Rediscovering the Red-Armed Bat - an updated monograph on Natterer's (Myotis nattereri)

A quarter of a century ago, eek, I completed fieldwork on the border of England and Wales that contributed to the knowledge base on Natterer's Bat and informed habitat management guidance for the species (https://cieem.net/resource/habitat-management-for-natterers-bats/).

Armed with a red Yugo (affectionally: ‘Don’t Go’), roof racks, a Dictaphone, and Yagi antennas, I spent many happy early-morning hours blundering through the countryside along the Welsh borders, alarming livestock and the local constabulary in equal measure, on the quest for a fix (of the radio-tracking kind). ?

Fast-forward to the time of this article, Natterer's is acknowledged as a widespread species, and the number of occurrence records for Britain has certainly grown, but relatively few new accounts of the species’ ecology have been published.

In this article, I bring together and reflect on research completed in the 21st century on the behaviour, ecology, and conservation of this very charming bat.

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1. Species Background and Distribution

Natterer's bat (Myotis nattereri) is a medium-sized vespertilionid bat with a body mass ranging from 4-13 grams and a wingspan between 245-300 mm.

Natterer's are characterised by a combination of morphological features. The dorsal fur is typically a light buff-brown, contrasting sharply with the pure white ventral fur, the face and ears are notably bare, and the limbs often exhibit a pinkish hue, a trait contributing to its colloquial name, the ‘red-armed bat’.

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The ears are long and narrow, extending beyond the tip of the muzzle when laid forward, and possess a distinctive notch along the outer margin. The tragus, a prominent flap of skin within the ear, is slender and lanceolate, extending beyond the ear notch. A key diagnostic feature is the presence of a dense fringe of stiff, downward-pointing bristles along the outer edge of the uropatagium, the membrane between the hind legs. This feature, combined with the bat's S-shaped calcar (a cartilaginous spur on the heel), aids in distinguishing Natterer's from the similar vespids, Geoffroy's and Bechstein's bats.

Natterer's as a species is widely distributed across Europe, extending into parts of Asia and North Africa. In Britain, Natterer's is also widespread, though with occurrence becoming much patchier in Scotland, but considered under-recorded. The species is found in various habitats, including woodland, farmland, and urban areas, but demonstrates a preference for broadleaved woodland, particularly those with diverse tree species and abundant edge habitats, and river corridors. This preference is likely due to the availability of suitable roosting sites in tree cavities and the abundance of insect prey. However, the bat has also been found to utilise other less expected habitats, such as coniferous plantations, highlighting an adaptability to different environments.

Natterer's is listed as a European Protected Species under Annex IV of the European Habitats Directive and is protected in Britain under the Wildlife and Countryside Act 1981. This legal protection prohibits the deliberate capture, killing, or disturbance of the bats, as well as damage or destruction of their roosts. However, despite limited population data, as the species is recorded across a large area of Britain and there is no evidence of a contraction of the geographical range over the last 20 years (see section 3), the conservation status of Natterer's is currently listed as Least Concern on both the IUCN and British Terrestrial Mammals Red Lists.

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2. Life History and Demography

Natterer's exhibit a life history strategy typical of many bat species: they are long-lived, with individuals recorded up to 20 years old, but have a low reproductive rate. The combination of longevity and low fecundity makes them particularly vulnerable to population declines caused by environmental changes and anthropogenic disturbances. The life history strategy means populations may take a long time to recover from declines, emphasising the importance of proactive conservation measures to protect the species.

Natterer's typically give birth to one offspring per year. The reproductive cycle is closely linked to hibernation patterns, with mating occurring primarily in the autumn during swarming. Ovulation and fertilisation are paused until spring, after the bats emerge from hibernation. This strategy, known as delayed fertilisation, allows females to conserve energy during the winter and time their reproduction with the peak in insect availability in the spring and summer. The gestation period lasts for several weeks, and the young are generally born June-July. The timing of parturition (birth) can be influenced by weather conditions, with colder temperatures potentially leading to delayed births and slower juvenile growth rates.

Several factors are revealed in millennial research that influence the reproductive success of Natterer's. Increased precipitation during the critical spring period can strongly reduce the probability of successful reproduction in first-year females (females that returned from their first hibernation). This may be due to the energetic constraints faced by young, inexperienced females during the early stages of pregnancy, when they may be less efficient at foraging and more susceptible to adverse weather conditions. Additionally, body size plays a role in reproductive success, with larger first-year females having a higher probability of successful reproduction; larger females likely have greater energy reserves after hibernation, enabling them to better cope with the demands of pregnancy and lactation.

Survival of Natterer's bats is influenced by age, sex, and environmental conditions. Adult survival is generally higher than juvenile survival, a pattern commonly observed in many bat species. However, contrary to the general trend in mammals where females typically exhibit higher survival rates, in Natterer's bats, males appear to have similar survival probability as females. Annual survival rates have been estimated of 0.79-0.87 for adult females and 0.38-0.59 for juveniles in a commercial coniferous plantation in Scotland. The study was unable to estimate male survival rates due to challenges in tracking their movements and potential for higher dispersal rates. However, other studies have estimated the survival rate of male Natterer's to be 0.86 (95% CI: 0.66-1.00), suggesting that approximately 50% of males survive to 5 years old, with some individuals living over 10 years.

Hibernation is a crucial survival strategy for Natterer's in the British temperate environment, allowing them to conserve energy during the winter months when food resources are scarce. During hibernation, bats enter a state of torpor, characterised by a significant reduction in metabolic rate and body temperature, allowing individuals to minimise energy expenditure and rely on stored fat reserves to survive the winter. However, Natterer’s hibernation is not a state of continuous dormancy. Bats periodically arouse from torpor to perform essential physiological functions such as maintaining water balance, excreting metabolic waste products, and potentially foraging. The frequency and duration of these arousals can vary depending on several factors, including ambient temperature, body condition, and the availability of food resources. Hibernating Natterer's in southern England arise frequently from torpor, suggesting that the bats may be actively foraging during winter, which has important implications for their energy balance and survival. Frequent awakenings can deplete energy reserves, but the ability to forage during winter may provide Natterer's with a competitive advantage over other bat species that rely solely on stored fat reserves during hibernation.

The timing of awakenings in Natterer's bats appears to be influenced by both endogenous and exogenous factors. Most occur coincident with the time (relative to sunset) typical for bats emerging from summer roosts to forage, indicating that Natterer’s may maintain a circadian rhythm of activity even during hibernation, potentially increasing their chances of encountering prey during periods of peak insect activity.

In addition to foraging, social interactions may also influence arousal patterns in hibernating bats. In one study, a captive colony exhibited synchronised activity, suggesting that social cues may play a role in triggering awakenings. However, the mechanisms underlying this synchronised behaviour remain unclear and require further investigation.

Another important aspect of hibernation in Natterer's bats is their huddling behaviour. Huddling significantly reduces evaporative water loss during torpor, and the behaviour is likely an adaptation to cope with challenges such as drier environments or periods of low humidity. Huddling has implications for the conservation of hibernacula, indicating the importance of maintaining suitable microclimatic conditions to minimise water loss and energy expenditure during hibernation.

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?3. Sensory Ecology and Foraging Behaviour

Natterer's are primarily nocturnal insectivores, relying heavily on echolocation for navigation and prey detection. The bats echolocation calls are characterised by broadband frequency-modulated (FM) signals, predominantly composed of the first harmonic. The peak frequency of these calls is around 50 kHz, with a range of 16 to 135 kHz and an average duration of 4 ms. These calls are adapted for foraging in ‘messy’ environments like dense vegetation, where the bats must distinguish prey echoes from background clutter.

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In the field, Natterer's vary their echolocation calls depending on their distance from vegetation. When flying close to vegetation, they emit short, broadband calls with a high starting frequency and a steep frequency modulation. Such calls are well-suited for detecting and localising prey among the clutter, providing high resolution and minimising the overlap between echoes. As the bats move further away from vegetation, their calls become longer, narrower in bandwidth, less steeply modulated and so better suited for detecting prey in open spaces, having a longer range and less susceptibility to atmospheric attenuation.

The ability to hunt close to vegetation using broadband echolocation calls thus allows Natterer's to exploit a niche that is less accessible to other bat species with narrower bandwidth calls, enabling the ability to snatch prey as close as five centimetres to plant surfaces.

Natterer's also take acoustic advantage of hunting at low heights above water. The smooth surface of water produces clearer prey echoes than rough surfaces like vegetation, as it reflects sound waves more uniformly.

The echolocation calls of Natterer's are not only used for navigation and prey detection but also for prey selection. The bats are selective in their choice of prey, preferring medium-sized insects (5-15 mm body length). By selecting prey of a certain size, individuals can optimise energy intake and minimise the handling time of prey. Echolocation calls become more frequent and intense as the bats approach prey, a behaviour known as a 'feeding buzz', which helps to precisely locate and capture.

While the diet of Natterer's bats varies geographically and seasonally, studies have identified common prey trends. In both Scotland and southern England, a significant proportion of their diet consists of non-flying invertebrates, such as spiders, beetles, harvestmen, and lepidopteran larvae. This suggests the adoption of gleaning, especially when aerial prey is less abundant. Natterer's adaptability in foraging strategies, switching between aerial hawking and gleaning, allows them to exploit a broader range of prey items and habitats.

The gleaning behaviour of Natterer's is a key aspect of their foraging ecology, allowing them to exploit a dietary niche. Gleaning involves the capture of prey directly from surfaces, such as leaves, branches, the ground, and the surface of water bodies. The behaviour is facilitated by the bat's specialised morphology, including its large ears, broad wings, and fringed tail membrane. Natterer’s primarily glean prey from the underside of leaves, using a combination of echolocation and passive listening to locate and then the tail membrane to scoop up prey.

DNA techniques have been deployed to analyse Natterer’s diets, providing a more comprehensive and detailed understanding of feeding habits. Some 40 prey species from six taxonomic orders have been identified, including several species of moths that overwinter as larvae. Natterer's are likely to be providing a beneficial regulatory ecosystem service, as some of the identified prey species are known to be agricultural pests.

While Natterer's are often associated with broadleaved woodland habitats, their foraging behaviour demonstrates a broader range of habitat utilisation, including woodland edges, hedgerows, tree lines, pastures, and even coniferous plantations. Adaptability in habitat use is likely a key factor in the bat's widespread distribution across Britain, however, broadleaved woodland with abundant edge habitat is preferred, likely due to the availability of suitable roosting sites in tree cavities and the higher abundance and diversity of insect prey. The structural complexity of mature woodlands provides ample opportunities for both aerial hawking and gleaning.

Studies have revealed that Natterer's exhibit foraging site fidelity and individual specialisation in their habitat use. Individual bats repeatedly exploit specific foraging areas and exhibit individual preferences in habitat use, suggesting a degree of territoriality. Such behaviour may be driven by the bats' ability to learn and remember the location of profitable foraging sites, as well as their social interactions.

The implication for conservation efforts is important, in that a need to protect a mosaic of habitats has been highlighted, rather than a single preferred habitat, to cater to the diverse foraging preferences of individual bats.

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4. Population Dynamics and Genetics

In Britain, the Hibernation Survey index for Natterer’s has increased significantly since the baseline year of monitoring (1999), however the Roost Count index shows no significant change since baseline year (2002). Neither index shows any significant change since 2017. As the Roost Count is not considered a reliable measure of Natterer's bat population trends due to frequent roost-switching behaviour (intra-year), the population of Natterer's bat in Great Britain is considered to have increased over the long term (since 1999) and have been stable in the short-term (since 2017).

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The estimated population size for Natterer's bats in Great Britain is around 400,000, with a plausible range of 15,000 to 2.6 million. Roughly 75% of the population resides in England. The wide estimated range is due to uncertainties in roost density and the extent to which bats roosting within woodlands use other habitats. The population estimate may also be affected by factors not included in the analysis, such as the sex ratio in maternity colonies and the potential underestimation of the range in Scotland due to low recording effort.

Research suggests that swarming sites play a crucial role in facilitating gene flow between populations, as evidenced by the recapture of bats from distant summer roosts at these sites. Swarming sites have extensive catchment areas, with bats travelling up to 63 km, underscoring the importance of protecting not only the swarming sites themselves but also the surrounding landscape that supports the bats' movements and dispersal. There is evidence for natal philopatry in both sexes, with limited dispersal from natal colonies, suggesting that swarming sites are vital for maintaining genetic diversity within populations.

One study further investigated the genetic structure within a maternity colony, finding low overall relatedness but multiple maternal lineages. Colony formation may therefore be driven by the benefits of group living, such as thermoregulation and information sharing, rather than kin selection, which is traditionally viewed as the primary driver of sociality in bats.

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Human disturbance, particularly at sensitive sites such as roosts and swarming sites, can negatively affect populations of Natterer's. The impact of disturbance can manifest in various ways, including increased energy expenditure, disruption of behaviour, and reduced reproductive success. The vulnerability of Natterer's to disturbance is further compounded by their slow life history and low reproductive rate which, as mentioned in section 2, can limit ability to recover from population declines.

A study on the impact of human disturbance on hibernating Natterer's found that while hibernation counts can cause a slight increase in bat activity, the effect is minimal and unlikely to threaten winter survival, particularly on warmer days when bat activity is naturally higher. However, the authors acknowledge that the study was limited to a single hibernaculum and did not consider the potential cumulative effects of repeated disturbances or the potential for variation in disturbance sensitivity.

The potential for human disturbance to negatively impact bat populations is more evident in the response of Natterer's to exclusion from roosts in historic churches. A study assessing such exclusions found increased roost switching and potential energetic stress, which can be inferred to negatively impact survival and reproductive success. The study also observed that the bats did not readily adopt artificial roosts provided, suggesting that mitigation measures that involve providing alternative roosts may need to be reconsidered.

Swarming sites are particularly vulnerable to human disturbance, as they are often associated with woodland, caves and mines that are also used by humans for recreational or other purposes. Potential impacts include the disruption of mating behaviour and the displacement of bats to less suitable sites.

Disturbance at roosts, particularly during the maternity season, have significant consequences. Natterer’s exhibit high inter-annual fidelity to roosts, with certain roosts being reused for the same purpose in consecutive years. The loss or disturbance of roosts disrupts the bats' reproductive cycle and potentially leads to reduced reproductive success. Roosts of high conservation priority include attic maternity roosts and heavily timbered barns.

Inter-annual fidelity is particularly pronounced for maternity colonies, however, Natterer's is also known for roost-switching behaviour within a given season or year (intra-annual). This behaviour is considered a strategy to reduce parasite loads or respond to changes in environmental conditions or disturbance.

5. Conservation Status and Threats

Natterer's is currently classified as a species of Least Concern on both the global IUCN Red List and the UK Red List. Despite the overall positive population trend suggested by population monitoring (section 4), Natterer’s bat is not insulated from the multitude of threats that face the wider British bat community, and that could potentially impact its long-term sustainability, including:

Habitat loss and fragmentation - primarily due to agricultural intensification and urbanisation, the loss of broadleaved woodland and tree lines, which provide important roosting and foraging sites, can significantly affect Natterer’s survival and reproductive success. Removal of landscape features, such as hedgerows and tree lines, can further exacerbate the effects of fragmentation, disrupting commuting routes and foraging patterns. The loss of mature trees with suitable cavities for roosting is a particular concern, as these roosts are essential for maternity colonies and can be difficult to replicate.

Intensive land management - use of pesticides and herbicides has reduced insect prey abundance and contaminated foraging habitats, with an increasing risk of food shortages and potential health problems for Natterer’s. The general decline in insect populations due to agricultural intensification has been identified as a major contributing factor to the decline of many bat species globally.

Climate breakdown - changes in temperature and precipitation patterns can affect the timing of seasonal events, such as insect emergence and bat hibernation, potentially leading to mismatches in phenology and reduced foraging success. For example, if insect emergence occurs earlier due to warmer spring temperatures, but the bats' emergence from hibernation is not similarly advanced, they may miss the peak in prey availability, leading to reduced foraging success and potential impacts on their reproductive output and survival. Additionally, extreme weather events, such as droughts and floods, can directly impact the bats' survival and reproductive success by affecting roosting conditions and prey availability. The vulnerability of young females to adverse weather conditions during the critical early stages of pregnancy highlights the potential impacts of climate breakdown on the reproductive success and population dynamics of Natterer’s.

Human disturbance - particularly at roosting and swarming sites, disturbance can negatively affect Natterer's populations (see section 4).

Other threats - Natterer's may also be impacted by other factors. Predation by domestic cats has been documented for Natterer's; mortality of bats through collision with road vehicles; white-nose syndrome is known to be a disease which Natterer’s may contract, but have seemingly developed resistance. Competition with other bat species for roosting and foraging resources is expected to become more intense as habitats become increasingly fragmented and degraded.

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Effective conservation and management requires a multifaceted approach that addresses the threats and challenges faced by the species. A fundamental aspect of this approach for Natterer’s is habitat conservation and restoration, focusing on providing suitable roosting and foraging sites. Important foraging habitat for conservation action includes broadleaved woodland, tree lines, and riparian habitats. High inter-annual fidelity to roosts indicates priority should be given to identifying and preserving these structures for the bat's reproductive success.

The design and placement of artificial roosts need careful consideration, considering the specific preferences of Natterer's for warmer roosts during late gestation and early lactation. Incorporating access points into bespoke roost spaces within suitable property may be an effective tactic, as it allows bats to continue roosting within a building while preventing access to the interior spaces where conflicts with humans are most likely to occur.

Mitigating the impact of anthropogenic threats, such as roads and traffic, is also important. Implementing measures such as bat gantries and underpasses can help to reduce bat mortality and maintain connectivity between habitats. However, the effectiveness of these measures for Natterer's specifically remains to be evaluated. Reducing the use of pesticides and herbicides in agricultural landscapes will likely increase insect prey abundance and improve foraging habitat quality.

Addressing human-bat conflicts, such as those occurring in historic churches, requires finding solutions that balance the conservation of bats with the protection of cultural heritage. The use of deterrents, such as high-intensity ultrasound and artificial lighting, can be effective in moving bats away from specific roosting sites within churches, thus reducing damage. However, the use of deterrents should be carefully monitored and regulated to avoid negative impacts on bat welfare.

Certain woodland management practices, such as clear-felling and the use of chemicals, can negatively impact Natterer’s. Implementing sustainable forestry practices that maintain habitat diversity and retain old-growth stands, and minimise the use of chemicals, can help to mitigate these impacts.

Recent technology advances have significant potential as conservation tools. Passive acoustic monitoring (PAM) has emerged as a valuable tool for monitoring bat populations, particularly for species that are 'difficult to survey', such as Natterer's (due to late roost emergence behaviour, soft echolocation calls). Developments in automated sound classification systems are expected to greatly enhance the efficiency and accuracy of monitoring efforts.

The importance of engaging and educating the public in bat conservation has long been acknowledged. Natterer's is a good bat species for raising public awareness and fostering participation in conservation efforts, with its ‘red-armed bat’ nickname, and its relative accessibility for bat walks and educational talks and for citizen science involvement in monitoring.

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6. Research gaps and future directions

Research output has been slow and steady over the last 25 or so years on the ecology of Natterer's in Britain but key knowledge gaps appear to remain:

1. Impact of environmental change - while studies have examined the influence of weather conditions on foraging activity, the long-term impacts of climate change on prey availability and foraging behaviour remain unclear.
2. Mechanisms of foraging site fidelity and individual specialisation - underlying mechanisms driving foraging site fidelity and individual specialisation in Natterer's are not fully understood. Future research could explore the role of memory, learning, social interactions, and territoriality in shaping these foraging patterns; understanding these mechanisms is crucial for predicting how the bats might respond to changes in their environment.
3. Foraging behaviour and habitat utilisation of males - most studies on the foraging ecology of Natterer's bats have focused on females, due to prevalence in maternity colonies and the ease of capture at roosts. However, the foraging behaviour of males may differ significantly from that of females, and understanding these differences is crucial for comprehensive conservation planning.
4. Role of social calls in foraging - recent studies indicate that bat social calls may play a role in food source defence. Future research could investigate whether Natterer's use social calls during foraging and how these calls might influence their foraging behaviour and interactions with other bats.
5. Long-term monitoring - the National Bat Monitoring Programme has provided valuable data on the population trends of Natterer's, but continued long-term monitoring is essential for understanding the long-term impacts of various threats and the effectiveness of conservation measures. Future research should focus on refining monitoring techniques, such as the use of passive acoustic monitoring and eDNA analysis, to improve the accuracy and efficiency of population assessments.

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Summing up - the Red-Armed Bat 25 years on

Despite the encouraging evidence of population stability and the positive trend in hibernation counts, the conservation of Natterer's Bat cannot be taken for granted.

Research during the 21st century has highlighted the bats adaptability to various habitats, sophisticated echolocation and foraging strategies, and complex social interactions. While Natterer’s is currently listed as Least Concern, the species faces increasing challenges ahead from habitat loss, intensive land management practices, and climate change. Effective conservation strategy requires a multi-faceted approach, encompassing habitat restoration, mitigation of anthropogenic threats, expanded monitoring, and public engagement. Continued research is needed to address knowledge gaps and refine conservation efforts for this widespread but understudied species.

The journey from my early fieldwork to the present day underscores the dramatic evolution of bat conservation research. What was once a quest with a Yugo and a Yagi antenna, now draws on the efforts of many committed people and organisations deploying citizen science and cutting-edge technologies like satellite mapping, PAM and eDNA analysis. Yet, the underlying goal remains the same: to understand, protect, and treasure the natural world, one red-armed bat at a time.

Views in this article represent the author’s personal opinions only.

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