Analysis of feeding behavior in the newt Notophthlmus viridescens

JAMES B. MARTIN, NANCY B. WITHERSPOON, AND MILES H. A. KEENLEYSIDE Department of Zoology, University of Western Ontario, London, Ontario

Received July 17, 1973

MARTIN, J. B., N. B. WITHERSPOON, and M. H. A. KEENLEYSIDE. 1974. Analysis of feeding behavior in the newt Notophthalmus viridescens. Can. J. Zool. 52: 277-281.

Feedin behavior of newts (Notophthalmus viridescens) was studied by comparing the responses of individd animals to visual, chemical, and tactile cues from mosquito larvae. Feeding responses were most strongly stimulated by visual cues; less so by chemical and tactile cues in that order.

MARTIN, J. B., N. B. WITHERSPOON et M. H. A. KEENLEYSIDE. 1974. Analysis of feeding behavior in the newt Notophthalmus viridescens. Can. J. Zool. 52: 277-281.

On a Ctudie le comportement alimentaire de larves de tritons (Notophthalmus viridescens) en comparant les reactions de divers individus tt des stimuli visuels, chimiques et tactiles venant de larves de moustiques. Ce sont les stimuli visuels qui causent les rhctions alimentaires les plus marquees; viennent ensuite les stimuli chimiques, puis les tactiles. [Traduit par le journal]

Introduction Materials and Methods If live mosquito larvae are placed in an

aquarium containing several dozen hungry newts (Notophthalmus viridescens), activity among the newts increases rapidly. Those on the bottom begin to walk about faster and frequently press their snout into the substrate. Those out of contact with the bottom increase their swimming speed. The newts frequently snap at larvae, but often miss them. Occasionally a newt bites at the limbs or tail of another newt, especially when there has just been tactile contact between them. In short, there is a flurry of locomotor and feed- ing behavior. If only the water from a culture of mosquito larvae is placed in the aquarium, simi- lar increased locomotor activity, nosing into the substrate and snapping behavior, occur.

Observations such as these led us to examine the relative importance of different sensory modalities in eliciting feeding responses in N. viridescens. Since they are small animals, are easy to keep in the laboratory, withstand hand- ling well, and perform a number of unequivocal motor patterns in response to food, they are good experimental animals for such a study. Indeed, other workers studied feeding responses in N. viridescens for much the same reasons several decades ago (Reese 1912; Copeland 1913). These authors established that both visual

Experimental animals were obtained from the Carolina Biological Supply Co.. Burlington, North Carolina, and belong to the subspecies Notophthalmus viridescens dorsalis. They were kept on frozen adult brine shrimp (Artemia). Three different methods were used to present food stimuli to individual newts. They are described here; further details of each experiment are given with the Results.

I. Tube Method Several circular plastic bowls, diameter 20 cm, depth

9 cm, were lined with opaque green polythene sheeting and filled to a 2.5-cm depth with aquarium gravel and 6.0-cm depth with aged tap water. A glass tube, bore diameter 0.9cm, outer diameter 1.2cm, and length 30cm, with the bottom end either open or sealed shut was suspended vertically in the center of each bowl, with the bottom of the tube below the surface of the gravel.

Individual newts were presented with one or more of the next four stimulus situations.

(a) Visual stimulus only-Fifty mosquito larvae were placed in a tube sealed at the bottom end.

(b) Chemical stimulus only-From the mosquito larvae culture tank, 250 ml of water was filtered through several layers of cheesecloth and was added to the water in the test bowl.

(c) Chemical and visual stimuli-Two hundred and fifty milliliters of filtered mosquito culture water was added to the water in the bowl, and 50 mosquito larvae were placed in a tube open at the bottom but with a small asbestos plug inserted near the bottom end. The plug retained the larvae but allowed water to pass between tube and bowl (confirmed with dye).

(d) Control-An open tube was used, and aged tap water onlv was laced in both tube and bowl.

and chemical cues from food can evoke typical The teit expisure time to each stimulus situation was

feeding behavior. Our aim was to examine 5 min. Experimental bowls and tubes were thoroughly rinsed with aged tap water before each test.

further the relative effectiveness of visual, chemi- Open~Bovl Method cal, and tactile stimuli in the feeding behavior The me plastic bowls but without gravel or a glass of this species. tube were used. When tactile stimulation from moving

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278 CAN. J. ZOOL. VOL. 52, 1974

food organisms was required, 150 mosquito larvae were placed in a bowl. When a visual stimulus was needed, experiments were run in the laboratory with overhead room lights on; when not needed, they were run in darkness in a photographic dark room. For a chemical stimulus, 250 ml of filtered mosquito culture water was added to the water in the test bowl. When chemical cues were not required, no culture water was added and the live larvae were washed thoroughly in aged tap water before being placed in the bowl.

3. Beaker Method An 80-liter aquarium filled to a 5-cm depth with gravel

and a 20-cm depth with aged tap water was used. A single test newt was placed on the gravel inside an inverted 500-ml glass beaker. One hundred mosquito larvae were confined in a 25-ml plastic, tightly capped vial which was placed vertically on the gravel at measured distances from the beaker. When chemical cues were required, 1000 ml of filtered mosquito culture water was added to the experimental aquarium.

A total of eight experiments was run with these three h s i c designs. For each exprimen$, newts were selected randomly rrom the holding aquaria, were held without food for 2 or 3 days. and were then tested singly. Different aniinals were used roc replicates of each experiment, but the same animal may have been tested in more than one experiment. The various stimulus situations and controls within any experiment were presented in random sequence to each newt. For methods 1 and 3, the test newt was placed in the container facing directly away from the stimulus. Data were examined statistically by analysis of variance, except where otherwise noted.

Behavior Patterns Recorded From the total repertoire of motor patterns associated

with newt feeding behavior the following were selected as indicators of positive feeding responses.

I . Orientation-The animal turned towards a localized stimulus (contained in a glass tube or vial) so that the long axis of its body was no more than 30" to either side of an imaginary line from the stimulus to the center of the newt's body. Orientation was not recorded in tests using the open-bowl method because food stimuli were scattered throughout the test bowl.

2. Snout contact-This was recorded each time the anterior tip of a newt's snout made contact with a glass tube.

3. Bite-This consisted of a relatively slow opening of the mouth followed by rapid and forceful closing. It was distinguishable from respiratory movements by the wider opening and faster closing of the mouth. It was recorded regardless of the location of the animal in the test con- tainer.

4. Head turn-This was recorded when a newt moved its head laterally by bending the neck, without noticeably moving its body and tail. Head turns typically ranged up to about 66)" to either side of the long body axis, and were recorded regardless of the relationship between direction of turning and location of stimulus.

5. Line cross-In one experiment, the bottom of the test bowl was sectioned into quadrants by wax pencil lines on the underside. One line cross was recorded each time the test newt's head and body, at least as far back as the vent area, crossed one of these lines.

Results Experiment 1

The relative effectiveness of visual and chemi- cal stimuli in eliciting feeding responses was first assessed by testing 24 newts with the tube method as follows. Each animal was observed during a 5-min exposure to each of the four basic stimulus situations (visual, chemical, visual and chemical, and control) presented in random sequence. Frequencies of orientation, snout con- tact, and bite were recorded, and results are summarized in Table 1.

The data suggested that visual cues alone elicited frequent attempts at feeding, whereas chemical cues alone did not. However, when the two types of stimulation were combined, the frequency of snout contacts and bites was higher than with visual cues alone, showing that per- ception of chemicals associated with food in- creased the attempted feeding responses when the newts were also able to see their prey.

Experiment 2 The effect on feeding behavior of prey move-

ment as a conspicuous component of the visual stimulus situation was then assessed by compar- ing the responses of 12 newts to 50 dead and 50 living mosquito larvae, using the tube method of testing and tubes with the bottom end sealed. Larvae were killed by slight heating. The results indicated that movement was an important com-

TABLE 1

Responses of newts to four stimulus situations, using the tube method of testing. Values are mean responses

per 5-min test; n = 24

Behavioral Chemical response Control Chemical Visual + visual

Orientation 0 0 2.1 2.7 Snout contact 0 0 6.7 ** 9.6 Bite 0.6 1.2 ** 7.0 ** 13.2

* P < 0.05. **P < 0.01.

TABLE 2 Responses of newts to moving and non-moving visual cues, using the tube method of testing. Values are mean

responses per 5-min test; n = 12

Behavioral response Control Dead larvae Live larvae

Orientation 0 0.3 3.4 Snout contact 0 0.1 ** 6.8 Bite 0.3 0.3 * 4.5

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MARTIN ET AL.: FEEDING BEHAVIOR IN THE NEWT 279

ponent of the visually mediated stimulation of feeding behavior (Table 2). The newts showed very little response to the tube containing dead larvae, even though the larvae were clearly visible in the tube.

Experiment 3 visual cues for feeding were further studied

by comparing the responses of 16 newts to 2, 10, and 30 live mosquito larvae presented in random sequence with the tube method.

The results (Table 3) show that the number of larvae had little effect on response intensity. The newts responded strongly to all three prey densities.

Experiment 4 Since response latency might be important in

determining the success rate of a hunting newt, this experiment compared the effects of visual cues and visual plus chemical cues on both the time to initial orientation towards the stimulus (latency) and the frequency of feeding responses during the 2 min immediately after initial orientation. Ten newts were tested with the tube method.

The data (Table 4) show a significant reduc- tion in response latency when chemical and visual cues were present together ( t test, P < 0.05). However, subsequent feeding responses to the two classes of stimulation did not differ (analysis of variance, P > 0.05).

TABLE 3 Responses of newts to different numbers of live larvae,

using the tube method of testing. Values are mean responses per 5-min test; n = 16

Behavioral response Control 2 larvae 10 larvae 30 larvae

Orientation 0 2.3 1 .9 1.6 Snout contact 0 18.7 19.2 19.0 Bite 0 13.9 13.8 12.5

TABLE 4 Time to initial orientation (latency) and mean responses during the next 2 min to two stimulus situations, using

the tube method of testing; n = 10

Behavioral Visual + response Visual chemical

Latency, s 67 * 39 Orientation 2 . 2 1.7 Snout contact 3.3 2.8 Bite 2.5 4 . 2

Experiment 5 The relationship between the type of food

stimulus presented and response latency was examined further with the beaker method of testing. A single newt in an inverted beaker was presented either with visual cues only (n = 53) or with visual plus chemical cues (n = 64) with the vial containing mosquito larvae placed at varying distances from the beaker. The time for initial orientation towards the vial and the dis- tance between newt and vial were recorded in each trial.

DISTANCE - CM

FIG. 1. Relationshi between time to initial orientation towards pre and Bstance between newt and prey. Visual stimui only. n = 53.

300

240 - I

m - . ... . I 180 - 0 . ' . . w - 0 : . . . z - I- 120 ' . . . . * ' . . . . . . . 6 0 - . . =+*= '

* . . a .

DISTANCE - CM

FIG. 2. Relationshi between time to initial orientation towards pre and dstance between newt and prey. Visual and clemical stimuli present. n = 64.

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280 CAN. J. ZOOL. VOL. 52, 1974

The results are presented as two scatter dia- larvae on the numbers of larvae eaten was next grams (Figs. 1 and 2). In both cases, there assessed with the open-bowl technique. Twelve appears to be a positive relationship between newts were tested with four different stimulus distance to food and latency for orientation; situations, two including visual cues and two i.e., the closer a newt was to the larvae when excluding visual cues (tests run in a darkened released. the quicker i t turned towards them.

Regression analysis showed no significant ditrerence in slope hetween the two sets of points. Howebes, visual inspection of the points of Fig. 2 suggested they might fall into two groups, corresponding to distances between newt and larvae of 0-20 cm and over 20 cm. On this basis, separate regression equations were calculated for the points in these two ranges, Tor both experi- mental ponps of animals. A comparison of the two regression lines in the 0- to 20-cm range showed that :IL these distances the newts oriented more quickly towards the larvae when both chemical and visual cues were present (Fig. 2) than when only visual cues were present (Fig. 1, P < 0.05). This differential response did not hold for distances over 20 cm (P > 0.05).

Experiment 6 Since experiments 4 and 5 both showed that

response latency decreased when mosquito cul- ture water was added to the test containers, it appeared that chemical stimuli were increasing the appetitive searching for food. To examine this possibility further, the frequencies of head turns (In any direction) and of line crosses (as a measure of general activity) were recorded when newts were presented with chemical or visual

room). since in each trial the newts ate some larvae, animals were left without food for 3 days between trials. All trials lasted 15 min.

The data of Table 6 show that some larvae were eaten in the absence of visual cues, and that the addition of chemical cues increased the rate of feeding in the dark only slightly (comparison of columns 1 and 2, P > 0.05). However, when able to see their prey, the newts ate significantly more larvae (columns 2 and 3, P < 0.01), and this feeding rate increased even further when chemical cues were added to the test bowl (columns 3 and 4, P < 0.05).

Experiment 8 Finally, the effects on feeding responses of

eliminating visual cues by another technique was measured. Five newts served as untreated controls; the eyes of another five were rubbed quickly and lightly with cotton soaked inacetone, and a thin layer of vaseline was applied over their eyes: the eyes of five others wcre rubbed with acetone and then coated with vaseline mixed with lampblack. The efl'ectiveness of this experimental blinding was examined by placing all IS newts in a low holding tray in a dark room with a single light source. At the end of 30 min, all five of the vaseline- and lampblack-treated newts

feeding cues by the tube method. were still in the tray, while-all but 1 of the other The data (Table 5) show that chemical stimuli 10 had left the tray in the direction of the light.

alone increased the rate of head turning over the Each animal was then tested individually for control situation, and that visual stimuli alone 15min with the open-bowl method and the increased it even further. Line-cross frequency "tactile plus chemical plus visual" stimulus did not vary significantly among the three test situation (the situation was only "tactile plus situations. chemical" for the five blinded newts). The num-

bers of bites and of larvae eaten were recorded Experiment 7 for each newt. The role of tactile cues from living mosquito The results (Table 7) show that in

TABLE 5 Mean number of head turns and line crosses per 5 min TABLE 6 in response to three stimulus situations, using the tube Mean number of larvae eaten per 15-min test under four

method of testing; n = 6 stimulus situations, using the open-bowl method; n = 12

Behavioral Tactile + response Control Chemical Visual Tactile + Tactile + chemical +

Tactile chemical visual visual Head turn 10.2 ** 19.8 ** 34.7 Line cross 10.3 17.5 6.5 22.0 28.1 ** 61.3 * 86.0

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MARTIN ET AL.: FEEDING BEHAVIOR IN THE NEWT 28 1

with the controls both biting and consumption of larvae were significantly less frequent (P < 0.05) among "blinded" newts but were not changed among vaseline-treated newts (P > 0.05).

Discussion The data presented here clearly show that

visual cues are important in the elicitation of feeding attempts by Notophthalmus viridescens, and that chemical cues are also important, but less so than visual ones. Tactile cues mav also evoke feeding, although the experimental proce- dure did not permit clear separation of chemical and tactile stimulation. Several experiments demonstrated that visual perception only of live mosquito larvae will elicit approach and biting responses. Movement of the potential prey is essential for this response, but intensity of the response did not vary with three different larval densities. These results support the findings of Reese (1912) and Copeland (191 3) that move- ment is an important component of visual stimulation of N. viridescens feeding behavior.

A number of experiments showed that chemi- cal stimuli, eitheralone or together with visual stimuli, elicit feeding responses in N. viridescens (Tables 1 , 4, 5, and 6). Chemical cues appeared to be more effective in some situations than in others. For example, chemical plus visual stimuli produced shorter latency time to initial orienta- tion towards prey than visual stimuli alone when the prey wascloser than 20 cm, but not when it was further away (Figs. 1 and 2). This suggests that perception of chemical cues from live mosquito larvae leads to a heightened sensitivity

TABLE 7 Mean number of bites and of larvae eaten per 15-min

test under three stimulus situations, using the open-bowl method; n = 5

Behavioral Vaseline + response Control Vaseline lampblack

Bite 42.6 41.8 30 .O Larvae eaten 20.2 13 .O 6.2

NOTE: See text for resuits of statistical analysis.

to visual cues from those larvae when they are nearby, but not when they are some distance away. Thus, although chemicals from prey are stimulatory, the effective localization and ulti- mate capture of prey is influenced greatly by perception of visual cues. The increased rate of head turning in the presence of chemical stimuli from food (Table 5) could also enhance the likelihood of visual detection of food organisms.

In tests with light stimuli eliminated, either by running the tests in the dark, or by temporarily blinding the newts, some mosquito larvae swim- ming freely in the test container were eaten. This suggests the newts were able to detect the larvae by tactile stimulation; presumably, as both newts and larvae moved about in the container they occasionally made contact, thus setting off snapping movements by the newts.

It is difficult to relate these experimental findings to the natural situation. The major components of the diet of N. viridescens are well known; as aquatic adults they are carnivorous, feeding on aquatic insects and their larvae, crustaceans, worms, molluscs, leeches, amphibian and fish eggs, and occasionally small fish (Morgan and Grierson 1932; Bishop 1941; Noble 1954; Ries and Bellis 1966). However, the time of day when they feed most actively is not known. ~ e n c e , we conclude from our data that N. viridescens probably rely on visual, chemical, and tactile cues, in that order, to detect and capture their prey. Any of these sensory modalities can be effective alone, but vision seems to be the most efficient.

BISHOP, S. C. 1941. The salamanders of New York. Bull. N.Y. State Mus. 324: 1-365.

COPELAND, M. 1913. The olfactory reactions of the spotted newt, Diemyctylus viridescens (Rafinesque). J . Anirn. Behav. 3: 2 6 2 7 3 .

MORGAN, A. H., and M. C. GRIERSON. 1932. Winter habits and yearly food consumption of adult spotted newts, Triturus viridescens. Ecology, 13: 54-62.

NOBLE, G. K. 1954. The biology of the Amphibia. Dover, New York.

REESE, A. M. 1912. Food and chemical reactions of the spotted newt, Diemyctylus viridescens. J. Anim. Behav. 2: 190-208.

RIES, K. M., and E. D. BELLIS. 1966. Spring food habits of the red-spotted newt in Pennsylvania. Herpetologica, 22: 152-155.

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