Studying Gorilla Behavior and Ecology

The Dian Fossey Gorilla Fund International is the leading organization studying mountain gorillas, and supports studies of Grauer’s gorillas as well. This research focuses on gorilla behavior and ecology, and is used to: inform conservation policies, enrich our knowledge of the great apes, and to learn how to protect them from extinction.

Studying mountain gorillas at Karisoke™

Since Dr. Dian Fossey established the Karisoke™ Research Center and began studying mountain gorillas in 1967, we have acquired an unparalleled amount of field expertise and data about mountain gorillas and their habitat, as well as the biodiversity of the region.  Each year, Karisoke™ staff spend thousands of hours collecting basic information about the mountain gorillas, including their ranging patterns, changes in group composition (such as births, deaths, transfers), feeding and social behavior, health status, and major events (such as interactions among groups, group fissions, and dominance shifts).  Our databases are some of the longest running and largest in the world and are used by us and our colleagues in Rwanda and internationally to answer critical questions concerning the basic biology of the subspecies and practical issues regarding their survival.

Much of what the scientific community knows about gorillas has come from the early research at Karisoke™ conducted in the 1970s and 1980s.  Some of these discoveries include:

• Mountain gorilla groups are structured around a dominant male, multiple females and their offspring.  In the Virungas, about 40% of groups are multi-male but always led by a single, dominant male.
• Both males and females often disperse from their birth group, with females immediately joining another group and males remaining solitary until they can attract females away from other males.
• Group cohesion is maintained primarily through strong bonds between adult males and females, with relatively weak relationships between the sexes.  If the dominant male dies and there is no other male in the group to take his place, the group will usually disband.
• Dominance relationships between females are relatively weak as a result of the low levels of feeding competition.
• In groups with multiple males, the dominant male controls access to females, resulting in male-male competition being most apparent between rather than within groups.
• Infanticide is a significant cause of infant mortality and is a reproductive strategy used by males to increase their reproductive success.

In recent years, the composition of the groups monitored by Karisoke™ has changed considerably from those first observed by Dian Fossey and early researchers. They have grown larger in size (from an average of 10-15 individuals to up to 65 individuals) and often contain significantly more males (up to 8 silverbacks as compared to 2 or 3 in the 1970s and 1980s).  Consequently, we still have much to learn about gorilla social behavior and the factors influencing it.  For example, how do larger groups affect the vegetation?  Is there more competition for food in larger groups?  Why do some males and females remain in their birth groups whereas others choose to leave?  Does the dominant male still control breeding when there are a large number of other males in the group?

We are currently undertaking a number of studies using both our long-term databases and new data currently being collected to answer some of these questions. And we still have much to learn about other aspects of gorilla biology. Some of the recently completed and ongoing Karisoke studies of mountain gorillas include:

• Factors Affecting Transfer Decisions in Male and Female Mountain Gorillas
  Karisoke™ scientists recently completed a series of papers, in conjunction with the Max Planck Institute for Evolutionary Anthropology, examining why female gorillas transfer between groups.   The availability of the more than 40-year demographic database made this one of the most comprehensive studies on ape life- history decisions. The results differed from those of earlier studies, highlighting the importance of long-term data in illuminating this subject for long-lived species like gorillas.
  
  Karisoke™ researchers have also published several papers on male gorillas, including male reproductive behavior in large, multi-male groups.  A genetic analysis to see how the pattern of copulations and actual siring of offspring match up is currently underway, again in conjunction with Max Planck.

• The Effect of Extreme Conservation Measures on the Virunga Mountain Gorilla Population’s Growth and Survival
  Our long-term demographic data have been merged with data on the remainder of the population of Virunga mountain gorillas to look at the growth patterns of the population as a whole over the last 40 years. The project has also investigated the role that extreme conservation measures, such as veterinary intervention and daily dawn-to-dusk protection, have played in saving this population. This project represents a collaboration among multiple research and conservation entities, including the Max Planck Institute, the International Gorilla Conservation Program, the Mountain Gorilla Veterinary Project, and the national park authorities in Rwanda, Congo and Uganda.

• Feeding Ecology and Competition Among Mountain Gorillas
  The goals of this study  are two-fold. Firstly, to evaluate the potential ecological basis for the higher density of gorillas in the Karisoke™ area, which now supports twice the number of gorillas as it did in the 1970’s and ‘80’s. This is done by comparing the density and abundance of plant foods consumed in the Karisoke area now to earlier studies (1970’s, late 1980’s and early 1990’s). Secondly, to examine the behavioral consequences of increased group size on group dynamics: specifically group spread, day journey length and female social relationships (dominance interactions and feeding competition). This is done by comparing females in groups of different sizes and assessing the links between social status, group size, and food intake. 

• GIS Analysis of Gorilla Ranging Patterns,  Vegetation Classification, and Illegal Activities in the Virunga Conservation Area
  Karisoke™ Research Center staff collect GPS records on the location of gorillas as well as the location of snares, bamboo cutting, and other illegal activities in the park. These GPS points, along with historical information and maps, are entered into a computerized Geographic Information System (GIS), which helps us to understand how both gorillas and humans use the land. By analyzing poaching patterns in the Virungas since 1978, we can define optimal or suitable gorilla habitat and its implications for habitat management. These data are transferred each month to the national park authority (Rwanda Development Board, formerly ORTPN), and are used to inform park management activities.

• Paternity Analysis
  To determine the paternity of individual gorillas, Karisoke™ trackers collect fecal samples from all individuals within the research groups for DNA analysis. This collaborative study with the Max Planck Institute will help us to answer many questions about mountain gorillas' reproductive behavior and mating patterns.

• Social Dynamics and Reproductive Strategies of Male Mountain Gorillas
  In recent years, the number of male gorillas — blackbacks and silverbacks — within individual groups has risen. One of the questions we’re asking is: What is the impact of these larger multi-male groups on male behavioral patterns? And can we consider these changes to be adaptive? This study utilizes both traditional animal behavior data collection techniques and fecal and urine sampling to investigate male gorilla social dynamics and reproductive strategies.

• Maternal Behavior and Infant Development

• Gorilla Personality
  Each gorilla has a different personality. This study looks at the relationships between individual gorilla personality and a variety of behaviors and life history patterns. For example – How does personality relate to different styles of group leadership or dominance? Do rank differences among adults correlate with personality differences? Does fertility or reproductive success correlate with personality? An understanding of mountain gorilla personality will allow us to better predict this species' capacity to adjust to population change (population size, number of groups, group density), and habitat change (change in food supply, loss of habitat, degree of human presence), and ultimately enable us to better conserve and manage this small, fragile population.

• Census of the Mountain Gorilla Population (Conducted in 2003, to be repeated in 2010)

• Tourism Impact
  Over 70 percent of the Virunga mountain gorilla groups have been habituated and many are visited by tourists on almost a daily basis. Although a number of specifically designed rules help to minimize the impact of regular tourist visits on the mountain gorillas, no studies had been conducted on the potential impact that tourists may have on their behavior and long-term survival. This study uses both behavioral and physiological data (urine and fecal samples) to assess the impact of tourism on the Virunga mountain gorilla population. It also looks at the impact our researchers might have on the three research groups they study. The first phase of the Tourism Impact Study found clear evidence of a negative impact of tourist visits on gorilla behavior. The gorillas were more aggressive and exhibited a number of stress-related behaviors during the one-hour tourist visits. During the visits, gorillas ate less, spent more time moving and traveled greater distances. The gorillas also spent less time doing their normal activities and a greater percentage of time watching the tourists, rather than interacting with one another. Although the long-term impact of these visits is still not known, the possibility that gorillas are experiencing higher stress levels due to the tourist visits suggests that this may result in reduced reproductive success and slower population growth. This study has provided the park management authorities the scientific information needed to guide sustainable long-term management of the gorillas in the face of increased economic pressure to include more gorilla groups in the tourism program and to increase the number of visitors and visits per day to each group.

Studying Grauer’s gorillas in the Congo

In comparison to mountain gorillas, our knowledge of the Grauer’s subspecies is staggeringly little. Scientists do not even have a good estimate of the number of Grauer’s gorillas left. Estimates range from approximately 6,000 to approximately 26,000.  Thus, one of our primary goals is to document the distribution and density of the Grauer’s gorillas.  We work in both a national park — Maiko National Park — and surrounding community reserves to conduct censuses aimed at providing data on where in their range Grauer’s gorillas are still located and how many there are remaining.  Thus far, our work has found several populations of Grauer’s gorillas that were previously unknown.   We are also working with one of the community reserves, the Tayna Gorilla Reserve, to habituate two groups of gorillas for more intensive study.  It is hoped that we will be able to observe these groups as we do the mountain gorillas and gain insights into their social organization, behavior, and ecology.