RHEINISCHE FRIEDRICH-WILHELMS-UNIVERSITY OF BONN Infrared sense in snakes – behavioural and anatomical examinations (Crotalus atrox, Python regius, Corallus hortulanus) Dissertation to obtain the graduation of Doctor rerum naturalium (Dr. rer. nat.) presented to the biological department of the faculty of mathematics and sciences by Jill Ebert Bonn, September 2007 Angefertigt mit der Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn 1. Gutachter: Prof. Dr. Horst Bleckmann 2. Gutachter: PD. Dr. Helmut Schmitz Tag der mündlichen Prüfung: 18. Dezember 2007 Diese Dissertation ist auf dem Hochschulschriftenserver der ULB Bonn http://hss.ulb.uni-bonn.de/diss_online elektronisch publiziert. Erscheinungsjahr: 2008 Index i Index 1 Introduction...........................................................................................................................1 1.1 Infrared radiation..............................................................................................................1 1.2 IR perception in the animal kingdom...............................................................................1 1.3 IR sense in snakes............................................................................................................2 1.4 Differences in pit morphology.........................................................................................2 1.5 Scale surface structure above IR receptors......................................................................3 1.6 Morphology of IR receptors and the neural pathway of IR reception.............................4 1.7 Background of the objectives and working hypothesis....................................................5 2 Material and methods...........................................................................................................9 2.1 Natural history of the study species.................................................................................9 2.1.1 Western diamondback rattlesnake (Crotalus atrox)..................................................9 2.1.2 Ballpython (Python regius).....................................................................................10 2.1.3 Amazon tree boa (Corallus hortulanus)..................................................................11 2.1.4 Animal husbandry of the study species...................................................................12 2.2 Behavioural experiments................................................................................................13 2.2.1 Snake preparation....................................................................................................13 2.2.2 Experimental set-up.................................................................................................14 2.2.3 Snake exposure protocol.........................................................................................15 2.2.4 Evaluation criteria of behavioural responses..........................................................17 2.2.5 Control experiments for both species......................................................................18 2.2.6 IR images of mice...................................................................................................19 2.2.7 Statistics..................................................................................................................19 2.2.8 Calculation of the irradiance contrast......................................................................20 2.3 Anatomical investigations..............................................................................................20 2.3.1 Succinate dehydrogenase staining...........................................................................20 2.3.2 Light microscopy and transmission electron-microscopy.......................................22 2.3.3 Scanning electron microscopy................................................................................24 2.3.4 Tracing experiments................................................................................................26 2.3.5 Cast of a boa head as a functional model................................................................28 3 Results..................................................................................................................................30 3.1 Behavioural experiments................................................................................................30 3.1.1 Behavioural responses of Crotalus atrox................................................................30 3.1.1.1 Control experiments.............................................................................................32 3.1.1.2 Control of response assessment............................................................................33 3.1.1.3 Habituation...........................................................................................................33 3.1.2 Behavioural responses of Python regius.................................................................34 3.1.2.1 Control experiments.............................................................................................36 3.1.2.2 Habituation...........................................................................................................37 3.1.2.3 Video analysis versus direct response assessment...............................................38 3.2 Anatomical investigations..............................................................................................38 3.2.1 Succinate dehydrogenase staining experiments......................................................38 3.2.2 Light microscopic and transmission electron microscopic.....................................40 3.2.3 Scanning electron microscopic examination of the labial scale surface.................49 3.2.3.1 Examination of the surface structure of control scale areas.................................54 3.2.4 Tracer experiments..................................................................................................55 3.2.5 Investigation of the IR visual field..........................................................................57 Index ii 4 Discussion.............................................................................................................................66 4.1 Behavioural experiments................................................................................................66 4.1.1 Detection distance of the IR stimulus and behavioural responses..........................66 4.1.1.1 IR detection threshold of C. atrox........................................................................67 4.1.1.2 IR detection threshold of P. regius.......................................................................68 4.1.2 Possible influences on the results............................................................................69 4.1.3 Considerations on the results...................................................................................71 4.1.4 Pythons’ versus rattlesnakes’ IR sensitivity and its implications...........................72 4.2 Anatomical investigations..............................................................................................74 4.2.1 Examination of a possible directional representation with tracing experiments.....74 4.2.2 Structure and location of the IR receptors...............................................................75 4.2.3 Surface structure of the receptor-bearing labial scales............................................78 4.2.4 Considerations to the IR visual field and environmental relevance........................81 5 Summary..............................................................................................................................86 6 References............................................................................................................................88 7 Abbreviations.......................................................................................................................96 8 Appendix..............................................................................................................................97 8.1 Recipes...........................................................................................................................97 8.1.1 SDH staining...........................................................................................................97 8.1.2 Light microscopy and transmission electron-microscopy...........................................97 8.1.2.1 Epon embedding procedure..................................................................................97 8.1.2.2 0.05% Toluidine blue-borax-solution..................................................................98 8.1.2.3 Uranyl acetate and lead citrate staining................................................................98 8.1.3 Tracer injections......................................................................................................99 8.1.3.1 ABC-Elite-Kit......................................................................................................99 8.1.3.2 DAB solution........................................................................................................99 8.1.3.3 Neutral red staining procedure...........................................................................100 8.1.4 Head cast...............................................................................................................100 8.2 Additional data.............................................................................................................100 9 Acknowledgements............................................................................................................102 1 Introduction 1 1 Introduction The overall scope of this study was to examine behavioural and morphological aspects of the infrared (IR) sense in three different snake species. For this purpose a pitviper and a python were behaviourally tested on their IR detection threshold. Furthermore, the structure of the IR sensitive labial areas of a pit-bearing boa was morphologically investigated. The following introduction provides the necessary background knowledge. 1.1 Infrared radiation Electromagnetic radiation of wavelengths longer than that of visible light, but shorter than that of radio waves, is defined as infrared radiation. The spectrum spans three orders of magnitude with a wavelength between approximately 750 nm and 1 mm (e.g. Bergmann and Schaefer, 1987; Meschede, 2002). The IR spectrum is often subdivided into smaller sections, e.g. near infrared, which is the region closest in wavelength to the radiation detectable by the human eye (0.78-3 µm), followed by mid (3-50 µm) and far infrared (50-1000 µm) (Bergmann and Schaefer, 1987). However, international standards for the subdivision of the spectrum are not available. The source of IR radiation is heat or thermal radiation, produced by the motion of atoms and molecules in an object. Some animals have expanded their sensory information input by developing IR radiation detection mechanisms, in addition to other senses, e.g. the visible electromagnetic radiation detection mechanisms (vision). This enables them to additionally 'see' emitted thermal radiation. 1.2 IR perception in the animal kingdom Temperature is one important variable of the environment. Within the animal kingdom some species are known to perceive and utilize the presence of and the changes in thermal energy in their environment (Barrett et al., 1970). IR receptors enable them to detect the changes in thermal energy. The IR sense is employed in various ways, e.g. in prey detection, thermoregulation or protection from heat damage. Among mammals, the common vampire bat (Desmodus rotundus) is discussed in the context of IR reception (Kishida et al., 1984; Kuerten and Schmidt, 1982). In the world of insects, some blood-sucking bugs (e.g. Rhodnius prolixus) are discussed to be IR sensitive (Schmitz et al., 2000). Additionally, pyrophile beetles (e.g. Melanophila acuminata, Merimna atrata, Acanthocnemus nigricans) are known to possess an IR sense (e.g. Evans, 1964, 1966; Evans and Kuster, 1980; Kreiss et al., 2005; 1 Introduction 2 Mainz et al., 2004; Schmitz and Bleckmann, 1998, Schmitz et al., 2002). Furthermore, two families of the snake taxa also include IR sensitive representatives (e.g. Barrett et al., 1970). Depending on the species, the IR receptors are of diverse structure and position, e.g. in the snout region of snakes, in the case of beetles on the ventral side of the thorax (M. acuminata, A. nigricans) or of the abdomen (M. atrata). 1.3 IR sense in snakes The IR sense evolved independently within two distantly related families of snakes: in the primitive Boidae (boas and pythons) and in the subfamily Crotalinae (pitvipers including rattlesnakes, copperheads and bamboo vipers) of the advanced Viperidae (e.g. Barrett et al., 1970; Bullock and Diecke, 1956; Kluge, 1991; Molenaar, 1992; Nobel and Schmidt, 1937). The integration of both IR and visual information enables IR sensitive snakes to achieve a unique portrayal of their surrounding by 'seeing' different regions of the electromagnetic spectrum. They image visual light with their eyes and also perceive images of their thermal environment by detecting IR radiation at wavelengths centred around 10 μm, which matches the IR emission peak of homoeothermic animals, i.e. potential prey (e.g. Gamow and Harris, 1973; Grace et al., 1999). Despite their structural differences, the IR sensitive organs of crotaline and boid snakes are analogous and serve similar biological functions (de Cock Buning, 1984), i.e. primarily the detection of mainly homoeothermic prey and accurate predatory targeting, even in the absence of visual cues (e.g. Barrett et al., 1970; Bullock and Barrett, 1968; Bullock and Diecke, 1956; de Cock Buning et al., 1981; Goris and Nomoto, 1967; Grace et al., 2001; Hartline, 1974; Kardong and Mackessy, 1991; Kardong, 1993, 1996). Furthermore, it is known that pitvipers also use their IR sense for spatial orientation, e.g. finding basking places for thermoregulation (Krochmal and Bakken, 2000, 2003; Krochmal et al., 2004). In addition, their IR sense is discussed in the context of selecting a den site (Sexton et al., 1992) or avoiding predators (Greene, 1992). 1.4 Differences in pit morphology Pitvipers possess a forward-facing loreal pit situated between the nostril and the eye on each side of the head (see 2.1.1 Fig. 1 for a representative). These (1-5 mm deep) cavities (Barrett et al., 1970; de Cock Buning, 1985) expand into a depression of the maxillary bone (Dullemeijer, 1959). The pit opening is narrower than the base. A 10-15 μm thick IR receptor 1 Introduction 3 bearing membrane suspended near the cavity back divides the latter into an inner and outer chamber (e.g. Barrett et al., 1970; Molenaar, 1992). This membrane consists of three layers: the richly innervated nervous layer sandwiched between two cornified (0.5-1.5 μm thick) epidermal layers (Bullock and Fox, 1957; Lynn, 1931; Nobel and Schmidt, 1937). The nervous layer is associated with a high concentration of vascular beds (Barrett et al., 1970). The loreal pits function similarly to a pinhole camera. The radiation of an IR source impinges on a certain part of the TNM bearing membrane, which allows a determination of the angle (therefore position) of the IR source (Bakken and Krochmal, 2007; Gamow and Harris, 1973; Newman and Hartline, 1982). Due to the frontal position of the loreal pits the IR visual fields of each pit overlap strongly and cover the frontal region. In contrast to the pitvipers, the pit-bearing boid snakes show a wide variety in shape, size and positioning of more simply constructed pits (Lynn, 1931; see 2.1.2 Fig. 2 for a python and 2.1.3 Fig. 3 for a boa representative). Unlike pitvipers, boids usually possess their IR receptors within simple, shallow depressions in (pythons) or between (boas) specialised labial scales (Noble and Schmidt, 1937; Ros, 1935; Warren and Proske, 1968). The position, number, size and shape of these supra- and/or infralabial depressions differ from species to species (Maderson, 1970; Molenaar, 1992). Some IR sensitive snakes (e.g. Boa constrictor) even lack labial depressions altogether (Barrett et al., 1970; von Duering, 1974; von Duering and Miller, 1979). In this case the IR receptors are located in the labial scales (von Duering, 1974). In all boids the receptors are located in the outer epithelium of the labial scales and are supported by epithelial cells (Amemiya et al., 1996b; Molenaar, 1992). In pit-bearing boids each pit functions similarly to a pinhole camera. However, the information of several pinhole cameras is computed in a specific brain area (tectum opticum, see below) (Newman and Hartline, 1982). Due to the arrangement of the pits not only the frontal but the lateral regions are covered in the IR visual field. It is assumed that the variation of forms of the pits or labial depressions and the distribution of the IR receptive areas influence image formation. However, this has not yet been examined for any pit-bearing boa so far. 1.5 Scale surface structure above IR receptors Not only does the scale morphology have an influence on the IR perception, but also the surface structure of the scale above the IR receptors might influence the perception mechanism. The surfaces show an array of microscopic pores, which seems to be unique in IR sensitive snakes (Amemiya et al. 1995). According to Amemiya et al. (1995) and Campbell et 1 Introduction 4 al. (1999) they appear to function as a sort of optical grating to reflect the visible light while facilitating the passage of IR rays. These microscopic pores are always associated with the IR receptor-bearing scales (Amemiya et al., 1995). 1.6 Morphology of IR receptors and the neural pathway of IR reception The IR receptors of all IR sensitive snakes are similar (Molenaar, 1992). The IR receptors are unmyelinated free nerve endings, so-called terminal nerve masses (TNMs), which contain dense concentrations of mitochondria and oxidative enzymes, e.g. succinate dehydrogenase and ATPase (Bleichmar and de Robertis, 1962; Meszler, 1970; Meszler and Webster, 1968). The function of the TNMs as IR receptors has already been proven in previous investigations (e.g. de Cock Buning et al., 1981 a, b; Warren and Proske, 1968). The transduction process is unknown so far. The TNMs are closely associated with a capillary bed, which is discussed in the context of energy and oxygen supply and cooling (Amemiya et al., 1996a; Molenaar, 1992). The TNMs arise from palmate preterminal swellings and extend between the epithelial cells (Bleichmar and de Robertis, 1962; Bullock and Fox, 1957; von Duering, 1974). The dendrites of the TNMs converge to nerves whose bundles lead to the Vth cranial nerve (Nervus trigeminus). The pit organs of pitvipers are innervated by branches of the ophthalmic and maxillary nerve; thereby each branch supplies a different area of the membrane (Barrett et al., 1970; Bullock and Fox, 1957). The innervation of the boids IR receptors also involves the mandibular branch of the trigeminal nerve (Molenaar, 1992). In comparison to pitvipers in boids the innervation pattern is more complex and even varies among species. According to de Cock Buning (1985), in pythons the first rostral pits of the supralabial scale are innervated by the ophthalmic branch whereas the other supralabial pits are innervated by the maxillary branch. Molenaar et al. (1979) found distinct groups of IR receptors in Python reticulatus, which are innervated by distinct nerve bundles. Moreover, individual pits seem to be innervated by the same branches that diverge from bundles to neighbouring pits of the same group (Molenaar, 1992). The trigeminal system of IR sensitive snakes consists of the common trigeminal system (equivalent to the one found in other vertebrates) and the lateral descending trigeminal system, which is placed lateral to the first mentioned in the medulla oblongata (Kishida et al., 1984; Molenaar, 1992; Newman and Hartline, 1982). The primary afferent fibres of those sensory branches of the trigeminal nerve that lead to the TNMs build the Lateral trigeminal tract descendens (lttd) and terminate ipsilateral in the Nucleus of the lateral trigeminal tract descendens (LTTD) in the medulla oblongata. There, the projections from the different 1 Introduction 5 branches of the Nervus trigeminus are still topographically distinguishable (Molenaar, 1978). This LTTD is unique to all IR sensitive snakes (Molenaar, 1974; Schroeder and Loop, 1976). From there on the neural processing pathways of IR information differ in the crotaline and boid snakes. In boids, the efferent projections from the LTTD reach the nuclear complex of the nuclei N. rotundus and N. pararotundus, located in the dorsal thalamus and pretectum (Molenaar, 1992), and then directly ascend to the tectum opticum in the mesencephalon (Molenaar, 1992; Molenaar and Fizaan-Ostveen, 1980; Welker et al., 1983). In pitvipers, there is an intermediate nucleus (RC: Nucleus reticularis caloris) between the LTTD and the Tectum opticum (Kishida et al., 1980; Newman et al., 1980; Stanford et al., 1981). The Tectum opticum of IR sensitive snakes is not only the target of the ascending IR pathways, but also of the visual information processes by revealing IR, visual and visual-IR neurons terminating in a similar region (Kass et al., 1978; Terashima and Goris, 1975, 1976). Cross-modality interactions take place in IR and visual neurons (Molenaar, 1992). Similar to in boids, the efferent fibres in crotalines run from the Tectum opticum to the Nucleus rotundus in the diencephalon and from there to the ADVR (anterior dorsal ventricular ridge), a processing area in the telencephalon (Berson and Hartline, 1988). 1.7 Background of the objectives and working hypothesis Several behavioural studies have addressed IR perception in snakes (e.g. Grace et al., 2001; Grace and Woodward, 2001; Theodoratus et al., 1997). Early studies revealed the pit organs to be heat sensitive organs (e.g. Ros 1935; Noble and Schmidt 1937). Other behavioural investigations focused on prey detection (Chiszar et al., 1986; de Cock Buning et al., 1981a; Grace et al., 2001; Grace and Woodward, 2001; Haverly and Kardong, 1996; Kardong, 1996; Kardong and Mackessy, 1991) and the functional usage of the IR sense in the context of thermoregulation (Krochmal and Bakken, 2000, 2003; Krochmal et al., 2004). Young and Aguiar (2002) used behavioural changes during stimulus presentation as evidence for sensory perception in rattlesnakes in order to investigate the auditory system. In this study a similar methodical approach was used to investigate the IR sense. Unlike an electrophysiological approach (which can only record a part of the neural processing mechanisms, depending on the methods, e.g. evoked potentials, single cell recordings or multi-unit recordings), the behavioural approach does not only take the entire neural processing of stimulus perception into account, but also adds information on the relevance of the perceived. Up to now, no study has used this advantage of a behavioural approach to explore the IR detection range, i.e. the IR sensitivity of snakes. In one part of this dissertation