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About the Author
Andrew Froud is an ‘Ecologist’ working for the City of London, within Epping Forest (Essex). He is a licensed bat worker with over 10 years experience. In 2006, together with Keith French, he set up a 6 year research project, within a small part of Epping Forest, to examine habitat usage by bats in conjunction with the impact of habitat changes brought on by the sites Conservation Management Plan. Andrew’s initial interest in social calls came about when he and Keith discovered the first known lekking Nathusius’ pipistrelle roosts for Essex on their site. Since then, he has been fascinated by the subject, and recorded many social calls across a wide range of species. In recent years Andrew has delivered presentations on this subject to course delegates, local bat groups and natural history groups.
Keith French has over 10 years’ experience with bat related work, and is a licensed bat worker, both living and working within Epping Forest (Essex), where he is employed by the City of London as ‘Head Forest Keeper’. Having a driven associate like Andrew Froud has helped fuel his enthusiasm to delve deeper into this complex and relatively un-researched subject. After recording their first Nathusius’ pipistrelle lekking in an ancient oak pollard, the search was on to record social calls from other species, especially Noctule and Myotis. Keith has also been keen to pass on the knowledge he has gained by giving guided walks, presentations and allowing volunteers the chance to improve their skills. He is well travelled, having been fortunate enough to observe and record bats in some exotic locations such as the Amazon, Croatia, Costa Rica, USA and several African countries.
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An Overview of Bats within Britain and Ireland
Much has been written about the bat species occurring within Britain and Ireland, and it is not the intention here to reproduce what is easily accessible elsewhere. However, it is useful to cover a small number of more general areas relating to bats in order that the reader can relate the main subject matter of this book (i.e. social calls) to the species present in the area and their typical seasonal behaviours.
1.1 Species diversity
Within Britain and Ireland there are 17 species of bat regarded as being resident and breeding, represented by two families, comprising seven genera (see Table 1.1). In addition to these, other bat species have occasionally been found to be present, though these are thought to be occurring either purely as migrants or incidentally as vagrants (i.e. population not established and/or breeding not considered to be taking place). Throughout the world there are many families of bats; however, only two are known to be capable of true hibernation (Ransome, 1990; Altringham, 2003). Not surprisingly, due to our temperate climate producing much colder conditions in winter and hence less insect food availability, all bats occurring within Britain and Ireland belong to these two families.
1.2 Life cycles
In order to help understand the social aspects of bats it is useful to consider their life cycles. Table 1.2 describes the typical annual life cycle of the species which are resident within Britain and Ireland. Bear in mind that the information shown is of a genera; nature and therefore not species or location specific. There is considerable variation among species regarding their behaviors and differences occurring, within species groups, geographically within the area covered by this book. Furthermore, changing and unpredictable weather patterns are almost certainly having an impact upon bat behaviour.
1.3 Roosting behaviour
Throughout the year bats use a variety of different rooting locations (including alternatives) taking account of their specific needs, the season, unseasonable changes in weather conditions, disturbance and depending upon whether they are male or female. Table 1.3 provides examples of the different roost types that are typical for most of the bat species occurring in Britain and Ireland. Depending upon the species involved, a number of these roosting requirements could potentially occur within a single location. In reality, however, more often they tend to be separate from each other, either in the same locality, or for some species further away.
1.4 Commuting and foraging
All of the bat species occurring within Britain and Ireland use echolocation in order to help them commute and forage. Echolocation is a sophisticated system through which bats build up a sound picture of their surroundings, by producing a rapid repetition of a series of high-frequency pulses ([??] 1.1), and then listening for the returning echoes. Using this system a bat is able to establish its proximity to features, locate prey and navigate to and from roosting locations and foraging sites. The information that bats seek to gather as a result of echolocation is greatly aided, especially for some of our smaller species, by the presence of linear features along which bats can navigate and orientate themselves in the dark (Russ and Montgomery, 2002; Downs and Racey, 2006). Furthermore, these linear features provide a degree of protection from poor weather conditions and potential predation, and can also act as valuable foraging locations in their own right. The following are a small number of examples of features used by bats whilst commuting:
Waterways (e.g. rivers/canals/lake shores)
Woodland tracks and rides
Within Britain and Ireland, all of the bat species present are insectivorous and require large quantities of invertebrate prey (insects and, in some cases, arachnids), in order to fuel their high metabolisms. As such it is important that good healthy invertebrate populations thrive within the environment in which bats are present, and that these food resources are accessible to bats throughout their periods of activity.
1.5 Habitat use model
To summarise, in order to interact naturally with their surroundings, establish viable populations and indeed survive, bats need suitable roosting habitat, foraging locations and additional features that can assist with orientation in the dark, thus allowing them to navigate successfully between roosts and foraging areas. Taking account of all of this, the behaviours for many of the bat species occurring within Britain and Ireland can be described as shown in Figure 1.1.
Introducing Bat Social Calls
Bearing in mind their small size, bats are relatively long-lived mammals, with low reproductive rates. They live alongside other bats of the same species, at least for large parts of their life cycle, and occasionally with different species. In some respects (e.g. maternity colonies) large numbers of bats are living together in a shared roost for a common purpose (e.g. the rearing of young). Being long-lived and co-habiting as part of a colony, it would be fair to assume that their social lives are, relatively speaking for small mammals, complex. Therefore they would appear to have a need for effective communication (social calls), either in the context of their roosting behaviour or while airborne, for example during mating-related activity or while foraging.
With regards to social calls within the context of this book it should be understood that all calls deliberately or instinctively made by a bat are being considered, other than what is known to be regarded as echolocation. This means a whole range of scenarios are being covered, on the basis that if a bat makes a call (e.g. due to distress) it is doing this with the consequence of, either intentionally or inadvertently, alerting or communicating a message to other bats in the vicinity, which consequentially has the potential to impact upon the behaviour of other individuals.
In addition to oral communication whereby these animals have the ability to make sound and just as importantly hear, react and respond to each other, the ability to sense and recognise smell (olfactory perception), as well as the sense of touch (tactile perception), are also considered to be very important for bats (Fenton, 1985). These aspects, at least to some degree, are relevant when bats are interacting closely, for example within roosts (de Fanis and JOnes, 1995a; Bloss et al., 2002), mother and pup liaisons (Loughry and McCracken, 1991) and during the mating season (Bartonicka et al., 2010). Olfactory and tactile perceptions play important roles in bat-related social interaction. Although these areas are not covered within this book, it is important to make the reader aware that there are other things contributing towards bat social interactions than purely sound.
2.2 Echolocation in context
Sound is an extremely important resource for bats and is used in order to allow them not only to communicate, but for most species worldwide, including all species of Vespertilioniformes and many species of Pteropodiformes (Hutcheon and Kirsch, 2006), to operate effectively during darkness using a system called 'echolocation'. The bat species represented within Britain and Ireland use this system, which involves the rapid emission of high frequency (ultrasonic) pulses
(Briggs and King, 1998; Schnitzler et al., 2003), in order to assist with navigation and foraging (Ahlen, 1990).
Echolocation is not considered to be used primarily for communicative purposes, although it has been shown that bat behaviour can be influenced by the echolocation calls of other individuals. By listening to the echolocation pulses produced by other bats, they have been shown to 'eavesdrop' (Barclay, 19682; Fenton, 2003) and differentiate each other's behaviours through listening to the echolocation calls of conspecifics (Balcombe and Fenton, 1988; Yovel et. al., 2009). Studies have also shown, at least for some species (e.g. greater mouse-eared bat), that bats are able to learn the echolocation call characteristics of conspecifics, thus allowing them to recognise each other, in order to ascertain information about features such as location or foraging prospects (Yovel et al., 2009). Further study has also indicated that bat echolocation calls can contain social information that other bats from the same species could react to, and therefore could play a role in communication (Knornschild et al., 2012). Bearing all this in mind, it is not difficult to see how such scenarios could have implications upon their social behaviour, for example as a preamble for territorial disputes, or through interpretation of other bat activity for mutual benefit during foraging bouts. In a social context, therefore, it has been shown that echolocation could, at least to a certain degree, impact upon social interaction for some species.
Some of the studies that have been carried out to examine if and how echolocation plays a part in the social interactions of bats have also considered whether or not echolocation parameters can assist researchers in determining more than just the identification of a species – e.g. can these parameters be used to determine sex or age? Because of the primary function of echolocation and how it is influenced by the immediate habitat within which a bat finds itself (Holderied et al., 2006), it needs to be borne in mind that such studies are not easy to carry out. Nonetheless, to a certain degree it has been shown that such thoughts have some merit. This is exemplified by a study carried out in Bulgaria relating to horseshoe species, where it has been shown that the resting frequency echolocation calls produced by Blasius's horseshoe could assist in the differentiation of age, size or condition (Siemers et al., 2005). Other studies have also demonstrated similar findings, for example in the ageing of greater horseshoe bats (Andrews et al., 2011) and the sex of big brown bats (Kazial and Masters, 2004).
So, in summary, some research has been carried out to date regarding the impact that echolocation may be having on the social behaviour of bat species. This research has produced results demonstrating, at the very least, potential social implications for the very small number of species studied in this respect. Echolocation is not, however, considered as the primary method for bats communicating orally with each other; hence the use of social calls within bat communities.
2.3 Social calls
Bats use vocal communication (social calls) in a wide range of circumstances (Fenton, 2003) and these social calls, in comparison with echolocation, are more complex, louder and usually at a relatively lower frequency, thus allowing the sound to carry and be heard over greater distances (Lawrence and Simmons, 1982). Table 2.1 provides a comparison between echolocation and social calls, and, although not exhaustive, it is useful in beginning to understand the differences between the two. Owing to the manner in which bats behave and operate it is fairly commonplace to encounter both social calls and echolocation simultaneously or within the same environment.
2.4 Settings for social interaction
It is feasible that bats could become involved with social interaction in numerous ways. For example, two individuals may purely need to come into close association with each other for there to be a social engagement. During the authors' own studies, advertising males of both Nathusius' pipistrelle and noctule, from a call perspective, appear to behave differently when other species are flying in the vicinity of their mating roost. For example, at a study site within Epping Forest (Essex, England) two of the authors were observing a male Nathusius' pipistrelle in songflight within an area of wood pasture. This particular individual had been patrolling up and down the same short route, constantly social calling with echolocation occurring only as the bat turned at either end. As the evening progressed a noctule flew into the area, at which point the pipistrelle proceeded to chase the noctule, eventually appearing to force it to leave the area. Thereafter the pipistrelle resumed its songflight behaviour. A small sample of the ensuing confrontation is contained within [??] 2.1 and [??] 2.2.
There are, however, more commonly encountered settings within which bats may be heard to produce social calls (e.g. maternity roosts, mating sites), and the following sections tackle these in more detail.
2.5 Generic roost sites
Anyone that has stood close during the emergence of a larger bat roost will be familiar with the constant chatter and apparent jostling that takes place prior to bats emerging. Social interaction is surely occurring during these emergences ([??] 2.3, [??] 2.4 and [??] 2.5) when the bats are preparing to leave the roost in order to commence foraging. What precisely is happening at that moment, and on a night after night basis, is difficult to fully understand. Is there a strict pecking order, a semi-ordered queue, or is it just a free for all? One thing is certain, it is noisy! – and noise surely means some sort of social interactions.
It is considered that at least some aspects of sound produced within roosts relates to threatening behaviour (e.g. posturing) between bats, as such engagement is most often noticed when individuals are in close contact with each other, notably while in roosts. Therefore the larger the roost, the more likely it would be expected to encounter such engagement, and hence it is noted that social calls designated as 'threat calls' (Pfalzer and Kusch, 2003) occur more often in bat colonies with many individuals. The calls are prolonged and low in frequency, and being of this nature can travel a relatively long distance, meaning they can potentially reach beyond the roost itself. Pfalzer and Kusch (2203) suggested that these calls may therefore also potentially play a part in communicating roost location to other passing individuals. Of course this effect may not be the original intention of the call, but an indirect consequence. If it was having the ability to be noticed by other bats, would the call act as an enticement or a deterrent? Indeed it may even induce a different reaction from the 'passer-by' dependent upon species, sex or the individual's relationship with the colony involved.
These calls are most often encountered by bat workers when they are catching and handling bats, and for many species within Britain and Ireland the calls produced sound and look quite similar (Russ, 2012). However, it is worth noting that bats may get themselves into other situations that could cause them distress. But first, the difference between threatening (e.g. as discussed earlier in the context of roosting bats interacting with each other) and distressed should be considered. Put simply, an aggressor may come across as threatening, and if the aggressor is perceived as a real danger, the animal on the receiving end may indeed feel threatened, putting it into a state of distress. In a human context it is perhaps similar to the difference in emotional state between you shouting aggressively at someone (i.e. being threatening), as opposed to screaming for help (i.e. being in distress.). The first is an aggressive action, but the latter is a defensive reaction. Usually when discussing distress calls in bats, as opposed to threat calls, it is the latter, defensive reaction, that is being referred to.
There are a number of reasons as to why animals make distress calls as a result of feeling threatened by something occurring to them directly or within their immediate environment. A study relating to the distress call response behaviour of pipistrelle species within the UK (Russ et al., 2004) summarised what was considered to be the main rationale for the occurrence of such calls in a wider context, making reference to a number of studies that had previously been carried out in relation to bird species. From the options discussed within that study, and with regards to the behaviour of bats within Britain and Ireland, the following appeared to be the more likely intentions of the bat, or the indirect consequences of its behaviour:
(i) Alerting others to the presence of danger (e.g. a perceived predator) in order to advise them to be cautious and/or stay away.
(ii) Sending of a message requesting assistance from others, which may result in other bats coming into the area and mobbing the predator in order to dissuade the predator from its behaviour, or distract the predator from its victim.
Excerpted from "Social Calls of the Bats of Britain and Ireland"
Copyright © 2014 Neil Middleton, Andrew Froud and Keith French.
Excerpted by permission of Pelagic Publishing.
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Table of ContentsForeword by Dr Sandie Sowler
About the Authors
Chapter 1 – An Overview of Bats within Britain and Ireland
Chapter 2 – Introducing Bat Social Calls
Chapter 3 – Classification of Social Calls
Chapter 4 – Considerations in Survey Design
Chapter 5 – Analysis of Social Calls
Chapter 6 – Species Groups