The Influence of Weather Patterns on the Activity Levels of the Mantled Howling Monkey
The following is part of a research project from a Primate Behavior and Ecology course at La Suerte biological field station in Costa Rica.
Introduction
Tropical wet forests occupy low altitude zones within 23° 27’ latitude of the equator. Mean annual daily temperatures range between 18 and 24°C, with more diurnal temperature variation than seasonal. Yearly percipitation totals exceed 2,000mm and no month receives less than 100mm of rainfall [Sanford, 1994]. In the tropics and sub-tropics, where most non-human primates are found, the most important effect of weather on primate populations is indirect according to Richard [1985] in that the availability of food is strongly influenced by weather conditions, particularly by pattern and quantity of rainfall.
Direct influences such as a need to maintain the balance of temperature between the organism and its environment, may cause the individuals to react with certain behavioral mechanisms such as seeking out shady spots in heat or positioning themselves in the sunlight or huddling together in times of cold [Smith,1992]. Chivers noted in a study of Alouatta villosa at La Selva, Costa Rica that howlers were inactive until mid-morning on cloudy, wet mornings, whereas on hot, sunny days there was a considerable amount of feeding during the morning [1969]. Howler calls have also been noted as being affected by the change in season. The calls are more abundant in the dry season when the availability of food is lower [Chiarello, 1991].
Little information is available on the indirect influences of weather on howler’s daily activity and even less on the direct effect on changing weather patterns. Studies such as Richard [1970], which focused on the general activity patters of howling monkeys point out that howlers do tend to follow habitual daily patterns, but the paper only spends one line to mention that heavy rain suppresses such daily activities.
This lack of data focusing on the interactions of howling monkeys, as well as primates in general, with their current microclimates presents an opportunity for study. As shown, the amount of data collected to date on climate interactions leans towards the indirect effects of weather on food resources influencing primate behavior. It has been proposed that animals have adapted behavioral mechanisms to account for certain direct effects of climate, such as heat loss. Yet the current data does not proceed to show quantities of research focusing on the direct effects of weather on primate behavior.
Purpose
In my research, I further investigated the effects of weather on the daily activity of howling monkeys (Alouatta palliata). I hypothesized that the proportion of time howlers involve themselves in various activities is indirectly affected by daily weather changes as well as the aforementioned direct effects. I proposed that weather would have a direct effect due to extreme conditions such as heat, cold, or intense rain, which would affect the comfort levels of individuals. This would prevent primates from devoting as much time to feeding, foraging, traveling or social behaviors such as playing and grooming. In examining this hypothesis I hoped to answer several question:
- Is the amount of time devoted to foraging and feeding affected by changes in weather throughout the day?
- Are activities lessened in the presence of heavy rain?
- Do individuals make an effort to thermoregulate in the presence of intense heat or cold?
- Are vocalizations connected to changes in weather?
- Do periods of intense heat require greater time devoted to rest?
METHODS
Subjects
I focused my research on the two groups of mantled howling monkeys in the "small forest" of La Suerte Biological Field Station in NE Costa Rica (20ha, lat. 10° 26’30"N, long. 83° 46’15"W).
Data Collection
Data were collected on three weather variables: temperature, cloud cover, and rainfall, as well as several behaviors. The weather was recorded from a location outside the forest edge at 4:15am, 12:30pm, and 4:15pm each day in order to note all three measurements from the same site (this is where the rain gauge was located). Could cover was measured as overcast: no blue sky seen, partly cloudy: blue sky and clouds seen, or clear: no clouds seen. Temperature was taken with a centigrade thermometer. Rainfall was measured with a millimeter rain gauge.
Notations of these three weather conditions were also made within the forest at 10 minute intervals. Rain was measured within the forest as heavy: near impossible to look up and view the canopy, light: somewhat difficult to view the canopy, and minimal: not affecting view of the canopy. These measurements were taken in order to account for the microclamictic influences on the howlers. Conditions outside the forest do not always reflect those inside.
Behavioral data were collected in the form of five minute group-scan samples in order to determine how the howlers designate their time. The sex and age category of each individual in sight was also recorded: male, female, adult, subadult, juvenile, infant. Behavioral categories were:
- resting
- absence of directional movement; body in fixed position
- traveling
- movement of body from one point to another, be it across a single branch or through many trees
- foraging
- actively moving hands through branches; fixed position
- feeding
- placing food in mouth; chewing; ingesting
- howling
- an extending, resonating, vocal sound, not including the dawn chorus (recorded only after 6:00am)
- vocalizing
- any vocal sound other than howling (excluding dawn chorus)
- fighting
- interaction due to conflict of interest
- grooming
- physical contact between individuals; hands moving over
- playing
- movement of individuals, usually juveniles, not pertaining to any other behavioral category; may be in a fixed position or exhibiting directional movement
Data Analization
To analyze data gathered histograms were prepared comparing rainfall, cloud, cover, and temperature with three main activities: feeding/foraging, resting, and traveling. These activities were chosen as a result of a prepared activity budget which showed a large portion of time being devoted to these three behaviors. Comparisons were made between weather and rates of vocalizations and thermoregulation were charted. Due to the minimal number of bouts viewed where howlers made clear efforts to keep cool through body positioning, only back and body position in relation to heat avoidance were charted.
Results
Fortynine total hours were spent in the field, forty-one of which were in contact with the howlers. Time spent observing in each weather pattern follows: clear skies - 5hrs, partly cloudy skies - 17hrs and 35mins, overcast skies - 8hrs and 25mins, 18-22°C - 5hrs and 20mins, 23-27°C - 27hrs and 35mins, 28-32°C - 8hrs and 5mins, minimal rain - 1hr and 35mins, light rain - 15mins, heavy rain - 0mins.
The proportion of time designated to specific activities: 47% resting, 21% traveling, 14% feeding, 12% foraging, 5% howling and vocalizing, and 1% other (grooming, fighting, and playing).
In relation to temperature, 49% of each hour within a range of 18-22°C was spent feeding or foraging, 29% traveling, and 22% resting. Hours within a range of 23-27°C, howlers designated 56% of their time to resting, 23% to feeding or foraging, and 21% to traveling. During a range of 28-32°C, 55% of time was devoted to resting, 27% of time to feeding or foraging, and 18% to traveling.
Activity levels in relation to cloud cover: hours during clear skies - 42% of time designated to feeding and foraging, 32% to traveling, and 26% to resting. Partly cloudy skies - 60% to resting, 20% to feeding and foraging, and 20% traveling. Overcast skies - 47% feeding, 29% resting, 24% to traveling.
Activity levels in relation to rainfall: minimal - 53% resting, 31% feeding and foraging, 16% traveling. Light rain - 100% resting. There were no hours of observation during heavy rain.
Thermoregulatory behavior (back/body position) rates were minimal or zero in rain, a temperature of 28-32°C, and overcast skies. Individuals positioning back to the sun did not occur during clear skies. There was a frequency of 4 bouts per hour of individuals placing back towards sun during partly cloudy skies, and 16 bouts of body placement in the sunlight. During clear skies 13 bouts per hour of placing body in sun were recorded.
At a temperature range of 18°-22°C 7 bouts of back positioning per hour and 22 bouts of body positioning. Tempertures ranging between 23 to 27°C caused howlers to place their backs in the sun at a rate of 3 bouts per hour and their bodies were placed in the sun at a rate of 23 bouts per hour. There were no thermoregulatory bouts recorded during temperature ranges of 28 to 32 degrees Celsius.
DISCUSSION
Howling monkeys spend the majority of their time resting. Leaves are typically low-energy foods which require slow digestion. These long periods of rest are necessary in order to aid digestion.
Large parts of howler's time are also devoted to foraging and feeding. Low-energy levels in leaves also require that mass quantities be consumed. Furthermore, howlers are highly selective foragers and feeders. Mature leaves are often overlooked for the less cellulose-laden younger leaves. When young leaves are scarce, the tips or petioles of mature leaves are eaten, with the remainder being discarded.
Traveling also made up a significant portion of the howler's day, although less than either resting or feeding activities. This research was conducted during the dry season, when food resources are scarce and clumped. When food is not as plentiful, more time is required to travel between patches.
Feeding and foraging was most often observed during periods of temperature ranging from 18 to 22°C. There was more feeding and foraging observed in minimal rain than in light, and observed most often in periods without rain. There was no correlation between feeding and cloud cover. In warm temperatures howlers must devote more time to conserving energy, than feeding and foraging. In periods of rain, movements may be hindered, and energy/heat loss greater therefore howlers may need to be focused on these problems than consuming great quantities of food.
Howlers did lessen their activities in the presence of rain during the course of this study. However, it must be kept in mind that out of the seven days of data collection, it only rained a few hours on two evenings. This is not enough time to assume any long-term conclusions about the activity of howlers in rain.
Thermoregulatory behaviors did not often include positioning backs towards the sun. This is not easily understood since a howler’s back is its greatest surface area, and therefore a key spot for heat absorption. Howlers most often placed there entire bodies in the sun during cooler temperatures and when there was sunlight available. This is a possible form of thermoregulaiton, and a negative correlation was observed in relation to temperature and body positioning.
Vocalizations occurred most often during a temperature range of 23 to 27°C and partly cloudy skies. Vocalizations were not heard during rain, nor was a distinct dawn chorus heard on the mornings followed by rain. This may imply that howlers are more likely to vocalize when they are neither too hot, too cold, or too wet. Furthermore it may be of greater benefit to conserve energy and maintain an optimum body temperature during extremes in weather and not expend energy vocalizing.
Time devoted to rest increased with both temperature and rainfall. There was no correlation between resting and cloud cover. This may again imply that howlers must focus more during these times on conserving energy and maintaining optimal body temperatures, perhaps in order to better gain the energy potentials from their food sources.
This research has shown correlations between the activity levels of howlers and changing weather. However, due to the brief amount of time allotted to the study it is difficult to come to any solid conclusions. Weather will vary much more over a period of weeks or months than over the course of a few days. Also, the relatively small size of observations during rainfall creates an even larger problem in accurately discussing the affect rain has on howler’s activities. It is further difficult to distinguish whether or not a behavior correlates to weather or if behaviors are exhibited in relation to temporal changes. Howler’s are habitual creatures, not often varying from their sequence of activities.
For example, most activities occurred during periods of partly cloudy skies, 23 to 27°C, and an absence of rain. These conditions also made up a large part of the study period. A longer study would show more variation in weather, and therefore show an affect on behavior more accurately. Further planned study could also incorporate a more technical means of measurements, and give a more accurate representation of specific changes in weather.
A longer study may also find more solid correlations between activity and weather. The indirect effects of weather on howlers, in fact many animals, have been documented. However direct effects, such as overheating, and chilling should also affect the time howlers have to allot to their many activities, as this study has to an extent shown. When an animal is concerned with maintaining optimal energy levels and body temperatures, there is less time for socializing within the group in the form of resting together, grooming, or playing. These social interactions may be beneficial in stress reduction, in turn stabilize the health of the group.
Also, a better understanding of the direct effects of changing weather on primates may benefit efforts for conservation. Not only is it important to understand how weather affects food recourses, but the animals themselves. It is the individuals ability to adapt to various changes in weather, thereby ultimately increasing their reproductive success, and continuation of the species. In the ever changing world of global warming and increasing pollution, it is vital to understand the basic mechanisms animals use in order to adapt to direct effects as well as indirect. This will allow more accurate planing for the conservation of the worlds species.
CONCLUSIONS
- Feeding and Foraging was most often observed during temperatures of 18 to 22 degrees Celsius and an absence of rain. There was no correlation between feeding activities and cloud cover.
- During minimal amounts of rain (100.4mm/observation period) most of howler’s time was spent resting, followed by feeding/foraging, and then traveling. During light rain (200.1mm/observation period) all of howler’s time was devoted to resting. No data was recorded during heavy rain.
- Howler’s were most often observed placing their entire bodies in the sun during periods of partly cloudy skies and temperatures of 18 to 27 degrees Celsius. Placement of back towards sun was not a significantly used thermoregulatory mechanism.
- Vocalizations occurred most often during temperatures of 23 to 27 degrees Celsius and partly cloudy skies. No vocalizations were heard during rain.
- Time devoted to resting increased with temperature and quantity and pattern of rainfall. There was no correlation between resting and cloud cover.