1. Introduction
Enrichment is a term used by animal keepers to describe the introduction of environmental stimuli into the environments of captive animals. The intention of enrichment is to improve the psychological and physiological wellbeing of the animals (Brummer, 2010; Coelho, 2011; Reading, 2013). Some examples of environmental enrichment types are: sensory stimuli to promote natural behaviour (Wells, 2009), food manipulation to encourage foraging (Andrews, 2014; Vasconcellos, 2012), keeping animals in group structures that mimic their wild groups, in order to improve socialisation (Amrein, 2014) and larger enclosures to allow for more locomotion (Brummer, 2010; Coelho, 2011). Providing enrichment to captive animals can be time consuming and expensive for the businesses that house these animals (Newberry, 1995). The benefits and drawbacks of enrichment varies widely as there are many forms of enrichment available for animal keepers to use. Some concerns of enriching animal environments include the spread of disease in social housing, the danger to human safety from loose housed animals and the ethical dilemma of feeding captive predators live prey (Newberry, 1995).
In this article I will examine the four types of environmental enrichment mentioned above, discuss the challenges in providing these and speculate whether the benefits are worth the expenditure of such projects. To do this I will refer to research carried out on enrichment and will give focus to Ireland’s two premier wildlife facilities: Dublin Zoo and Fota Island Wildlife Park.
2. Sensory stimuli
Sensory enrichment is the use of environmental enrichment to stimulate the senses of an animal. The three sensory stimuli that are used on captive animals are auditory, visual and olfactory (Wells, 2009).
2.1 Auditory Enrichment
Auditory enrichment is the use of sounds and music as a form of environmental stimulation (Kelling, 2013; Robbins, 2014; Wells, 2009). Music has been used in modern human medicine for decades (Underwood, 1947). It has also been used on domesticated, agricultural and zoo animals in more recent times. The results have been mixed depending on the species. Kennelled dogs responded to classical music, cattle to country music (Wells, 2009) and gorillas preferred natural sounds heard in the wild (Robbins, 2014). In one study, a recording of a lion’s own roar was played back to the lion to encourage it to be more vocal (Kelling, 2012). A side effect of providing auditory enrichment to animals is the negative effect it may have on those within range of the recordings. This is why it is necessary to not only study the effect the auditory enrichment has on the targeted animal, but to also study the effect it has on nearby animals and on human visitors. In the study on lion vocalisation just mentioned, the researchers also examined visitors and ungulates. The increase in lion vocalisation and the sound of the recordings attracted visitors to the lion enclosures and cause them to stay for longer periods. The ungulates that were within range did not exhibit any negative behaviours during the recordings either (Kelling, 2012). Further study into the positive and negative reactions of different species towards environmental stimuli would deliver useful information to personnel who design and plan the layout of animal enclosures.
2.2 Visual Enrichment
The use of visual enrichment in animals can range from surrounding enclosures with foliage, choosing colours that improve an animal’s mood or using physical props such as televisions, photos and mirrors (Newberry, 1995; Wells, 2009). Placing an animal in an enclosure with the wrong coloured walls or feeding them with the wrong coloured food can have negative effects. There is a trend amongst many animals of having negative reactions to the colour red and positive reactions to soft colours such as blue or green. Domestic chickens, pigeons, monkeys and mice have all been found to respond in such a manner (Wells, 2009). This is less likely to have the same effect on nearby animals as auditory enrichment has, because animals are unlikely to see inside adjacent enclosures. If animals can view other enclosures, then surrounding their enclosures with vegetation will not only act as a barrier to unwanted external visuals, but will also provide a source of visual enrichment itself and provide a sense of privacy. Dublin Zoo surrounds their housing units with vegetation. The zoo has a keen interest in enrichment and refers to their enclosures as “habitats” on their website (Dublin Zoo-Enrichment, 2014). The creation of visual distractions and barriers are also effective when an animal perceives another animal as its predator. Separate studies performed on amphibians and on the bird species, Parus major, demonstrate an induced hormonal response to viewing their predators (Narayan, Cockrem, Hero, 2013; Cockrem, 2002).
2.3 Olfactory Enrichment
Olfactory enrichment is also carried out in Dublin Zoo (Dublin Zoo-Enrichment, 2014). Keepers in the zoo have planted specific flowers that have attractive aromas to their captive residents and also used enriching objects that are scented (Dublin Zoo-Enrichment, 2014). A challenge facing those that intend on using olfactory stimuli is understanding the importance of scents to different species. Species that are similar to humans rely more on their sight than sense of smell. As a result olfactory enrichment is not as effective on great apes, such as chimpanzees and gorillas, as it is on animals that use smell to locate their prey, mates and food (Wells, 2009). To use olfactory enrichment on the wrong species would be a waste of time and resources. Focusing olfactory enrichment on the correct species contrarily, can reveal positive results. When African wild dogs are exposed to the odours: lion, cattle or gazelle blood, they illicit behavioural and hormonal changes to the gazelle blood, but not to the lion or cattle blood (Rafacz, 2014).
3. Food Manipulation
Creating novel feeding methods is a form of enrichment that encourages animals to forage for food, behave more naturally and increase activity levels. Fota Island Wildlife Park in County Cork provides food for their cheetahs by pulling its food along a run at 65km/h. According to their website this is done to motivate their cheetahs to exercise. Cheetahs do not exercise unless they are required to in order to feed (Fota Island Wildlife Park, 2014). As a result, this form of enrichment is necessary for the health of the cheetahs. It is also not a challenging task for the park to complete and is likely to entertain visitors. Altering feeding methods is not a costly or time consuming method of enrichment in general. Devices to scatter food in different locations at different times are commercially available. They are also highly beneficial; when a study was carried out on ursids using timed scatter feeders it was found to increase foraging time for the bears, promote a wider range of behaviours and reduce repetitive behaviours by 50% (Andrews, 2014). For many animals, while they are in their natural habitats, foraging accounts for the majority of their time (Newberry, 1995). It has been discovered that maned wolves still forage for their food when given the option of readily available food on a tray and food that is scattered amongst foliage. This contradictory behaviour is referred to as contrafreeloading. It is theorised that wolves continue to forage as it allows them to gain information on their environment and to enact a species-typical response (Vasconcellos, 2012). Scattering food is the most natural feeding technique used by most zoos. Feeding predators live animals would be seen to be unethical (Reading, 2013). This is a challenge conservationists still need to overcome as it is difficult to reintroduce a captive predator to the wild if it is unable to hunt and feed itself.
4. Social Grouping
Housing animals in groups that are similar in composition to their wild social groups has proved to be a successful method of enrichment. The ability to socialise reduces stress and improves cognitive abilities. In certain cases species do not behave naturally unless they are grouped appropriately. Male cheetahs pair-up to form coalitions in the wild. Replicating such coalitions in captivity has been found to improve their behaviour and their probability of successfully reproducing with females (Chadwick, 2013). Animals benefit from suitable group structures as well, as many juvenile animals require the direction and training of more mature parental figures (Reading, 2013). The relevance of group structure increases in species that take longer to develop and are more intelligent (Reading, 2013). Orangutans are intelligent great apes that live in fission-fusion social systems in the wild, where their groups are dynamic and unstable. In captivity however, they are large constant social groups. A study on Sumatran and Bornean orangutans highlighted the differences in the required social grouping of each species. As Sumatran orangutans are more social and live in larger groups than their Bornean relatives, they experienced less stress in larger captive groupings (Weingrill, 2011). Maintaining healthy social structure is more challenging than providing food and sensory enrichment. Controlling populations is complicated as animals require relocation over large distances, which is expensive and involves increased stress from sedation procedures (Weingrill, 2011). Culling the animals is a far less popular option than sending them to another zoo, as many species are critically endangered and visitors become attached to the animals, causing negative media attention if they disagree with a zoo’s action (Dell’Amore, 2014).
5. Enclosure Size
Creating an enclosure large enough to house a group of large intelligent animals is perhaps the most costly source of environmental enrichment. In order to allow an animal experience natural locomotion, an enclosure must be large enough both horizontally and vertically (Reading, 2013). Vertical space is necessary for animals that participate in arboreal locomotion. This includes most great apes and monkey species. Flying species also need a certain degree of height along with an enclosed roof to prevent escape. Fota Island Wildlife Park is a free-range environment for most animals. This allows for a greater degree of movement in a more natural setting. On the other hand this poses a threat to visitors and employees. To overcome this challenge, large predators are not free-ranging (Fota Island Wildlife Park, 2014). Conversely Dublin Zoo is composed of separate “habitats” for different species, which the zoo has expanded on a number of times in its history. One of the most recent expansions was a 12,000 square metre “Gorilla Rainforest” in 2011, which houses a breeding group of seven gorillas (Dublin Zoo-Gorilla Rainforest, 2014). In 2010 a study of Coyotes in three different size enclosures found that the larger their enclosure, the more similarly they behaved to wild coyotes. Additionally, heart rate response to activities was also affected by enclosure type, implying that coyotes in larger housing units are fitter than those in smaller units (Brummer, 2010).
6. Evaluating Enrichment Data
In order to establish the success of these enrichment projects researchers can study the animal’s behaviour and physical condition. Behavioural studies observe and record the actions carried out by the subjects. Species are timed carrying out these actions and are compared to their healthy counterparts in the wild. Repetitive actions and stereotypic behaviours, such as pacing back and forth, are seen as negatives and are typical of subjects who are mentally unhealthy (Robbins, 2014; Brummer, 2010; Coelho, 2011; Reading 2013; Vasconcellos, 2012). As behavioural studies are largely observational they are not financially restrictive. However, challenges can arise in drawing conclusions from such studies, for example, when a species is extinct in the wild, there are no natural subjects to compare with the captive subjects. Physical conditions that a study examines can focus on physical fitness indicators, such as heart rate (Brummer, 2010) or stress levels measured by hormonal changes. Glucocorticoids are commonly used as a measurement of stress. They can be measured using faecal cortisol metabolite enzyme-immunoassays (Narayan, 2013; Weingrill, 2011). Running physical diagnostics demands the use of laboratory equipment and physical samples, which require more tools and finances than behavioural studies.
7. Discussion
Through enrichment animals are less likely to experience negative effects such as: finding their captive environment stressful (Robbins, 2014; Coelho, 2011; Reading, 2013; Weingrill, 2011), showing signs of aggression (Doane, 2013) or signs of stereotypic behaviour (Andrews, 2014; Robbins, 2014; Brummer, 2010; Coelho, 2011). They are more likely to have increased activity levels (Robbins, 2014; Reading, 2013), exhibit natural behaviours (Robbins, 2014; Brummer, 2010; Coelho, 2011; Reading 2013; Vasconcellos, 2012) and display higher cognitive function (Coelho, 2011; Reading, 2013). As a result of the improved mental and physical abilities of animals exposed to enrichment, the reintroduction of captive animals to the wild has become more successful (Coelho, 2011; Reading, 2013). While enrichment improves an animal’s survival rate after reintroduction, a captive animal that has been exposed to enrichment is still less likely to be as successful as an animal born in the wild (Coelho, 2011; Reading, 2013). However, many wild species population numbers are too low, so wildlife managers must rely on individuals that are born in captivity if they are to reintroduce these species (Coelho, 2011; Reading, 2013).
Last year the World Wildlife Fund issued the 10th edition of “The Living Planet Report”. This report performed an assessment of the planet’s health and claimed that over the past 40 years vertebrate populations have declined by 52% (Rohr, 2014). If this trend continues then using captive animals to repopulate wild ecosystems will be obligatory. The four enrichment types mentioned in this review are vital for maintaining physically and psychologically healthy animals. While enrichment can be time consuming, financially costly and can often be complicated, the loss of ‘ecosystem services’ is far more costly and worthy of using our resources to preserve captive populations.
Fortunately zoos and parks like Dublin Zoo and Fota Island concentrate their attention towards species that are on the IUCN red list. Of Dublin Zoo’s mammal species, twelve are endangered (african hunting dog, bongo, chimpanzees, elephants, golden lion tamarin, gibbons, red ruffed lemur, sulawesi macaque, spider monkey, orang-utan, red panda and snow leopard), four are critically endangered (rodrigues fruit bat, gorilla, sumatran tiger and amur tiger) and one, the oryx, was previously extinct in the wild (Dublin Zoo-Mammals, 2014; IUCN, 2014). Furthermore, both Dublin Zoo and Fota Island Wildlife Park are members of the European Zoo Breeding Programme. (Dublin Zoo, 2014; Fota Island Wildlife Park, 2014). As such, it is highly likely that the descendants of animals housed in these two Irish institutions will be the source of future reintroductions.
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