For this study used both a survey and CCTV footage was used to analyse the different factors, especially different types and quantities of enrichment, that may be affecting breeding success in European mink.
Survey
To better understand factors that might influence breeding success and potentially improve the welfare of European mink, a survey on European mink husbandry and management was created by the European mink European Association of Zoos and Aquaria (EAZA) Ex situ Programme (EEP) coordinator (Kristel Nemvalts, at Tallinn Zoo, Estonia) and one of the Small Carnivore Taxon Advisory Group research coordinator (María Díez León). The survey was sent across Europe to centres that house European mink or coordinate their breeding.
The survey contained 131 questions, divided into eleven sections. A total of 72 questions were used in the analysis. Open-ended responses were standardised and manually reformatted. Some answers from open-response questions were fitted into categorical groups (see Table 1) for analysis. The only numerical information (per facility) used was the number of mink (this variable was assessed both numerically and categorically), number of holding types, number of days between cleaning the nest box with a litter (not counting the first time since birth delay), and most common number of dens/nest boxes per holding type.
Table 1 Description of categorical factors of the survey
| Categorical factor | Description of subcategories |
| Population size | Small if the number of mink at the facility is less than or equal to 10, large if there are more than 10 |
| Breeding | Breeding success was at least one offspring born, yes or no Breeding attempts in the past 10 years, yes or no Breeding procedure: natural, artificial insemination, or sent to another facility Introduction to mate soft, or hard introduction Females mate choice, yes or no Tracking oestrus by measuring vulva or behavioural changes Male present for parturition, yes or no Females have the choice out of multiple nest boxes, yes or no Nesting material: wood chips, hay, grass and straw |
| Size of enclosure | Most common enclosure size across holding types: small if the area of the enclosure is less than or equal to 20 m2, large if over 20 m2 |
| Barrier | Visual barriers present, yes or no |
| Type of substrate | Natural: grass, soil/earth, rock, woodchip, shavings, sand, straw, gravel, other (e.g. root, branches, limestone) Artificial: concrete, metal Both: combination of natural and artificial |
| Type of furnishings | Natural: tree climbable or not, shrubs, vegetation, rocks, burrows, water features Artificial: climbing structure, platform, other (e.g. plastic pipes, rope, wood crates) Both: combination of natural and artificial |
| Public view | If the public can see into 0-25%, 26-50%, 51-75%, or 76-100% of the enclosure |
| Access to the enclosure | Access to the outdoor enclosure is continuous 24 hours a day, or continued but restricted for cleaning and management Access to an additional enclosure, yes or no |
| Cleaning | Frequency: daily, weekly, fortnightly |
| Feeding | Frequency: once, twice, or three times per day Time of feeding: random, morning, evening, or both morning and evening Regular starving days, yes or no Food source: commercial, local supplier, or other Food type: carcass, meat, rodent, bird, chick, eggs, fish, rabbit, live prey, cat pellets, or both (i.e. carcass and commercial diet) Food frozen or cooked Add vitamins or supplements, yes or no |
| Drinking | Water source: running or standing water Water is treated, yes or no |
| Enrichment | Provide enrichment, yes or no Frequency: once per day, two or three times per week, weekly, fortnightly Enrichment type: foraging, olfactory, manipulable Enrichment with best outcomes: structural, foraging, olfactory, manipulable, feed, live prey |
| Training and keepers | Perform training, yes or no Least and most experienced keeper: less than a year, between one and five years, five years, between six and ten years, more than ten years Keeper routine: dedicated keeper, rotating Regularly entering enclosures, yes or no |
| Monitoring behaviour | Method: live observation during opening hours, live observation outside of opening hours, camera 24 hours a day |
| Abnormal repetitive behaviours (ARBs) | Most common frequency of route tracing, stationary, over-grooming ARBs: never, rarely, sometimes, always, unknown Most common time of day performing ARBs: morning, midday, afternoon, evening, random, unknown Most common season performing ARBs: winter, spring, summer, autumn, all seasons, unknown |
| Transport and restraint | Transport material: wooden/metal crate, crush/squeeze cage, pet carrier, nest box Adult and juvenile restraint type: physical, chemical (medicinal), or both Cause for restraint: specific medical procedures, transport, hormonal cycle determination, annual vaccinations and health checks |
Data analysis
Fisher’s exact tests were employed to analyse the effects of all the factors in Table 1 on breeding success and ARBs. Answers to the survey were divided into groups, and contingency tables (2×2) were created for the individual variables The groups were: population size, enclosure design, breeding success, abnormal repetitive behaviours, and keepers. Bonferroni correction was performed to adjust the p-values and account for multiple testing, due to many individual variables compared in the analysis.
Video
Video footage was collected from CCTV from Tallinn Zoo, in Estonia, off-show exhibit containing adult European mink for breeding and release. Only adult males were used since their behaviour is an important indicator for breeding success.
Ten males were used in the study, four individuals in group A and six in group B. I was kept blind when recording behaviour (i.e. I was not informed which group represented successfully breeding males) during the behavioural sampling and the data analysis, and was only informed after the analysis was completed. The CCTV recorded videos via motion activation in the enclosure. I analysed the recordings from 22:00-3:00 (five hours) over five days, since European mink are nocturnal and they are most active during this time especially during mating season which is from early March to the end of April. Note the first day of video data starts 00:00 and the fifth day of video data ends at 14:00, therefore on the final day only used three hours (00:00-3:00) of data for the analysis. In total the mink were observed for 23 hours each. The enclosures (2 x 4 m, eight meters squared in area) contained different types of enrichment (e.g. solid plastic tube, clear plastic bendable tube, bamboo bridge, plastic bag, contain, stuffed animal) and varying quantities of enrichment. The locations and type of enrichments was changed every one to two days. However, the minimum amount of enrichment always remained two types, solid plastic tubes and a plastic bag. The video data for European mink enrichment interactions and overall behaviour was analysed manually using the ethogram in Table 2. I recorded the count of behaviours observed, additionally counting the number of seconds mink interacted with enrichment.
Table 2 Ethogram describing the behaviours recorded
| Behaviour | Description |
| Out of sight | Animal is not in view of the camera after being seen at least once in the recording. |
| Sleeping | Sitting or lying still with eyes closed. |
| Resting | Sitting/laying still or with minimal movement (moving less than 2cm) with eyes open. |
| Alert | Standing or sitting still with head looking side to side or upwards, may include ear movement and or sniffing. This may be performed with weight on hind legs, with their torso upright and front legs up (or front legs on a higher object/structure). |
| Locomotion | Animal is walking or running on the ground or flat platform (at the fence line) in the enclosure. |
| Climbing | Animal is moving at a vertical angel with at least two paws on non-platform structures such as a tree trunk, branch, plank, fencing, or the edge of the water basin. |
| Intake behaviour | Animal is eating or drinking from a food bowl. This may include raising their head to the ceiling to help swallow water after each sip. |
| Foraging | Digging or scratching at subtracts or groups of items on the ground. |
| Pacing stereotypic behaviour | Walking or running using the approximate same route without obvious function (moving more than one body length) repeating the same action three or more times. This typically occurs near the borders of the cage but may include visits to the net box. |
| Static stereotypic behaviour | Animal is moving its body without obvious function, repeating the same action three or more times, with no or little horizontal leg movement. This typically includes vertical movement. This may also be identified as the animal investigating something or an area without obvious function for more than three seconds without diverting their focus. |
| Interacting with fence | Animal is biting, scratching, sniffing, vocalising, or looking directly (without moving their head) against the borders of their enclosure (i.e. fence). |
| Investigating | Sniffing or looking at an item (without touching) or specific location, usually facing their head downwards or against a structure. |
| Investigate enrichment | Animal performs “investigating” behaviour as described above but directing their focus at the enrichment exclusively. |
| Inside enrichment | Animal is inside the enrichment. This may include locomotion through the enrichment or manipulation/investigation behaviour while inside the enrichment. |
| Climb-on enrichment | Animal has at least two paws on top of the enrichment. This may include locomotion, alert, resting, and or sleeping. |
| Manipulate enrichment | Animal uses their paws or face to interact with the enrichment to move or bite for example. |
| Grooming | Licking or gnawing at own fur, scratching themself with their paws. |
| Elimination behaviour | Animal urinates and or defecates. |
| Vocalising | Animal orally emits sound such as screaming or hissing. |
| Digging | Animal uses both front or back legs simultaneously to displace soil or foraging material. The motion often starts with the front legs and is followed by the back legs. |
| Rubbing | Animal is rubbing their torso/body against the ground or a structure. |
| Shake | Animal shakes body or only their tail from side to side. This behaviour is often observed after rubbing. |
| Basin | Any behaviour done inside the water basin, such as swimming. The animal is often out of sight while in the water basin. |
| Other | Any behaviour not previously described. |
| Inactive | Animal is not seen in any recording during a full hour. |
Data analysis
Models were fitted separately for each behaviour type, and inference was based on the fixed-effect estimates from the mixed-effects models. A Fisher’s exact test was used to compare the categorical variables of group by enrichment quantity. While binary logistic regressions were used to separately test if enrichment quantity (numerical variable) or total enrichment interaction duration could predict which breeding group the mink belonged to.
To address the issue of repeated measurements, mink individual identity was included as a random intercept in all models. The model consists of a Poisson response distribution with mean λi, linked to the linear predictor via a log link function:
BehaviourCounti ∼ Poisson (λi)
log (λi)= β0 + β1BreedingGroupi +β2Enrichmenti + β3BehaviourTypei + bIndividual(i)
where λi is the expected count of the behaviour in observation i, the βi are fixed-effect regression coefficients, and bIndividual(i) is the random intercept associated with the identity of the mink observed in observation i, assumed to be normally distributed with mean zero and variance σ2Individual. “BreedingGroup” is either Group A (successful breeders) or Gorup B (unsuccessful breeders). “Enrichment” indicates either enrichment type (Standard or Additional) or quantity (numerical). “BehaviourType” is the specific behaviour measured from Table 2.