. We developed a method to convert population isotope variances into a standardized index of individual specialization (WIC/TNW) that can be compared among populations, or to gut-content variation. We applied this method to diet and carbon isotope data of four species of frogs of the Brazilian savannah. Isotopes showed that gut contents provided a reliable measure of diet variation in three populations, but greatly overestimated diet variation in another population. Our method is sensitive to incomplete sampling of the prey and to among-individual variance in fractionation. Therefore, thorough sampling of prey and estimates of fractionation variance are desirable. Otherwise, the method is straightforward and provides a new tool for quantifying individual-level diet variation in natural populations that combines both gut-content and isotope data.

Daily activity schedule, gregariousness, and defensive behaviour in the neotropical harvestman Goniosoma longipes (Opiliones: Gonyleptidae).

In this paper we provide a field account of some aspects of the behavioural biology of Goniosoma longipes (Roewer), a harvestman which commonly occurs in caves in South-east Brazil. During daytime, solitary and aggregated individuals can be found resting on the cave walls. Just after sunset, however, many individuals leave the cave to forage for live and dead arthropods. Foraging individuals return to the cave before dawn. Aggregations of G. longipes contain on average 34 individuals (range 7-200), and the groups are usually found close to the water source and away from the cave entrance. The main predators of G. longipes are the spider Ctenus fasciatus Mello-Leitao (Ctenidae) and the opossum Philander opossum (L.) (Didelphidae). Upon disturbance solitary and aggregated individuals may either flee, or drop from the cave wall or vegetation. The harvestmen can also release a repugnatory liquid upon manipulation, and aggregated individuals collectively discharge this secretion toward the aggressor before fleeing. The activity schedule of G. longipes shows that individuals need to leave the cave periodically to forage, and therefore the population can be considered trogloxene. Data on the food items collected by G. longipes indicate that the harvestman is a generalist predator which also feeds on dead animal matter. Gregarious behaviour is considered relatively common among harvestmen and has been interpreted in several ways. We suggest that gregarious behaviour in G. longipes may be related with the choice of more suitable microconditions in the cave habitat and/or with group chemical defence.

Conditional male dimorphism and alternative reproductive tactics in a Neotropical arachnid (Opiliones)

In arthropods, most cases of morphological dimorphism within males are the result of a conditional evolutionarily stable strategy (ESS) with status-dependent tactics. In conditionally male-dimorphic species, the status’ distributions of male morphs often overlap, and the environmentally cued threshold model (ET) states that the degree of overlap depends on the genetic variation in the distribution of the switchpoints that determine which morph is expressed in each value of status. Here we describe male dimorphism and alternative mating behaviors in the harvestman Serracutisoma proximum. Majors express elongated second legs and use them in territorial fights; minors possess short second legs and do not fight, but rather sneak into majors’ territories and copulate with egg-guarding females. The static allometry of second legs reveals that major phenotype expression depends on body size (status), and that the switchpoint underlying the dimorphism presents a large amount of genetic variation in the population, which probably results from weak selective pressure on this trait. With a mark-recapture study, we show that major phenotype expression does not result in survival costs, which is consistent with our hypothesis that there is weak selection on the switchpoint. Finally, we demonstrate that switchpoint is independent of status distribution. In conclusion, our data support the ET model prediction that the genetic correlation between status and switchpoint is low, allowing the status distribution to evolve or to fluctuate seasonally, without any effect on the position of the mean switchpoint.

Parental investment and the evolution of subsocial behaviour in harvestmen (Arachnida Opiliones)

The diversity of reproductive strategies within the arachnids rivals all other arthropod groups. However, with the possible exception of spiders and scorpions, evolutionary biologists have overlooked these organisms. The order Opiliones is divided in three suborders (Cyphophthalmi, Palpatores and Laniatores) with considerable differences in morphology, habits and behaviour. In this review we focus on the life history, sexual behaviour and ecology of Opiliones, and discuss the possible causes of the diversity of forms of parental investment found in this order. In the Cyphophthalmi the forms of parental investment are restricted to the choice of oviposition sites. Among the Palpatores there is selection of microhabitats for oviposition, egg-hiding, and discharge of repugnant secretions on the eggs. The most common form of parental investment in Laniatores is probably egg-hiding by females but several species show subsocial behaviour, including both maternal and paternal care. The scarcity of paternal care among harvestmen and the differences when it does occur suggest that male care evolved independently in different laniatorean lineages from a plesiomorphic state of non-care rather than from female care. Both male and female care reduce egg mortality. Egg predation, rather than infection by fungi may be the most effective pressure favouring the evolution of subsocial behaviour in harvestmen. Subsocial behaviour appears to be restricted to members of the Laniatores since they show a conjunct of preadaptations that may favour prolonged associations between the parental and the offspring such as, restriction of the reproduction effort to specific periods and places, considerable adult longevity, and aggressive mechanisms of defence. On the other hand, many of these preadaptations mentioned are absent in the Palpatores. Reduced fecundity and increased egg size accompanied the evolution of subsocial behaviour in laniatorean harvestmen. Palpatores, which show very simple forms of parental investment, have significantly higher fecundity and a smaller egg size than Laniatores, which show subsocial behaviour.

Resource defense polygyny shifts to female defense polygyny over the course of the reproductive season of a Neotropical harvestman

Although studies classify the polygynous mating system of a given species into female defense polygyny (FDP) or resource defense polygyny (RDP), the boundary between these two categories is often slight. Males of some species may even shift between these two types of polygyny in response to temporal variation in social and environmental conditions. Here, we examine the mating system of the Neotropical harvestman Acutisoma proximum and, in order to assess if mate acquisition in males corresponds to FDP or RDP, we tested four contrasting predictions derived from the mating system theory. At the beginning of the reproductive season, males fight with other males for the possession of territories on the vegetation where females will later oviposit, as expected in RDP. Females present a marked preference for specific host plant species, and males establish their territories in areas where these host plants are specially abundant, which is also expected in RDP. Later in the reproductive season, males reduce their patrolling activity and focus on defending individual females that are ovipositing inside their territories, as what occurs in FDP. This is the first described case of an arachnid that exhibits a shift in mating system over the reproductive season, revealing that we should be cautious when defining the mating system of a species based on few observations concentrated in a brief period.

Efficiency of uniparental male and female care against egg predators in two closely-related syntopic harvestmen

Although the benefits of maternal care have been investigated in many species, the caring role of males in species with exclusive paternal care has received less attention. We experimentally quantified the protective role of paternal care in the harvestman Iporangaia pustulosa. Additionally, we compared the effectiveness of paternal care against predation in this species with a syntopic harvestman with maternal care, Acutisoma proximum. We demonstrated that nearly one-third of the unprotected Iporangaia clutches disappeared entirely in 12 days, while the other two-thirds suffered a mean reduction of 55% in egg number. Conversely, 50% of the control clutches did not suffer any reduction, and only one was entirely consumed by predators. We also demonstrated that the mucus coat that covers Iporangaia clutches has an important deterrent role against predation by conspecifics: 58.3% of the clutches without mucus were attacked and three of them were entirely consumed, whereas only three clutches with mucus were attacked, suffering a reduction of up to three eggs. Iporangaia males were as efficient as Acutisoma females in protecting eggs. However, unattended Acutisoma eggs were attacked 20% more frequently than unattended Iporangaia eggs. Unattended Iporangaia eggs are protected by a mucus coat that prevents or decreases predation rate, whereas Acutisoma eggs are more susceptible to predation, probably because they lack this mucus coat. Thus, besides the fact that Iporangaia males efficiently protect the offspring against egg predators, females also contribute to egg protection by providing a mucus coat that deters egg predators.

Maternal care, defensive behavior, and sociality in neotropical Goniosoma harvestmen (Arachnida, Opiliones)

Summary:Goniosoma includes large and conspicuous species of harvestmen that occur exclusively in the Brazilian Atlantic forest. This paper reports on the parental activities, defensive behavior, and gregariousness of five species of Goniosoma, and also summarizes the published biological data for the genus. The behavioral patterns within the genus are discussed and a hypothesis for the emergence of gregariousness in the group is presented. Several Goniosoma live inside caves, mainly as trogloxenes. Although species of the genus comprise only 5% of the harvestmen fauna in Brazil, they account for 20.6% of all harvestmen species recorded in Brazilian caves. In addition to several morphological and physiological features that may favor the occupation of caves, species of Goniosoma also present subsocial behavior, which may confer a special advantage in this particular environment. Female protection is crucial for egg survival since predators may consume entire batches in a single night. Most species of Goniosoma form dense diurnal aggregations ranging from three to 200 individuals (mainly subadults and adults of both sexes). In a hypothetical scenario for the evolution of sociality in the Goniosoma, physiological constraints acting on individual harvestmen would lead to a behavioral response for the selection of sites with appropriate microclimatic conditions. The lack of cannibalism among adults and subadults may favor tolerance towards conspecifics, and may represent a pre-adaptation to gregariousness. The derived functions of gregarious behavior may include strengthening of the defensive signal through the collective release of a repugnant secretion, the dilution effect, and promptness in fleeing a predator attack as a consequence of the additional alarm role of scent gland secretions. These defensive functions may overcome the costs of group living and may be responsible for the maintenance of gregariousness in Goniosoma.

The ecological tale of Gonyleptidae (Arachnida, Opiliones) evolution: phylogeny of a Neotropical lineage of armoured harvestmen using ecological, behavioural and chemical characters

The large Neotropical family Gonyleptidae comprises nearly 820 species divided into 16 subfamilies. The majority of publications on harvestman ecology, behaviour and scent gland secretion chemistry have focused on this family. We used the information available in the literature and combined it with an intensive search for ecological, behavioural and chemical data to infer the phylogeny of the Gonyleptidae. We included 28 species belonging to 14 of the 16 gonyleptid subfamilies in the ingroup and four species belonging to the families Cosmetidae, Stygnidae and Manaosbiidae in the outgroup. We performed the analyses using equally weighted characters and coded 63 characters comprising 153 states, which makes this the largest non‐morphological, non‐molecular phylogenetic data matrix published to date. We obtained five most parsimonious trees, and the strict consensus resulted in six collapsed nodes. The results show that the monophyly of Gonyleptidae is equivocal because Metasarcinae is placed at a basal polytomy with the outgroups Cosmetidae and Stygnidae. Gonyleptinae, Pachylinae and Progonyleptoidellinae are polyphyletic groups, but the remaining subfamilies are monophyletic and have several synapomorphies. Based on the resulting topology, we discuss the performance of ecological, behavioural and chemical characters, and map a selected set of characters to discuss their evolutionary patterns in the family.

BEHAVIORAL REPERTORY OF THE NEOTROPICAL HARVESTMAN ILHAIA CUSPIDATA (OPILIONES, GONYLEPTIDAE)

Abstract In this study, we provide an ethogram for the harvestman Ilhaia cuspidata and describe the daily activity pattern of captive individuals. We also provide a comparison between the behavioral repertory of this species with that of the syntopic Discocyrtus oliverioi. Five females and four males of I. cuspidata were maintained in the same terrarium from November 1999–November 2000 for qualitative and quantitative observations. Twenty behavioral acts were recorded, classified in seven categories and the relative frequency of each was determined: exploration (69.8%), resting (16.7%), feeding (6.3%), grooming (4.4%), social interactions (2.6%), reproduction (0.1%) and others (0.3%). There was a marked difference in the frequency of the behavioral categories between sexes: females fed more frequently than males and males were involved in social interactions more frequently than females. During most of the daylight hours, individuals remained inside shelters and became active from 19:00–09:00 h. Although I. cuspidata and D. oliverioi showed almost the same behavioral acts, there were quantitative differences in their repertories: the relative frequency of behavioral categories “resting” and “social interactions” were higher for I. cuspidata whereas “reproduction” and “grooming” were higher for D. oliverioi. The main qualitative difference between these two species was related to the forms of parental care: females of D. oliverioi guard their eggs and first instar juveniles, whereas females of I. cuspidata scatter their eggs in time and space and do not actively protect their offspring. Since both species share the same habitats (sometimes in multi-species aggregations), the behavioral differences between them may be explained by particular morphological and physiological characteristics of the species, as well as by phylogenetic constraints.

Maternal care in the neotropical harvestman bourguyia albiornata (Arachnida: Opiliones): Oviposition site selection and egg protection

Summary Females of the harvestman Bourguyia albiornata oviposit almost exclusively inside the tube formed by the curled leaves of the bromeliad Aechmea nudicaulis . Oviposition is not correlated with rainfall or with temperature, but is negatively correlated with the number of individuals of A. nudicaulis emitting e owers. Because the ine orescence stalk occupies the tube of leaves of e owering bromeliads, oviposition site is not available for ovigerous females. During one year, 83.6% of the females produced only one egg-batch and the remaining produced two or three batches. The mean number of eggs per batch along successive reproductive events decreased signie cantly. A e eld experiment in which females were removed from the nest bromeliad showed that less than 7% of the unprotected eggs survived beyond two weeks. In a control in which females were maintained over the eggs, predators attacked only one egg-batch. Ants were the most important egg predators, and at the study site at least 20 species nest and/or forage in A. nudicaulis . Intense predation on eggs by generalist predators may be an important pressure promoting parental care in Neotropical harvestmen.