Long‐term effect of neutering age on body condition score and bodyweight in domestic cats

Abstract

Background

Neutered cats are more predisposed to obesity. However, limited research has explored whether body condition score (BCS) and bodyweight vary across lifestages as a function of age at neutering.

Methods

Longitudinal clinical records of BCS (n = 2410) and bodyweight (n = 2073) were paired with age at neutering and other owner‐reported auxiliary information before analysis using cumulative link mixed‐effects models and linear mixed models to explore which factors influence BCS and bodyweight as cats age.

Results

BCS and bodyweight were age dependent, increasing until 9 years of age and then declining. No differences in BCS or bodyweight were found between cats neutered at 4 months or younger, 5 months or 6 months. Cats neutered at 7‒12 months experienced a less rapid age‐related increase in bodyweight and BCS. Females had lower bodyweight and BCS than males, with differences more pronounced in the summer and autumn. Longhaired cats had a lower BCS than shorthaired cats, but hair length was not significantly associated with bodyweight.

Limitations

It was not possible to measure welfare impacts or incorporate diet and lifestyle measurements.

Conclusion

Cats neutered prepubertally showed no difference in bodyweight or BCS as cats aged compared to cats neutered at 5 or 6 months. Cats neutered at 7‒12 months had a lower risk of bodyweight and BCS gain overall. Postneutering weight management advice is recommended for all cats, and monitoring should utilise a combination of both BCS and bodyweight, particularly for longhaired cats.

INTRODUCTION

Neutering of owned cats (Felis catus) is widely accepted in the UK as good management practice for preventing accidental litters and thus reducing the number of unwanted cats free roaming and relinquished to shelters. ¹ Additionally, neutering has behavioural benefits ² , ³ and positive health outcomes for cats, resulting in increased longevity, ⁴ such as reducing the risk of mammary neoplasia ⁵ or reducing infectious disease transmission through a reduction in agonistic and copulatory interactions between conspecifics. ³ Despite high neutering rates in the UK, ⁶ around 80% of owned cat litters are accidental, ¹ resulting in substantial populations of unowned cats both free roaming ⁷ and within shelters. ⁸ As cats can reach sexual maturity at 4 months of age, it is recommended that neutering is carried out early enough to prevent unplanned litters. Recently, modelled data have demonstrated that neutering age has a profound impact on the number of unowned cats within the UK, even when neutering rates remain constant. ⁹

In 2006, the Cat Group published recommendations that the standard age of neutering of owned cats be reduced from 6 months to 4 months, ¹⁰ a practice typically referred to as prepubertal neutering. This term also encompasses the neutering of cats younger than 4 months, which is often appropriate and desirable within a shelter or trap‒neuter‒return context. ¹¹ , ¹² , ¹³ Despite prepubertal neutering being supported by both veterinary and animal welfare organisations, ¹⁴ , ¹⁵ , ¹⁶ not all UK veterinarians carry out prepubertal neutering or recommend the practice to owners. ¹⁷ , ¹⁸ Barriers to the adoption of prepubertal neutering by veterinarians include a lack of confidence in carrying out the procedure and the need for more training opportunities. ¹⁷ , ¹⁹ Veterinarians also report concerns around the lack of longitudinal studies on the impact of age of neutering on health and welfare, ¹⁷ including concerns of increased bodyweight in cats neutered prepubertally. ²⁰ While the body of evidence to date suggests that neutering cats prepubertally has similar effects on long‐term growth and development as neutering cats between 6 and 8 months, ²¹ and may even be beneficial in aiding healthy bodyweight management by reducing acute changes in feeding behaviour, ²² studies have been limited in either the duration of the follow‐up ²⁰ , ²² or the fact they are based on shelter populations only. ²¹ , ²³

Body condition score (BCS) and bodyweight are considered readily accessible metrics that may act as indicators of relative health for cats at any age. ²⁴ , ²⁵ Having a BCS associated with being overweight or obese has been implicated as a risk factor for a range of diseases, such as diabetes, ²⁶ , ²⁷ skin disease ²⁸ and osteoarthritis. ²⁹ It also has links to mortality ²⁷ and may impede normal feline behaviours such as grooming, playing or accessing high resting spots due to mobility issues. ³⁰ A recent study revealed that animal welfare experts identify obesity in owned cats as one of the top three current priority welfare issues in the UK, based on potential severity and duration of impact on cats’ health. ³¹ The reported prevalence of overweight and obesity in owned cats varies globally, with estimates of 11.5% in the UK, ³² 15.7% in Australia ³³ and 41% in the United States. ³⁴ A range of risk factors for obesity have been identified, such as a dry diet, ³⁵ , ³⁶ indoor‐only lifestyles, ³⁶ , ³⁷ certain breed types, ³³ acquisition source ³⁷ and neutering. ³⁸ , ³⁹ , ⁴⁰

Associations between neutering and increased bodyweight are well established and are posited to be caused by hormonal and metabolic changes leading to decreased activity levels and increased food intake postneutering. ⁴¹ , ⁴² Concerns have been raised that cats neutered prepubertally may experience greater long‐term impacts on their health compared to those neutered later. ¹⁷ While the literature so far finds no difference in obesity in later life between cats neutered prepubertally and those neutered between 6 and 8 months old, ²¹ , ²³ limitations within the current body of literature must be acknowledged. Few studies to date have considered both BCS and bodyweight. Where bodyweight is used in isolation, the range of what can be considered a healthy or normal bodyweight for each individual must be taken into account, as this may vary based on age, ⁴³ , ⁴⁴ sex ⁴⁵ , ⁴⁶ and breed. ⁴⁷ BCS is a measure of body fat mass scored by visual analysis and palpation of the ribs. While BCS is a validated measure of body fat mass ⁴⁸ , ⁴⁹ with a relatively high rate of intraobserver reliability for trained veterinary professionals, ⁵⁰ it is still subjective, with observer interpretation shown to be affected by confounding variables such as coat length. ³⁸ When measured by owners or untrained professionals, as is done in many studies due to the relative ease of accessing larger amounts of data in this way (e.g., as described by Rowe et al ³⁶ ), the reliability of BCS may be reduced or subject to reporter bias. ³⁸ , ⁴⁰ , ⁵¹ Additionally, analysis of BCS often either incorrectly treats the variable as numeric rather than categorical (e.g., as in Alexander et al ⁵² ) or distills the variation down to a binary measure of whether the cat is obese or not (e.g., as in Rowe et al. ³⁵ ), thus not understanding the full spectrum of body types and the probability of moving between each of them. Studies may also not take into consideration other cat attributes or contributory factors that may influence a cat's propensity to gain bodyweight or increase their BCS, such as season of measurement or sex. Finally, many studies utilise bodyweight or BCS measurements taken at a single time point or during a limited timeframe rather than exploring long‐term changes across a cat population.

Understanding the influence of age at neutering on the bodyweight and body condition of owned cats is important, with results from such studies providing veterinarians with the necessary understanding to tailor and guide bodyweight management advice for cat owners. As such, this study aimed to determine associations between age‐related increases in bodyweight and BCS and the age at neutering in owned cats. Specifically, using observational, longitudinal veterinary clinical measurements paired with owner‐reported auxiliary information, a range of potential risk factors was considered when asking whether the age at which a cat is neutered affects their BCS and bodyweight as they age.

MATERIALS AND METHODS

Data collection

Data for this study were obtained as part of the Bristol Cats Study, a longitudinal cohort study of cats in the UK. Cats and owners were recruited to the Bristol Cats Study between 2010 and 2013, when cats were between 8 and 16 weeks old. Regular questionnaires were sent to owners registered in the study as their cat reached specific ages until owners reported that they were no longer in possession of their cat (due to death, loss or other reasons) or opted to stop participation in the study. In addition to the owner‐reported data, cards to record BCS and bodyweight, alongside the date of recording, were sent to owners at the end of each year to be completed by a veterinarian at their cat's next veterinary visit, with records returned up to February 2023 included. The Bristol Cats Study received ethical approval from the University of Bristol Animal Welfare and Ethical Review Body (reference UB/17/049). For more detailed information on the Bristol Cats Study, see Murray et al. ⁵³

Data cleaning

The BCS system used by veterinarians was a five‐point scale. ⁴⁹ Although not validated against an objective measure of body fat, it has similarities with the nine‐point BCS scale, which has been validated. ⁴⁸ Responses provided on the nine‐point scale were excluded, due to a lack of validated conversion between the five‐ and nine‐point scales. As BCS is an ordinal, and therefore a non‐numeric scale, any responses that included a decimal point or a range were rounded down to the nearest whole number, given that such scores indicate that the cat does not fully meet the criteria for the higher bracket, for example, a score of 2.5 would be coded as 2, or a response ‘between 4 and 5’ would be coded as 4. Bodyweight (in kg) was included to two decimal places where provided. The dates at which the BCS and bodyweight measurements were recorded were also coded into seasons as follows: spring—March, April and May; summer—June, July and August; autumn—September, October and November; winter—December, January and February.

The data from the owner questionnaires were cleaned as follows. The cat's age at each BCS and/or bodyweight measurement was calculated using the date of recording and the date of birth. The date of birth was used as reported or, if unknown, calculated by subtracting the age reported at the time of the survey from the date of survey completion. Due to version differences in questionnaires, age at the time of neutering was used as reported where available; otherwise, it was calculated. Where calculated, the length of time owners reported had passed since the neutering operation was subtracted from the date of questionnaire completion to give a neuter date. Then, the date of birth was used with the neuter date to calculate age at neutering. The owner‐reported acquisition source was grouped into biologically relevant categories with adequate sample sizes, resulting in three separate categories: rescue centre/shelter, feral/stray and other (e.g., purchased online, acquired from a breeder). Hair length and sex were used as provided by the owner. As owner‐reported and veterinarian‐reported data were not collected at the same time point, all owner‐reported explanatory variables used were fixed variables, specifically, demographic details. Within questionnaires, owner‐reported time‐relative variables such as outdoor access, multicat households or diet did not align with the time of recording BCS and bodyweight and so were not included.

Data exclusions

From an initial dataset of 832 individual cats for whom veterinary recordings of BCS on a five‐point scale and bodyweight were available, 584 were taken forward for analysis. First, due to morphological differences in cats of different breeds, ⁴⁷ , ⁵⁴ , ⁵⁵ cats reported by owners as purebred, or those where the owners did not know if the cat was purebred, were excluded (n = 226). Second, cats reported to have been pregnant at any point were excluded (n = 5), as pregnancy may impact BCS and bodyweight both during and after pregnancy, ⁵⁶ while also predisposing them to be neutered at an older age. Data on other potentially confounding factors predisposing cats to being neutered at an older age were unavailable, precluding their exclusion. Next, cats whose neutering age was unknown were excluded (n = 15). Finally, BCSs recorded when cats were less than 1 year old were excluded from the analysis, which led to the removal of 22 cats from the dataset.

Data were checked to ensure sufficient sample sizes for categorical variables and that continuous variables were biologically reasonable. As only a single incidence of a BCS of 1 occurred throughout the entire dataset, this datapoint was excluded (n = 1); however, as this cat had other recorded BCSs above 1, the individual was retained within the dataset. Additionally, no hair length data for cats that were reported to have medium hair length were included in the analysis, both due to the small sample size of this group and the ambiguity in the interpretation of this category (n = 7).

Data analysis

Dataset amalgamation, coding, cleaning and exclusions were completed in Microsoft Excel for Microsoft Office 365 (version 2407). ⁵⁷ Descriptive statistics for demographic details, absolute counts of veterinary visits and BCSs, and means, medians and ranges for BCS and bodyweight measurements were also calculated in Microsoft Excel. All further data analysis was carried out in R (version 4.2.2). ⁵⁸ Cumulative link mixed‐effects models (CLMMs) were used to test variables for effects on BCS using the CLMM function within the package ‘ordinal’ (version 2022.11‐16). ⁵⁹ Each CLMM included a random effect of cat ID, nested within owner ID, to account for non‐independent correlated data across individual measurements and among cats within the same household. CLMMs are able to deal with the ranked variable of BCS, whereby BCS has an ordered but not numeric structure.

Linear mixed models (LMERs) with a random effect of cat ID nested within owner ID were used to test variables for effects on cat bodyweight using the lmer function within the R package ‘lme4’ (version 1.1‐30). ⁶⁰

For both analyses (CLMMs and LMERs), sex, age at neutering, acquisition source, hair length and season were incorporated as categorical fixed effects, and the age of the cat was incorporated as a continuous fixed effect within the model. Age was incorporated as a non‐linear function using a quadratic term to test whether bodyweight or BCS changes non‐linearly as cats age, as we may expect an increase early in life and a decline later in life.

Model validation

Within both CLMMs and LMERs, the significance of the coefficients was assessed with likelihood ratio tests (which use a chi‐squared statistic), while model comparison was made according to the Akaike information criterion (AIC), where the most parsimonious (lowest AIC) is favoured. Each predictor variable was considered separately, along with testing of individual interaction terms. Furthermore, a multivariable model using identified predictors was used to build a model via stepwise backwards elimination. To validate the age at neutering categories, we explored support for alternative categorisation as indicated by the data (Figures S1 and S2). Statistical comparisons were made between the same datasets that comprised complete records for the predictor of interest, which ranged between 94.8% and 100% complete (see Table S1 for further details).

To further check the validity of our model outcomes, we used a combination of approaches. For the LMER models, variables were checked for influential outliers, normality, homoscedasticity and collinearity using the ‘performance’ package (version 0.12.2), ⁶¹ with all terms showing low correlation according to the variance inflation factors (all variables VIF < 1.5). Modelled bodyweight data at 80 weeks were additionally compared to validated, published growth charts for both male and female, intact and neutered cats, ²⁰ , ⁶² and weights were comparable to anticipated predictions when considering the evidence that neutered cats typically weigh more than intact cats (see Table S2 for values).

Model checking procedures are not as developed for CLMMs. However, we performed multiple checks to test the sensitivity of our data. First, we checked for multicollinearity and influential outliers by recreating the model the lmer function within the R package ‘lme4’, assuming that the BCS was a numeric term and applying a similar approach to LMER model checks. Second, we checked the sensitivity of the findings to the inclusion of BCS 2 data, given that this represented underweight cats, which may have a disproportionate impact on our findings. Finally, comparison of results with weight analysis provides an additional form of sensitivity analysis, with alignment in results providing further model validation. The most supported models were then plotted using ggplot2 (version 3.4.2). ⁶³

RESULTS

Demographics

Following data cleaning, a total of 2410 veterinary visit data points were retained, with BCS recorded at each visit and bodyweight recorded at 2073 visits. The total dataset included 584 individual cats residing with 466 owners. Forty‐six percent (n = 270) of the cats were female, and 80% (n = 468) were shorthaired. Twenty‐five percent (n = 144) were homed from a shelter environment and 12% (n = 68) were previously stray or feral, with the remaining falling into the ‘other’ category. Twenty‐one percent (n = 120) were neutered at 4 months or under, 36% (n = 209) at 5 months, 33% (n = 194) at 6 months and 9% (n = 52) between 7 and 12 months. The dataset contained between one and 12 BCSs for each cat, with 73% (n = 426) having at least two recorded BCSs, and the median being three.

During the 2410 veterinary visits included in the study, a BCS of 3 (ideal) was recorded at 67% of visits (n = 1608), with 85% (n = 498) of cats having at least one recorded BCS of 3. In contrast, at 30% of veterinary visits (n = 714), signs of the cats being above ideal body condition were reported. Specifically, 48% (n = 279) of cats had at least one recorded BCS of 4 (overweight), comprising 26% (n = 628) of all veterinary visits. Nine percent (n = 52) of cats had at least one recorded BCS of 5 (obese), making up 3% (n = 86) of all veterinary visits. Additionally, 13% (n = 76) of cats had a BCS of 2 (underweight) recorded, comprising 4% (n = 88) of veterinary visits (Table 1). The bodyweights of 528 cats were recorded, with between 1 and 10 measurements per cat, and 74% (n = 392) of cats having at least two weight measurements recorded. The mean bodyweight recorded across all veterinary visits was 4.73 kg. The full details of the attributes of cats with BCS and bodyweight recorded at veterinary visits are shown in Table 1.

TABLE 1.

Descriptive statistics of 584 individual cats with 2410 body condition score (BCS) measurements and 2073 bodyweight measurements recorded by veterinary professionals, and associated sample sizes of different characteristics

	BCS (n = 2410)			Bodyweight (n = 2073)
Variables	N	%	Mean (range)	N	%	Mean (range)
Response variables
BCS
2	88	4
3	1608	67
4	628	26
5	86	3
Bodyweight (kg)						4.73 (2.20‒11.20)
Time‐varying explanatory variables
Age (years)			5.40 (1.04‒12.89)			5.89 (1.04‒12.89)
Season of veterinary visit
Spring (March‒May)	410	17		353	17
Summer (June‒August)	648	27		533	26
Autumn (September‒November)	657	27		560	27
Winter (December‒February)	695	29		622	30
Fixed explanatory variables
Sex
Female	1065	44		927	45
Male	1338	56		1139	55
Hair length
Short	1950	85		1673	85
Long	335	15		295	15
Source
Stray or feral	269	11		222	11
Shelter or rescue	676	29		602	29
Other	1425	60		1215	60
Neuter age
4 months or under	503	21		452	22
5 months	822	34		697	34
6 months	845	35		722	35
7‒12 months	217	9		185	9

Body condition score

In the best supported model, season and its interaction with sex (χ ²(3) = 7.94, p = 0.04, ΔAIC = 2), age and its interaction with age at neutering (χ ²(2) = 158.14, p < 0.001, ΔAIC = 154) and hair length (χ ²(1) = 4.50, p = 0.03, ΔAIC = 2.5) were predictive of BCS, according to both significance testing and AIC model selection. These findings were supported both with and without BCS 2 included in the model, so full results, including those for BCS 2, are discussed here. For details of all tested variables and associated statistics, see Table S3.

The age at neutering was a significant factor. Where four age at neutering categories (Figure S1) were included in the model, the probability of being a certain BCS overlapped for cats neutered at 4 months or under, 5 months and 6 months. These four discrete age at neutering categories did not significantly improve model fit (χ ²(6) = 5.74, p = 0.45), with the simplified model with two age at neutering categories (6 months and under and 7‒12 months) also supported by AIC selection (ΔAIC = 6.3). Figure 1 highlights the most supported results in terms of the effect of age at neutering on BCS.

FIGURE 1.

Predicted effect of the interaction between age at neutering and the age of a cat on the probability of attaining a body condition score (BCS) of (a) 2, (b) 3, (c) 4 or (d) 5, calculated for male and female cats. The fitted line represents the effect from the cumulative link mixed‐effects models, assuming that the BCS was recorded in winter for shorthaired cats

BCS changed non‐linearly with age, increasing in earlier life and decreasing in later life; however, the rate of increase was significantly less for cats neutered between 7 and 12 months (Figure 1). Male and female cats had significantly different BCSs, and this was associated with season. Males had an increased probability of a higher BCS; however, females had a higher probability of having a decreased BCS in summer and autumn (Figure 2). Additionally, cats with shorter hair had an increased probability of having a higher BCS.

FIGURE 2.

Predicted effect of the interaction between sex and the season of measurement of body condition score (BCS) on the probability of attaining a BCS of (a) 2, (b) 3, (c) 4 and (d) 5 at different ages. The fitted line represents the effect from the cumulative link mixed‐effects models, assuming that cats were neutered before the age of 7 months and were shorthaired

Bodyweight

For predictors of bodyweight, season and its interaction with sex (χ ²(3) = 9.36, p = 0.02, ΔAIC = 3.3) and age and its interaction with age at neutering (χ ²(2) = 14.21, p < 0.001, ΔAIC = 10.3) were found to be important, according to both significance testing and AIC model selection. For details of all tested variables and associated statistics, see Tables S4 and S5.

The age at neutering was a significant factor. Modelling that included four age at neutering categories suggested this was driven by cats neutered between 7 and 12 months having a lower bodyweight. As with the BCS analysis, the modelled bodyweights of cats neutered at 4 months or under, 5 months and 6 months overlapped (Figure S2). Using four age at neuter categories did not significantly improve model fit (χ ²(6) = 8.47, p = 0.21), and the simplified model containing only two age at neutering categories (6 months and under and 7‐12 months) was also supported by AIC selection (ΔAIC = 3.5). Figure 3 highlights the most supported results in terms of the effect of age at neutering on bodyweight.

FIGURE 3.

Predicted effect of the interaction between (a) age at neutering and age on the bodyweight of male and female cats, assuming that bodyweight was recorded in winter and (b) sex and the season of measurement on the bodyweight of cats at different ages, assuming that cats were neutered before the age of 7 months. The fitted line represents the results from the linear mixed models. Raw bodyweight values are also shown for (a)

Age was a significant factor, following a non‐linear relationship whereby bodyweight increased in early life and decreased in later life. Cats neutered after 7 months weighed less and their bodyweight did not increase as much as that of cats neutered at 6 months and under. Additionally, male cats were significantly heavier than female cats, and bodyweight was affected by season, with females having lower bodyweight in summer and autumn (Figure 3).

DISCUSSION

In this large contemporary cohort, several principal risk factors governing the BCS and bodyweight of companion cats have been described. BCS and bodyweight were dependent on age, increasing up to around 8‒9 years of age and then decreasing. No appreciable difference between cats neutered at 4 months or under, 5 months or 6 months was found, with all three age groups following similar bodyweight trajectories throughout their lifetime. However, cats neutered between 7 and 12 months did not experience such a rapid rate of increase as those neutered at 6 months or under. Additionally, the results corroborated that sex is an important factor, along with its interaction with season. These findings were consistent across separate analyses of BCS and bodyweight. However, hair length was associated with BCS only, potentially due to the subjective nature of BCS measurements. ³⁸ , ⁴⁰ , ⁵¹

Concerns over the lack of long‐term clinical research on the risks and benefits of prepubertal neutering have been highlighted as a barrier to uptake by some veterinarians. ¹⁷ Addressing this, this longitudinal study found no difference in BCS or bodyweight between cats neutered at 4 months or under, 5 months or 6 months. These findings add to the growing body of literature suggesting cats neutered prepubertally are at no greater risk of developing obesity than those neutered at around 6 months old. ²⁰ , ²³ , ⁶⁴ Therefore, from a weight management perspective, proactive dietary management and other lifestyle approaches recommended within the veterinary literature ¹⁸ should be encouraged for all cats neutered at 6 months old or younger.

Differences were observed between cats neutered at 6 months old or younger and cats neutered between 7 and 12 months old, with cats neutered later experiencing a slower rate of growth in early life, and typically weighing less throughout adulthood. This result is comparable to a study that found cats neutered between 29 weeks (∼6.7 months) and 18.4 months old had reduced growth in early life following neutering compared to cats neutered prior to 29 weeks. ²⁰ While it is possible that neutering at an older age may be protective against excess weight gain later in life, it is not yet clear why cats neutered later may show size differences postneutering when compared to cats neutered before 7 months old. Changes postneutering have been shown to include increased fat and lean mass, as well as increased ribcage length and abdominal girth when compared to unneutered populations, ²⁰ potentially due to changes in growth hormones or satiety hormones and consequently food intake. However, an early study highlighted that neutering at around 4 months compared to 7 months may be advantageous for weight management by reducing the likelihood of acute changes in eating behaviour. ²² As decisions on age at neutering also need to account for the positive impact of prepubertal neutering on population control, ⁹ and thus subsequent welfare, of domestic cats, in addition to other individual benefits, the current balance of evidence supports prepubertal neutering, especially given that proactive dietary management can be used to maintain a healthy weight.

It is possible that age at neutering may be a proxy measure for the physiological state of an individual at the time of neutering, with potential key physiological differences present in cats aged 6 months and under compared to those aged 7‒12 months. As neutering norms typically result in most cats being neutered by 6 months old (approximately 90% within this study), neuters taking place between 7 and 12 months may have been performed out of necessity, such as cats being smaller initially or having health concerns. Some physiological confounders in relation to age at neutering were accounted for by removing cats reported to be or have been pregnant; however, further exploration of other health‐related factors, such as illness in early life, would require access to clinical records. The reverse may also be true, in that cats neutered earlier may have reached a greater weight sooner than others in the cohort. Both human ⁶⁵ and feline studies ⁴⁶ , ⁶⁶ suggest that bodyweight in early life may be predictive of bodyweight throughout adulthood, with excess juvenile weight reported as a risk factor for adult obesity. However, it must be considered that the underlying mechanisms for this are likely complex and include genetic and epigenetic factors, as well as resource availability. Future studies with a larger sample size for cats neutered at 7 months or older may choose to explore potential explanations for why cats neutered later than 6 months appear to weigh less throughout their lives, and whether this may have any detrimental impact as cats age into their senior years and typically lose weight. ³³ , ⁶⁷ , ⁶⁸

It should be noted that the accuracy of the owner‐reported date of birth for their cat and subsequently the calculated age at neutering and age at the time of each BCS and bodyweight measurement may not be exact due to potential recall errors. ⁶⁹ This was reduced as far as possible by using the first incidence of owner‐reported date of birth and date of neutering to increase recall accuracy or likelihood that the initial date provided would have been referenced against an appointment or calendar. Additionally, cats with an unknown date of birth were excluded from the study. Age at neutering was also grouped and analysed as a categorical variable, rather than a continuous numerical variable, to further reduce any impact of inaccurate reporting.

Age at time of record was a significant predictor of BCS and bodyweight, with BCS and bodyweight increasing from birth, peaking at around 8‒9 years, and then beginning to decrease. This finding agrees with other studies reporting decreasing bodyweight in older cats, ³³ , ⁶⁷ , ⁶⁸ which has been documented as starting at around 8 years of age. ⁷⁰ In some individuals, this decrease may be due to sarcopenia (i.e., the gradual loss of lean body mass that occurs due to age). While well documented in people, little literature currently exists around sarcopenia in cats; however, it has been reported to be seen in 37.6% of cats over 7 years of age. ⁷¹ These decreases in BCS or bodyweight in later life are thought to be the result of changes to metabolism ⁴⁴ and activity levels. ⁷² Decreased bodyweight in older cats may also be symptomatic of diseases such as hyperthyroidism ⁷³ or chronic kidney disease (CKD). ⁷⁴ While 8 years is younger than the average age of diagnosis for these diseases, it has been suggested that low levels of weight loss may be apparent 3 years before a diagnosis of CKD. ⁷⁵ Additional longitudinal studies following cats throughout their midlife and into their senior years and recording any incidences of disease may be useful in evaluating whether this weight decrease is predictive of disease.

Sex was found to be significant, whereby males generally weighed more than females at any given time point. It is likely that this weight difference is due to overall size differences between the sexes. This finding is in concordance with the literature, which suggests that sexual dimorphism is present in the species, with male cats being typically larger and heavier than females. ⁵⁵ , ⁶² , ⁶⁸ , ⁷⁶ , ⁷⁷ The BCS of male cats was also typically greater than that of female cats. Previous literature has shown inconsistency regarding BCS and associations with sex postneutering, with studies suggesting that both males ³⁸ and females ²⁰ may be at greater risk of higher BCS postneutering. The present study revealed that differences were seemingly driven by seasonal effects, with female cats displaying lower BCSs and bodyweights in the summer and autumn, while the BCS of male cats remained more consistent throughout all seasons. This interaction of sex and season may help to explain inconsistencies in the results of other studies that have not used longitudinal data to explore BCS or bodyweight trends through time or considered the effect of season.

Seasonal changes may be due to variation in food intake throughout the year. While the literature is inconsistent regarding reported levels of food intake and the correlation with season, likely due to factors such as variation in environment, age and lifestyle, some evidence has reported that cats consume more food during colder months, ⁷⁸ although this has not been shown to correlate with bodyweight changes where investigated. ⁷⁹ There is also evidence that this may be sex specific. Alegría‐Morán et al. ⁸⁰ found that female cats significantly increased their energy intake in colder months (autumn and winter), although the effect on bodyweight was not explored. A lag effect of this increased intake may explain the increased bodyweight during the winter and spring. Activity levels and energy expenditure in cats have also been found to vary seasonally, with increased activity levels sometimes demonstrated during summer months, when there is an increased photoperiod ⁸¹ and weather is more favourable. ⁸² Hunting may also vary seasonally due to differences in prey abundance and type ⁸³ , ⁸⁴ ; however, any impact on bodyweight is unclear and is likely to be complex due to changes to both energy expenditure and intake of prey and human‐provided food. Due to timing differences in the collection of owner‐reported versus veterinarian‐reported information, it was not possible to link bodyweight and BCS directly with parameters such as diet, outdoor access or multicat households, and as such, these data were not included as explanatory variables within the models. However, with improved understanding of potential lag effects of energy intake or expenditure on bodyweight and BCS, future models may better accommodate these temporally and individually variable data.

Hair length was the only variable not predictive of both BCS and bodyweight. Hair length was only predictive of BCS, potentially due to the subjectivity of BCS. Wolf and Drobatz ⁸⁵ reported that long hair in dogs was a risk factor for veterinary professionals overestimating bodyweight for lower‐weight dogs and underestimating bodyweight for higher‐weight dogs. In cats, long hair is a reported risk factor for being underweight, ³³ likely due to owners presuming their cat is larger than they are. In this study, cats with long hair were more likely to be given a lower BCS than shorthaired cats. As no bodyweight association was found, it is likely that this represents inaccurate BCS estimates due to the thicker layer of fur. This finding is in concordance with Courcier et al., ³⁸ who found that owners were more likely to underestimate the BCS of longhaired cats than shorthaired cats. Long hair may distort the perspective of cat body shapes, leading both owners and veterinary professionals to perceive the fur to be responsible for more of the cat's size than it is. These findings highlight the benefits of using both BCS and bodyweight in clinical assessments and when formulating appropriate recommendations on nutrition and bodyweight management for cats.

It should be noted that while the five‐point BCS system was used in this study, the nine‐point scale would have been more ideal if available. It is possible that veterinarians habitually round down when deciding between rankings on a BCS chart, leading to underreporting of cats being overweight or obese. This is somewhat evidenced by the number of veterinarians who provided BCSs that were not a whole number, that is, ‘3 to 4’ or ‘3.5’. Alongside being a validated measure of body fat mass, the nine‐point scale means veterinarians can be more nuanced in their grading, which may reduce reporting bias. No literature is currently available to give an indication as to the extent of this potential reporting bias; however, it is possible that BCS in this study is underestimated.

Overall, this study highlights that bodyweight gain is not greater for cats neutered prepubertally when compared to cats neutered at the historically recommended 6 months; however, the future risk of BCS and bodyweight increase was least in cats neutered between 7 and 12 months. Despite this, given the number of veterinary records that scored cats within this population as overweight or obese, veterinarians should endeavour to provide postoperative weight management advice to owners. Although BCS and bodyweight may be useful as general health indicators when triangulated with other metrics, this study cannot infer the welfare of any of the cat subpopulations. However, timely neutering is known to have beneficial impacts on population control, long‐term prevention of disease and the reduction of owner‐reported undesirable behaviours. This, coupled with the current lack of evidence of any long‐term negative impacts on health, provides a case for prepubertal neutering at the general population level as a positive cat welfare intervention.

In conclusion, this study has provided evidence of the influence of various exogenous and endogenous factors on BCS and bodyweight, including age, sex, seasonality and coat length. The practicalities of recording BCS in longhaired cats and the importance of season when assessing bodyweight changes for female cats in clinical practice have also been highlighted. The underlying mechanisms by which these factors influence BCS and bodyweight are largely unknown but are likely mediated by the diet and activity levels of individual cats. More research into the mechanisms by which BCS and bodyweight are impacted by exogenous and endogenous factors would be beneficial, along with the wider consideration of how these factors translate to welfare implications across a cat's lifetime.

AUTHOR CONTRIBUTIONS

Conceptualisation, data curation, methodology, writing—original draft preparation and writing—review and editing: Rae Foreman‐Worsley. Resources and writing—review and editing: Emily Blackwell. Conceptualisation and writing—review and editing: Lauren R. Finka and Elizabeth Skillings. Conceptualisation, formal analysis, methodology and writing—review and editing: Jenni L. McDonald.

CONFLICT OF INTEREST STATEMENT

The authors declare they have no conflicts of interest.

ETHICS STATEMENT

The Bristol Cats Study received ethical approval from the University of Bristol Animal Welfare and Ethical Review Body (reference UB/17/049).

Supporting information

Supporting Information

Foreman‐Worsley R, Blackwell E, Finka LR, Skillings E, McDonald JL. Long‐term effect of neutering age on body condition score and bodyweight in domestic cats. Vet Rec. 2025;e5433. 10.1002/vetr.5433

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.