Hedgehog admissions increased over the period studied in all admission groups. The overall increase was 2.4% per year, similar to a 2.86% annual increase between 1996 and 2017 observed in data from the Jersey Hedgehog Preservation Group (N Reeve-personal communication, Molony 2007). These figures are comparable to the simultaneous rate of population decline (Roos et al 2012), and probably represent different expressions of the same threats to hedgehog survival. The increase in admissions might be due to improved public awareness of the plight of the hedgehog, but if this were so, one might expect that over time, animals would be admitted at an earlier stage, with fewer dead or moribund on arrival, higher admission weights and reduced 48-h mortality, none of which proved to be the case.
Because admission diagnoses in this study represented a limited, non-random subset of animals, they must be interpreted with caution. With due allowance for this, proportions of animals diagnosed with trauma were unchanged over the study period in adults, early and late litter juveniles. These proportions, however, only relate to sub-lethal injury, and thus may not reflect population loss through lethal trauma such as roadkill (Wembridge et al. 2016). Trauma admissions diminished across the breeding season, almost exclusively in adults, perhaps because the ranging behaviour of male hedgehogs lessens (Reeve 1982). Admissions for malnutrition increased during the study period, perhaps reflecting progressive habitat changes and reduced food availability (Hof and Bright 2009). Malnutrition appeared mainly to affect adults, including postpartum females which may be emaciated (Jackson 2006), and late litter juveniles, both having to gain weight rapidly to survive hibernation (Morris 1984; Bunnell 2009), which is consistent with the increase in malnutrition admissions across the breeding season.
Admissions of juveniles showed a bimodal distribution during the breeding season, with a higher narrower peak (weeks 26–31) in June July and August, followed by a lower, wider peak (weeks 39–46) in September, October, November and December. There was no change in the timing of admission peaks for early and late litters over the 13 year study period. This is perhaps too short a period to detect the possible phenological effects of climate change (Jackson 2006).
Median peak admission of early litters in early July (week 27–28) fits with previous research (Jackson 2006; Bunnell 2009). In the later litters, median peak admission of early October (Week 40) correlates with Bunnell’s finding of mid-October, but differs from findings in the Hebrides (Jackson 2006), where the latest births recorded were in mid-September, resulting in the latest estimated achievement of this weight to be mid-October, in contrast to the latest found here at the end of November. This may be due to the different summer length between England (the location of RSPCA WRCs) and the Hebrides and would support research that claims breeding is triggered by sustained periods of warmth (Fowler 1988; Reeve 1994). Summer is considerably shorter in the Hebrides, and to have any hope of having successful late litters whether as 1st litters, or 2nd, they must breed much earlier on. This is supported by research suggesting that 2nd litters are not attempted in Sweden, which has even shorter summers (Kristiansson 1981).
Admission weights were higher in late than early litters, consistent with the observations of Bunnell (2009). Orphans were commoner in early than late litters (77% versus 23%), but malnourished were commoner in late litters (42% vs 4%). This may be as orphans admitted following nest disruption (and so unweaned) are likely to be younger and lighter, whereas if the orphans are old enough to be part-weaned and have begun to forage on their own (Reeve 1994; Bunnell 2009), failure to successfully feed themselves might cause the malnutrition which ultimately brought them into the WRCs. The mean admission weight for late litters was 181 g, which although generally lower than one would hope to see in a weaned hedgehog, is consistent with an underweight juvenile.
In both litters, 95% confidence intervals for admission weights span only 4 g, and the difference between means is substantial (65 g), supporting representation of the 2 litters as discrete entities. 95% confidence intervals in adult admission weights spans only 9 g. The weight difference between the litters is statistically significant (44%, p < 0.00005) and along with the poorer survival of the later litters suggests possible specific developmental stages with increased vulnerability in juvenile hedgehogs, where the timing of these stages during the year may be critical.
There are several possible explanations for the admission weight and survival differences between early and later litters. It may be that a proportion of late litters may be a second attempt to breed after a failed early litter, or a 1st litter for some young females maturing later in the Summer (Reeve 1994). Jackson (2006) also noted that the reasons for failed breeding attempts were mostly unknown but observed two attempts (13% of known failures), both in the early season, which failed prior to birth. On two other occasions the mother died during the lactation period, apparently from exhaustion. Additionally, it might be argued that if the 2nd litter follows a failed 1st, the female may have an underlying health issue or weakness causing her 1st litter to fail but in the 2nd litter does not appear or develop until later, thus causing the eventual mortality of orphans. Likewise, if a 2nd litter follows a successful 1st, the female’s resources may be depleted, and she is unable to give the 2nd litter the same support. Hedgehog milk is full of crucial antibodies and passes on passive immunity (Landes et al 1998; Morris 1961) so if not fully supplied the 2nd litter would be weaker and more vulnerable, and so less likely to survive despite intervention from Rescue Centres.
In Reeve and Husijer’s study (1999), the percentage of animals released ranged from 40 to 75%, in the Jersey study 67% (N Reeve-personal communication, Molony 2007; Reeve and Huijser 1999), and in our study, 58%. If animals surviving less than 48 h are excluded, Molony reports a 53% release rate, our figure being 78%. These figures are broadly comparable, and compare favourably with release rates, across all species, of 42% in RSPCA centres, and 40% in BWRC centres (Grogan and Kelly 2013). Given recent research into the survival overwinter of WRC releases compared to wild animals found it to be roughly equal (Yarnell et al. 2019), this is very encouraging, and suggests that the guidelines on release set by the RSPCA are successful.
Survival increased by 2.2% per year, (26% over the study period), particularly in juveniles, in which survival increased by 36%. This did not appear to be due to improvement in the condition of animals on admission; there was no change in proportions of animals dead or moribund on arrival, in admission weight, or in 48-h mortality over the study period. Nor did it appear to be due to progressive change in the proportions of adults, early litters, and late litters, since this was unaltered during the course of the study. Increased malnutrition admissions, associated with better survival, may have made a contribution. Without any other obvious factors, it appears that care at the centres is more successful. The RSPCA protocol has not significantly changed during the study period, however the individual experience and expertise of the staff there can only have improved, especially as wildlife care is a relatively recent field.
Reeve and Huijser report causation and seasonal variation similar those in our sample with deaths due to “unnatural causes” such as trauma predominating in the early part of the breeding season, and due to “natural causes” including malnutrition in the later part, and similar patterns were seen in the Jersey study (Reeve-personal communication, Molony 2007; Reeve and Huijser 1999). In our study, survival was greater in early litters than in late litters and adults. Orphan admissions were commoner in early than late litters (77% versus 23%), malnutrition admissions commoner in late than early litters (42% versus 4%) and commoner still in adults 53%. Late litter animals have to sustain a higher growth rate than early litters (Bunnell 2009) with a diminishing food supply, and thus being perhaps in a less healthy condition than orphans, be less likely to survive. The view that there is a high mortality in late litters related to an inability to reach adequate hibernation weight has been challenged by Bunnell (Bunnell 2009), but the higher mortality of late litters in our sample, and their tendency to malnutrition tend to support it. In adults, postpartum females are vulnerable to malnutrition (Jackson 2006) which would also explain the increased malnutrition later in the year. Likewise, trauma admissions, with relatively poorer survival, were commonest in adults (73% in adults, 27% in juveniles), which is consistent with their expected greater motility, thus greater exposure to unnatural threats.
Within admission groups, survival was independent of admission weight, endorsing Molony’s findings (Molony 2007). The tight clustering of admission weights around the mean, however, makes it less likely that such an association would be found. It is encouraging to those inclined to resuscitate animals of very low weight, that even those below 40 g had reasonable survival rates, but it is likely that some selection bias is present, especially as those arriving at very low weights and in poor condition are more likely to be euthanised within 48 h in accordance with RSPCA protocol.
Mortality of adult animals diminished across the breeding season. This may reflect the diminishing effect of trauma, which declines as the year goes on, as male adults cease to roam in search of mates (Reeve 1982). Mortality of juveniles increased throughout the breeding season. This may reflect a transition from early litter orphan admissions with better survival to late litter malnutrition admissions associated with higher mortality.