Ethics statement
All work was conducted under a Natural England Licence (20121788) and following approval by the ethical review committees of the School of Animal, Rural and Environmental Sciences, Nottingham Trent University (ARE10), and the School of Pharmacy and Biomolecular Sciences, University of Brighton.
Study sites and animals
Rehabilitated hedgehogs were from the Royal Society for the Prevention of Cruelty to Animals (RSPCA) wildlife rehabilitation centres. Rehabilitated hedgehogs’ admission weights ranged from 0 g (born in captivity) to 530 g. Hedgehogs that weighed under 150 g were bottle fed and weaned at between 150 and 175 g (n = 4), and all others ate independently. No distinction was made between hand-reared and weaned hedgehogs in the analysis. All individuals had passed a veterinary health check prior to release. Where possible, hedgehogs were released back to sites where they were found. All other hedgehogs were released at sites deemed suitable for hedgehogs; selection criteria included available suitable habitat located away from main roads (to reduce the chances of road mortality) and for the site to have minimal or no local badger (Meles meles) activity or reported badger setts nearby, as previous hedgehog releases in such areas resulted in high rates of predation (Doncaster 1992; Morris and Warwick 1994).
The study took place at five study areas across Southern and Central England over four winters. The rehabilitation release sites, UK grid references and winters were as follows: Pett (TQ8741 1402) (2010-11); Seddlescombe (TQ7832 1817) (2011-12); a local nature reserve in Brighton (TQ 3262 0723) (2011–2013) and a suburban area of Brighton (TQ3130 0415) (2013–2014). Wild hedgehogs were also studied in Brighton (TQ2627 0694) (2013–2014) and at Nottingham Trent University’s Brackenhurst Campus near Southwell, (SK 6946 5243) (2012–13 and 2013–14). In Brighton, wild hedgehogs were located in residential gardens following reports from a local hedgehog community group. At Brackenhurst, wild hedgehogs were located during nocturnal searches using one million candle power spotlights. The majority of wild hedgehogs were caught in October prior to winter hibernation, while the rehabilitated individuals were released throughout autumn and winter. Release of rehabilitated hedgehogs was timed so that mean night temperature was above 0 °C for at least 2 days before release and forecast for minimum temperature to be above 0 °C for 5 days post release. Rehabilitated hedgehogs were ‘soft’-released at dusk, by placing a nest-box which contained bedding and food, at the release site, from which the hedgehog could leave and re-access. The boxes were checked daily after release to check the hedgehog had moved, and if no hedgehog was present, the box was removed. Hedgehog ages were classed as either ‘adults’ (known to have survived at least one winter season) or ‘young of year’ (yet to experience a winter season), and gender was recorded.
Radio tracking
All hedgehogs were tagged with VHF radio transmitters weighing approximately 7 g (Biotrack, Wareham, Dorset) following the methods described by Young et al. (2006). All radio transmitters weighed less than 5% body weight (range 0.54–1.79%), meeting the guidelines of the American Society of Mammalogists (Sikes and Gannon 2011). Radio-tracking took place using a Sika radio tracking receiver fitted with a flexible Yagi three-pronged antenna (Biotrack). Hedgehogs were also individually marked with coloured heat shrink tubing to aid identification and weighed prior to release using an electronic balance (Glasby and Yarnell 2013). Rehabilitated hedgehogs were located via radio-tracking once per day for 3 days after release. All hedgehogs were located in their nests at least once per week between their release date and the following spring, and the location of each nest was recorded using a Garmin GPSmap 62 handheld GPS. The radio-tracking survey period was split into three-time periods in an attempt to identify periods of differing survival over autumn, winter and spring. These were ‘autumn’ (before 31st of October), ‘winter’ (between 1st of November and 31st of March) and ‘spring’ (after 1st of April). From the 1st of March, location data was taken at least three times a week to identify hedgehog activity after hibernation. Once a hedgehog has been identified as moving nests after the 1st of March, nocturnal radio tracking commenced in order to relocate and weigh hedgehogs and to remove their radio-transmitters. Weight change over winter was calculated as the difference between the weight when first radio-tagged and released, to the date the animal was first caught after the 1st of March. The last weight of the animal would depend on when hedgehogs became active after hibernation in relation to when they were recaptured, so that individuals that had been active for some time are likely to have recovered weight lost over winter, whereas those that were captured soon after becoming active are likely to weigh less than before winter, increasing variation in this variable.
Analysis
Hedgehog fate was determined as either ‘surviving’, ‘died’ or ‘unknown’. Survival during a survey period (autumn, winter or spring) was confirmed if an individual was relocated alive or had moved nests in the following season. An animal was recorded as having died if it was found dead during the study period. If an individual was not re-caught due to tag loss/failure, or animal disappearance, then the fate was recorded as being unknown. The total number of days alive was calculated for each hedgehog between the first date of radio-tag attachment and release, to the last date known to be alive (i.e. the date that the hedgehog was recaptured, and the tag removed or the last date of recorded nest movement). The total number of days alive was also calculated for each individual during the study and also for each survey period. The first day alive was either the day the individual was first radio-tagged and released, or if already tagged, as the first day of the study period (autumn, winter or spring). The last day alive was either the day the animal was found alive and had its tag removed, or in the situation where the animal could not be relocated due to disappearance or tag failure and loss, the day which the individual was known to have moved nest or was last seen alive. The number of days alive was used in right-censored Kaplan-Meier survival analysis to compare survival rates of wild and rehabilitated individuals (Kaplan and Meier 1958). That is, only hedgehogs that were known to have died were recorded as such, and for hedgehogs where fates were unknown, they were assumed to have survived up to the last movement between nests and assumed to have dropped out of the survey at that point in time. Kaplan-Meier survival analysis was conducted for all hedgehogs pooled, and by all hedgehogs pooled across season, by treatment (wild or rehabilitated) and by treatment up to 150 days into the study. The survival curves of wild and rehabilitated were compared up to 150 days as the sample sizes (number of individuals still being tracked) beyond this time period was greatly reduced (mostly to due tag removal, loss or disappearance of individuals), making statistical comparisons dependent on a few individuals. Individual hedgehogs were treated independently in each survey period for this analysis, so that a hedgehog tracked over the entire survey period was included in the survival estimates across all three survey periods. Survival rates between the two groups were tested with a log-rank test. Survival rates of all hedgehogs were compared between seasons to ascertain whether season influenced survival as this is likely to inform release protocols. All survival analysis was conducted in R (R Core Team 2014) using the ‘survival’ (Therneau 2015) and ‘survminer’ (Kassambara and Kosinski 2018) packages.
Changes in hedgehog weight (g) were assessed using variables: ‘start weight’ (weight of individual at the start of the radio-tracking period); ‘last weight’ (last known recorded weight of individual during the study); ‘weight change’ (the difference in ‘start weight’ and ‘last weight’); ‘daily weight change’ (‘weight change’ divided by the number of days between the start and last weights were recorded); and ‘percentage weight change’ (proportional weight loss over winter per individual). Nesting rate (number of nests used by individuals per 100 days) was recorded as the number of nests reported as being used by each individual over the duration of the radio-tracking period ∗ 100. The total number of days individuals were radio-tracked was also recorded and compared between wild and rehabilitated individuals.
To test whether overwinter behaviour of hedgehogs varied by age or gender, a preliminary analysis of only the wild hedgehogs was conducted to determine whether these variables required factoring into analyses. All data was tested for homogeneity with a Bartlett test, and for normality using a Shapiro-Wilk test. To compare differences in over winter behaviour between gender (male versus female), age (adults versus young of year) and wild versus rehabilitated hedgehogs, independent t tests were used where assumptions of normality and homogeneity were met, and Mann-Whitney U tests used otherwise. Unless specified all values represent means ± 1 standard error. All analysis was conducted in R (R Core Team 2014).