cercetare hranire broaste

Frog farming Investigation of biological and mechanicalagents to increase the consumption of pelleted foodby adult Rana temporaria

Jonathan Miles James Williams Adrian Hailey1

School of Biological Sciences University of Bristol Woodland Road Bristol BS8 1UG UK1Corresponding author Present address Department of Life SciencesThe University of the West Indies St Augustine Trinidad and Tobago West Indiese-mail ahaileyfsauwitt

Abstract The requirement of moving prey for eliciting feeding behaviour is typical of anurans andis one of the major dif culties of rearing frogs This dif culty can potentially be solved by usingmechanical devices to move the food or by mixing inert food with live prey This study investigatedconsumption of pelleted food by adult common frogs Rana temporaria when moved by a mechanicalstirrer or by y larvae The mechanical device did not produce higher consumption than inert pelletsalone Fly larvae signi cantly increased consumption of food pellets whether these were mixedtogether or the larvae were isolated below the pellets by a exible membrane Consumption wassimilar whether the membrane was sealed or perforated so that movement of the pellets rather thanthe scent of the y larvae was the stimulus that increased pellet consumptionFrogs did not apparentlylearn to feed on pellets with no increase in consumption through the experiment of either control inertpellets or of pellets moved by y larvae Consumption (of dry mass) of pellets was similar to that oflive crickets by frogs in the same conditions Frogs consumed signi cantly more xenopus pellets(produced for aquatic Xenopus toads) than trout pellets from an equal mixture both by number andmass The two types of pellet had similar nutritional composition but differed in texture smell sizeand mass and colour xenopus pellets being larger (82 mg and 50 mg respectively) and darker Anycombination of these differences could have caused the difference in consumption with colour beingmost likely darker pellets presenting greater contrast against the white background of the feedingdishes The difference in consumption shows that optimisation of pellet characteristics such as theincorporationof a dark dye could be important in large-scale frog culture

Key words Farming feeding frog pelleted food Rana ranaculture

Introduction

There is substantial demand on a number of anuran species for both scienti cand culinary purposes France alone imports 34 tonnes of frogsrsquo legs each year

copy Koninklijke Brill NV Leiden 2004 APPLIED HERPETOLOGY 1 271-286Also available online - wwwbrillnl

272 Jonathan Miles James Williams Adrian Hailey

to be sold as expensive delicacies (Stebbins and Cohen 1995) At present mostof these animals are collected directly from the wild Alternatively on smallerscales frogs are reared on live prey and sold as expensive research specimensHigh economic costs and concerns about sustainability (Cooke et al 1990) havepromoted an interest in the possibility of large-scale rearing of frogs (ranaculture)both farming (strictly of captive-bred specimens) and ranching (from wild-caughtstock) Ranaculture has received interest for over a century (Chamberlain 1897)There were peaks of interest in the USA in the 1930s for edible frogs (Herriman1933 Schorsch 1933 Anonymous 1938 Stearns 1939) and from the 1970s forlaboratory specimens (Priddy amp Culley 1971 Culley 1976 Culley 1991) Currentinterest in the culture of frogs as a food source largely involves replacement speciesfor European green frogs (Beebee 1996) in the developing world (Lima et al 1986Hardouin 1991 1995 1997 Chifundera 1996 Somsueb and Boonyaratpalin2001 Mushambanyi 2002) though there is some continuing interest in ranaculturein Europe (Martinez et al 1996)

Apart from susceptibility to disease (Rodriguez-Serna et al 1996 Weng et al2002 Huys et al 2003) the major problem in ranaculture is getting frogs to eatRanid frogs consume live prey in nature and are usually refractory to inert foodin captivity (Modzelenski and Culley 1974 Holyoak 2002) Reliable consumptionof inert food is rare so that for example studies of digestibility have required thefrogs to be force-fed (Braga et al 1998 Castro et al 1998 2001) Ranid frogs aregeneralist predators of any moving prey of suitable size (Itaumlmies amp Koskela 1970Houston 1973 Blackith and Speight 1974 Hodar et al 1990 Hirai and Matsui1999) Prey capture by anurans is triggered mainly by visual cues (Maturana et al1960 Kramek 1976 Borchers et al 1978 Duellman and Trueb 1986) althoughmechanical stimuli are important for ingestion (Anderson and Nishikawa 1996Deban et al 2001) Many frogs capture prey at night under very low illuminationand in such conditions the movement pattern of the prey appears to be the mostimportant visual stimulus (Roth et al 1998) Moving prey elicit feeding responsesbecause moving edge receptors or bug detector cells in the retina of a frogrsquos eyeonly respond to small dark moving objects (Ewert 1980 1987) Many anuranspecies follow this type of visually mediated feeding behaviour which has majorimplications for ranaculture

To be economic on a large scale cultivated frogs would have to be fed on read-ily available prepared foods rather than live prey Research has shown that it ispossible to use moving visual stimuli to elicit prey capture behaviour even with-out the presence of live prey Moving models have long been used in neurologi-cal studies to elicit prey-capture responses (Ewert 1976) Recently Japanese re-searchers have been able to induce frogs to eat silk worm pupae by using mo-torized trays that mechanically roll the pupae back and forth to simulate liveprey (Helfrich LA Neves RJ Parkhurst J 2001 Commercial frog farminghttpwwwextvtedupubs sheries420-255420-255pdf) It is therefore possiblethat the strong visually mediated feeding behaviour of frogs could be exploited to

Feeding frogs pelleted food 273

increase the attractiveness and thus the consumption of prepared foods such as pel-lets

A novel method of moving food pellets was used by M Real AJ LabordaFJ Purroy and R Alvarez Nogal to stimulate feeding by young Rana perezi inexperiments on growth in relation to diet composition following a suggestion ofLima and Agostinho (1984) This work and more recent studies on R temporaria(R Alvarez personal communication) used live y larvae (maggots) mixed withfood pellets in the food dish Movement of the maggots caused the pellets to moveapparently stimulating their consumption by frogs It is possible however thatenhanced growth of frogs was due to consumption of the maggots themselves ratherthan the food pellets If the presence of maggots did increase pellet consumption itis also possible that this was due to accidental consumption of pellets with maggotsor to olfactory cues rather than to movement of the pellets

The present study investigated methods of moving food pellets offered to adultfrogs including both live maggots and mechanical feeding trays The hypotheseswere that the consumption of pellets would be increased by moving the food andthat movement rather than olfactory cues would be the more important stimulusThe experiments thus focused on the consumption of pellets rather than on resultantgrowth of the frogs with two advantages First the trials could be of much shorterduration of days rather than the months required to measure growth reliably bodymass change in frogs is generally slow (Ryser 1989) and easily obscured by watergain or loss Second consumption of maggots by the frogs was not a problem sinceit is their effect on pellet consumption that was of interest not on growth rates

The nal questions examined were whether frogs increased consumption ofpellets over time and whether consumption differed between types of pellet Long-term exposure to a particular prey can elevate the feeding response of amphibiansto that prey (Jaeger and Barnard 1981) which would also have implications forrearing frogs on pellet food The properties of prey have an important effect onits recognition by amphibians Gruumlsser and Gruumlsser-Cornehls (1968) found thatthe size con guration and contrast of the prey and its background were importantparameters in the attractiveness of a prey object We therefore used two types ofpellet both to maximise the chance of producing high consumption and to examinewhether a preference existed between them and thus whether pellet characteristicscould be important in ranaculture Rana temporaria was used as a model anuranthis species is no longer widely used for food or research

Methods

Adult common frogs of body length 55-72 mm were obtained commercially(Blades biological supplies Cowden Edenbridge Kent) and maintained in anapproximately 2 m2 plastic enclosure with net cover in a licensed controlledenvironment facility (12plusmnC and 12L12D photoperiod) Frogs had large waterdishes allowing total immersion in conditioned water (limescale extracted and


Table 1 Characteristics of the two types of foodpellets Values are shown sect SD where measured(N D 5) otherwise they are from manufacturersrsquoinformation

Xenopus Trout

Length (mm) 61 sect 08 47 sect 04Diameter (mm) 47 sect 03 37 sect 03Mass (mg) 82 sect 4 50 sect 3Colour Dark brown Pale brownOil () 19 20Protein () 48 42Ash () 9 8Fibre () 1 2Vitamin A (iu giexcl1) 12 12Vitamin D3 (iu giexcl1) 02 2Vitamin E (iu giexcl1) 018 012

ltered through sand and carbon) rocks and cut vegetation for cover and were fedmealworms and crickets A total of 25 individuals were used recognised from theirdorsal pattern using colour photographs although only 20 or 16 frogs were used ineach phase of experiments

Frogs were housed individually in 36 pound 60 pound 31 cm glass aquariums withsheet plastic covers during experiments to avoid competitive feeding interactions(Boice and Witter 1969 Boice and Williams 1971) The aquariums were coveredexternally with newspaper to reduce disturbance to the frogs The aquariumscontained a food tray water dish and a resting shelter constructed from bricks Thefood tray was housed within a feeding shelter to prevent the frog jumping throughthe tray and displacing the pellets this shelter surrounded the feeding dish aboveand on three sides

Experiments occurred in three phases In each phase each individual frog expe-rienced each of the trial conditions for a 48 h period being returned to the animalhouse for 24 h between these trials Each frog experienced each trial condition onlyonce in each phase of the experiment Each phase thus took about two weeks duringwhich time frogs were only fed in the aquariums The frogs were kept in the con-trolled environment between phases for up to three weeks and fed invertebrates

Two types of commercially-produced pellets were used with 25 of each type inthe food dish at the start of each trial These were trout pellets (Trout Elite 40Trouw Aquaculture Longridge Preston) and xenopus pellets (No 3 pellets Bladesbiological supplies) The latter were designed for feeding Xenopus toads which areexceptional among anurans in feeding aquatically on benthic macroinvertebrates(Schoonbee et al 1992) and readily taking pelleted food The characteristics ofthe pellets are shown in table 1 Pellets remaining after 48 h were counted and thenumber eaten converted to mass for statistical analysis


Figure 1 The main feeding tray designs a Pellets on their own (treatment 1) on a white cloth coverin a 10 cm plastic Petri dish 25 xenopus and 25 trout pellets were used in each treatment b Pelletsmoved mechanically (treatment 2) by an electric motor rotating a stirring arm with a plastic ballon each end under the white cloth cover c Maggots under pellets (treatment 3) separated by a thinsheet of white plastic perforated with air holes d Maggots sealed under pellets (treatment 7) with anunperforatedwhite plastic cover and an air tube leading outside the frogrsquos cage

The rst phase involved 20 frogs each exposed to each of ve stimulus treat-ments

1 Pellets alone not manipulated in any way (the basic control) The food dishwas a 10 cm plastic Petri dish with white cloth cover to give high contrast tothe pellets and increase the likelihood of their detection as food ( g 1a)

2 Pellets moved mechanically The aquarium was modi ed with a raised oorbelow which was a slow-moving 12 V DC electric motor attached to a stirringarm with a plastic ball on each end which moved the white cloth cover of thefood dish ( g 1b) The balls moved the pellets in an apparently similar way tothe biological agents (maggots)

3 Maggots under pellets Live blow y maggots (Lucilia sp obtained from shing bait suppliers) were placed in the food dish and covered with asoft white plastic sheet Movement of the maggots moved the pellets above


( g 1c) The sheet was perforated with air holes so that scent could passthrough but the frog could not feed on the maggots

4 Maggots mixed among pellets The situation was the same as the control(treatment 1) except that ve maggots were placed in the food dish amongthe pellets to move them about It was possible for the frogs to feed on thesemaggots

5 Crickets without pellets (the live control) Three live adult house crickets(Gryllus bimaculatus) were placed in the aquarium This treatment was used asan additional control of the effect of the experimental conditions on feeding bythe frogs for two possibilities First if no pellets were consumed this controlprovided a test of whether all feeding activity was inhibited in the aquariumsSecond if pellets were consumed this control provided a comparison of theintake of frogs feeding on live versus pelleted food

The rst phase experiments veri ed that frogs would eat pelleted food The livecontrol was therefore discontinued and the second and third phases both involvedonly four treatments and 16 individual frogs The treatments in the second phasewere as follows

3 Maggots under pellets This was the most successful treatment from the rstphase and became the control in the second phase against which the othertreatments were evaluated

6 Scented pellets moved mechanically This was identical with treatment 2 of the rst phase except that the pellets were pre-exposed to the scent of maggotsThis was designed to show whether the greater effectiveness of pellets movedby maggots in the rst phase compared to pellets moved mechanically wasdue to the scent of maggots The pellets were scented by keeping them withmaggots in a small container overnight before the trial

7 Sealed maggots under pellets This was similar to treatment 3 except thatthe plastic covering the maggots was not perforated There was an air tube toallow the maggots to respire ( g 1d) leading to the outside of the aquariumto remove scent stimuli Comparison of treatments 3 and 7 was thus designedto test whether movement or scent was responsible for the effectiveness ofmaggots under pellets in the rst phase of experiments

8 Dead maggots under pellets This was also similar to treatment 3 but in thiscase the maggots had been killed (by pinching their anterior ends) Treatments7 and 8 were thus complementary in testing the stimuli responsible for theeffectiveness of maggots under pellets Treatment 8 provided scent but notmovement while treatment 7 provided movement but not scent

The third phase of experiments also involved four treatments all of which hadbeen used previously treatments 1 3 7 and 8 The three latter treatments wererepeated to check the results from the second phase of trials Treatment 1 (pellets ontheir own) was repeated to show the effectiveness of the nal treatments against thebasic control The comparison of treatment 1 in the rst and third phases was also


designed to test whether the frogs had learned to feed on pellets during the courseof the trials

Numbers of pellets were converted to mass consumed by multiplying by themean dry mass of pellets of each type (after 24 h drying in an oven at 70plusmnC)Crickets were also dried at 70plusmnC for comparison with consumption of pelletsThe data included many zero values and were thus not normally distributed ortransformable to a normal distribution Non-parametric tests were therefore used(and error bars are not shown on gures) Kruskal-Wallis tests were used as analternative to analysis of variance to compare results across all treatments duringeach phase of experiments Two-tailed Mann-Whitney tests were then used tocompare speci c pairs of treatments within and between phases these comparisonswere made according to a priori hypotheses as indicated in the descriptions ofthe treatments above rather than unplanned comparisons of all possible treatmentpairs Dimensions of xenopus and trout pellets were compared with two-samplet tests Statistical analysis was performed with Minitab This program gives W

as the Mann-Whitney test statistic and uses its distribution to estimate P directly(McKean and Ryan 1977) rather than indirectly through U hence values of W arereported here

Results

First phase

There was a signi cant difference between the mass of pellets consumed under thefour treatments (Kruskal-Wallis test H D 163 df D 3 P D 0001) indicatingthat manipulation of pellets did in uence feeding by the frogs The mean dry massof pellets (or crickets) consumed in each treatment is shown in g 2 treatment 3(maggots under pellets) gave the highest consumption and treatment 1 (pellets ontheir own) the lowest

Mann-Whitney tests were used to compare each of the test treatments (2-4)with the basic control (treatment 1) Treatments 3 and 4 were both signi cantlydifferent from the control W D 2870 P D 00007 and W D 3275 P D 0022respectively The mechanical treatment (2) was not signi cantly different from thecontrol (W D 4180 P D 0829) The rst phase thus suggested that live maggotsprovided the best form of pellet movement giving signi cantly better feeding thaninert pellets which the mechanical device did not

The consumption of crickets (treatment 5) was similar to that of pellets indicatingthat the frogs were showing a reasonable appetite when fed on prepared foodcompared to live prey in the same conditions This comparison was in terms of drymass which is likely to limit food consumption in ectotherms where the durationof digestion is substantial (Hailey et al 1998) Mann-Whitney tests showed thatconsumption of crickets was not signi cantly different from that of pellets abovemaggots (treatment 3 W D 4630 P D 0151) but was signi cantly different from


Figure 2 Mean dry mass of pellets or crickets consumed per frog in the rst phase trials Treatmentsare 1 Pellets alone (the basic control) 2 Pellets moved mechanically 3 Maggots under pellets4 Maggots mixed among pellets 5 Crickets without pellets (the live control)

the basic control (treatment 1 W D 2900 P D 00009) Inert pellets alone thusresulted in food consumption that was signi cantly lower than that of live prey

Second phase

There was again a signi cant difference between the mass of pellets consumedunder the four treatments (Kruskal-Wallis test H D 119 df D 3 P D 0008)despite the absence of the basic control It is thus clear that different forms ofpellet manipulation did affect food consumption of the frogs The mean values areshown in g 3 treatment 3 (maggots under pellets) gave the highest consumptionas in the rst phase The mechanical treatment (6) again gave low consumptionbut in this case the pellets had been scented by previous exposure to maggotsThe poor performance of the mechanical treatment (2) in the rst phase relativeto the treatments involving maggots was therefore not due to different scent stimuliScenting the pellets with maggots actually signi cantly decreased the amountconsumed compared to unscented pellets in the rst phase (treatments 2 and 6W D 2335 P D 0019)

Mann-Whitney tests were used to compare each of the other treatments withthe best-performing treatment (3 mdash maggots under pellets) as this had givenconsistently high consumption in both the rst and second phases In the secondphase there was a signi cant difference between treatments 3 and 6 (mechanical


Figure 3 Mean dry mass of pellets consumed per frog in the second phase trials Treatments are3 Maggots under pellets 6 Scented pellets moved mechanically 7 Maggots sealed under pellets8 Dead maggots under pellets

W D 3465 P D 0006) There were however no signi cant differences betweentreatment 3 and either treatment 7 (sealed maggots under pellets W D 3065P D 0098) or treatment 8 (dead maggots under pellets W D 3000 P D0164) The second phase thus allowed the rm conclusion that maggots were moreeffective than the mechanical device in stimulating feeding by the frogs It remainedunproved however whether scent or movement of the maggots was the major factorin increasing pellet consumption

Third phase

There was again a signi cant difference between the mass of pellets consumedunder the four treatments (Kruskal-Wallis test H D 121 df D 3 P D 0007)The results in g 4 show low consumption in the basic control (treatment 1 pelletson their own) and treatment 8 (dead maggots under pellets) and high consumptionin the treatments involving live maggots under the pellets (3 and 7 the latter sealed)

Mann-Whitney tests showed that the control treatment 1 was signi cantly dif-ferent from both treatments 3 and 7 (W D 1945 P D 0007 and W D 2095P D 0035 respectively) but not from the dead maggots under pellets (treatment 8W D 2565 P D 0777) These results suggest that it was the movement rather thanthe scent of maggots which stimulated increased pellet consumption This sugges-tion is con rmed by the direct comparison of live and dead maggots under pellets


Figure 4 Mean dry mass of pellets consumed per frog in the third phase trials Treatments are1 Pellets alone 3 Maggots under pellets 7 Maggots sealed under pellets 8 Dead maggots underpellets

(treatments 3 and 8) which were signi cantly different (W D 3350 P D 0007)Treatments 3 and 7 (maggots under pellets the latter sealed) were not signi cantlydifferent (W D 2455 P D 0493) as in the second phase so that scent had noeffect on pellet consumption

A Mann-Whitney test was also used to compare treatment 1 (pellets on their own)in the rst and third phases of experiments There was no signi cant difference(W D 2800 P D 0595) suggesting that frogs did not learn to take pellets fromthe food tray during the course of the experiments This was supported by a Kruskal-Wallis test comparing treatment 3 (maggots under pellets) across all three phaseswhich also showed no signi cant change during the experiments (H D 476 df D 2P D 0092)

Comparison of pellet types

The frogs consumed more xenopus than trout pellets in most of the treatmentsthroughout the three phases of the experiment ( g 5) There were signi cantdifferences in the numbers of xenopus and trout pellets consumed in all three phases(1-sample Wilcoxon tests of the difference between the two types table 2) Thesigni cance was even greater when consumption was expressed as mass since thexenopus pellets were larger than the trout pellets (table 2) Xenopus pellets were


Figure 5 Mean numbers of pellets of two types consumed per frog in each treatment and phase of theexperiment Filled circles D xenopus pellets open circles D trout pellets

Table 2 Wilcoxon tests of the difference in consumptionof xenopus and trout pellets between individual frogs bynumbers and by dry mass N is the number of frogs N fortest is the number used in interpretation of the test statisticexcluding frogs that consumed equal amounts of the twotypes (ie where the difference was 0 the same as the nullhypothesis median)

W N (N for test) P

Phase 1 Numbers 1810 20 (19) 0001Mass 2090 20 (20) lt0001

Phase 2 Numbers 1000 16 (15) 0025Mass 1190 16 (16) 0009


signi cantly longer (t D 347 df D 8 P D 0005) and wider (t D 577 df D 8P lt 0001) than trout pellets

Discussion

This study found that it was possible to increase the consumption of pelleted foodby making the pellets move using live maggots but we were unable to producea mechanized feeding tray that would increase consumption There are severalpossible reasons why the mechanical method failed Firstly the type of movement


that caused by the mechanized feeding tray was continuous while pellet movementcaused by maggots was random and interrupted Small objects that move in a jerkyfashion are more likely to trigger an attack than continuously moving ones in thetoad Bufo bufo (Borchers et al 1978) The speed of pellet movement was alsofaster in the mechanized treatment than that caused by maggots The speed of preymovement has been identi ed as another parameter which affects prey captureresponses in amphibians (Beaquin and Gaillard 1998) Another possible factoris that the rotating arm had a large effect on the cloth on which the pellets layand the perception of moving prey by frogs only occurs where the background isstill (Burghagen and Ewert 1983) In treatments with live maggots these causedrelatively subtle movement of the tight plastic cover and minimized the movementof the background in these feeding tray designs It is also possible that soundsproduced by moving maggots alerted the frogs to the presence of food (Martof1962) The louder sounds and vibration caused by the mechanised feeding traysmay have stressed the frogs and thus limited consumption of pellets from thesetrays

Treatments using live maggots signi cantly increased consumption of foodpellets giving a similar consumption of dry mass to that of live cricket prey Thedifferent treatments in phases two and three con rmed that it was the movementrather than the scent of the maggots which was effective There is evidence fora role of olfaction in feeding of anurans (Heusser 1958 Shinn and Dole 19781979 Dole et al 1981 Rossi 1983) Nevertheless maggots sealed beneath pelletswere equally effective in increasing pellet consumption in this study con rming thewidely accepted view that olfactory cues are of secondary importance to visual cuesin anuran feeding behaviour (Dawley 1998) Olfaction may have been importantin the consumption of pellets in the control treatment Other evidence suggeststhat olfaction can be important in amphibians when prey is not moving and atnight when visual clues are less clear (Placyk and Graves 2002) This study doesnot preclude the use of olfactory cues but suggests that those from maggots wereno more attractive than those from the pellets themselves These treatments alsoshowed that consumption of pellets was not simply an accidental result of ingestingmaggots but also occurred when the maggots were inaccessible to the frogs beneaththe cover of the feeding tray

Consumption of pellets in the control treatment 1 and the best treatment 3 didnot change between the phases of the experiment indicating a lack of learning oracclimatization David and Jaeger (1981) reported that long-term exposure to a par-ticular prey type can elevate the feeding response to that prey but such preferencesare believed only to develop shortly after metamorphosis Work on salamanders hasshown that the period 1-2 months after metamorphosis was important in develop-ing experience-dependent preferences which were not reversible (Luthardt-Laimer1983) Wake et al (1983) also reported that salamanders reared on xed prey learntto eat immobile objects The implication is that frogs reared during ranacultureshould be fed pellets immediately from metamorphosis


Xenopus pellets were consumed signi cantly more frequently than trout pelletsA number of factors may have caused this difference The similarity of nutritionalcomposition and the consistent difference in consumption throughout the experi-ment ( g 5) both suggest that learning or optimisation of diet selection was notinvolved although anurans can learn visual features of food (Brower et al 1960)Xenopus pellets were signi cantly larger than trout pellets in both length and widthFrogs have an optimal or preferred prey size (Ewert 1983) and the xenopus pelletsmay have been closer to this size for adult R temporaria There was also a distinctcolour difference between the pellets and the darker xenopus pellets would havegreater contrast against the white background on which pellets were presented an-other important factor in eliciting feeding in anurans (Ewert 1980) The two typesof pellet also differed noticeably in scent at least to humans when presented in bulkAlthough the reason cannot be proved the fact of differential consumption showsthat optimisation of pellet characteristics such as the incorporation of a dark dyecould be important in large-scale frog culture

Although maggots were more successful than the mechanical method the latterprobably has more potential in practical ranaculture Further work should be doneon developing and testing a range of mechanical methods for moving pellets Pelletmanipulation in the absence of live prey would greatly reduce the cost of rearingfrogs commercially The method of Holyoak (2002) with a suction air ow systemwhere a moving arm picks up a pellet and transfers this to the frog container isperhaps rather complex and dif cult to adapt to large scale production Maggotswere effective although only producing small movements of the pellets whichsuggests that the duration of movement may be the most important factor movingfor long enough to excite feeding but not continuously

Further work is needed on the long-term growth of frogs fed pelleted foodConsumption rates of about 100 mg dry mass dayiexcl1 ( g 2) suggest that frogs willgrow substantially faster than in the wild Loman (1979) reported feeding rates ofadult R temporaria of 375 mg wet mass dayiexcl1 in the wild equivalent to 2-4of body mass dayiexcl1 Captive R arvalis consumed only 13 of body mass dayiexcl1

(Loman 1979) similar to values for this species in the wild (Chlodny and Mazur1969) Captive frogs fed on high-energy concentrated food thus have the potentialto grow rapidly if reliable ways to induce them to feed can be perfected

Acknowledgements This work was stimulated by an unpublished study by M RealAJ Laborda FJ Purroy and R Alvarez Nogal We thank Rafael Alvarez Nogal forcommunicating his recent work in this eld and Robert Massie and Paul Court fortechnical assistance This study was carried out under Project Number UB02 B002


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Jaeger RG Barnard DE (1981) Foraging tactics of a terrestrial salamander choice of diet instructurally simple environments Am Nat 117 639-664

Kramek WC (1976) Feeding behavior of Rana septentrionalis (Amphibia Anura Ranidae)J Herpetol 10 249-251

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Accepted December 30 2003


to be sold as expensive delicacies (Stebbins and Cohen 1995) At present mostof these animals are collected directly from the wild Alternatively on smallerscales frogs are reared on live prey and sold as expensive research specimensHigh economic costs and concerns about sustainability (Cooke et al 1990) havepromoted an interest in the possibility of large-scale rearing of frogs (ranaculture)both farming (strictly of captive-bred specimens) and ranching (from wild-caughtstock) Ranaculture has received interest for over a century (Chamberlain 1897)There were peaks of interest in the USA in the 1930s for edible frogs (Herriman1933 Schorsch 1933 Anonymous 1938 Stearns 1939) and from the 1970s forlaboratory specimens (Priddy amp Culley 1971 Culley 1976 Culley 1991) Currentinterest in the culture of frogs as a food source largely involves replacement speciesfor European green frogs (Beebee 1996) in the developing world (Lima et al 1986Hardouin 1991 1995 1997 Chifundera 1996 Somsueb and Boonyaratpalin2001 Mushambanyi 2002) though there is some continuing interest in ranaculturein Europe (Martinez et al 1996)

Apart from susceptibility to disease (Rodriguez-Serna et al 1996 Weng et al2002 Huys et al 2003) the major problem in ranaculture is getting frogs to eatRanid frogs consume live prey in nature and are usually refractory to inert foodin captivity (Modzelenski and Culley 1974 Holyoak 2002) Reliable consumptionof inert food is rare so that for example studies of digestibility have required thefrogs to be force-fed (Braga et al 1998 Castro et al 1998 2001) Ranid frogs aregeneralist predators of any moving prey of suitable size (Itaumlmies amp Koskela 1970Houston 1973 Blackith and Speight 1974 Hodar et al 1990 Hirai and Matsui1999) Prey capture by anurans is triggered mainly by visual cues (Maturana et al1960 Kramek 1976 Borchers et al 1978 Duellman and Trueb 1986) althoughmechanical stimuli are important for ingestion (Anderson and Nishikawa 1996Deban et al 2001) Many frogs capture prey at night under very low illuminationand in such conditions the movement pattern of the prey appears to be the mostimportant visual stimulus (Roth et al 1998) Moving prey elicit feeding responsesbecause moving edge receptors or bug detector cells in the retina of a frogrsquos eyeonly respond to small dark moving objects (Ewert 1980 1987) Many anuranspecies follow this type of visually mediated feeding behaviour which has majorimplications for ranaculture

To be economic on a large scale cultivated frogs would have to be fed on read-ily available prepared foods rather than live prey Research has shown that it ispossible to use moving visual stimuli to elicit prey capture behaviour even with-out the presence of live prey Moving models have long been used in neurologi-cal studies to elicit prey-capture responses (Ewert 1976) Recently Japanese re-searchers have been able to induce frogs to eat silk worm pupae by using mo-torized trays that mechanically roll the pupae back and forth to simulate liveprey (Helfrich LA Neves RJ Parkhurst J 2001 Commercial frog farminghttpwwwextvtedupubs sheries420-255420-255pdf) It is therefore possiblethat the strong visually mediated feeding behaviour of frogs could be exploited to






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cercetare hranire broaste

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