Simon Ripperger

From radio telemetry to ultra-low-power sensor networks: tracking bats in the wild

Sensor networks have successfully been used for wildlife monitoring and tracking of different species. When it comes to small animals such as smaller birds, mammals, or even insects, the current approach is to use extremely lightweight RF tags located using radio telemetry. A new quantum leap in technology is needed to overcome this limitation and enable new ways to observe larger numbers of small animals. In an interdisciplinary team, we are working on the different aspects of such a new technology. In particular, we report on our findings on a sensor- network-based tracking solution for bats. Our system is based on integrated localization and wireless communication protocols for ultra-low-power systems. This requires coding techniques for improved reliability as well as ranging solutions for tracking hunting bats. We address the technological and methodical problems related to system design, software support, and protocol design. First field experiments have been conducted that showcase the capabilities of our system.

Frugivorous Bats Maintain Functional Habitat Connectivity in Agricultural Landscapes but Rely Strongly on Natural Forest Fragments

Anthropogenic changes in land use threaten biodiversity and ecosystem functioning by the conversion of natural habitat into agricultural mosaic landscapes, often with drastic consequences for the associated fauna. The first step in the development of efficient conservation plans is to understand movement of animals through complex habitat mosaics. Therefore, we studied ranging behavior and habitat use in Dermanura watsoni (Phyllostomidae), a frugivorous bat species that is a valuable seed disperser in degraded ecosystems. Radio-tracking of sixteen bats showed that the animals strongly rely on natural forest. Day roosts were exclusively located within mature forest fragments. Selection ratios showed that the bats foraged selectively within the available habitat and positively selected natural forest. However, larger daily ranges were associated with higher use of degraded habitats. Home range geometry and composition of focal foraging areas indicated that wider ranging bats performed directional foraging bouts from natural to degraded forest sites traversing the matrix over distances of up to three hundred meters. This behavior demonstrates the potential of frugivorous bats to functionally connect fragmented areas by providing ecosystem services between natural and degraded sites, and highlights the need for conservation of natural habitat patches within agricultural landscapes that meet the roosting requirements of bats.

Automated proximity sensing in small vertebrates: design of miniaturized sensor nodes and first field tests in bats.

Abstract Social evolution has led to a stunning diversity of complex social behavior, in particular in vertebrate taxa. Thorough documentation of social interactions is crucial to study the causes and consequences of sociality in gregarious animals. Wireless digital transceivers represent a promising tool to revolutionize data collection for the study of social interactions in terms of the degree of automation, data quantity, and quality. Unfortunately, devices for automated proximity sensing via direct communication among animal‐borne sensors are usually heavy and do not allow for the investigation of small animal species, which represent the majority of avian and mammalian taxa. We present a lightweight animal‐borne sensor node that is built from commercially available components and uses a sophisticated scheme for energy‐efficient communication, with high sampling rates at relatively low power consumption. We demonstrate the basic functionality of the sensor node under laboratory conditions and its applicability for the study of social interactions among free‐ranging animals. The first field tests were performed on two species of bats in temperate and tropical ecosystems. At <2 g, this sensor node is light enough to observe a broad spectrum of taxa including small vertebrates. Given our specifications, the system was especially sensitive to changes in distance within the short range (up to a distance of 4 m between tags). High spatial resolution at short distances enables the evaluation of interactions among individuals at a fine scale and the investigation of close contacts. This technology opens new avenues of research, allowing detailed investigation of events associated with social contact, such as mating behavior, pathogen transmission, social learning, and resource sharing. Social behavior that is not easily observed becomes observable, for example, in animals living in burrows or in nocturnal animals. A switch from traditional methods to the application of digital transceiver chips in proximity sensing offers numerous advantages in addition to an enormous increase in data quality and quantity. For future applications, the platform allows for the integration of additional sensors that may collect physiological or environmental data. Such information complements social network studies and may allow for a deeper understanding of animal ecology and social behavior.

Low-weight wireless sensor network for encounter detection of bats

In this paper a compact low weight wireless sensor node for proximity detection is presented. The energy-efficient system design enables an operation time of more than 9 days at a total weight of the tag of less than 2 g. Test measurements in a colony of bats proof the applicability of the system for encounter detection of individual bats based on received signal strength measurements.

Monitoring Bats in the Wild: On Using Erasure Codes for Energy-Efficient Wireless Sensor Networks

We explore the advantages of using Erasure Codes (ECs) in a very challenging sensor networking scenario, namely, monitoring and tracking bats in the wild. The mobile bat nodes collect contact information that needs to be transmitted to stationary base stations whenever they are in communication range. We are particularly interested in improving the overall communication reliability of the wireless communication. The mobile nodes are capable of storing a few 100kB of data and to exchange contact information in aggregated form. Due to the continuous flight of the bats and the forest environment, the wireless channel quality varies quickly and, thus, the communication is in general assumed to be highly unreliable. Given the very strict energy constraints of the mobile node and the inherently asymmetric channels, conventional techniques such as full data replication or Automatic Repeat Request to improve the communication reliability are prohibitive. In this work, we investigate the tradeoff between reliability achieved and the cost in form of additional transmissions, that is, the additional energy costs. Our energy measurements on a real platform combined with larger-scale simulation of the wireless communication clearly indicate the advantages of using ECs in our scenario. The results are also applicable in other configurations when unreliable communication channels meet tight energy budgets.

System design for encounter detection of distributed wireless sensors

In this paper a system for proximity detection of small sized animals like bats is presented. The system is based on a time slotted communication between compact low weight mobile nodes. For precise time synchronization of the nodes a scalable grid of base stations is proposed that distributes a time reference signal using a time division multiple access (TDMA) scheme. The system parameters are chosen considering the timing constraints caused by the severe requirements to the mobile nodes. In a power consumption analysis the system operation time limited by the 1.8 g nodes is calculated to be 7.4 days. First field test results are shown which proof the functional capability of the systems principle.

Low-weight wireless sensor node for animal encounter detection and dual-band localization

In this paper the feasibility of encounter detection and simultaneous localization of bats in their natural habitat is demonstrated. Furthermore a compact design of an enhanced mobile node exhibiting a weight of only 1.85 g is presented. The new design allows for an increased robustness of the localization by implementing dual-band signal transmission at 2.4GHz and in the sub-gigahertz range. The prolongation of operation time up to 14 days and beyond is discussed in detail.

Home Range of Noack's Round-Leaf Bat (Hipposideros aff. ruber) in an Agricultural Landscape of Central Ghana

Thirteen individuals of Noacks round-leaf bat, Hipposideros aff. ruber, were radio-tracked for 38 nights in an agricultural landscape in Kwamang, Ashanti Region, Ghana. Local convex hulls were used to estimate home range sizes of the bats. Based on 1,192 fixes, the mean (± SD) home range size was 36 ha ± 35 ha. Individual home range size ranged from six to 95 ha and frequently overlapped among individuals. The foraging area covered 50% of the home range while the core area formed 2%. The mean maximum foraging distance was 1.1 km, with individual distances up to 2.6 km, suggesting Hipposideros aff. ruber is capable of covering relatively long distances. Male bats returned to the cave more often than females during the night. Although the cave was the main roost, each bat also had individual night roosts on trees.

Proximity sensors reveal social information transfer in maternity colonies of Greater noctule bats

Bats are a highly gregarious taxon suggesting that social information should be readily available for making decision. Social information transfer in maternity colonies might be a particularly efficient mechanism for naïve pups to acquire information on resources from informed adults. However, such behaviour is difficult to study in the wild, in particular in elusive and small-bodied animals such as bats. The goal of this study was to investigate the role of social information in acquiring access to two types of resources, which are crucial in the life of a juvenile bat: suitable roosting sites and fruitful feeding grounds. We hypothesized that fledging offspring will make use of social information by following informed members of the social groups to unknown roosts or foraging sites. In the present study we applied for the first time the newly developed miniaturized proximity sensor system ‘BATS’, a fully automated system for documenting associations among individual bats both while roosting and while on the wing. We quantified associations among juveniles and other group member while switching roosts and during foraging. We found clear evidence for information transfer while switching roosts, mainly among juveniles and their genetically identified mothers. Anecdotal observations suggest intentional guidance behaviour by mothers, indicated by repeated commuting flights among the pup and the target roost. Infrequent, short meetings with colony members other than the mother indicate local enhancement at foraging sites, but no intentional information transfer. Our study illustrates how advances in technology enable researchers to solve long-standing puzzles. Miniaturized proximity sensors facilitate the automated collection of continuous data sets and represent an ideal tool to gain novel insights into the sociobiology of elusive and small-bodied species.

Two dispersers are better than one: a \'bird-fig\' attracts bats via nocturnal scent

Abstract The plant genus Ficus is a keystone resource in tropical ecoystems. One of the unique features of figs is the diversity of fruit traits, which in many cases match their various dispersers, the so-called fruit syndromes. The classic example of this is the strong phenotypic differences found between figs with bat and bird dispersers (color, size, and presentation). The ‘bird-fig’ Ficus colubrinae represents an exception to this trend since it attracts the small frugivorous bat species Ectophylla alba at night, but during the day attracts bird visitors. Here we investigate the mechanism by which this ‘bird-fig’ attracts bats despite its fruit traits, which should appeal solely to birds. We performed feeding experiments with Ectophylla alba to assess the role of fruit scent in the detection of ripe fruits. Ectophylla alba was capable of finding ripe figs by scent alone under exclusion of other sensory cues. This suggests that scent is the main foraging cue for Ectophylla alba. Analyses of odor bouquets from the bat- and bird-dispersal phases (i.e. day and night) differed significantly in their composition of volatiles. The combination of these two findings raises the question whether E. alba and F. colubrinae resemble a co-adaptation that enables a phenotypically classic ‘bird-fig’ to attract bat dispersers by an olfactory signal at night thus maximizing dispersal.The plant genus Ficus is a keystone resource in tropical ecoystems. One of the unique features of this group is the modification of fruit traits in concert with various dispersers, the so-called fruit syndromes. The classic example of this is the strong phenotypic differences found between figs with bat and bird dispersers (color, size, and presentation). The ‘bird-fig’ Ficus colubrinae represents an exception to this trend since it attracts the small frugivorous bat species Ectophylla alba at night, but during the day attracts bird visitors. Here we investigate the mechanism by which this ‘bird-fig’ attracts bats despite its morphology which should appeal solely to birds. We performed feeding experiments with Ectophylla alba to assess the role of fruit scent in the detection of ripe fruits. Ectophylla alba was capable of finding ripe figs by scent alone under exclusion of other natural sensory cues. This suggests that scent is the key signal in the communication between Ectophylla alba and Ficus colubrinae. Analyses of odor bouquets from the bat- and bird-dispersal phases (i.e. day and night) differed significantly in their composition of volatiles. This indicates that an olfactory signal allows a phenotypically classic ‘bird-fig’ to attract bat dispersers at night thus to maximizing dispersal.