A special berry
A tiny but sweet fruit’s incredible attraction for bears
It’s late July when the curtain falls on the bears’ breeding season. A solitary animal determinedly crosses an amphitheatre of high-altitude clearings and scree, pausing to investigate some bushes.
Small clusters of dark red, fleshy, spherical fruits emerge from their leaves; the bear sniffs them insistently, then moves on to the next bush. These are alpine buckthorn bushes and we are in an area of buckthorn scrub. There is a sense of industrious anticipation in the air ahead of the coming months. Depending on the season and slope, in a few days or weeks the berries will turn black and become appetising for bears. Around the same time and in much the same way year after year, the bears in the Park come to these sunny rocky areas from all directions. For the bears, it’s buckthorn time.
The phenomenon is very similar to what happens in the salmon rivers of North America, although of course the scale is different. There, you can see more than sixty bears feeding together and tolerating each other in the same area. But even in the park, it’s not that rare to see up to ten bears taking turns among the bushes a few dozen metres apart.
Elisabetta
For more than 10 years, PNALM researchers and technicians have been spending their summers observing bears feeding in the buckthorn scrub. The first bears start to arrive as early as the first week of August, but the peak of the gatherings occurs between mid-August and the first week of September. Every year, males and females of all ages and females with cubs or juveniles in tow have between thirty and sixty days to feed on these fruits, depending on the weather (rain and temperature) which influences their ripeness and persistence. As the weeks pass, the bears move from one amphitheatre to another following the ripening of the berries, staying for almost a month in some particularly buckthorn-rich sites. From mid-September, sightings become very rare and the bears move down to lower altitudes in search of pears, apples, acorns or beechnuts, depending on the year.
Why bears eat buckthorn
Alpine buckthorn (family: Rhamnaceae) is a perennial, pioneer shrub typically found in the altitude band above beech (between 1125 and 2225 metres). The bushes, just a few metres high, grow on arid, sun-exposed calcareous rocky slopes, in rock crevices, on ledges, on the edges of beech forests and in clearings. The fruits are grouped in clusters of 3-8 drupes and may contain up to 2-3 seeds. In a territory of about 1800 km2, the buckthorn grows in an area of no more than 30 km2, forming more or less continuous populations varying in size from 3 to 13 km2. Each fruit has a calorie content of about 2.8 kilocalories of metabolisable energy. Assuming that the bears feed at maximum efficiency, using ingestion efficiency values recorded in captivity and a defecation rate of 7 droppings per day, each containing up to 5000 buckthorn seeds (corresponding to about 1700 fruits), a bear could consume over ten thousand fruits per day, corresponding to a meal of about 8000 kilocalories (enough to feed a person for two whole days!).
Buckthorn can contribute more than half a bear’s daily energy needs – an excellent food for gaining weight before winter.
This berry, just a few millimetres in diameter and weighing just 0.26 grams, seems to have all the characteristics needed to enable bears to fatten up and prepare for winter. In late summer, bears can choose from more than nineteen different species of fruit, but most of the nourishment and energy comes from eating this one berry. In some years, it contributes more than 70% of their energy intake and even in years when there is an abundance of sugar and fat rich nuts such as beech nuts, buckthorn alone contributes almost half the summer diet. A study carried out between 2006 and 2008 using various techniques (direct observations and genetic and radiotelemetric monitoring) found that more than half the Park’s bears regularly visit these areas each year between August and September. A diet consisting only of fruit could have high metabolic costs for a bear, but with their reduced requirements, Apennine bears are less constrained by an essentially frugivorous diet than larger bears. Young bears and in particular females, sometimes weighing considerably less than half the weight of an adult male bear, can easily accumulate fat and lean mass by eating mainly fruits.
Bears don’t miss a beat when it comes to food. For a bear, buckthorn is a tasty morsel obtained without excessive effort.
The ease with which bears find, handle and assimilate nutrients from the buckthorn makes this fruit an ideal food during the period of hyperphagia. The bushes are distributed in groups throughout the territory, making this resource easy to look for and locally abundant. Bears concentrate more than half their feeding and resting activities in an area of just a few hundred metres around the buckthorn scrub. In practice, bears sleep and eat in the same place. The bushes are just a few tens of centimetres high and the berries are clearly visible grouped in clusters at the tip of the branches, so they can be easily grasped and manipulated. Often in a sitting position, the bears bend the branches apparently effortlessly and grasp the fruit with their lips. They do not waste even a single bite and, in fact, only berry residues (skin and seeds) are found in the excrement, with no leaves or twigs.
Buckthorn acts as a magnet and bears move the centre of their activity to these areas between August and September. During this season between 2006 and 2010, twelve male and female bears were monitored with tracking collars. As the buckthorn ripens, the females in particular restrict their movements to an area of a few tens of square kilometres (around 30 km2) and the buckthorn scrub can occupy up to 40% of the territory used by the bears during this period. But each bear obtains energy and nourishment from its environment in a different way. There is no single recipe for all bears. Age, gender, individual history, the proximity of the buckthorn scrub to the bear’s habitual territory and the availability of other natural foods, not to mention competition with other bears or animals, are all factors that can influence a bear’s daily choice of how and where to feed. Some bears spend no more than three days in the buckthorn scrub, while others remain there for more than a month. But most animals move between the bushes for an average of at least eighteen days, with no difference between males and females.
Buckthorn lays down the law. It takes a lot of time for a bear to satisfy its hunger with fruits weighing less than a gram. Several factors may change bears’ behaviour during this season, including the presence of tourists and other bears.
In summer, neither males nor females have exclusively nocturnal rhythms. Females limit their movements between seven in the morning and three in the afternoon, while males are more twilight animals and only start moving again after five in the afternoon. However, during the long years of research and monitoring carried out to date, it has not been uncommon to see young bears and females with cubs moving through the bushes even in the middle of the day, perhaps taking advantage of a cloudy sky. The bears’ hours of inactivity coincide with the times when those areas are most heavily frequented by tourists and also the hottest hours of the day. In the Park, the choice of when and for how long to feed therefore seems to represent a balance between the costs of feeding at high temperatures and the risk of encountering humans. The number of hours/day available for feeding can, on the other hand, significantly limit the amount of berries a bear is able to ingest. Females with cubs and juveniles, the groups with special needs and able to benefit most from a strictly frugivorous diet, also face the most risks. Going out during the day can not only ensure an optimal meal, it also lets you enjoy the peace of mind of eating away from potentially aggressive adult males. And, in fact, the infanticide of two cubs by a male bear was documented for the first time in 2007 right in an area of buckthorn scrub.
After hundreds of hours of observation, we can now say that bears spend most of their time eating in buckthorn scrub. Sometimes they even seem to ignore each other. In fact, they are always very alert and react to any noise. Greater tension is observed when they first emerge from the woods, a behaviour they share with deer or hares. They stop, scour the area and keep alert, sniffing the air and craning their necks to the left and right. It is probably the conditions of reduced peripheral visibility that induce this behaviour. This enables the bears to assess the presence or absence of possible sources of danger. However, the most reactive are the females with cubs, who may threaten other bears with vigour and determination if they feel they’re in danger.
Elisabetta
Life in the buckthorn scrub is a big change for bears in the Park. Bears move from being alone to all-out gatherings and therefore have to deal with the tensions that can arise from a life that is a little too social. But they do not come unprepared and show great ability in overcoming the problems of coexistence that may arise from this forced sociability. All this is the result of a long process of mutual understanding established over the years. In more than 14 hours of observation between 2006 and 2007, bears were seen to spend more than 70% of their available time feeding almost exclusively on buckthorn (and more rarely on ants, grass and roots), moving around the area from bush to bush. Bears spend no more than 5% of their time on the alert and only 1% in social interaction, mostly involving states of vigilance or alertness towards other bears or parental interactions in the case of females with yearling cubs.
But life in the buckthorn scrub has many more twists and turns than you might think. Bears provide a vital link in the transport of nutrients from high altitudes to the valley floor. Because of their mobility and relatively long intestinal retention time (over 5 hours in the case of the berries), bears can play a key role in seed dispersal between distant environments. Droppings with buckthorn have also been found at low altitudes (~3-5 km away and 1000 m below the feeding grounds). The seeds may then be dispersed again by small mammals and birds, which in turn derive a source of nourishment. In practice, bears are like farmers, planting seeds everywhere and growing a plant community that feeds both themselves and other animals, while at the same time providing food to meet the energy needs of many species.
Why you shouldn’t disturb a bear while its eating
During a study carried out in the Park between 2006 and 2007, without interference and from a distance, researchers observed the behaviour of bears in the buckthorn scrub, in the absence or presence of hikers or people watching them. The study involved more than 500 hours of observation and 16 interactions were observed. On all occasions, when the bears were surprised by sudden noises (rolling stones) or people talking, especially if in large groups and even from as far away as 300 metres, or when approached at less than 100 metres including on an official footpath, the bears became alarmed or fled and did not return to feed, at least during daylight hours. All this has costs for a bear. For every minute a bear does not feed, it can lose up to 6 kcal, plus the energy costs associated with stress and escaping. According to a study conducted on black bears, to gain weight on berries alone, a bear would need to feed uninterruptedly for at least 12 hours. Time is a tyrant for bears living in environments such as the Apennines with a heavy human presence, making them adopt more twilight and nocturnal habits. So bears may not have ways or time to compensate for the effects of repeated disturbance. Bears may also become accustomed to people if their experiences are not excessively negative. And this has, in fact, been observed in a number of the more touristy areas of buckthorn scrub. But habituation is not a panacea for a bear, because appearances can sometimes be deceptive. A lack of reactivity may conceal a state of tension. As in humans, if stress persists over time it can cause serious physical and behavioural disorders.
All the information gathered underlines the exclusive importance for bears of buckthorn bushes and scrub within the Park. Any factor adversely affecting the level of berry productivity or the times available for the bears to feed can significantly reduce the energy provided by this food. The worse affected are mainly females with cubs and juveniles. However, although remote, these areas are today a destination for hikers and enthusiasts, a phenomenon in expansion in recent years, especially associated with the possibility of seeing bears. The high incidence of both wild and domestic ungulates observed feeding in the buckthorn scrub could also have some effect on the future productivity of the various populations.
Maintaining areas of buckthorn scrub where bears are free from human interference or other disturbance is a guarantee for the reproductive success of bears in the Park.