44 [8, 9] including the new coronavirus strain, SARS-CoV-2 [10]. In laboratory animals 45 exposed to scientific procedures, a standard approach to monitoring health status is to 46 measure body weight. Weight loss is a key indicator of health problems, and therefore 47 understanding the factors that contribute to natural variation in body weight is critical for 48 correctly monitoring an animal's condition. Ferrets undergo significant variation in their 49 body weight seasonally; however, there is currently no normative data available to provide 50 a benchmark for the expected seasonal weight changes. Seasonal variations may mask 51 or exaggerate changes in body weight due to an experimental procedure or change in 52 health status and thus must be integrated into assessments of a ferret's health status.
Seasonal weight changes have been demonstrated in multiple species 54 independent of diurnality, including monkeys [11, 12] , raccoons [13] , hamsters [14] and 55 rodents [15] . There are a range of potential factors that elicit seasonal weight changes, 56 but temperature and day length are key triggers, which are ultimately crucial for survival.
Ferrets are members of the mustelid family and have been domesticated from 58 European polecats, a species which was native to western Euroasia. Seasonal weight 59 changes have been observed in polecats and other closely related species such as mink.
60 These weight changes are seen as adaptations to the differing energy intake and 61 expenditure requirements of winter and summer [16, 17] . In animal care facilities, daylight 7 126 terms or relative to each animal's long-term average (calculated from all available data).
127 To examine day-light triggered weight changes, data from summer and winter were 128 considered independently in terms of weeks from the transition to shorter/longer days. (Fig. 1A) . The pattern of weight change for one ferret over the collection 8 147 period is shown in Figure 1B . All ferrets conformed to a similar seasonal pattern of weight 148 change with weight greatest in April (when lights were altered to their summer day length)
149 and lowest in October (when the light cycle was switched to winter day lengths, Fig. 1C ).
150 The observed decreases in weight during the summer period and increases in weight 
Modelling the contribution of season and fluid regulation to body weight 226 To determine the relative contribution of season and water regulation duration on body 227 weight, we fitted General Linear Models to weight data, using the following predictors:
228 starting weight (the first measurement of weight made after the light transition, this data 229 point was excluded from the modelling), the number of weeks since light transition, and 230 the day of water regulation. We again considered summer and winter data separately, 13 231 and for each, found the best fitting model using a significant drop in deviance as the 232 criterion for including parameters (using the F statistic to compare models, p<0.05). In 233 each case the best fitting mode retained each of the three main effects (week since 234 transition, day of water regulation and starting weight) as well as the two-way interaction 235 between the week and starting weight (see table 1 ). To illustrate the key features of these 236 models, we used the fitted models to simulate the changes in weight that would occur 237 over a 20-week period in summer and winter for animals of 750g and 900g (Fig. 4) . 
While our models could recapitulate key trends in the data, and for some animals 253 the predicted and observed weights were closely aligned ( Figure 5A ), others were much 254 less well predicted (e.g. Figure 5B ). Moreover, to be useful as a diagnostic measure of 255 healthy or abnormal weight changes, the model should be able to estimate an expected 256 weight of an animal given factors such as the season and its starting weight. To test this, 257 and determine whether this model had any utility as a diagnostic measure, we applied the 
We found that that although predicted and observed weights were correlated 294 shows a rapid weight loss that stabilises at a baseline, whereas Animal F1705 continues to gain weight 295 after the weight change, before beginning to lose weight for the remainder of the season.
Given the observation that weight loss/gain with season could be fit as a linear 298 model ( Figure 2E ,F), but that there was significant inter-animal variation we adapted our 299 modelling approach to address three questions about the changes in body weight 300 between weeks that occurred with water regulation that were common across animals:
301 First, was the change in weight between weeks constant, or did it vary through the 302 season? Second, does the change in weight between weeks, expressed as a proportion 303 of the animal's weight, depend on the size of the animal? Third, does some of the 304 additional variability we observed relate to whether animals have been on water 305 regulation the previous week? To address this, we first considered the change in weight 306 that occurred from each week (week i) to the next (week i+1) (using only the body 307 weight measurements from the first day on water regulation, i.e. Monday mornings), 308 expressing this as a % change in weight relative to bodyweight measured in week i. We 309 then fitted a GLM to these % values, using three predictors: (1) the number of weeks 310 since seasonal transition (i.e. summer to winter), (2) the animal's mean weight, and (3) 311 whether the animal had been on water regulation the previous week (week i-1). As 312 before, data for summer and winter were modelled separately.
313 When modelling weight changes in the winter the intercept was significant (beta = 1.38, 314 table 2), suggesting that typically animals gained 1.38% of their body weight weekly in 315 winter, and the week was also a significant predictor (beta = -0.065) indicating that 316 animals initially gained weight more rapidly. In summer the intercept was again significant 317 (beta = -0.73) and the only other significant predictor was whether the animal had been 318 on regulation the previous week (beta = -0.82, indicating that animals who had access to 319 free water in the previous week lost more weight than those who had not had access to 320 free water). In summary therefore, it is possible to estimate typical patterns of weight loss 321 / gain that should occur over the course of a week but data such as that shown in Figure   322 6C,D demonstrate that each animal must really be assessed individually (ideally in 323 comparison to its own historic data) in the context of other factors. Table 2 326 Final models for estimating the % change in body weight from week n to n+1. The number of weeks since 327 transition, the animals mean body weight and whether the animal had been water regulated in week n-1 20 328 were considered as factors, with only the number of weeks since transition being predictive for the 329 summer data, and whether the animal had been on water regulation or not the previous week influencing 330 the summer data.
In addition to body weight data, we also had the timing of oestrus for each animal 368 typically consume in a 24 hour period). Since the key contributor to weight loss in water 369 regulated animals is thought to be reluctance to eat dry food (rather than dehydration per 370 se) providing animals with water combined with ground pellet diet to form a mash [8] 371 appears to be successful at ensuring weight loss does not exceed more than a few 372 percent. Food and water restriction are common methods used as motivation to train 373 many laboratory animals including ferrets, rats and mice in tasks for research [35] and 374 weight loss is a key marker of health status. Understanding how seasonal fluctuations 375 interact with these effects is therefore important to refine health assessment and ensure 376 the highest standards of animal welfare.
In addition to body weight data, we also had the timing of oestrus for each animal 
The animals used in this study were all classed as young to middle aged [29] with 386 the oldest animal being 4 years old. Ferret weights also vary across the lifespan, which 387 is on average between 6-8 years [29] . At birth, female ferrets weigh 6-12g and grow