Behavioral Enrichment and Tool Use - Denver Zoo

"But how do you tell the two females apart?"  Ann Rademacher smiled as she pointed out some obvious differences.  She knew it wouldn’t be long before I could easily distinguish Bibi from Maguba, even if only part of her butt was visible through the doorway into one of the backrooms.  Joe was easy – he was twice the size of either female, with the silvered hair of a mature male.  It was 1989, and Ann was introducing me to the gorilla group at the Denver Zoo. She was their keeper, and I was going to do behavioral enrichment research with them.  Ann would continue to be generous with her time and her knowledge, and the gorillas would change my life.    

On this first day, Joe eyed me with mild suspicion, appropriate for his role of group leader and protector.  None of the gorillas looked directly into my eyes, only an occasional furtive glance. Bibi was disinterested; humans weren’t important to her unless they had food.  But Maguba was checking me out and keeping close tabs on Ann and me as the tour of the exhibit continued. The gorilla exhibit was small – a main indoor room with two adjoining back rooms and a tiny outdoor yard.    

All three gorillas were “founders” – they had been wild caught in Africa and were genetically very valuable to the captive population.  Unfortunately, however, none of them had any surviving infants.  In order to capture these gorillas, their mothers and fathers had been killed, and likely other members of their family groups.  Gorillas are very attached to their infants and will do whatever they can to protect them, even sacrificing their own lives.  Before gorillas were classified as an endangered species, this is how zoos acquired them for their collections. 

Bibi had come to the Denver Zoo in 1971 when she was 3 years old.  She was then housed with a male named Thomas and used to ride around on his back.  They produced one infant, who did not survive.  Bibi was a sweet, somewhat portly gorilla, who seemed very serene and comfortable with her life.  The hair on top of her head had the distinctive reddish tint frequently seen in western lowland gorillas, and her liquid chocolate eyes were completely guileless.

Joe (Kabako) was a handsome silverback who, as a group leader should, spent most of each day keeping tabs on what was going on around the gorilla exhibit. The silverback is of utmost importance to his group, protecting them, leading them, and refereeing family squabbles.  Joe had been raised by humans at the zoo in Birmingham and had come to Denver in 1986 at the age of 22.  Although the two female gorillas had also been raised in unnatural environments, Joe seemed to suffer more adverse effects.  He lacked some basic gorilla social skills and was sexually attracted to humans, especially men with beards.  Maguba and Bibi both tried their best to seduce Joe, each in her own way, but to no avail. Later Joe would be sent to the Brownsville zoo in hopes the female there would have more success.

There was something special about Maguba.  Her heavy brow ridge overshadowed her eyes, her face had angular features, and she was not what you would call a beautiful gorilla.  But she was so bright and keenly aware of her environment.  She seemed to want to be a part of whatever was going on. Maguba had arrived at the Denver Zoo in 1971 when she was less than a year old and was initially raised by humans in a nursery area near the other gorillas.  She became very attached to her caretakers and absolutely did not want to join Bibi and Thomas, crying and clinging to her caretaker’s leg when that time came in 1972.  I often had the impression that, given the choice, Maguba would prefer to be with humans.       

One evening while housed in the nursery Maguba was stolen, but evidently proved to be too much for her captors to handle.  At 5:00 in the morning a man was walking down a nearby street and out of the darkness Maguba leapt onto his back.  He went to a house, desperately seeking help, and the resident let them in and called the police, who notified the zoo.  In the meantime Maguba had turned the front door knob and was back outside, heading up the street! Eventually the director of the zoo came to rescue Maguba, who seemed to have enjoyed her adventure. 

Before I left that first day, Maguba gestured toward me.  She’s asking for a treat, said Ann.  "I don’t have anything today, but hopefully in the future."  Maguba seemed to understand.  She would be my mentor. 

Ernie's story is different - he's a gorilla's gorilla, being born at the Oklahoma City Zoo and raised by his mother in a family group.  He arrived in Denver in 1989 when he was 7 years old (photo at right), and thus did not participate in my entire study.  Although Ernie was very well adjusted, he did exhibit some hair plucking, an abnormal behavior seen in some captive gorillas. It's my understanding he learned this by watching his father.     

Ernie was introduced to Bibi the next year, after his 8-month quarantine period. When gorillas are transferred between zoos, they are initially housed separately to make sure they don't introduce any disease to their new group.  Fortunately Ernie could see, hear, and smell the other gorillas. The introduction went smoothly, with Ernie and Bibi quickly bonding with each other. He was ecstatic to be back with another gorilla.  Bibi was nervous at first, but she sensed Ernie's longing and began to touch him.  He was quivering as her hands ran over his head and body. For Ernie it was love at first sight. 

If wild animals are to be kept in captivity, we humans have an ethical and moral responsibility to enrich their lives.  Enrichment for captive animals is divided into social, environmental, and behavioral.  Social enrichment refers to housing the animals in a group which simulates that found in the wild, e.g. one male with multiple females or bachelor group for gorillas. Environmental enrichment involves providing a physical environment that resembles that in the wild. Behavioral enrichment provides the animals with opportunities for species-typical behaviors, such as foraging for food, problem solving and making choices, nesting.

Zoo officials used to think that enrichment objects needed to look "natural" (as if living in captivity is natural).  Fortunately, for the most part, this is no longer the case! 

photo: Ubuntu, Knoxville Zoo, 2016 

photo credit: Tennessee Trails Photography

For me personally, the main goal of my research was to provide behavioral enrichment for the Denver gorillas.  I would then document how their activity budgets changed, e.g. less inactivity.

Tool behaviors include both tool use and tool manufacture.  Tool use involves using an object from the external environment to achieve a specific goal (photo of Ernie).  There is an extremely tiny number of animal species, besides humans, who use tools, such as sea otters, some birds, elephants, and chimpanzees.  Tool use provides a window into the animal's mind, as it must visualize a problem and then a way to solve that problem.  Tool manufacture involves modifying the object so it can be better used, which is even further mentally from the goal.

Only rarely has tool use by gorillas in the wild been documented  (unless you include nest building, which I do 😉): 

https://onlinelibrary.wiley.com/doi/abs/10.1002/ajp.22351

https://onlinelibrary.wiley.com/doi/abs/10.1002/ajp.20436 https://www.sciencedirect.com/science/article/abs/pii/S0376635713002052

https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.0030380  

However, I knew of tool use by captive gorillas.  For example, a female gorilla who disliked a new keeper began to manufacture spears by using her teeth to modify branches, which she then threw at the keeper!

The scientific goal of my research was to stimulate tool behaviors and document them for the first time in captive gorillas.

For all phases of this study, I observed and recorded behaviors from the public viewing areas, focusing on one gorilla at a time and scoring a behavior every minute.  As you can imagine, there often were logistic challenges as I weaved my way through the crowds in all sorts of weather. The behavior categories were: feeding, locomotion, abnormal behaviors (regurgitation and reingestion of food, coprophagy [eating feces], ear covering, hair plucking), public interaction (clapping hands, snapping fingers, vocalizing, displaying), solitary play (includes manipulation of objects), inactivity, social interactions, and miscellaneous.     

Since I wanted to document behavior changes, I first needed to collect baseline data on the gorillas' behaviors before adding any enrichment.  During this phase, I collected 30 hours of data (1800 data points) on each gorilla.  I've got data out the wazoo, but here are the gorillas' basic activity budgets:      

Bibi:  feeding 47%, locomotion 7%, regurgitation and reingestion (R&R) 2%, solitary play 4%, inactivity 35%, misc. 5%

Joe:  feeding 34%, locomotion 3%, R&R 2%, public interaction 3%, solitary play 2%, inactivity 54%, misc. 2%

Maguba:  feeding 53%, locomotion 5%, R&R 1%, solitary play 2%, inactivity 35%, misc. 4%

It is interesting to mention that during all these 90 hours I only scored 5 social interactions - 3 affiliative (friendly) between Joe and Maguba and 2 agonistic (aggressive) by Joe toward Bibi (see photo of Bibi avoiding Joe).  In addition to these, Joe displaced Bibi 45 times, and he displaced Maguba 24 times. Displacement occurs when one individual moves toward another, and that individual moves away.  It's a good indicator of dominance.      

Ann was already providing enrichment for the Denver gorillas, and the activity budgets of the two females did not differ too dramatically from that of gorillas in the wild.  But it seemed desirable to increase feeding time and decrease inactivity for some individuals.  Also, zoos are always interested in decreasing abnormal behaviors.   When I saw how much Ann was already doing for the gorillas, I realized my main contributions would be to regularly provide certain enrichments and opportunities for tool use, and to document their effects on behavior.

Tool behaviors require both cognitive and physical abilities, plus a reason to use a tool.  One theory proposes that tool use can evolve as an adaptation for accessing "embedded" foods, such as a nut in a shell or honey inside a bee nest.       

This second phase of my study involved presenting to the gorillas yummy new foods (e.g. dried fruit, rice cakes, nuts, novel fruits) inside simple containers - cardboard boxes (photo of Joe), paper sacks, cardboard tubes, burlap bags (photo gallery).  Maguba's favorite treat was live mealworms, which she relished. When I offered a mealworm to Bibi, she knocked it out of the cage and looked at me with a "would you eat that?!" expression on her face.

Not only would these packaged treats familiarize the gorillas with embedded foods, but they would present a challenge and prolong feeding time.  Studies have shown that captive animals will choose to work for food, even when free food is available.  The gorillas then could use the packaging itself as enrichment, perhaps as a blanket or a chair, or even a cleaning rag (see photos below).  They especially enjoyed draping burlap over their bodies (photos below). All captive great apes like to drape with objects (see photo gallery below).    

During this phase I observed each gorilla for 29 hours (1740 data points), collecting a total of 87 hours of data.  Here are their activity budgets, as well as differences from Baseline data:     

Bibi:  feeding 53% (+6%), locomotion 6% (-1%), R&R .5% (-1.5), solitary play 7% (+3%), inactivity 30% (-5%), misc. 3.5% 

Joe:  feeding 22% (-12%), locomotion 3% (same), R&R 0% (-2%), public interaction 2.5% (-.5%), solitary play 3% (+1%), inactivity 68% (+14%), misc. 1.5% 

Maguba:  feeding 38% (-15%), locomotion 4% (-1%), R&R 1% (same), solitary play 3% (+1%), inactivity 48% (+13%), misc. 6% (see social interactions below)     

We were hoping for positive behavior changes - specifically more feeding and solitary play, less inactivity and abnormal behaviors.  Bibi's behaviors perfectly fulfilled this expectation (bless her heart).  However, some of Joe and Maguba's behavior changes were puzzling.  For both of them, solitary play increased and abnormal behaviors decreased, but both also had increases in inactivity and decreases in feeding.       

Data collection during this phase was temporarily halted when Maguba had major abdominal surgery (photo gallery) to harvest eggs for a gorilla reproduction study.  After she was returned to the gorilla group, she still had a period of recovery and lethargy, which seemed to be reflected in her increased inactivity. Maguba was Joe's favorite female, and I wondered if the close relationship (and empathy?) between Joe and Maguba had something to do with their similar behavior changes.         

There were dramatic changes in social interactions, as the gorillas seemed more comfortable together.  Aggressive behavior by Joe toward Bibi stayed low at 2 instances, and there were about half as many scores (21) for Joe displacing Bibi.  She seemed more relaxed around Joe and even solicited him for mating!  I was surprised and thrilled that recorded friendly social interactions increased during this phase:  16 (+13) between Joe and Maguba, plus 2 between Bibi and Maguba. The majority of these interactions involved enrichment objects.     

Some of the packaging, especially burlap bags, was used by the gorillas as a bridge to facilitate social interactions.  I had first noticed this type of behavior at another zoo where a female gorilla and a silverback had recently been introduced.  She was attracted to him, but still nervous, and she would sit behind him and touch his back with a stick - very sweet.  In a photo below, Maguba approached Joe prior to snapping him with the burlap like we might do with a towel!

For gorillas, who tend to avoid direct eye contact, a camera is probably like a big eye staring at them.  Maguba once threw feces at my video camera after I had set it up on a tripod!  I therefore appreciate the gorillas allowing me to take photos of them.

Finally it was time to introduce opportunities for tool use!  Zoo maintenance folks worked with us to construct three "fishing" boards with holes and flanges for attachment of capped pvc pipe, into which food (e.g. banana, cooked oatmeal, yogurt) could be placed.  Two of the boards were attached to the mesh at the back of the inside exhibit and one outside (see photo).  When providing enrichment it is imperative that all group members have the opportunity to participate. Some food could be accessed with fingers or tongues, but tools were necessary to "fish" for most of the food.     

Although browse was sometimes provided by the keepers throughout my research, during this phase I gathered it for the gorillas on a regular basis. Branches from bushes (e.g. forsythia) and trees (e.g. crabapple, elm, willow) not only provided a natural activity as the gorillas ate the leaves, flowers, fruit, and bark, it also could be modified to make tools.  I cut most of the browse on zoo grounds, after identifying the plants and consulting with the zoo horticulturist to make sure they were not toxic. When the gorillas saw me coming with browse, they often sang joyfully. Gorillas make a loud "singing" vocalization when they are excitedly anticipating something. Maguba got excited when she saw me coming with the pvc pipes.  She would find a tool and be waiting for me at the board.     

During this phase I collected 20 hours of data (1200  data points) on each gorilla.  Here are the gorillas' activity budgets, as well as how their behaviors differed from the previous phase:     

Bibi:  feeding 67% (+14%), locomotion 8% (+2%), R&R .6% (+.1%), solitary play 4% (-3%), inactive 16% (-14%), misc. 4%     

Joe:  feeding 30% (+8%), locomotion 4% (+1%), R&R 0% (same), public interaction 5% (+2.5%), solitary play 2% (-1%), inactive 55% (-13%), misc. 4%      

Maguba:  feeding 71% (+33%), locomotion 5% (+1%), R&R 3% (+2%), solitary play 2% (-1%), inactive 14% (-34%), misc. 5%

As you can see, these data strongly demonstrate the benefits of behavioral enrichment.  Feeding and locomotion increased for everyone, with a corresponding decrease in time inactive.  For all the gorillas, moving from fishing board to fishing board played a role in increasing locomotion. Browse consumption (photos below) took up a lot of time for all three gorillas (Bibi 17%, Joe 6%, Maguba 22%).  Although Maguba was the only gorilla to exhibit tool behaviors (photos below), she spent 13% of her day using tools at the fishing boards.  And she didn't waste any time - she began using tools on the first day and was modifying her tools by the second day.     

Maguba also generalized her tool use to other situations.  One day Bob Hamill, who had become lead keeper when Ann moved to another zoo, was digging in the planter behind the gorilla exhibit.  Maguba watched intently (photo below). She then found a stick which she repeatedly pushed through the mesh and into the planter dirt, licking the dirt off the stick.      

Even though tool use is not a species-typical behavior for gorillas in the wild (as far as we know), it can provide those in captivity with the opportunity to spend time foraging for food, as well as challenge them mentally.  Puzzle feeders are becoming more common in zoos (see photo below). The public was amazed by Maguba's tool use, frequently making positive comments.      

The fishing boards also seemed to lead to a slight increase in  R&R, as most of Maguba's R&R happened while she was working at the fishing boards.  This suggests that feeding enrichment (all their enrichments involved novel or preferred foods) can actually increase R&R.  Which makes perfect sense, if you think about it.  Who wouldn't want to experience yummy food again? However, the good news was that none of the gorillas gained weight, which is always a concern for captive animals, who are more sedentary than their wild counterparts.     

There were also positive changes in social behaviors.  Displacements stayed about the same.  Joe was aggressive toward Bibi twice, but he didn't injure her.  Joe had a history of chasing Bibi around the exhibit as she screamed bloody murder, which seemed to entertain him.  Not much fun for Bibi, and when he caught her he often held her down and bit her.  These wounds were not serious enough to require medical attention, but it obviously was not a desirable situation. It was wonderful to observe the change.  Bibi was more relaxed and confident, even engaging in chest-beating displays.  I saw one amazing and lengthy play session between Bibi and Joe where he was play-chasing her around and around the back rooms.  During this play they exchanged possession of a burlap bag, which Bibi would smack on the floor to invite Joe to resume the chase - so much fun!       

Joe and Maguba were often close to each other (photo below).  There were 23 scored friendly interactions (7 more than the previous phase) between them, many of which involved an enrichment object.  Among these interactions, 16 scores were Maguba soliciting Joe for mating (photo below).  Female gorillas exhibit "proceptive" behavior, which means they make the first move and often have to work at convincing the male to mate.  Maguba did her best, but Joe didn't seem to understand.     

Formal research was halted when Bibi became ill with an abdominal abscess and was separated for treatment.  When she recovered, she was introduced to Ernie (who had arrived during Phase 1).  Joe was later transferred to Brownsville, and Maguba was introduced to Bibi and Ernie.  This was all very interesting to observe, but it would be thirteen months before I resumed formal data collection.

When it came time to move Ernie and Bibi across the keeper aisle to the main gorilla exhibit, they were placed in separate rooms to recover.  As Ernie woke up from the anesthesia and realized he was alone, his hoots and cries echoed mournfully throughout the building.  But all was well when Bibi rejoined him the next day.  Of course Bibi was familiar with the exhibit, but it was new to Ernie and initially he went everywhere with Bibi, his arm tightly around her.  Joe and Maguba shared half the exhibit, with Ernie and Bibi in the other half.  Joe and Ernie played through the two separating doors, each drumming on his side of one metal door, then rushing to the other door to continue their play.        

After Joe was transferred to Brownsville, Ernie and Bibi were introduced to Maguba in November 1990.  Ernie knew Maguba was next door and was very curious (photo gallery).  He did a classic double take when the door slid open and he saw Maguba.  The intro was rowdy and joyful, with hours of rambunctious displays, as the gorillas sorted out their relationships.  The air was filled with pleasure rumbles and hay dust, as the three slid and chased and twirled.  Ernie was in heaven.  Bibi made pleasure vocalizations to Maguba; she was obviously happy to be reunited with her.     

The new group was given a couple months to get comfortable with each other (see photo).  During this time, Maguba gracefully ceded dominance to Bibi.  Maguba's prior dominance evidently had a lot to do with being Joe's favorite female, which is typical. Although Maguba's dominance had been subtle, Bibi had more of a "freight train" style, and Ernie and Maguba became adept at getting out of her way.  Also, in the absence of a silverback, Maguba seemed to assume the role of "watcher," keeping track of what was going on around the gorilla exhibit (photo below). Normally the females don't have to be concerned with this.     

Because of all the social changes, it was necessary to collect baseline data on Ernie, as well as new baseline data on Bibi and Maguba.  All of the previous enrichments were continued - packaged treats, browse, burlap, and fishing boards - but we didn't add any new enrichment.  I collected 20 hours of data (1200 scores) on each gorilla.  Here are their activity budgets, plus differences from Phase 3 for the females:     

Bibi:  feeding 55% (-12%), locomotion 7% (-1%), R&R 3% (+2%), solitary play 5% (+1%), inactivity 26% (+10%), misc. 4%    

Maguba:  feeding 38% (-33%), locomotion 10% (+5%), R&R 5% (+2%) and ear covering 1%, public interaction 4%, solitary play 2% (no change), inactive 36% (+22%), misc. 4%      

Ernie:  feeding 44%, locomotion 7%, solitary play 12% (8% with enrichment objects), inactive 30%, misc. 7% (see social play below)     

When a gorilla was in one of the back rooms, s/he could be out of view of the public (and me). These scores were always subtracted from the total data points for each gorilla.  It is critical for captive animals to have visual barriers from the public.  This seemed especially important to Bibi and Ernie, who were out of view 14% and 15% of the time respectively.  I imagined they were doing all sorts of wonderful things back there!     

The decrease in feeding for Bibi and Maguba may have been due to the removal of "henscratch," a mixture of grain and seeds.  During previous phases this was a routine part of husbandry that resulted in much foraging through the hay.  This removal could also have been related to their increase in R&R, which Ernie greatly admired and tried unsuccessfully to copy.     

Bibi spent 27% (+10%) of her day eating browse, Maguba 8% (-14%), and Ernie 15%.  Maguba spent 15% (+2%) of her day using tools at the fishing boards, and Ernie 6%.  I was able to document observational learning and the transfer of a cultural behavior, as Ernie learned to use tools by watching Maguba (photo below).  Very exciting!!     

Maguba's behavior changes seemed to reflect her new position in the group.  Her new public interaction consisted of playful displays directed at me.  Her increased locomotion (which included pacing back and forth) and ear covering seemed to indicate that she was uncomfortable in this new group.  Her feeding decreased dramatically, and she lost a few pounds.     

Ernie introduced new types of solitary play behavior - turning somersaults, play biting himself, galloping and sliding around the exhibit - all typical of young gorillas.  Ernie also was a consummate nest builder.  This is a natural gorilla behavior which I had not seen in any of the three adult gorillas.  Ernie constructed large nests from hay (photo below), often incorporating other objects such as burlap.  

Ernie's nests were frequently appropriated by Bibi.  Eventually, if he saw Bibi approaching, he sometimes scooped up the entire nest in his arms and moved it, waddling bipedally.  This was very exciting for me, regarding the "is a nest a tool" question, as a more strict definition of a tool is that it is "detached from the substrate" prior to use. Yea Ernie!!     

There were 76 friendly social behaviors scored, much more (+53) than during the previous phase. Bibi and Ernie were very playful with each other, wrestling, play biting, tickling, and sexual exploration.  Maguba was involved in playful displays with both Bibi (7) and Ernie (34). Display behavior involves one gorilla posturing at or charging past another and can be in either a playful or aggressive context. During the time I was observing Ernie, I scored 56 playful interactions, mostly with Bibi.  Aggressive behaviors (8) also increased (+6), all but one of which were directed by Maguba against Ernie, as she rebuffed his attempts to make contact with her.

Although Bibi's feeding time approximated that of gorillas in the wild, it seemed desirable to increase feeding time for both Maguba and Ernie.  And since Bibi wasn't very interested in the fishing boards, I wanted to explore another device that might be more appealing to her. Onward to the next and last enrichment phase!

Portable treat boards were added in this phase (see photo).  During Phase 3, Bibi seemed nervous to work at the stationary fishing boards, constantly looking over her shoulder, probably because she was the subordinate gorilla in the group.  This was an impetus to add another portable enrichment activity.   

Initially the maintenance folks made these boards from wood, but the gorillas did a good job of tearing off pieces of these (photo gallery).  So we added indestructible plastic boards.  Holes were drilled partially through the boards, and I stuffed a variety of foods into the holes.  The gorillas could access some food with their fingers and tongues (photos below), and Maguba soon began to use tools (photo gallery).  One day Ernie was intently watching Maguba and, when she finished and left her board and tools behind, Ernie picked them up and used those tools exactly as Maguba had (photo gallery).  Another exciting example of observational learning!        

In another example of social learning, one day we gave the gorillas a roll of paper towels.  They unrolled the paper and used it in all sorts of ways - displaying, draping, playing.  Bibi used some of the paper to wipe the front glass of their exhibit (photo below), working with great concentration and stretching to reach way up high.  (I assumed she had seen keepers doing this.) Ernie watched and eventually mimicked this behavior (photo below), with an "I don't see what's so great about this" expression on his face. 😊  Even Maguba checked it out, when the other gorillas weren't looking (photo gallery).     

I collected 20 hours of data on each gorilla, and here are their activity budgets, plus differences from Phase 4:     

Bibi:  feeding 54% (-1%), locomotion 5% (-2%), abnormal behaviors 3% (same), solitary play 7% (+2%), inactive 25% (-1%), misc. 6%    

Maguba:  feeding 46% (+8%), locomotion 4% (-6%), abnormal behavior 4% (-1%), public interaction 1% (-3%), solitary play 2% (same), inactive 41% (+5%), misc. 2% 

Ernie:  feeding 69% (+25%), locomotion 4% (-3%), abnormal behavior 1% (+1%), solitary play 6% (-6%), inactivity 18% (-12%), misc. 2%   

There were only insignificant changes in Bibi's activities, and it was good to see that Maguba seemed to be settling in with the new group.  Ernie's feeding time increased dramatically, along with a large decrease in inactivity.  I did, however, score two new abnormal behaviors for Ernie - hair plucking and R&R - and his solitary play, both with objects and without, decreased by almost half.     

All three gorillas, especially Bibi and Ernie, spent less time eating browse. Animals can become habituated to enrichment, which underscores the need for variety. Maguba spent slightly less time using tools at the fishing boards, but Ernie spent a whopping 29% of his day doing this (photo below), 23% more than during the previous phase.  Ernie had mastered the art of tool use and tool manufacture!!     

The treat boards were a hit with the gorillas.  Ernie spent 18% of his day extracting food, sometimes using a tool, and 2% transporting or playing with the boards.  The boards were also popular with Bibi (9%, 2%) and Maguba (8%, 1%), who also used boards to make her displays more impressive (photo below).  One day Ernie took two treat boards up on a shelf, and when Maguba approached he dropped one down to her, thus avoiding a confrontation - another example of gorilla intelligence, as well as gorilla politics!     

Bibi and Maguba spent 30% and 23% of their days respectively engaged in enrichment activities. Ernie's percentage jumped to an amazing 58%!  Behavioral enrichment can be easy and cheap, and it fills a captive gorilla's day with challenging and fun stuff.     

Most of the social interactions in this phase involved displays, and affiliative behaviors decreased a lot - a total of 31 scored instances (-45).  Bibi and Ernie had friendly interactions 15 times, Maguba and Ernie 6, Bibi and Maguba 6, and all three gorillas 4 times.  There were only 7 (-1) scored aggressive interactions, with those between Maguba and Ernie decreasing the most. It appeared the group was settling into a comfortable groove.        

On June 24, 1991, Maguba was transferred to the Dallas Zoo.  Maguba and I had become friends, and it was a sad day for me.  I sat beside her transfer crate at the Denver Zoo clinic and reassured her as she came out of the anesthesia.  I watched as she was loaded into a truck for her journey to Dallas.  Bob accompanied Maguba to help her settle in, and I would see her soon.

After Maguba left the group, we decided it would be instructive to collect more data on Bibi and Ernie.  All types of enrichment were continued, and nothing new was added.  The data would be used to compare their behaviors with and without Maguba in the social group.  I collected 20 hours of data (1200 data points) on each remaining gorilla, and it was a fun phase for all of us. Here are their activity budgets, as well as differences from the previous phase:    

Bibi:  feeding 49% (-5%), locomotion 4% (-1%), abnormal 1% (-2%), solitary play 4% (-3%), inactive 34% (+9%), misc. 8% (see social behavior below)     

Ernie:  feeding 63% (-6%), locomotion 2% (-2%), abnormal .3% (-.5%), solitary play 7% (+1%), inactive 18% (same), misc. 10% (see social behavior below)  

Enrichment activities continued to be important - Bibi 30% (same) of her day and Ernie 52% (-6%). Bibi spent 16% (same) of her time eating browse, but Ernie's 13% was an increase of 6%. Bibi manually extracted food from the treat boards 13% (+4%) of her day; Ernie extracted this food 20% (+2%) of his day, using tools about 2/3 of the time.  Both gorillas spent less time at the fishing boards:  Bibi .6% (-.1%) and Ernie 19% (-10%).  Ernie's decrease could have been due to his more proficient use of tools, as his technique had improved dramatically.  Practice makes perfect! He spent about 32% of his day using tools!!    

The most striking difference occurred in total social behaviors, with an increase of 96 data points. Bibi displayed aggressively toward Ernie a very low 3 times (+1), always in the context of food. There were 110 scores for play between Bibi and Ernie, which was an increase of 95!  I didn't catch these with my camera, so I've included above a picture of Bibi and Ernie playing after they were first introduced in 1990.  These affiliative interactions included play chase, play bite, tickle, wrestle, holding hands, and lots of wonderful, breathy laughter.  I think Maguba's presence had inhibited their play, and they could now abandon themselves to each other.

I had two main objectives with this research project:  1) provide new enrichment and measure its effect on behaviors, 2) stimulate tool behaviors.  Since there were many variables that could have affected the gorillas during the final three phases, let's compare Baseline behaviors for the original three gorillas with their behaviors during Phase 3.     

Feeding increased significantly for Bibi and Maguba, but decreased slightly for Joe.  Locomotion stayed the same for Maguba, but increased somewhat for Bibi and Joe.  Solitary play stayed about the same for all three gorillas.  Although Joe's idle time stayed about the same, for both Bibi and Maguba it was less than half as much as during Baseline.  Bibi and Joe's R&R decreased by half, but Maguba's R&R increased from 1% to 3%.  I guess you could say abnormal behaviors were pretty much a wash. ​ 

So, certainly for these female gorillas, we can emphatically say:  BEHAVIORAL ENRICHMENT ROCKS!!!  This has been proven countless times during the 30-odd years since my research project.  And I learned that BURLAP (or other fabric) stimulates both solitary and social play, and is an essential enrichment for captive gorillas.        

Although Joe never expressed an interest in using tools, this opportunity was enthusiastically embraced by Maguba and later by Ernie.  Even Bibi eventually used tools (see photo).  One day (after my study was completed!) Bob handed Bibi a stick, and she began inserting it in the fishing board.  Soon she was using tools with the treat boards too.  Even though TOOL USE does not seem to be a species-typical behavior for gorillas in the wild, it can be very effective at stimulating captive gorillas and increasing their feeding time.         

The surprise result and most awesome, unpredicted change was in SOCIAL BEHAVIORS.

Keeping in mind that my data during Baseline only captured three friendly interactions, for Joe and Maguba there were many during Phase 3.  Joe's aggression toward Bibi stayed extremely low - 2 scored instances.  Subjectively, Bibi was more relaxed.  During the first part of the study Bibi had multiple healed/healing bite wounds on her back, but as the enrichment progressed Joe stopped wounding her.  It seemed that the enrichment alleviated Joe's boredom and his tendency to entertain himself by chasing Bibi.  And Bibi solicited Joe for mating!  And I even saw Joe playing with Bibi!!     

The behavior changes discussed above strongly suggest an improvement in the PSYCHOLOGICAL WELL-BEING of Bibi, Joe, and Maguba, which is what we should strive for.  "The overall impression was of a more cohesive social group, centered around a more benevolent male leader."  (from my dissertation)

I am forever grateful to the gorillas and their keepers.  Ann, Bob, and Jody were devoted to the gorillas.  They allowed me into their space and patiently answered my endless questions.  Jody Hodges became lead keeper when Bob retired, and we would spend countless hours with Denver gorillas in the years to come.  Bibi, Joe, Maguba, and Ernie graciously tolerated my constant observation and picture taking.  These amazing gorillas allowed me into their lives. When I first met them, I was incredibly naïve.  But these four gorillas taught and inspired me, and changed the course of my life.