Activity budget of Bengal Monitor Varanus bengalensis: Effect of daytime, season, age and temperature

Abstract

The Bengal Monitor (Varanus bengalensis) is a large, ecologically flexible species and commonly found in many ecological settings including human-modified habitats. We conducted a study on the activity budget of this species employing the focal sampling method in the Chandpur district of Bangladesh. The Bengal Monitor spent the highest proportion of time in resting (33.09%) and the least proportion (3.75%) of time in anti-predatory behavior. The proportion of time spent on each activity by Bengal Monitor varied significantly between age classes and seasons except anti-predatory behavior. The highest proportion of basking activity was recorded for adult and sub-adult individuals in the dry season. Meanwhile, resting and foraging behavior was higher during the wet season. Locomotion activities were higher in sub-adults than in adult individuals. The duration of the activities did not differ significantly for daytime and season. However, an increase of air temperature significantly impacted lowering basking and rising locomotion activities among all behaviors of adult Bengal Monitor. The present findings suggest that the activities of the Bengal Monitor varied according to temporal and seasonal changes in the habitat's temperature, and age may crucially affect the allocation time for different activities.

1. Introduction

Like all organisms, reptiles are intricately linked to their environment, and their behavior is profoundly influenced by the environmental conditions they encounter. For example, being ectothermic, temperature profoundly affects their metabolic rates, activity patterns, and overall behavior [1]. Other crucial environmental factors include resource availability such as food, water sources, and shelter options [2]; the presence or absence of nesting sites [3] can alter foraging, locomotion, and reproductive behaviors. Furthermore, habitat alteration and anthropogenic activities profoundly impact reptile behavior by disrupting natural environmental cues, leading to altered activity patterns [4]. The environmental change and alteration are expected to be intensely severe on ectothermic animals due to their limited ability to maintain their body temperature [5]. An imbalance between the changes in ambient conditions and the ability to adapt these changes may significantly impact their activity pattern and fitness.

Among the ectothermic animals, monitor lizards or monitors are highly adaptive, habitat generalists and opportunistic carnivores with a few exceptions (e.g., Varanus bitatawa, V. indicus) [ [6,7]]. The distribution of this diverse reptilian group extends throughout Africa, south Asia, southeast Asia and Australasia [8]. Bangladesh is home to three monitor lizard species; among them, the Bengal Monitor Varanus bengalensis (Daudin, 1802) is found in all habitats throughout Bangladesh [9]. Its global distribution ranges from southeastern Iran to southern Asia [10], with variation in ecology and natural history in different habitats and geo-climatic zones [ [6], [9]].

The Bengal Monitor is around 180 cm in snout-vent length (SVL) with a 100 cm tail. It is terrestrial in habit, but young individuals can be arboreal [11]. It is primarily an omnivorous species [12]; however, scavenging can be seen frequently. This species can be crucial in cleaning the environment [13]. Some research on this species suggests that it can play an essential role in the ecosystems through scavenging and being a top predator in their respective habitats [ [10,14]]. Despite their potential importance in the ecosystem, there is still a lack of data regarding the activity budget of Bengal Monitor in Bangladesh. Research on activity budget provides information regarding animalsʹ necessities, likes and dislikes, and bio-energetic budgets, which are essential for assessing environmental and human influences on particular species [15]. Several studies on activity budgets have been conducted for a number of species (e.g. Refs. [[17], [28]]) including Water Monitor (Varanus salvator; [16]) in Bangladesh. However, to the best of our knowledge, a study on activity budget has yet to be conducted on the Bengal Monitor in Bangladesh except for some studies from other countries (e.g., Srilanka) [ [17,18]].

Thus, this study was initiated due to a need for more adequate data on activity budget of Bengal Monitor in Bangladesh. The study aimed to determine the activity budget for different daytime, seasonal variations, and age categories. We also investigated the impact of temperature on activity budget since temperature is considered the most significant factor for reptiles [1]. Bengal Monitor is both globally and locally Near Threatened (NT) species and its' population is decreasing (https://www.iucnredlist.org/species/164579/1058949). Bangladesh is facing habitat changes, so studying their behavior will help protect them by understanding their activity during different times and seasons. This study looked at some factors related to Bengal Monitors' activity which would help prioritize conservation efforts. The result on activity budget and behavior of Bengal Monitor from other countries would not be similar since temperature, season and daytime are different. This ecological data concerning requirements, preferences and dislikes will help implement future conservation and management strategies.

2. Methods

2.1. Study sites

We studied the Bengal Monitor lizard species in Faridganj, Chandpur district (23°07′30.00″ N, 90°44′54.96″ E) which is located in the southern part of Bangladesh about 12 m asl. The study area is primarily a plain land with permanent water bodies like ponds, ditches, lakes, canals, and marshes, enriched with natural grasslands and depleted habitats like agricultural lands. The primary vegetation in this area includes large trees (e.g. Koroi Albizzia sp., Mahagani Swietenia mahagoni, Cool mat Schumannianthus dichotomus, Bamboo Bambusa sp., Burflower-tree Anthocephalus kadamba), bushes, thickets, and homestead forest.

2.2. Behavioral observations and data collection

We observed Bengal Monitor behavior throughout the study area from May to December 2020, covering both the wet (May to August) and dry (September to December) seasons [19]. The activity patterns of monitors were studied by the focal sampling method [ [20,21]]. The observation was done four days per week in each month in three shifts (morning- 7:00–11:00, noon- 11:00–15:00 and afternoon- 15:00–19:00). After the detection of individuals, we recorded our observation every 5 min to keep consistency and uniformity. One person watched the lizard, another wrote down the lizard's activities and double-checked the time to maintain accuracy and efficiency. We recorded observations for all individuals randomly, avoiding individual recognition of Bengal Monitor. The number of adult and sub-adult individuals were 16 and 15 respectively. We estimated the age based on body size i.e., sub-adult- total length ≥100 cm and adult-total length <100 cm [11]. To determine the age of the Bengal Monitor, each observer screened the individual very carefully. In case of difficulties differentiating between the adult and sub-adult individuals, the observer took photographs and compared them with reference images [22].

We recorded the activities of focal individuals in detail as well as behaviors directed at an individual by others. We added or removed monitor lizards from observation record when they entered or left the observation site. Each observation site covered about a ∼20 m radius from the observation point. To avoid influencing their activity, we followed the individuals from hidden points with the naked eye. We recorded the duration of each observed behavior in a notebook since time is a vital factor for calculating the time-activity budget. The activities of the Bengal Monitor were recorded in two distinct categories: active and inactive, with five sub-categories (Table 1). We collected air temperature (2 m above ground) data at the start and end of each observation period using a standard mercury thermometer with an accuracy range of ±1.5 °C.

Table 1.

Ethogram and description of activities displayed by Bengal Monitor (Varanus bengalensis) in Faridganj, Chandpur, Bangladesh adapted from Rahman et al. (2017).

General category	Sub-categories	Description of activities
Inactive	Basking	sunbathing or thermoregulatory behavior to heat-up the body, whole body exposed especially dorsal area
	Resting	when the individuals do nothing but lying in shaded areas like under tree
Active	Foraging	any activity that is directed towards the procurement of food/nutrients
	Locomotion	any types of movements like walking, swimming, or climbing
	Anti-predatory	minor activities like fighting, hiding, escaping and avoiding that were seen or observed rarely

2.3. Statistical analyses

The proportion of time spent on each activity was calculated in percentage following the formula T_f = n_f × 100/N, where, n_f is the time spent for a specific behavior; N is the total time spent for all behaviors [23]. We then performed a non-parametric ANOVA (Kruskal-Wallis test) to compare the proportion of time spent on activities in relation to daytime, season, and age. Using linear regression, we also assessed the relationship between mean air temperature and activity budget. In this case, the independent variable is the average air temperature for each time slot on any given day and the dependent variables are the Bengal monitor's activity budget for that time slot on that day. We used RStudio version 3.5.1 (RStudio Team 2015) for all statistical analyses with a significance level of P ≤ 0.05.

3. Results

During observations from May 2020 to December 2020, we recorded 1219 activity records totalling 5633.683 min of Bengal Monitor behavior in the study area. The mean ambient air temperature (±SE) during observation periods was 26.89 ± 0.211 °C in the dry and 30.15 ± 0.068 °C in the wet season. However, the Wilcoxon test didn't show any significant difference among seasons (W = 7, p = 0.886) (Fig. 1). Overall, Bengal Monitor in the study area spent the largest proportion of activity time in resting (33.09%), followed by basking (26.02%), foraging (22.79%) locomotion (14.34%) and anti-predatory (3.75%) (Fig. 2A). According to our observation, the adult Bengal Monitor used its maximum time in inactive behaviors (i.e., resting- 30.12%, basking-29.49%) than active behaviors. In contrast, the resting behavior of sub-adult individuals accounted for nearly half (44.49%) of the total proportion (Fig. 2C). Fig. 2A-D includes data on the total time spent in each behavior for daytime, age and season.

Fig. 1.

― Average seasonal and monthly air temperature during the study period in Faridganj, Chandpur, Bangladesh.

Fig. 2.

― Time spent by Bengal Monitor (Varanus bengalensis) in different activities considering daytime, seasons and age categories in Faridganj, Chandpur, Bangladesh. A. Total time spent (in minute) in different activities, B. Time spent in different daytime, C. Time spent by different age classes, D. Time spent in different seasons.

3.1. Proportion of activities in relation to season and age

We found statistically significant differences on all proportional activities between age classes and seasons except anti-predatory behavior (Fig. 3). In the dry season, both adult (H = 18.057; df = 1; P = 0.00002) and sub-adult (H = 12.974; df = 1; P = 0.00032) Bengal Monitor spent more time in basking than in wet season. In the wet season, however, only adult Bengal Monitor rested (H = 5.271; df = 1; P = 0.02169) and foraged (H = 3.988; df = 1; P = 0.04582) more. During the dry season, the sub-adult Bengal Monitor spent more time in locomotion than the adult Bengal Monitor (H = 3.845; df = 1; P = 0.0499). Though the adult monitor spent more time in locomotion during the wet season (H = 11.093; df = 1; P = 0.00087), we did not see a similar significant increase in the case of sub-adult monitors (Fig. 3). Finally, we did not find any significant difference in time spent in anti-predatory behavior across seasons and age classes.

Fig. 3.

― Activity budget of Bengal monitor in relation to season and age classes. Boxes represent the interquartile ranges; horizontal lines within the boxes represent the medians; whiskers indicate minimum and maximum values. Different letters indicate significant differences between groups.

3.2. Proportion of activities in relation to season and daytime

During the dry season, the Bengal Monitor spent proportionally more time basking than during the wet season at noon and in the morning (Fig. 4). In contrast, resting behavior was higher in the afternoon during the dry season compared to the wet season. During the dry season, foraging activity was the highest in the morning, but during the wet season, they foraged more in the afternoon. In both seasons, locomotion was the highest in the morning and lowest at noon. In the afternoon, we observed more anti-predatory behavior. Though we observed a noticeable difference in activity budget across the daytime, we did not find a significant difference using the non-parametric ANOVA test (P > 0.05).

Fig. 4.

― Activity budget of Bengal monitor in relation to daytime and season. Boxes represent the interquartile ranges; horizontal lines within the boxes represent the medians; whiskers indicate minimum and maximum values.

3.3. Impact of air temperature on the activity budget of Bengal monitor

Average air temperature had a significant and large effect on the basking and locomotion activity of adult Bengal Monitor (Table 2; Fig. 5). As the temperature increased, the adult Bengal Monitor used more time in locomotion (positive relationship) and basked less (negative relationship). However, air temperature did not significantly impact the activity budget in the sub-adults case. Though a significant relationship was not observed, a large effect size (F-statistics) was observed in the case of basking and locomotion activity (Table 2).

Table 2.

Regression analysis showing the relationship between average air temperature and activity budget of adult and sub-adult Bengal monitor.

Age category	Activity	Adjusted R2	F-statistics	P value
Adult	Basking	0.093	6.267	0.015
	Resting	−0.006	0.707	0.405
	Foraging	0.036	2.904	0.095
	Locomotion	0.109	7.231	0.01
	Anti-predatory	−0.009	0.537	0.467
Sub-adult	Basking	0.085	3.046	0.096
	Resting	0	0.998	0.329
	Foraging	−0.025	0.471	0.5
	Locomotion	0.051	2.186	0.154
	Anti-predatory	−0.046	0.038	0.847

Fig. 5.

― Relationship of activity budget of adult and sub-adult Bengal monitor with average air temperature.

4. Discussion

The present study is the first to detail the variation of activity levels for wild sub-adult and adult Bengal Monitor in natural habitat. We found inactive behaviors proportionally dominated Bengal monitor activity. Previous studies about the activity budget of reptiles showed that they spent more time in inactive activities like basking and resting [ [24], ]. Abayaratna and Mahaulpatha [24] showed that the Bengal Monitor uses the morning period mainly for basking, at the same time, we found they preferred to bask more in the morning and noon in the dry season (Fig. 4). The difference might be caused by the fact that we divided the time into three while they analyzed activities in four periods. However, we did not find a significant difference in basking activity among daytimes. Both adult and sub-adult Bengal Monitor basking activity was higher during the dry season, and the seasonal variation is similar to other monitor lizard species (e.g., V. salvator [16]). The average temperature in the dry season (i.e., September to December) is low, which compels Bengal Monitors to spend more time basking. This also supports the impact of air temperature on basking activity (Table 2). Varanids use solar heating to keep their core body temperature within a small range [25]. It is well known that monitor lizards' body temperatures fall below ambient during the night, and they must increase them the next day by basking before starting foraging and other activities [24].

Like basking, most monitor lizard species adjust their core body temperature to recover strength by resting in cooler habitats. For relaxation, sleep, or strength recovery, monitor lizards spend a significant portion of time in resting activity [ [10,26]]. Resting activity was proportionally higher in the morning (36.4%) and noon (42.37%) to avoid the heat and hot weather of the wet season (Fig. 4). For this, they used the shaded areas under large trees or lying with limbs outstretched in cooler substrates. The adult individuals used comparably more time in resting (Fig. 2C). We observed only a single event of fighting or aggression behavior of sub-adult individuals. The adult V. bengalensis are almost solitary in their natural habitat and try to avoid conflict. During the breeding season, when males fight for mates, monitor lizards are more likely to interact with one another [14]. This study observed anti-predatory behavior for the food competition predominantly in the wet season (Fig. 2D).

During the study period, Bengal Monitor activities began early in the morning and ended just before dusk. Of the active behaviors observed, foraging accounted for the most significant proportion of time, similar to the previous studies from Bangladesh [13] and Srilanka [24]. However, the temporal variation is remarkably different from the previous studies. They found the most active foraging time at noon (12:00–15:00 h). In contrast, this study resulted in the peak foraging period in the morning and afternoon in both dry and wet season (Fig. 4). The overall activity budget indicates that the Bengal Monitor is most likely to bask at noon to increase body temperature and use the morning and afternoon period for foraging activity.

Monitor lizard activities vary depending on the weather patterns and food availability in their particular habitat [13]. Foraging activity was relatively higher in hot and wet seasons in the adult Bengal Monitor than in the sub-adults (Fig. 3). This may have occurred because sub-adults need to fulfil their daily feeding requirements for growth irrespective of the environmental conditions. Similar foraging behavior was observed in Varanus salvator [16]. Foraging activity was less in dry winter since food availability and temperature were low in the study area. The foraging behavior of Bengal Monitor consists of a slow, forward movement with progressive head swaying from side to side and regular tongue flicking to detect the presence of prey which is similar as of V. mertensi [27].

During our study, the Bengal Monitor moved throughout aquatic, arboreal and wetland habitats. Most Bengal Monitors used the same places during the day to bask, shade, forage and other activities. During observations, locomotion behavior was mainly prominent at morning and noon (Fig. 4). Like foraging activity, locomotion was predominantly higher in the wet season (Fig. 4). It is likely that monitors moved more in the morning and noon due to increase of air temperature, consequently resulting return from foraging or basking ground. The sub-adults are proportionally more active in movement than adults (Fig. 3). We observed that searching for a secure place without interference (e.g. human or animal) caused the sub-adults to move frequently.

In conclusion, we found deviations in the activity budget of adult and sub-adult Bengal Monitors living in the studied area, possibly influenced by the air temperature, availability of foods and the unseen anthropogenic pressures imposed by the locals. However, other circumstances (such as the presence of mammals such as the Jungle Cat and Asiatic Golden Jackal, habitat loss, and other environmental factors) could have influenced Bengal Monitor behavior. As a result, future research should focus on the consequences of anthropogenic pressures on these species' behavior, habitat use, demography, and genetics. These studies are critical in conservation biology because we urgently need to understand how monitor lizards adapt in the presence of humans, as animal contact is an unavoidable reality today.

Data availability statement

Data will be made available on request.

CRediT authorship contribution statement

Mohammad Firoj Jaman: Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization. Sultan Ahmed: Writing – review & editing, Writing – original draft, Visualization, Validation, Investigation, Formal analysis, Data curation. Sakhawat Hossain: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Md Fazle Rabbe: Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Formal analysis, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.