Abstract
The size of urban cat colonies is limited only by the availability of food and shelter; therefore, their population growth challenges all known population control programs. To test a new population control method, a free-roaming feral cat colony at the Zoological Park in the city of Rio de Janeiro was studied, beginning in 2001. The novel method consisted of performing a hysterectomy on all captured female cats over 6 months of age. To estimate the size of the colony and compare population from year to year, a method of capture-mark-release-recapture was used. The aim was to capture as many individuals as possible, including cats of all ages and gender to estimate numbers of cats in all population categories. Results indicated that the feral cat population remained constant from 2001 to 2004. From 2004 to 2008, the hysterectomy program and population estimates were performed every other year (2006 and 2008). The population was estimated to be 40 cats in 2004, 26 in 2006, and 17 cats in 2008. Although pathogens tend to infect more individuals as the population grows older and maintains natural behavior, these results show that free-roaming feral cat colonies could have their population controlled by a biannual program that focuses on hysterectomy of sexually active female cats.
Although humans and cats are known to have shared space and resources for at least 9000 years, 1 feline urban populations are recognized as public health hazards. 2−4 Therefore, many efforts are spent to prevent or control feline population by surgical methods, 3,5−7 removal, and culling. 8−10
Although previous methods of controlling feral cat populations, including neutering of both females and males have been inefficient over the long term in many places, 3,5−7,11−13 in the United States castrating and spaying have been shown to control the number of feral cats in a closed environment. 14 In practical applications, trap-neuter-return (TNR) methods have been used with some degree of success by animal protectionists who developed independent programs with the cooperation of their local veterinarians, 14−18 and it has been suggested that those programs must be matched with an effective educational campaign directed to citizens to reduce the risk of owned-cat abandonment in order to reduce immigration. 19
When neutering techniques which preserve the hormone production are used to control a colony population growth it has been hypothesized that the immigration rate was low, especially when compared to gonadectomy neutering techniques. 9,10 However, in the long-term there may be significant changes in the structural composition of the colony as a consequence of the increased number of annual estrus and matings, which will result in disputes among adult males. 20−23 These wrangles may not only hurt the cats but also cause nuisance to humans, especially because cats are nocturnal and noisy when fighting or mating.
Although results only allow different authors to hypothesize, many have speculated that capture and culling as well as TNR and adoption methods initially reduced resident populations in many cases, but other feral cats could join the colony. Failure observed in some neutering programs was attributed to be most likely the result of the hormone influence on feline social behavior; once neutered, male cats tend to be benevolent and share their territory, shelter, and food with immigrating cats. This may eventually result in the neutered cat being banished by intact immigrant males. 9,10 Therefore, neither neutering nor culling seems to control migration into the colony site, possibly resulting in repopulation of the colony. 13
It is known that neutering serves to enhance the welfare of animals once it reduces birth and death rates. 18,20−24 When female cats are hysterectomized, the social structure of the colony tends to be retained, even though reproduction is halted, resulting in maintenance of the resistance offered to intruders hoping to share local resources. 25
Therefore, considering that: (1) domestic feral cats can behave and present population dynamics characteristics similar to the wild felids, which are well studied models; 26,27 (2) the reproductive rate in a polygamic species with superposed cycles is defined by the number of active reproductive females; 28 (3) natural selection favors those individuals which contribute more to the future of their population; 29 (4) the reproductive rate of mammals increases with age when they are young and decays later during life span; 30 (5) that young individuals present low chances of survival up to reproductive age; 31 (6) young females first litters present low survival rates 22,30 and; (7) that in matrilinear societies sexually mature females inhibit the mating chances of young females, 32 the present model was elected mainly because hysterectomy determines reduction of the intrinsic rate of natural growth of the population in two ways. First, it preserves the dominant female sexual behavior and second, halts the recruitment.
The first study of hysterectomy of adults as a method of population control in a free-roaming cat colony was conducted from 2001 to 2004. 25 Results of that study suggested that the program could have beneficial results if cats are captured and females are hysterectomized on a biannual basis. 25 Therefore, the present study followed the same colony biannually, performing hysterectomy and estimating the population at each capture.
Materials and methods
Free-roaming feral domestic cats from the Rio de Janeiro Zoological Park, Brazil (23°54′S and 43°13′W) during 2006 and 2008 were included. The zoo occupies an area of 13.8 hectares and is surrounded by a slum (west), a park (east and north) and by a penitentiary (south). All animals captured that were previously known from the study conducted from 2001 to 2004 were called inhabitants 25 and the unknown cats, immigrants.
Population size and structure of the feral feline colony were estimated according to techniques developed for wild animal studies, as previously described. 25 The feline population was estimated by a method described by Lincoln–Petersen in 1930, which was modified by Chapman. 33 This method consists of marking all animals caught in a first capture and releasing them back into their population. In the second capture (recapture), the number of marked and unmarked (new) individuals is counted and recorded. The method is based on two combined and consecutive samplings. 34,35 The upper and lower limits of the 95% confidence interval were calculated following the binomial method. 35
In 2006 and 2008, cats were trapped, microchipped, ear-tipped, released, and recaptured. Trapping was always performed from June to September, using the same methodology previously described. 25 After sedation and anesthesia, hysterectomy was performed in young–adult or adult females (≥6 months of age), as previously described. 25 Only these females were submitted to surgery because most litters die before achieving reproductive age. 22,31,36 All trapped animals were released in the same place as they had been trapped. All males, young females (<6 months), and previously hysterectomized females were released immediately after identification and vaccination (rabies and polyvalent vaccine for panleukopenia, calicivirosis, rhinotracheitis and chlamydiosis). Females that were hysterectomized at the time of capture were held for 24 h.
Actual and estimated cat numbers were compared to evaluate the population. The real number (not estimated) of trapped animals by gender and age range was used to evaluate the impact of hysterectomy on the population.
Results
In 2006 and 2008, 27 individuals were captured. Three cats were captured both years (one tom cat; and two adult females – one hysterectomized in 2004 and the other hysterectomized in 2006). Seventeen individuals were trapped in 2006, including three queens (all immigrants) that underwent hysterectomy. Kittens comprised 6% of the captured cats in 2006. Seven of the cats captured in 2006 had previously been captured and had been submitted to hysterectomy before (2001–2004, when a total of 97 cats were captured — 11 female kittens; five male kittens; 45 adult females and 36 adult males). The total cat population in 2006 was estimated to be 26.
Ten adults were trapped in 2008, including five immigrant queens that were hysterectomized that year. The population estimate for 2008 was 17 cats. The structure of the population was relatively stable during both capture years, with a ratio of three males to five females in 2006 and approximately two males to three females in 2008. The number of new kittens was reduced from one in 2006 to zero in 2008 (Table 1; Fig 1).
Distribution of domestic cats (Felis catus L 1758) living in the Zoological Park of Rio de Janeiro by age, gender, and population status at capture.
Est=estimated number; Imm=number of immigrant cats; Res=number of resident cats.
Structure of the estimated domestic cat (Felis catus L 1758) population living in the Zoological Park of Rio de Janeiro, Brazil, from 2001 to 2008. Results of 2001, 2002, 2003 and 2004 were published in Mendes-de-Almeida et al. The impact of hysterectomy in an urban colony of domestic cats (Felis catus Linnaeus 1758). Int J Applied Res Vet Med 2006;
Immigrant adult females represented 30% (3/10) of the cats captured in 2006 and 71% (5/7) in 2008.
Discussion
Hysterectomy of adult female resident cats apparently reduced the number of kittens born; less kittens could be captured as the program was maintained. When the colony started to decrease to lower than what seemed to be the equilibrium numbers (the minimum number of individuals that avoided immigration), five adult females joined the colony. Neither one of these immigrant females gave birth, suggesting that even before hysterectomy, their reproductive activity might have been suppressed due to interference of dominant autochthonous females, as commonly observed in wild felines. 18,37
There is a gradual decline in the estimated colony population in 2006 (26 cats) and 2008 (17 cats) compared with the initial number found before the first intervention in 2001 (59 cats). 28 When the growth of the population is compared among all years, the number of births fell dramatically in 2006 and 2008 from that recorded in 2001. 25 In 2001, eight kittens were captured, and in 2002–2004 a maximum of four was captured by year (range=1–4). In 2006, only one kitten was captured, and no kittens were captured in 2008. The estimated numbers are different from the captured numbers because the cats were all feral and very suspicious of humans. That is the reason why the chosen technique, which can in all cases be influenced by cats’ capacity to learn, was one largely used to estimate the population of wild felids in the nature.
It is possible that missing cats may have moved away from the colony, died of diseases or accidents. 18 In the present study, the wild residents of the zoo may have caught cats straying in their cages.
Therefore, as suggested before, a biannual program to submit females to hysterectomy can be recommended as an effective and feasible method for controlling free-roaming feral cat colonies in areas where nuisance due to the noise produced by cats mating and fighting is not a limiting factor. 25 Furthermore, it is important to consider that when hormones are intact, animals will fight, mate and also gather in groups which in the long term will enhance transmission of pathogens.
Conclusion
A biannual program of feline population control by submitting sexually mature females to hysterectomy restricted growth of the free-roaming feral cat colony. This method reduced the number of kittens born in the colony and decreased the immigration of other feral cats into the colony while the equilibrium numbers seemed to be maintained.
Conflict of interest
Flavya Mendes-de-Almeida worked for Fundação RIOZOO for 7 years and received microchips from Avid for this study. Norma Labarthe has served as expert testimony for Idexx Laboratories, Merial Saúde Animal, Novartis Saúde Animal and Pfizer Saúde Animal.
Footnotes
Acknowledgements
The authors thank Kathleen Newcomb for brilliantly reviewing the manuscript and Paulo Barata for helping interpretation of data. The present study had no funding.