Key stimuli of prey for toads (Bufo bufo L.): Configuration and movement patterns

Summary The ability of the Common ToadBufo bufo (L.) to distinguish between a worm-like and an antiworm-like (e.g., a bar at right angles to its direction of motion) moving stripe is not generally altered by movement dynamics, such as velocity of motion or by particular movement patterns—so far as has been investigated. A small square stimulus, however, with its indifferent shape is more attractive as prey for the toad when displaced in a stepwise manner, rather than moved at corresponding continuous (constant) velocity. Step frequencies of 1 to 2 cps were found to have optimal releasing values.Supported by the Deutsche Forschungsgemeinschaft Ew 7/6     

Behaviour and body temperature of hibernating common toads Bufo bufo

Hibernation behaviour of six common toads Bufo bufo was studied. Transmitters, generating pulses with a temperature dependent repetition rate, implanted in the toad's abdominal cavity, were used to measure the body temperature. Measurements of body temperature and soil temperature at various depths were used to establish the hibernating depth. All toads moved vertically between a few preferred depths and hibernating depth appeared to vary for individual toads. Upward movements mostly were apparent reactions on changes of the soil temperature gradient at the depth of the toad.     

Growth response of testis tissue to partial castration in toads, Bufo bufo bufo

The growth response of the remaining intact testis or testis fragments to partial castration was studied as a function of the duration of the postoperative period, the amount of testis mass excised, as well as the functional state of the testes at the time of operation. Excision of about 90% of the testis mass caused a growth response that increased from slight after eight weeks to pronounced after 14 weeks. After 14 weeks the growth response was slight tounilateral excision of 75% of a testie and pronounced to bilateral excision of 75% of each testis. Subtotal castration caused formation of new seminiferous tubules within the remaining testis tissue when the operation was performed early in the annual testis cycle, whereas the growth response late in the cycle was primarily caused by increased spermatogenetic activity within existing tubules. Partial castration stimulated oocyte formation within fragmented testes, but not in a remaining intact testis. Oocyte formation within a testis fragment was independent of the presence of the Bidder's organ.     

Question of “head preference” in response to worm-like dummies during prey-capture of toads,Bufo bufo

If a black worm-like dummy is moving against a white background, toads fixate and snap at the leading end of the stimulus. This “head preference” phenomenon is — within limits — independent of (i) background structure, and (ii) stripe length. “Head preference” can be disturbed by reducing the amount of the stimulus background contrast as well as by point structures incorporated in the worm-like shape of the stimulus. If the stimulus-background contrast of the worm dummy is reversed, toads exhibit a clear preference in fixating and snapping for the trailing end of the stimulus. This “tail preference” is independent of changes in (i) and (ii). The neural basis of “head preference” or “tail preference” respectively, is discussed.     

Behavioral reactions of Bufo bufo L. tadpoles to chemical excretion of adult toads and changes during larval development

The reactions of toad (Bufo bufo L.) tadpoles to chemical excretes of adults were studied. In double-choice test aquarium tadpoles demonstrated avoidance reactions (swimming away from the place of stimulus delivering, reduction time of spent in stimulus part of aquarium and reduction of movement activity). Efficiency of stimulus reduced when its concentration decreased. The thresholds of tadpoles sensitivity estimated according to the reduction of time spent in stimulus part of aquarium decreased in onthogeny: they were near 500 x 10(-5) g hour/l at 36-39 developmental stages and 500 x 10(-4) g.hour/l at 40-41 stages.     

斯洛维尼亚池塘中大蟾蜍的交配模式

为了解大蟾蜍( Bufo bufo)的配对成功是否与身体大小有关,在蟾蜍产卵之前,我们在斯洛文尼亚捕捉和测量了2 224只成体样本。在这些捕获的个体中, 1 772只为雄性, 452只为雌性,其中355对处于抱对状态。雌性的体长(从吻端到泄殖腔的距离)一般比雄性稍长。无论雄性还是雌性,抱对者的体长均大于未抱对者。抱对的雄性和雌性的体长具有显著的正相关。我们所发现的大小匹配的交配模式,与从其它普通蟾蜍种群得到的结果一致[动物学报51 (3) : 513 -515 , 2005]。     

Molecular phylogenetics and historical biogeography of the west-palearctic common toads (Bufo bufo species complex)

In most pan-Eurasiatic species complexes, two phenomena have been traditionally considered key processes of their cladogenesis and biogeography. First, it is hypothesized that the origin and development of the Central Asian Deserts generated a biogeographic barrier that fragmented past continuous distributions in Eastern and Western domains. Second, Pleistocene glaciations have been proposed as the main process driving the regional diversification within each of these domains. The European common toad and its closest relatives provide an interesting opportunity to examine the relative contributions of these paleogeographic and paleoclimatic events to the phylogeny and biogeography of a widespread Eurasiatic group. We investigate this issue by applying a multiproxy approach combining information from molecular phylogenies, a multiple correspondence analysis of allozyme data and species distribution models. Our study includes 304 specimens from 164 populations, covering most of the distributional range of the Bufo bufo species complex in the Western Palearctic. The phylogenies (ML and Bayesian analyses) were based on a total of 1988 bp of mitochondrial DNA encompassing three genes (tRNAval, 16S and ND1). A dataset with 173 species of the family Bufonidae was assembled to estimate the separation of the two pan-Eurasiatic species complexes of Bufo and to date the main biogeographic events within the Bufo bufo species complex. The allozyme study included sixteen protein systems, corresponding to 21 presumptive loci. Finally, the distribution models were based on maximum entropy. Our distribution models show that Eastern and Western species complexes are greatly isolated by the Central Asian Deserts, and our dating estimates place this divergence during the Middle Miocene, a moment in which different sources of evidence document a major upturn of the aridification rate of Central Asia. This climate-driven process likely separated the Eastern and Western species. At the level of the Western Palearctic, our dating estimates place most of the deepest phylogenetic structure before the Pleistocene, indicating that Pleistocene glaciations did not have a major role in splitting the major lineages. At a shallow level, the glacial dynamics contributed unevenly to the genetic structuring of populations, with a strong influence in the European-Caucasian populations, and a more relaxed effect in the Iberian populations.     

The antidromic activation of tectal neurons by electrical stimuli applied to the caudal medulla oblongata in the toad,Bufo bufo L.

In order to specify the tectal projection to the bulbar/spinal regions, the antidromic responses of the physiologically identified tectal neurons as well as the gross antidromic field responses in the optic tectum to electrical stimuli applied to the caudal medulla were examined in the paralyzed common toad, Bufo bufo. The antidromic field potential was recorded in the optic tectum in response to electrical stimuli applied to the ventral paramedian portion of the contralateral caudal medulla (where the crossed tecto-spinal pathway of Rubinson (1968) and Lázár (1969) runs), but generally not when they were applied to various parts of the ipsilateral caudal medulla. The antidromic field potential was largest at the superficial part of Layer 6 or at the border between Layers 6 and 7 of the optic tectum, indicating that neurons in these layers project to the contralateral caudal medulla. Mapping experiments of the antidromic field potential over the optic tectum showed that the antidromic field potential was recorded mainly in the lateral part of it, indicating that this part of the optic tectum is the main source of projection neurons to the contralateral caudal medulla. Various classes of tectal neurons as well as retinal ganglion neurons were identified from the characteristics of the response properties to moving visual stimuli and the properties of the receptive fields. Of these, the Class T1, T2, T3, T4, T5(1), T5(2), T5(3), and T5(4) tectal neurons were activated antidromically by stimuli applied to the contralateral caudal medulla. Only a limited proportion of the Class T5(1) neurons was activated antidromically by stimuli applied to the ipsilateral caudal medulla. On the other hand, the Class T7 and T8 neurons, as well as the Class R2, R3, and R4 retinal neurons, were not activated antidromically by stimuli applied to the caudal medulla of either side. These results suggest a possibility that these tectal neurons which project to the medullary regions form the substrate of the sensorimotor interfacing and contribute to the initiation or coordination of the visually guided behavior, such as prey-catching.     

Neuronal responses are differentially affected by the polarity of tectal DC stimulation in the toad Bufo bufo

Male toads were tested behaviourally for their prey catching responses to worm-like stimuli before being prepared for visual unit and slow potential shift (SPS) recording from the optic tectum. The neuronal responses of toads to a prey-like visual stimulus reflected their motivational tendency prior to operations. One second of DC stimulation to the tectum was followed by an SPS of reversed polarity during which time a visual prey-like stimulus was presented. A negative SPS following positive DC stimulation was associated with enhanced neuronal responses to a visual stimulus. The positive SPS that followed negative stimulation was associated with a decline in neural responses below background when a visual stimulus was additionally given. The SPS was largely a result of DC stimulation that interacted with the motivational tendency to produce enhanced neuronal responses, while the potential was negative and vice versa.     

Negative effects of Rhabdias bufonis (Nematoda) on the growth and survival of toads (Bufo bufo)

Summary The growth and survival of juvenile toads, Bufo bufo, infected with a common lung nematode, Rhabdias bufonis, were studied. Toads were raised from tadpoles in the laboratory and infected 2 months after metamorphosis. Individual toads were exposed to doses of 10, 40, 80 or 160 larvae, which enabled examination of the hypothesis that parasite-induced mortality is affected by worm numbers. Growth of infected toads began to diverge from that of uninfected controls at 6 weeks post infection (WPI) and by 12 WPI the most heavily infected toads were approximately half the mass of controls. No controls died throughout the experiment; however, mortality of infected toads was strongly affected by parasite density. A mechanism for mortality is suggested by the significant negative relationship between parasite density and dietary intake. This parasite-induced anorexia was detected at 3 WPI and persisted up to 9 WPI. Patterns of reduced host growth, survival and dietary intake provide experimental evidence of the negative consequences of parasitic infection in a natural parasite-host system which may also be present under natural conditions.     

The effects of drift and selection on latitudinal genetic variation in Scandinavian common toads (Bufo bufo) following postglacial recolonisation

Clinal variation is paramount for understanding the factors shaping genetic diversity in space and time. During the last glacial maximum, northern Europe was covered by glacial ice that rendered the region uninhabitable for most taxa. Different evolutionary processes during and after the recolonisation of this area from different glacial refugia have affected the genetic landscape of the present day European flora and fauna. In this study, we focus on the common toad (Bufo bufo) in Sweden and present evidence suggesting that these processes have resulted in two separate lineages of common toad, which colonised Sweden from two directions. Using ddRAD sequencing data for demographic modelling, structure analyses, and analysis of molecular variance (AMOVA), we provide evidence of a contact zone located between Uppland and Västerbotten in central Sweden. Genetic diversity was significantly higher in southern Sweden compared to the north, in accordance with a pattern of decreased genetic diversity with increasing distance from glacial refugia. Candidate genes under putative selection are identified through outlier detection and gene–environment association methods. We provide evidence of divergent selection related to stress response and developmental processes in these candidate genes. The colonisation of Sweden by two separate lineages may have implications for how future conservation efforts should be directed by identifying management units and putative local adaptations.Subject terms: Evolutionary genetics, Genomics, Genetic variation     

Visual unit,EEG and sustained potential shift responses to biologically significant stimuli in the brain of toads (Bufo bufo)

Summary Visual unit activity, EEG changes and sustained potential shifts (SPS) were recorded from the toad tectum whilst the animal was presented with a visual stimulus. Telencephalic EEGs were also recorded.On the surface of the tectum, retinal unit activity preceded a sustained negative shift in potential and an increase in the amplitude and dominant frequency of the EEG. In deeper layers of the tectum, T5 units with configurational selectivity for wormlike stimuli were found. The activity of these units followed a pronounced SPS and EEG change.Visual unit activity was most pronounced during the negative-going phase of the synchronised EEG, when there was also a small decrease in amplitude of neuronal spikes. Similarities between the latencies and durations of EEGs and SPSs, and their response decrements, on repeated stimulus presentation, implies a close relationship between them not shared by the visual units studied. The specific activity of tectal units is discussed in relation to the correlated EEG and SPS changes, which may form part of an adaptive sensitizing mechanism.Abbreviations EEG electroencephalogram - ERF excitatory receptive field - SPS sustained potential shift - T4, T5 tectal neurons     

Stereological analysis of mitochondria in embryos of Rana temporaria and Bufo bufo during cleavage

Total numbers of mitochondria and their morphology have been quantitatively determined in mature oocytes and in cleaving embryos of two anuran species Rana temporaria and Bufo bufo using stereological methods. Surface densities of inner mitochondrial membranes for both studied species during cleavage ranged from 5.43 m2/cm3 to 7.53 m2/cm3, whereas volume densities of mitochondria did not exceed 1.65%. Since values of these parameters were low, thus embryos during cleavage may be considered as metabolically "silent". Transition of ultrastructural morphology of mitochondria towards that characterising actively respiring organelles occurs at stage 9 for R. temporaria and at stage 8 for B. bufo, correlated with blastula-gastrula and mid-blastula transition, respectively. The total numbers of mitochondria N(c) in mature oocytes are as high as 114.8 and 107.2 millions for R. temporaria and B. bufo, respectively, and during cleavage at late blastula stages they increase to 300 millions for both species under study. We suggest that an undefined mechanism might eliminate during cleavage those amphibian embryos which contain small number of mitochondria and low levels of nutrient substances.     

Relationship between nutritional state and testes function, together with observations on patterns of feeding, in the toad, Bufo bufo bufo

The effects of 10 weeks of starvation, restricted feeding, or ad lib. feeding on nutritional state and testes functions were investigated in male toads that had artificially hibernated in a refrigerator before being placed at room temperature. Food intake increased for about four weeks in both the fed groups. After this initial period the toads on restricted food ate their ration for the remaining part of the experiment, and they developed fat bodies of sizes characteristic of male toads during late summer in nature. The toads fed ad lib. increased food intake to about twice that of those on restricted food. After two weeks of high feeding rate food intake rapidly declined and remained low to the end of the experiment, when the fat bodies were larger than normally seen in nature. At the end of the experiment spermatogenetic activity had declined to a low level in the starving toads, whereas it was high in the fed toads. Starvation for 10 weeks did not affect the late stages of spermatogenesis. Interstitial cells and thumbpads remained reduced in the starving toads, but were restored in the fed toads. The importance of the annual cycle in nutritional state for gonadal cycles in male and female toads is discussed.