Sex-specific temperature dependence of foraging and growth of intertidal snails

Predicting the biological impacts of climate change requires an understanding of how temperature alters organismal physiology and behavior. Given differences in reproductive physiology between sexes, increases in global temperature may be experienced differently by the males and females of a species. This study tested for sex-specific effects of increased air temperature on foraging, growth, and survival of an intertidal snail, Nucella ostrina (San Juan Island, Washington, 48–30′44″N, 123–08′43″W). Snails exhibited periodic peaks in foraging. Subjecting snails to elevated low tide air temperatures did not alter the timing or magnitude of this pattern. Despite similar temporal patterns in foraging, females foraged more than males, even when the risk of thermal stress was high. While males and females appear to have a similar body temperature threshold for optimal growth, females were more likely to cross that threshold resulting in a loss of body mass when exposed to daily increases in air temperature. These results suggest that the consequences of a warming climate in the short term may be different for males and females of N. ostrina, but also imply longer-term costs of reduced reproductive output, abundance, and distribution of this ubiquitous intertidal predator. Generally, this study points to the possible significance of sex-specific responses in an increasingly warm world.     

Temperature, salinity and prey availability shape the marine migration of Arctic char, Salvelinus alpinus, in a macrotidal estuary

The influence of salinity, temperature and prey availability on the marine migration of anadromous fishes was determined by describing the movements, habitat use and feeding behaviours of Arctic char (Salvelinus alpinus). The objectives were to determine whether char are restricted to the upper water column of the inter-/subtidal zones due to warmer temperatures. Twenty-seven char were tracked with acoustic temperature/pressure (depth) transmitters from June to September, 2008/2009, in inner Frobisher Bay, Canada. Most detections were in surface waters (0–3 m). Inter-/subtidal movements and consecutive repetitive dives (maximum 52.8 m) resulted in extreme body temperature shifts (−0.2–18.1 °C). Approximately half of intertidal and subtidal detections were between 9–13 °C and 1–3 °C, respectively. Stomach contents and deep diving suggested feeding in both inter-/subtidal zones. We suggest that char tolerate cold water at depth to capture prey in the subtidal zone, then seek warmer water to enhance feeding/digestion physiology.     

Combining heat-transfer and energy budget models to predict thermal stress in Mediterranean intertidal mussels

Recent studies have emphasised that organisms can experience physiological stress well within their geographic range limits. Developing methods for mechanistically predicting the presence, absence and physiological performance of organisms is therefore important because of the ongoing effects of climate change. In this study, we merged a biophysical–ecological (BE) model that estimates the aquatic (high tide) and aerial (low tide) body temperatures of Mytilus galloprovincialis with a Dynamic Energy Budget (DEB) model to predict growth, reproduction and mortality of this Mediterranean mussel in both intertidal and subtidal environments. Using weather and chlorophyll-a data from three Mediterranean sites along the Italian coasts, we show that predictions of sublethal and lethal (acute) stress can potentially explain the observed distribution (both presence and absence) of mussels in the intertidal and subtidal zones, and the maximum size of animals in the subtidal zones. Importantly, our results suggest that different mechanisms limit the intertidal distribution of mussels, and that these mechanisms do not follow a simple latitudinal gradient. At the northernmost site (Palermo), M. galloprovincialis appears to be excluded from the intertidal zone due to persistent exposure to lethal aerial temperatures, whereas at the southernmost sites (Porto Empedocle and Lampedusa) sublethal stress is the most important driver of mussel intertidal distribution. Our predictions provide a set of hypotheses for future work on the role of climate change in limiting intertidal distribution of mussels in the Mediterranean.     

Depletion of heavy isotopes of oxygen and hydrogen in tissue water of intertidal plants: implications for water economy

Stable oxygen and hydrogen isotope ratios of leaf and thallus water of the intertidal seagrasses Phyllospadix scouleri and P. torreyi and the marine algae Egregia menziesii, Gelidium coulteri, and Corallina vancouverensis from three locations in California, USA, were determined in 1987. Compared with subtidal seawater, most plant-water samples were depleted in the heavy isotopes ¹⁸O and deuterium. Depletion of heavy isotopes was greatest in plants growing at the highest intertidal elevations. This was an unexpected result, because enrichment of heavy isotopes occurs during evapotranspiration in terrestrial plants. Two possible mechanisms for this isotopic depletion are proposed: direct uptake of heavy isotope-depleted water vapor and preferential diffusion of ¹⁶O and ¹H into littoral plants from water remaining in the intertidal zone at low tide.     

Distribution of Juvenile,subadult and adult Scylla serrata (Crustacea: Portunidae) on tidal flats in Australia

In Deception Bay, northern Australia, during 1979–1981, a study was made of the distribution of Scylla serrata (Forskal) in an area having a broad intertidal zone. Juveniles (20 to 99 mm carapace width) were resident in the mangrove zone, remaining there during low tide. The majority of subadult crabs (100 to 149 mm) migrated into the intertidal zone to feed at high tide and retreated to subtidal waters at low tide. Adults (150 mm and larger) were caught mainly subtidally and only small numbers were captured in the intertidal at high tide. Few crabs were captured in the coolest months (May to August). Adults were captured on the flats mainly in the warmest months (January to April), but subadults could be captured over the entire summer (September to March). Juveniles were found in the upper intertidal throughout the year.     

Regulation of intertidal food webs by avian predators on New England rocky shores

Although there is a large body of research on food webs in rocky intertidal communities, most of the emphasis has been on the marine benthic components. Effects of avian predation on highly mobile predators such as crabs, remains practically unstudied in rocky shore ecosystems. The crab, Cancer borealis, is an important component of the diet of gulls (Larus marinus, L. argentatus) at the Isles of Shoals, Maine, USA. C. borealis prey include the predatory gastropod Nucella lapillus L., the herbivore Littorina littorea, and mussels Mytilus edulis L. We hypothesized that gulls reduce abundance of C. borealis in the low intertidal and shallow subtidal, thereby allowing C. borealis prey to persist in high numbers. A study of crab tidal migration showed that C. borealis density nearly doubled at high tide compared to low tide; thus, crabs from a large subtidal source population migrate into the intertidal zone during high tides and either emigrate or are removed by gulls during low tides. Results from a small-scale (1 m2) predator caging experiment in the low intertidal zone indicated that enclosed crabs significantly reduced L. littorea abundance when protected from gull predation. In a much larger-scale gull exclusion experiment, densities of C. borealis increased significantly during low and high tides in exclosures relative to the controls. C. borealis density was inversely correlated with changes in the abundance of two mesopredators Carcinus maenas and Nucella lapillus, and with the space-occupier M. edulis. There was a similar negative correlation between abundance of C. borealis and the change in abundance of the herbivore L. littorea, but the trend was not significant. Mortality of tethered L. littorea was associated with C. borealis density across sites. However, preferred algae did not change in response to L. littorea density during the experiment. Thus, we found suggestive, but not conclusive, evidence for a three-level cascade involving gulls, crabs, and L. littorea. Our studies strongly suggest that gulls, as apex predators, generate three-level trophic cascades in rocky intertidal food webs by preventing the highly mobile subtidal predator, C. borealis, from establishing substantial populations in the low-mid intertidal zone thereby indirectly enhancing densities of two key mesopredators (N. lapillus, Carcinus) and blue mussels (M. edulis).     

Feeding strategies and diets of breeding grey-headed and wandering albatrosses at South Georgia

The foraging areas and diets of the grey-headed albatross Thalassarche chrysostoma and wandering albatross Diomedea exulans were studied in March/April 2000 at Bird Island, South Georgia, during their respective chick-rearing and brood-guard periods. Oceanographically, March/April 2000 was abnormal, with warm conditions close to South Georgia. These conditions affected albatross foraging behaviour, particularly that of grey-headed albatrosses. Both species tended to forage in different areas of the ocean, with significant differences in trip durations. Grey-headed albatrosses (n=9) foraged mainly in Antarctic waters (predominantly shelf waters of the South Shetland Islands and Antarctic Peninsula, and also in oceanic waters around South Georgia), feeding mainly on krill (Euphausia superba; 77% by mass). Foraging trips lasted 13.3 days (range: 5–26 days), far longer than the 1–3 days found in previous studies. Only one grey-headed albatross was associated with the APF (Antarctic Polar Front), a reported foraging area in recent studies. Wandering albatrosses (n=9) foraged in Antarctic (South Georgia Shelf) and Antarctic Polar Frontal Zone (APFZ) waters, with trips of 1–4 days trip duration (usual for this species), feeding on fish (46% by mass) and cephalopods (32%). One bird was associated with the APF, and two birds foraged on the shelf/shelf break over the Patagonian shelf. These findings suggest that sea surface temperature anomalies, produced by movement of the APF closer to South Georgia or by eddies, may have had an effect on the foraging strategy of grey-headed albatrosses that year (the main prey of grey-headed albatrosses in previous studies, the ommastrephid Martialia hyadesi, is known to be associated with the APF). Also, when both albatross breeding periods overlap, their foraging areas were complementary, which reflected the prey taken.     

Sibling species or poecilogony in the polychaete<Emphasis Type="Italic"> Scoloplos armiger</Emphasis>?

In marine invertebrates multiple modes of development, or poecilogony, may occur in a single species. However, after close examination, many of such putative cases turned out to be sibling species. A case in point may be the cosmopolitan orbiniid polychaete Scoloplos armiger, which inhabits marine shallow sediments. In addition to the well-known direct, holobenthic development from egg cocoons, pelagic larvae have also been described. Our culture experiments revealed a spatially segregated source of the two developmental modes. All females of an intertidal population produced egg cocoons and no pelagic larvae. All but 2 out of 15 females of an adjacent subtidal population produced pelagic larvae and no egg cocoons. Based on these results we performed a molecular genetic analysis (RAPD-PCR) on three intertidal and four subtidal populations in the North Sea. Selected samples from all sites were analysed also by the AFLP method. We found significantly higher genetic diversity within subtidal than within intertidal populations. This is consistent with a wider dispersal by pelagic larvae and a smaller effective population size when development is holobenthic. Total genetic divergence is not related to distance but to the intertidal/subtidal division. We suggest that S. armiger actually represents two sibling species.     

Seasonal and spatial differences in the fish assemblage of the non-estuarine Sulaibikhat Bay,Kuwait

In the period September 1986 to August 1987 fish were captured once a month, using an otter trawl, from the intertidal and subtidal regions of Sulaibikhat Bay, Kuwait. Correlation analysis revealed that numbers were not directly related to temperature at the time of sampling. The recruitment of large numbers of young of the year Liza carinata (Valenciennes) during March was related to low temperatures in November, the probable time of spawning of this species. Salinity at the time of sampling was inversely correlated with numbers. This result indicates that large numbers of 0+fish recruit to the Bay during the period of maximum fresh water outflow through the Shatt-al-Arab. Numbers of fish were significantly greater in the intertidal region, where they were present almost entirely as 0 group fish, than in the subtidal. The three dominant species of the assemblage are shown to use the two depth intervals in differnt ways. L. carinata was capture a almost exclusively in the intertidal region as 0+ fish. Pomadasys stridens (Forsskål) was captured as 0+ fish in both the intertidal and subtidal regions but in greater numbers in the intertidal region. Leiognathus brevirostris (Valenciennes) was captured as small, mostly 0+ individuals in the intertidal region and as larger fish in the subtidal region. The use made by the smaller fish of the intertidal region is related to the avoidance of sublittoral predators and reduction of intraspecific competition, whilst larger fish in the sublittoral region may be avoiding predation by piscivorous birds.     

No evidence for homeoviscous adaptation in intertidal snails: analysis of membrane fluidity during thermal acclimation, thermal acclimatization, and across thermal microhabitats

Many eurythermal organisms alter composition of their membranes to counter perturbing effects of environmental temperature variation on membrane fluidity, a process known as homeoviscous adaptation. Marine intertidal gastropods experience uniquely large thermal excursions that challenge the functional integrity of their membranes on tidal and seasonal timescales. This study measured and compared membrane fluidity in marine intertidal snail species under three scenarios: (1) laboratory thermal acclimation, (2) thermal acclimatization during a hot midday low tide, and (3) thermal acclimatization across the vertical intertidal zone gradient in temperature. For each scenario, we used fluorescence polarization of the membrane probe DPH to measure membrane fluidity in individual samples of gill and mantle tissue. A four-week thermal acclimation of Tegula funebralis to 5, 15, and 25°C did not induce differences in membrane fluidity. Littorina keenae sampled from two thermal microhabitats at the beginning and end of a hot midday low tide exhibited no significant differences in membrane fluidity, either as a function of time of day or as a function of thermal microhabitat, despite changes in body temperature up to 24°C within 8 h. Membrane fluidities of a diverse group of snails collected from high, middle, and low vertical regions of the intertidal zone varied among species but did not correlate with thermal microhabitat. Our data suggest intertidal gastropod snails do not exhibit homeoviscous adaptation of gill and mantle membranes. We discuss possible alternatives for how these organisms counter thermal excursions characteristic of the marine intertidal zone.     

Abalones and sea urchins in an area inhabited by sea otters

Abalones (Haliotis spp.) and sea urchins (Strongylocentrotus spp.) are part of the subtidal fauna in the kelp bed off Hopkins Marine Station, Pacific Grove, California, USA, a protected marine reserve. Although these animals have been preyed upon by sea otters for over 10 years, their densities are gubstantial (Haliotis spp.: 0.21/m²; Strongylocentrotus spp.: 0.22/m²), and two species, H. rufescens (Swainson) and S. purpuratus (Stimpson), have wide size distributions indicating broad age ranges. Most of the adult abalones and sea urchins occur clustered deep in crevices, either as a direct result of sea-otter predation or because ample drift of algal food reduces foraging activities. Abalones tend to occur in larger crevices than sea urchins, and competition for suitable crevice space may exist among these large, invertebrate, algal-drift feeders.     

The organization of work in Polybia occidentalis: costs and benefits of specialization in a social wasp

Summary Nest construction, a complex social activity requiring the coordination of 3 tasks (Fig.2), was compared in large (<350 adults) and small (<50 adults) colonies of Polybia occidentalis. The 3 tasks—water foraging, pulp foraging, and building—are performed by 3 separate groups of workers (Fig.4). Of the 8 acts comprising the 3 tasks, 5 regularly involve the transfer of water or pulp from one worker to another on the nest.Small colonies required nearly twice as long (35.4 worker-min) as large colonies (20.1 workermin) to complete a unit amount of construction work. Behavioral acts involving material transfer among workers were responsible for most of the increase in small colonies. In other words, the waiting times experienced by material donors and recipients were greater in small colonies. In small colonies workers switched among the three tasks more frequently than in large colonies (Fig. 4). This was the result of more frequent switching by generalists (workers that performed 2 or 3 of the tasks), rather than by a decrease in the proportion of specialists (workers performing only 1 task type) (Fig. 3).The series-parallel system by which Polybia occidentalis organizes nest construction has a major advantage over the series operation of solitary wasps. Pulp foragers collect and carry loads that are 6.1 times as large as builders can work with at the nest, and water foragers bring in loads that appear to be limited only by crop capacity and that provide all the moisture necessary for the complete processing of 0.74 of a foraged pulp load. As a result P. occidentalis can collect and process a given amount of nest material using 2.6 times fewer foraging trips than would be required by the series system. This in turn means that P. occidentalis not only achieves an energy saving that probably more than offsets the increased costs of material handling at the nest, but it reduces the exposure of its foragers to predators in the field.     

The contribution of seagrass beds (<Emphasis Type="Italic">Zostera noltii</Emphasis>) to the function of tidal flats as a juvenile habitat for dominant,mobile epibenthos in the Wadden Sea

Since the substantial loss of subtidal eelgrass (Zostera marina L.) in the 1930s, seagrass beds in the Wadden Sea are limited to the intertidal zone and dominated by Z. noltii Hornem. This study deals with the effect of vegetated tidal flats on quantities of mobile epifauna and proves empirically the function of seagrass canopies as a refuge for marine animals remaining in the intertidal zone at ebb tide. Drop-trap samples were taken in the Sylt-Rømø Bight, a shallow tidal basin in the northern Wadden Sea, on vegetated and unvegetated tidal flats during July and August 2002, and during the entire growth period of Z. noltii from May to September in 2003. The species composition in Z. noltii and bare sand flats showed minor differences since only two isopod species (Idotea baltica and I. chelipes) occurred on Z. noltii flats exclusively. Juvenile shore crabs (Carcinus maenas L.), brown shrimps (Crangon crangon L.) and common gobies (Pomatoschistus microps Krøyer) were also found abundantly on bare sand flats. However, the results showed significantly higher abundances and production of these dominant species on vegetated tidal flats. Additionally, the analyses of faunal size classes indicated higher percentages of small individuals in the seagrass bed during the entire sampling period. Despite drastic diurnal fluctuations of dissolved oxygen at low tide, faunal density in the residual water layer remaining in seagrass canopies at ebb tide was found to be consistently higher than that found in artificially created tide-pool units. Although species composition of mobile epifauna did not basically differ between vegetated and unvegetated tidal flats, Z. noltii beds are considered to contribute quantitatively to the function of tidal flats, as an extended juvenile habitat for some of the most important species of the Wadden Sea food web.Communicated by O. Kinne, Oldendorf/Luhe     

Disturbance by ice and life-history adaptations of the seagrassZostera marina

The formation of winter ice, and its movement with the tides, has had a major influence on the life-history parameters of shallow-water populations of the rhizomatous marine angiosperm eelgrass (Zostera marina) in Nova Scotia, Canada. In this region, and annual form of eelgrass inhabits intertidal and shallow subtidal mudflats and a perennial form is common in subtidal areas. Where lowtide water depth was greater than winter ice thickness (25 cm), ice movement removed much of the aboveground biomass from perennial ramets, but did not influence the density of ramets. Measurements of primary production showed that perennial plants allocated a greater proportion of their total production to below-ground structures than measured in all previous studies on eelgrass. Thus perennials are well anchored in the sediment, are more able to withstand removal by ice, and have reserves available for production of new leaves when ice melts. In the spring, seed germination was greatest in areas where ice had removed whole plants (or their aboveground parts) than among mature ramets which had survived winter intact. Survival of seedlings (new genets) was not affected by shading from the adult canopy, but a shading experiment showed that competition for light with mature ramets had a significant negative effect on morphology, growth and the allocation of seedling biomass to below-ground parts, thus reducing the ability of new genets to survive ice disturbance in the next winter. The annual form ofZ. marina was restricted to areas where low tide water depth was much less than winter ice thickness. Annual plants did not survive winter, had small investment in below-ground parts and high reproductive effort, and overwintered as seeds. The genetic status of the two forms and the restriction of the annual to very shallow water are discussed in the light of previous work on eelgrass     

Algal “gardening” behavior by nereid polychaetes: Effects on soft-bottom community structure

Nereid polychaetes (Nereis vexillosa and Platynereis bicanaliculata) attach pieces of drift algae to their tube surfaces. The presence of permanent algal cover increases the predictability of the food supply for at least the herbivores (including the nereids) and the deposit feeders, and modulates the temperature and salinity stresses of the marine intertidal soft-bottom environment. However, it may affect the access of organisms to the oxygenated water layers above the sediment surface. This is true for polychaetes that live head downwards in vertical tubes. It is suggested that the attachment behavior of the nereid polychaetes increases the ability of the plant to colonize habitats both temporally and spatially.     

Biogeographic patterns on shallow subtidal reefs in the western Indian Ocean

To resolve biogeographic limits and patterns on the east coast of Africa, the presence/absence and quantitative biomass data were collected from 55 shallow subtidal reefs along 4,800 km of coastline (5.2°–31.1°S). Multivariate analysis of distributions, trophic structure and biomass revealed two distinct marine provinces, the Tropical Indo-West Pacific and Subtropical Natal, with a transitional overlap (of ca. 120 km) located between Leven Point and Kosi Mouth on north-east South Africa. This region of overlap was one of three bioregions revealed by post hoc analyses. Biomass was unexpectedly highest in the tropics. ‘Auto-heterotrophs’ (species with autotrophic symbionts such as corals and clams), deposit feeders and grazers contributed significantly more biomass to the Tropical Indo-West Pacific Bioregion, whereas filter feeders dominated the Subtropical Natal Bioregion. ‘Auto-heterotrophs’ declined with latitude while filter feeders increased; soft corals made a defining contribution in the overlap bioregion. Possible underlying causes of these patterns include productivity, nutrient levels, riverine input, light penetration and temperature.     

Predators influence the tidal distribution of oysters (Crassostrea virginica)

Biotic and abiotic conditions can separately and synergistically influence the abundance and distribution of species and create vertical zonation patterns in marine systems. In Corpus Christi Bay, TX, USA, eastern oysters (Crassostrea virginica) are limited to intertidal habitats, while in adjacent estuaries, oysters not only grow subtidally, but thrive in these areas to the extent they are a viable commercial fishery. The purpose of this study was to assess how predators and abiotic conditions affect oyster mortality and growth at different tidal elevations. Anecdotal evidence suggests that abiotic conditions, primarily hypoxia and salinity, as well as oyster disease, limits oysters to intertidal areas. Yet, in Corpus Christi Bay, oysters are absent from subtidal areas where hypoxia is not known to occur. Infection by Perkinsus marinus (Dermo) is common in the study area, but previous work suggests that infection rates do not increase when oysters are transplanted subtidally. We investigated oyster tidal distributions by transplanting newly settled oysters into intertidal and subtidal areas. Predation on oysters was significantly greater in subtidal as compared to intertidal habitats. When protected from predators using cages, oyster survival significantly increased. Further, oysters in subtidal areas allocated significantly more resources to shell growth than did those in intertidal areas, and oysters are known to grow heavier shells in response to predators. Oyster settlement was not statistically different between inter and subtidal areas, and abiotic conditions measured during the study did not exceed known tolerance limits for oysters. Previous studies have shown that abiotic conditions influence oyster mortality and the success of restored oyster reefs. Our findings indicate that predators can also affect oyster distribution, and their effects should be evaluated when developing plans for oyster management and restoration.     

Niche segregation and habitat specialisation of harpacticoid copepods in a tropical seagrass bed

Several harpacticoid copepod species are adapted to an epiphytic lifestyle. Previous studies on tropical seagrass meiofauna mainly focussed on the epiphytic communities and neglected the benthic component. The present study aims to document the benthic harpacticoid copepod communities sampled from different sediment depth horizons adjacent to five seagrass species in the intertidal and subtidal zone of a tropical seagrass bed (Gazi Bay, Kenya). Two benthic copepod communities could be identified mainly based on the tidal position of the samples: a first community was collected near the intertidal seagrasses Halophila ovalis and Halodule wrightii; a second community occurred near the subtidal seagrasses Thalassia hemprichii, Syringodium isoetifolium and Halophila stipulacea. The first community was mainly determined by sediment characteristics (e.g. skewness), while the second community was split off based on organic matter content (% TOM), nutrient and pigment values. A subtle combination of horizontal and vertical niche segregation was reported for the dominant copepod families. Species of the families Thalestridae, Laophontidae and Diosaccidae were structured by tidal position and showed a strong preference for the subtidal zone. The opposite strategy, i.e. a clear preference for the intertidal zone, was found for copepods belonging to the families Paramesochridae and Canuellidae. In addition, Apodopsyllus africanus (Paramesochridae) was well-adapted to stress and was concentrated in the deeper sediment layers near the subtidal seagrasses. On the other hand, Canuellidae, as filter feeders, were concentrated in the upper centimetres of the sediment. The families Ectinosomatidae and Cletodidae did not show any vertical or horizontal segregation. On the species level, however, clear horizontal niche segregation was detected for the family Cletodidae. In addition to the reported ecological results, the study material was used to evaluate different niche definitions. We found tidal position to be the most important factor forcing harpacticoids to specialise. Sediment depth horizon was less powerful in dividing the families into different guilds (from specialists to generalists) based on standardised niche breadth. The present study documents the subtle habitat partitioning of co-existing species in a limited area and its role in sustaining high biodiversity in the community.     

Foraging behaviour and echolocation in the rufous horseshoe bat (Rhinolophus rouxi) of Sri Lanka

Summary In October 1984 foraging areas and foraging behaviour of the rufous horseshoe bat, Rhinolophus rouxi, were studied around a nursery colony on the hill slopes of Sri Lanka. The bats only foraged in dense forest and were not found in open woodlands (Fig. 1). This strongly supports the hypothesis that detection of fluttering prey is by pure tone echolocation within or close to echo-cluttering foliage. During a first activity period after sunset for about 30–60 min, the bats mainly caught insects on the wing. This was followed by a period of inactivity for another 60–120 min. Thereafter the bats resumed foraging throughout the night. They mainly alighted on specific twigs and foraged in flycatcher style. Individual bats maintained individual foraging areas of about 20x20 m. They stayed in this area throughout the night and returned to the same area on subsequent nights. Within this area the bats generally alighted on twigs at the same spots. Foraging areas were not defended against intruders. The bats echolocated throughout the night at an average repetition rate of 9.6±1.4 sounds/s. While hanging on twigs they scanned the surrounding area for flying prey by turning their bodies continuously around their legs. On average they performed one brief catching flight every 2 min and immediately returned to one of their favourite vantage points. Echolocation sounds may consist of up to three parts, a brief initial frequency-modulated (FM) component, a long constant frequency (CF) part lasting for about 40–50 ms, and a final FM part again (Fig. 4b, c). Adult males and females emitted pure tone frequencies in separate bands, the males from 73.5–77 kHz and the females from 76.5–79 kHz (Fig. 5). During scanning for prey from vantage points, the bats mostly emitted pure tones without any FM component (Fig. 4a). The last few pure tones emitted before take-off were prolonged to about 60 ms duration. The final FM part was therefore not an obligatory component of the echolocation signals in horseshoe bats. During flight and especially during emergence from the cave, most sounds consisted of a pure tone and loud initial and final FM sweeps. We therefore suggest that the initial FM part might also be relevant for echolocation. From our observations we conclude that the FM components are especially important during obstacle avoidance. In most sounds emitted in the field a fainter first harmonic was present. It was usually up to 30 dB fainter than the second harmonic, but in some instances it was as loud or even distinctly louder than the second one (Fig. 6a). Even within one sound the intensity relationship between the two harmonics may be reversed. We therefore suggest that the first harmonic is an integral part of the signal and relevant for information analysis in echolocation.