Gloriana Chaverri

Social calls used by a leaf-roosting bat to signal location

Social calls in bats have many functions, including mate attraction and maintaining contact during flight. Research suggests that social calls may also be used to transfer information about roosts, but no studies have yet demonstrated that calls are used to actively attract conspecifics to roosting locations. We document the social calls used by Spixs disc-winged bat (Thyroptera tricolor) to actively recruit group members to roosts. In acoustic trials, we recorded two sets of calls; one from flying individuals termed ‘inquiry calls’, and another from roosting bats termed ‘response calls’. Inquiry calls were emitted by flying bats immediately upon release, and quickly (i.e. 178 ms) elicited production of response calls from roosting individuals. Most flying bats entered the roost when roosting individuals responded, while few bats entered the roost in the absence of a response. We argue that information transfer concerning roost location may facilitate sociality in T. tricolor, given the ephemeral nature of roosting structures used by this species.

Range overlap and association patterns in the tent-making bat Artibeus watsoni

Space use and ranging patterns of individuals have traditionally been used to identify social systems, and the amount of overlap in the home ranges of individuals can provide indirect information about the likelihood of social interactions. We compared data on foraging and roosting range overlap of the tent-roosting bat Artibeus watsoni to assess how environmental and demographic factors may affect movement patterns and social interactions. We estimated the overlap of foraging and roosting ranges among simultaneously radiotagged dyads and calculated a simple association index among these individuals. We then used these data to determine whether relative population and roost abundance, as well as sex class, affected spatial and social patterns. Our results show that range overlap in A. watsoni significantly determines the degree to which individuals interact with each other. We also found that roosting range overlap may be a greater determinant of daytime social interactions than foraging range overlap, because roosting range overlap showed a stronger correlation with association patterns. In addition, the differences that we observed in roosting range overlap and association among dyads were a consequence of the effects of population and roost density. Bats sampled in areas of high-population abundance had significantly higher roosting range overlap than bats in areas of low-population abundance, and roosting ranges of bats sampled at sites with few roosting resources (i.e. tents) overlapped more than those of bats living in areas with more tents. Finally, we observed no overlap in roosting range and no association among adult males, supporting the hypothesis that males defend tents or roosting areas against other males to gain exclusive mating access to females using these structures.

ECOLOGICAL CORRELATES OF RANGE SIZE IN THE TENT-MAKING BAT ARTIBEUS WATSONI

Abstract We examined variation in foraging- and roosting-range size in the tent-making bat Artibeus watsoni. We radiotracked 42 individuals at 2 sites in the Golfito Wildlife Refuge and 6 sites at Corcovado National Park, southwestern Costa Rica. Average home-range size of A. watsoni was the smallest recorded for a bat species (3.6 ha). Site, age, reproductive status, habitat, and season all had significant effects on foraging ranges, but sex and population abundance did not. Bats from the most productive sites, such as primary forests, had smaller foraging ranges, and subadults ranged farther than adults. During the dry season, females in late pregnancy had the smallest foraging ranges of all, most likely as a result of flight constraints imposed by the large fetus and an increase in fruit availability. Conversely, lactating females sampled during the early rainy season had among the largest foraging ranges. Males had significantly smaller roosting ranges than females, and adult bats sampled from areas of low roost abundance and high population abundance had smaller roosting ranges. However, subadults appeared to be unaffected by such changes in resource abundance, perhaps because they often roost in unmodified leaves. Although differences in resource availability and competition were important in determining roosting-range size, these effects were concealed by effects of reproductive activities such as mating, pregnancy, and lactation. Reproductive condition, along with foraging and roosting resource abundance, were largely responsible for determining ranging patterns in A. watsoni, but factors influenced foraging- and roosting-range size in different ways.

Ecological Determinants of Social Systems: Perspectives on the Functional Role of Roosting Ecology in the Social Behavior of Tent-Roosting Bats

Abstract In this chapter, we compile ecological and behavioral data on tent-making bats to determine if the variation in social behavior observed in this group may be explained by any of the components of their roosting ecology. Results suggest that most of the variation in the social behavior within and among species may be explained by the quality, abundance, and distribution of roosting resources, such that larger, scarcer, and clumped roosts are typically occupied by more individuals. The abundance of tent-roosts may also influence group cohesion in most species, as very abundant resources apparently facilitate roost switching, and individuals that change roosts are often also more likely to change roost partners. In addition, the harem-like composition of roosting groups observed in most tent-making bats may be largely influenced by the role of males during tent construction and defense. We argue that collection of further ecological and behavioral data, coupled with more quantitative analyses, is needed for additional generalizations to be drawn.

MATING SYSTEM OF THE TENT-MAKING BAT ARTIBEUS WATSONI (CHIROPTERA: PHYLLOSTOMIDAE)

Abstract Vertebrate mating systems are influenced by ecological and phylogenetic factors, and the variation observed in mating behavior is frequently attributable to the extent to which male assistance in the rearing of offspring increases a females reproductive rate, size and stability of female groups, and density and distribution of females in space. In this study we evaluate patterns of association and parentage to describe the mating system of the tent-making bat Artibeus watsoni. During 16 consecutive months, we regularly surveyed 2 sites in southwestern Costa Rica and determined size, composition, and stability of social groups, and established patterns of parentage within roosts. We found female-biased, mixed-sex social groups formed by individuals located in several roosts within an area. Roosting associations were low (average across all sites: 31%), and the frequent changes between roosting partners suggests that males and females formed nonexclusive mating bonds, which was further supported by our finding that only 27% of young were sired by the male within a roosting group. Paternal sibships were only observed in 17% of dyads of young roosting together, indicating that few males copulated with >1 female of the roosting group during the mating season. However, paternal sibships were only observed in Golfito, which could be explained by greater group size and stability attributable to lower roost availability. Thus, our findings are consistent with a polygynandrous mating system (2 or several males sharing access to 2 or several females, and vice versa) in which males associated intermittently with females that frequently switched roosts, and where breeding opportunities for males seemed related to the defense of roosts, roosting territories, or both.

When genes move farther than offspring: gene flow by male gamete dispersal in the highly philopatric bat species Thyroptera tricolor

For species characterized by philopatry of both sexes, mate selection represents an important behaviour for inbreeding avoidance, yet the implications for gene flow are rarely quantified. Here, we present evidence of male gamete‐mediated gene flow resulting from extra‐group mating in Spixs disc‐winged bat, Thyroptera tricolor, a species which demonstrates all‐offspring philopatry. We used microsatellite and capture–recapture data to characterize social group structure and the distribution of mated pairs at two sites in southwestern Costa Rica over four breeding seasons. Relatedness and genetic spatial autocorrelation analyses indicated strong kinship within groups and over short distances (<50 m), resulting from matrilineal group structure and small roosting home ranges (~0.2 ha). Despite high relatedness among‐group members, observed inbreeding coefficients were low (FIS = 0.010 and 0.037). Parentage analysis indicated mothers and offspring belonged to the same social group, while fathers belonged to different groups, separated by large distances (~500 m) when compared to roosting home ranges. Simulated random mating indicated mate choice was not based on intermediate levels of relatedness, and mated pairs were less related than adults within social groups on average. Isolation‐by‐distance (IBD) models of genetic neighbourhood area based on father–offspring distances provided direct estimates of mean gamete dispersal distances ( r^ ) > 10 roosting home range equivalents. Indirect estimates based on genetic distance provided even larger estimates of r^ , indicating direct estimates were biased low. These results suggest extra‐group mating reduces the incidence of inbreeding in T. tricolor, and male gamete dispersal facilitates gene flow in lieu of natal dispersal of young.

Effects of roost specialization on extinction risk in bats

Understanding causes and consequences of ecological specialization is of major concern in conservation. Specialist species are particularly vulnerable to human activities. If their food or habitats are depleted or lost, they may not be able to exploit alternative resources, and population losses may result. We examined International Union for Conservation of Nature (IUCN) Red List bat data and the number of roosts used per species (accounting for phylogenetic independence) to determine whether roost specialization is correlated with extinction risk. We found a significant correlation between the IUCN Red List category and the number of roost types used. Species that use fewer roost types had a higher risk of extinction. We found that caves and similar structures were the most widely used roost types, particularly by species under some level of risk of extinction. Many critically endangered, endangered, or vulnerable species used natural roosts exclusively, whereas less threatened species used natural and human-made roosts. Our results suggest that roost loss, particularly in species that rely on a single roost type, may be linked to extinction risk. Our focus on a single life history trait prevented us from determining how important this variable is for extinction risk relative to other variables, but we have taken a first step toward prioritizing conservation actions. Our results also suggest that roost specialization may exacerbate population declines due to other risk factors, such as hunting pressure or habitat loss, and thus that management actions to preserve species under risk of extinction should prioritize protection of roosting sites.

Social calls produced within and near the roost in two species of tent-making bats, Dermanura watsoni and Ectophylla alba.

Social animals regularly face the problem of relocating conspecifics when separated. Communication is one of the most important mechanisms facilitating group formation and cohesion. Known as contact calls, signals exchanged between conspecifics that permit group maintenance are widespread across many taxa. Foliage-roosting bats are an excellent model system for studying the evolution of contact calling, as there are opportunities to compare closely related species that exhibit major differences in ecology and behavior. Further, foliage-roosting bats rely on relatively ephemeral roosts, which leads to major challenges in maintaining group cohesion. Here, we report findings on the communication signals produced by two tent-making bats, Dermanura watsoni and Ectophylla alba. We found that both species produced calls in the early morning near the roost that were associated with roostmate recruitment. Calling often ended once other bats arrived at the tent, suggesting that calls may be involved in roostmate recruitment and group formation. The structure and function of these calls are described and future research directions are discussed.

Cooperative signaling behavior of roost location in a leaf-roosting bat

Research suggests that social calls are important for conveying information about food and roost location in bats. However, no studies have specifically documented calls that are used to actively attract conspecifics to roosting locations. Here we describe the cooperative signaling behavior of roost location towards flying conspecifics in Spix’s disc-winged bat (Thyroptera tricolor), a species that uses a highly ephemeral roosting resource. Two types of calls were recorded during field experiments; one from flying individuals termed “inquiry calls”, and another from roosting bats termed “response calls”. Inquiry calls were emitted by flying bats immediately upon release, and quickly elicited production of response calls from roosting individuals. Most flying bats entered the roost when roosting individuals responded, while very few bats entered the roost in the absence of a response. During playback experiments, we found significant differences in response rates among individuals, which could be caused by diverse intrinsic and extrinsic factors. In addition, results of our ongoing field studies suggest that the cooperative signaling behavior of roost location is important in maintaining social cohesion, and that the use of a larger home range when resources are scarcer may decrease group stability by hindering communication.

Sound amplification by means of a horn-like roosting structure in Spix's disc-winged bat.

While sound is a signal modality widely used by many animals, it is very susceptible to attenuation, hampering effective long-distance communication. A strategy to minimize sound attenuation that has been historically used by humans is to use acoustic horns; to date, no other animal is known to use a similar structure to increase sound intensity. Here, we describe how the use of a roosting structure that resembles an acoustic horn (the tapered tubes that form when new leaves of plants such as Heliconia or Calathea species start to unfurl) increases sound amplification of the incoming and outgoing social calls used by Spixs disc-winged bat (Thyroptera tricolor) to locate roosts and group members. Our results indicate that incoming calls are significantly amplified as a result of sound waves being increasingly compressed as they move into the narrow end of the leaf. Outgoing calls were faintly amplified, probably as a result of increased sound directionality. Both types of call, however, experienced significant sound distortion, which might explain the patterns of signal recognition previously observed in behavioural experiments. Our study provides the first evidence of the potential role that a roost can play in facilitating acoustic communication in bats.