Varanus salva tor

Malayan Nature Joumal1993 46: 229

241

Notes of the feeding behaviour of the Water Monitor, Varanus salva tor CARLTRAEHOLT 1 Abstract: The present study reviews some behavioural aspects of the feeding behaviour of five individuals of the Malaysian Water Monitor lizard, Varanus salvator. Search patterns and hunting techniques are revealed which document that V. salvator exhibits area-concentrated foraging behaviour and that the principal hunting mode is "open pursuit" rather than stalking and ambushing. This species also shows a significant ten­ dency to seize live mice and rats in the head/chest region. Prey is always devoured anterior end first and the prey is normally killed before it is devoured. Social facilitation of feeding behaviour between individual lizards is common, and no attempts at piracy were observed. The species shows a clear circadian rhythm; however, the timing of peak activity shows inter-individual variability. Given the opportunity to feed on live fish the lizards showed no signs of being able to catch them.

INTRODUCTION The Water Monitor lizard Varanus salvator (Varanidae) is the most wide­ spread of all the species in its family. It is found from Sri Lanka, northern India, Bangladesh, Burma, Vietnam and Hainan to Peninsular Malaysia and the Indonesian islands (Smith, 1932). It is the only monitor lizard that is knoV\>'l1 to have colonized marine habitats successfully, and in particular its colonization of remote Indonesian islands might be due to its adapta­ bility towards both fresh water and salt water habitats. Individuals have been reported by Smith (1932) as swimming between islands more than 2 km apart, and it is likely that they are able to cover even longer distances as they frequent islands more than 20 km off-shore. The Water Monitor is a carnivorous species feeding on almost every­ thing it can handle (Deraniyagala, 1931). Whether it feeds mainly on carrion or captures live prey by active hunting is not known. The ability to survive on small islands makes it doubtful that they survive on carrion alone, as the mortality rate among potential prey items would be too low to support a population of such lizards. This suggests that they perform some kind of active hunting, be it for fish or terrestrial organisms (Duckett, 1986; Harrison & Lim, 1957). lInstitute of Population Biology, University of Copenhagen, Universitetsparken 15, 2100 Copenhagen, Denmark.

(Accepted for publication 2 February 1993)

229

Previous studies of other monitor lizards including V. komodoensis, V. gouldii and V. dumerili (Auffenberg, 1981; Pianka, 1970; Sprackland, 1976) indicate that monitor lizards in general are active hunters. Auffenberg (1981) and Burden (1928) even imaginatively suggest that V. komodoensis and V. salvator in the ecosystem of the Indonesian islands Sumba and Komodo represent an ecological equivalent to the lion in Africa and the tiger in Asia. The body temperatures of Australian monitors V. gouldii have been studied by Bartholomew & Tucker (1964), Johnson (1972) and Stebbins & Barwick (1968), revealing that peak activity in that species occurs at approximately 37° C. At what body temperature level the Water Monitor is most active is not known, although Gleeson (1981) has indicated that it may be about 35 to 36° C. Although Malaysian Water Monitors have been reported to be active both day and night (Harrison & Lim, 1957), most accounts describe them as diurnal. Descriptions of predation among monitor lizards have been limited and the few that exist deal mainly with Varanus komodoensis and Varanus bengalensis. Clearly, further understanding of the behavioural adaptations of, possibly, one of the most active reptilian hunters demands more com­ prehensive and detailed descriptions. This report is intended to reveal some aspects of the ecology of predation of the Water Monitor, V. salvator, a little studied species. METHODS Five juvenile Varanus salvator were used as the basis of this study. They were caught in an oil palm plantation in the state of Selangor (Peninsular Malaysia) and brought to the Copenhagen Zoo (Denmark). Their total length measured from 100-120 cm during the observations. They were kept in a 4 m 2 cage with access to two 150 litre water basins. The cage contained two 'caves' and some logs that made climbing possible. The floor was heated, which kept the water temperature constantly at 28°C, whereas the air temperature was 25° C. During the acclimatisation period they were fed with carrion, eggs, minced beef, herring and live mice. The lizards were given 6 mon ths to adapt to their new surroundings. To measure their diurnal rhythm they were recorded on video (NEC, PVC - TL 50E Timelapse Recorder) for 14 days. The results of "hunting tactic', "ingestion direction", "initial attack site" and "number of tongue flicks" were observed personally when the lizards were fed. The observations had to be done from behind a shelter since the lizards' behaviour differed if they were aware of human presence. The food during the observation periods consisted of herring and live mice. Since determining the sexes is extremely unreliable in most varanids (Davis & Phillips, 1991; Vikramanayake & Dryden, 1988), unless dissected, no observations on intraspecific or

230

intrasexual behavioural differences were made. Observations were made on 70 fceding bouts (60 mice and 10 fish). The video recordings were used to analyze special movements during hunting, feeding and searching. Other behavioural interactions were also noted and arranged in classes according to the motive and function of the particular act. After having starved the lizards for one week in order to increase their hunger and hence their foraging activity, five live Trichogaster laevi (freshwater fish) approximately 10 cm long were placed in one of the water hasins. The lizards were observed for 4 hours to see if and how they would catch the fish. RESULTS

Behavioural Observation Hunting Class 'Open pursuit': This was the most common way to capture live mice and rats. It is a technique in which the lizard sprints towards the prey item and catches it. If it fails the lizard will usually follow the prey until it succeeds in capturing it. If it loses sight of the prey, it will start searching. This hunting mode was very effective (Table 1). It is also used by V. bengalensis and (for small prey) by V. komodoensis (Auffenberg, 1981, 1982). 'Ambush': The lizard waits in the cave or water basin until prey gets within 10-15 cm distance. The lizard then catches the prey with a fast striking movement. This has only previously been reponed to occur in V. komodoensis (Auffenberg, 1981). 'Stalk': This was only used a few times when prey was near the water and the lizard in a cave. Instead of using the 'open pursuit' the lizards sometimes went into the water and approached the prey. It was, however, difficult to differentiate between 'ambush' and 'stalk' because of the limited space available to the lizards in the cage. This technique has been seen occasionally among V. bengalensis (Auffenberg, 1982).

Table 1. The distribution of hunting techniques used by Varanus salvator, the frequency and the success rate. Predation technique

Ambush

Stalk

Pursuit

Other

Frequency Missed Success rate

4 (6.7%)

2 (3.3%)

37 (61.7%)

17 (28.3%)

o

o

o

o

100%

100%

81%

53%

231

I

Prey Handling Class 'Shake': Captured live prey were always shaken violently and hit against either the substrate, cave or log. What it hit did not seem to be a deliberate act but rather a coincidence depending on the place where the 'shake' behaviour happened. This behaviour has previously been reported among V. bengalensis (Auffenberg, 1982; Loop, 1974), V. indicus (Dryden, 1965) and V. dumerili (Sprackland,1976). 'Scrape': Apart from the shake behaviour, the lizards also scraped prey against the substrate. This also occurs in V. indicus (Dryden, 1965) and V. bengalensis (Auffenberg, 1982; Loop, 1974). 'Crush skull and neck': Lizards mostly seized mice in the head/chest region (Table 2) which would normally kill the prey. If the prey was not initially grasped on this part of the body, the lizard would always turn it around until it was held in the head/chest region. The behaviour also occurs among V. dumerili (Sprackland, 1976) and V. indicus (Dryden, 1965). Search and Foraging Class 'Non-investigative': This is rather a passive behaviour, apparently the incidental discovery of food when a lizard was moving from one basking site to another. The travel path was usually direct, with no deviations. No tongue flicks "..ere observed during such activity. The behaviour has also described by Auffenberg (1982) in V. bengalensis, and Vogel (1979) observed this behaviour in wild V. salvator in a situation where one individual approached a carcass already being consumed by another individual. 'Investigative search': Searching for food was characterized by (1) regular tongue flicks at a rate of approximately two per minute, which increases after food has been swallowed (Fig. 2), (2) greater lateral and horizontal head and body movements, and (3) slower forward speed due to sinusoidal and circular trackway and casting about. The lateral movements seem to be an attempt to locate prey visually. This search behaviour can Table 2. The initial attack sites on mice and their distribution in percent. results are based on 60 attacks on live mice. The respective attack sites the defined as follows: Head:::: from front leg and anterior; chest:::: region between front legs and midpoint between front and hind legs; stomach region between hind legs and the midpoint between front and hind legs; tail:::: posterior to the hind legs. Initial attack sites Frequency

% of total attacks, n = 60

Head/Chest

Stomach

Tail

46

9 15.0%

5 8.3%

76.7%

232

overall be termed "scanning" (Curio, 1976). It is also seen in V. bengalensis (Auffenberg, 1982), V. dumerili (Sprackland, 1976) and V. niloticus (Edroma & Ssali, 1983). 'Casting about': During forward movements the lizard stops and searches a small area around its head, and sometimes the rate of tongue flicks increases. This behaviour may be related to the discovery of a scent track by the lizard. The behaviour also occurs in V. bengalensis (Auffenberg, 1982). 'Area-concentrated search': This search pattern is linked to an area where the lizard has previously made a prey capture. After it has finished ingestion the lizard will search intensively in the same area where it caught the prey. Others 'Regurgitate': When extremely full a lizard will regurgitate food if disturbed. This has also been found to occur in V. komodoensis (Burden, 1928). Table 1 shows the distribution and respective percentage of different kinds of prey capture. The result show that the principal \vay of capturing live mice is by "open pursuit" which counts for 37 (62%) of all the attacks (n = 60). This is consistent with the behaviour of V. bengalensis (Auffenberg, 1982). However, this was less successful (7 missed attempts = 18.9%) than the "ambush" tactic which did not have a single miss in six attempts. The term "other" in Table 1 refers to acts that cannot be classified as either open pursuit or ambush. The term covers two different acts: (1) the lizard behaves shyly in human presence and mice can run around on the lizards and even nibble the head without initiating any responses from the lizard; however, sometimes it becomes too strong a stimulant for a hungry lizard which grabs and kills the mouse without any further actions; and (2) the lizard is active but full and does not pay the mice any attention. l\"ever­ theless, it appears that live mice always induce feeding behaviour, and if a mouse approaches even a full lizard often it will eventually be captured and eaten. Whereas V. bengalensis exhibits no tendency to seize a live mouse at any particular site on the mouse's body (Loop, 1974), V. salvator shows a significant tendency to seize the mouse at the chest/head area compared to the stomach and tail (x 2 9.19, p < 0.01, df = a). The results are given in Table 2. The nine cases when a mouse was seized in the stomach region appeared to be in the "Open pursuits". Only in five cases (8.3% n = 60) were mice captured by their tail. This occurred only when the monitors had already eaten a few mice. It might have been due to the triggering of feeding reactions as explained above. Captured prey was normally killed with a bite in the neck. If this was not sufficient the prey was shaken

233

violently and scraped against the substrate. Similar behaviour is found to occur in V. bengalensis (Auffenberg, 1982). In general, a captured mouse was not eaten before it was dead or a least so injured that it was unable to struggle. According to Loop (1974), the size of the prey has a direct influence on how V. bengalensis treats it; the larger the prey the more violently will it be treated. To see whether this was also relevant among the water monitors they were presented with a full grown rat. Despite its size, the rat was attacked and killed by a single lizard exactly the same way as mice. In these cases a lizard struggling wi th a rat induced strong social facilita­ tion in the other lizards. All of them would gather around the feeding individual, bu t apparently no piracy attempt occurred, which is in contrast to the behaviour of V. bengalensis (Auffenberg, 1982). The individual that first caught the prey was not disturbed in its bout. However, if the prey got loose all of the lizards present would try to catch it. When a prey item has been caught and killed there is a significant tendency for the lizards to ingest the prey front end first (x 2 , p < 0.001, df = 1) (Table 3), which is similar to the tendency found among V. bengalensis (Loop, 1974) and V. dumerili (Sprackland, 1976). Loop (1974) suggests the ingestion direction is directly related to the size of the prey (discussed below). All the food/prey were swallowed in one piece and there were no attempts to tear prey apart and eat it bit by bit. After the captured prey has been swallowed the lizard starts an "area-concentrated" search behaviour where the rate of tongue flicks/minute increases from 2-3 flicks/min to 21 flicks/min. (Figure 1). This figure also shows that the mean rate under "investigative searching" is 2.2 flicks/min, and that the sudden increase in the number of tongue flicks after eating decreases with time. When fed with five live fish the lizards apparently located the prey by sight, from above the water surface. There were a few attempts at "open pursuit" which revealed that the lizards were unable to catch live fish. After a few such failed attempts a lizard carried ou t displacement activities before returning to the water basin and making another unsuccessfull capture attempt. This behaviour was kept up for two hours, after which the lizards lost interest in the fish. The Trichogaster laevi were still present five days after they were placed in the basin, and were then removed. Table 3. The ingestion direction of two different kinds of prey. Ingest direction

Anterior

Other

Mice (captured) Fish (scavenged)

60 (100%) 10 (100%)

o

o

234

'"

15

-"

~

'"c:"

0>

(2

10

5

B

TIme (mm. l

Figure 1. The number of tongue flicks/minutes. Letter A refers to the amount of tongue flicks after having eating and letter B under normal conditions (n = 8).

Circadian rhythm The captive V. salvator showed a diurnal rhythm, illustrated in Fig. 2, in spite of there being no night period in their cage. Fig. 2 indicates some difference in the circadian rhythm between the individuals. Lizards A, Band E conducted most of their activity during late afternoon to evening (which accounted for up to 450 minutes/time span in lizard E) and again in the morning, whereas lizard C was mostly active between 2400 hour and 0600 hour. The term "activity" for this analysis includes three main kinds of behavioural patterns, "search", "climb" and "break out". When the lizards moved between two basking/sleeping sites, this was not counted as "activity".

DISCUSSION

V. salvator exhibits two main kinds of prey capture, "open pursuit" and "ambush" (Table 1 and descriptions above). Even though the result in Table 1 indicate that the ambush tactic seemed to be the most successful (100% of attempts succeeding), this was not the commonest method of prey capture. Instead, it was the "open pursuit" which made up 61.7% of the total attempts. This hunting technique is also the most common one in

235

I

A

300

c

200

E

::

~! t;