Litter Size Variation in Polish Selected Small Dog Breeds

Abstract

In breeders’ general opinion small breed females produce less numerous litters. The aim of the study was to analyse the litter size and the frequency of the gender ratio in selected small dog breeds in view of their popularity in Poland. The data set comprised information on 639 litters (in total 2578 puppies) of eight breeds, which were born between January 2003 and end December 2014. The results were statistically analysed using statistical program SPSS 20.0. Medium-size litters were observed in the analysed small dog breeds (4.034±0.1). Comparison of the selected breeds of the Fédération Cynologique Internationale (FCI) Groups showed that the mean litter size in Group IX was higher (4.36±0.08) than that in Group III (3.87±0.14) and the differences were statistically significant. The study has confirmed the hypothesis that larger females produce more numerous litters, but there are large intra-individual variations in the number of pups born in individual breeds. Additionally, the gender ratio in the puppies born in the analysed breeds was equal, despite the fluctuations in the individual breeds.

KeyWords:

• Canine
• Reproduction
• Litter size
• Prolificacy
• Small dog breeds

Introduction

The number of dogs kept by inhabitants of cities as companion animals is increasing. Small breeds are best suited for this role. Besides their small size, small breeds often exhibit a peaceful nature and are not troublesome in everyday life even for inexperienced owners. Owing to their small size, these dogs are willingly kept by families with children and by the elderly. Small dog breeds comprise dogs with highly energetic temperament, requiring a certain dose of activity (e.g., small terriers), which are perfect for active people, as well as dogs with a very balanced temperament, preferring peaceful walks (companion and toy breeds). The term litter size is commonly used in dog breeding. In animal husbandry and in farm livestock breeding, scientific terms: prolificacy and fertility are used. According to the Lexicon of scientific terms from genetics and animal breeding, prolificacy is the number of offspring born by a female: in a single pregnancy, in a year, or in a lifetime. The concept of prolificacy refers only to females. This feature is specific to each species, e.g. usually a single descendant is born from one pregnancy in horses and cattle, while the litter of pigs consists of 8-12 offspring. Fertility is the tendency of animals to reproduce. Male fertility is expressed in the production of sperm capable of fertilization of the ovum, performing the sexual act and ejaculation. Female fertility is manifested by the ability to produce ova, conceive, to carry the pregnancy, and deliver and raise the offspring (Nowicki et al., 1994). Analysis of these definitions indicates that the term prolificacy is analogous to the term litter size. In terms of reproductive physiology in dogs, a female is able to bear from 1 to up to 17 puppies twice a year (Borge et al., 2011; Concannon, 2011). In purebred dogs, reproduction of bitches is limited by the Breeding Rules of the Polish Kennel Club to one litter a year (regardless of the number of puppies born in a single litter). In the general opinion expressed by breeders and veterinarians, small breed females produce less numerous litters (1-2 pups), which in turn affects the economic aspect of breeding. The aim of the study was to analyse the litter size and the frequency of the gender ratio in selected small dog breeds.

Materials and methods

The study material contained data on reproduction of small dog breeds derived from the breeding documentation of the Lublin Branch of the Polish Kennel Club and observations made in chosen breeding farms. Breeds in which adult body weight reaches up to 10 kg and the height at the withers up to 35 cm were classified as small breeds. These measurements are exactly defined by breed standards. Breeds from two FCI Groups (Fédération Cynologique Internationale, www.fci.be) were taken into account. From Group III: Terriers, we chose: Cairn Terrier (body weight according to the FCI standard: 6-7.5 kg, the height at the withers: 28-31 cm), Jack Russell Terrier (5-6 kg; 25-30 cm), West Highland White Terrier (6-8 kg; about 28 cm), and Yorkshire Terrier (up to 3.2 kg; 22-24 cm) were chosen. From Group IX: Companion and Toy Dogs we chose: French Bulldog (8-14 kg; 25-35 cm), Cavalier King Charles Spaniel (5-8 kg; 25-32 cm), Havanese (3.5-6 kg; 23-27 cm), and Shi Tzu (4.5-8 kg; to 27 cm) (fci.be) were selected. The criterion for the choice of the breeds was their popularity and delivery of at least 12 litters during the investigation period. Hence, the French Bulldog was included in the research group. French Bulldogs are classified as small breeds due to their low height, despite the fact that the maximum body weight of males can reach up to 14 kg. The selected breeds are the most numerous in the Lublin Branch of the Polish Kennel Club. The data for the Havanese and Jack Russell Terrier breeds were combined with observations carried out in the chosen breeding farms, registered in other Branches of the Polish Kennel Club. The data set comprised information on 639 litters (in total 2578 puppies) of eight breeds that were born between January 2003 and the end of December 2014. In the study, the number of puppies born live and reared up to 7 weeks of age was regarded as a litter size due to incomplete data available about stillborn puppies and mortality rates from birth to the litter audit, during which the breeder has to present puppies between 6 and 12 weeks of age (Regulamin Hodowli Psów Rasowych, 2014).

The results were statistically analysed using statistical program SPSS 20.0. The frequency of the gender ratio in the born puppies was analysed. The significance of differences between average litter sizes in individual breeds was calculated using one-way analysis of variance (ANOVA) and Tukey’s post-hoc HSD test. In order to compare the average litter size in two FCI Groups (III and IX), Student’s t- test was used. Levene’s test was used to validate the hypothesis of the homogeneity of variance,.

Results and discussion

The number of dogs registered in the Polish Pedigree Book of the Polish Kennel Club and the number of stud dogs in our country prove the popularity of a breed. These data are updated at the beginning of each year. To serve as a breeding animal, the dog must fulfil specific breeding requirements. Therefore, stud dogs in each breed represent around 40-50% of the population of registered dogs.

In female dogs, the litter size depends on the breed and dog size, with a tendency to have smaller litters by small breeds (Johnson, 2008; Scantlebury, 2000). Analysis of the average litter size in the eight examined breeds revealed the largest average number of pups per litter in the Cairn Terrier breed (6.61±0.4) and the lowest in the West Highland White Terrier (3.5±0.4) and the Yorkshire Terrier (3.57±0.1). These differences were statistically significant [F (7; 632) = 16.858; P<0.01; η²= 0.157 (Table 1). The largest variations were noted in the average litter size in the breeds of the same FCI Group (Terriers). Comparison of the selected breeds of the FCI Groups performed with Levene’s test showed that the mean litter size in Group IX (4.36±0.08) was higher than that in Group III (3.87±0.14) and the differences were statistically significant [t (637)=2.935; P<0.05].

Due to the absence of data on mothers’ weight in each breed, it was not possible to examine the statistical correlation between the litter size and female body weight. However, taking into account the maximum body weight specified in the breed standards of the analysed breeds, dogs in FCI Group III scored an average body weight of 6.2 kg and those in Group IX - 9 kg. This confirms the hypothesis that there is a relationship between mother’s body weight and the litter size.

Analysis of the pup gender ratio in the selected breeds showed the highest frequency of males (55.5%) in Cairn Terriers; similar frequency was reported in West Highland White Terriers (53.57%), Jack Russell Terriers (52.42%), and Shi Tzu (52.26%). Almost equal gender proportions were noted in French Bulldogs (50.93%). In turn, in the Yorkshire Terriers, Havanese, and Cavalier King Charles Spaniels, the male frequency was lower than 50% (48.11%, 47.22%, and 42.26%, respectively). However, taking into account all the analyzed small breeds, the frequency of born males was 50.04%.

In investigations carried out by Okkens et al. (1993) on five large breeds, the mean litter size was 6.8 (± 0.4). Similar results were obtained by Feldman and Nelson (2007), who recorded an average of 8 to 10 pups per litter in large breeds. In the study of reproduction parameters in Dogo Argentino (Caffaratti et al., 2013), the average litter size was 8.12 (SD±3.44). In medium-sized dog breeds, e.g., Nederlandse Kooikerhondje the mean litter size was 5.62 (SD±2.08) (Mandigers et al., 1994) and in the Beagle breed, the average litter size reached 5.7 (SD ±1.9) (Seki, 2010) and 5.5 (Shimatsu et al., 2007). The studies conducted by Borge et al. (2011) on 10810 litters of different-size dogs showed an increase in the litter size with the size of the breed; the average number of pups was 3.5 (±0.04) per litter in miniature breeds (body weight less than 5 kg) and 4.2 (±0.03) in small breeds (body weight 5-10 kg).

Noteworthy is the French Bulldog breed. It is one of the brachycephalic breeds, in which there are problems with natural birth (McGreevy and Nicholas, 1999; Wydooghe et al., 2013). In the present study, the litters of this breed were quite numerous (average 4.17), although the number was highly differed between mothers with a minimum of 1 puppy in the litter to a maximum of 10 puppies. In the study conducted by Wydooghe et al. (2013) concerning reproduction of the English Bulldog the average litter size was six pups but natural delivery was allowed only in 5.1% of females. Two breeds of FCI Group IX, i.e., the Shi Tzu and Cavalier King Charles Spaniel, have a partially brachycephalic and mezocephalic skull structure. The average litter size in the Cavalier King Charles Spaniel breed was 3.74 (±0.4) with a maximum of 7 whereas in the Shi Tzu breed it was 4.57 (±0.2) with a maximum of 8 puppies per litter (in 27 of the 78 analysed litters, there were 6 or more puppies) (Table 1). These results were slightly different from those obtained by Borge et al. (2011), where the highest average litter size was noted in the French Bulldog (4.7±0.3), then in the Jack Russell Terrier and in the Cairn Terrier (4.4±0.1) breeds. On average, values of more than four puppies per litter were noted in the Havanese (4.3±0.2), Shi Tzu (4.2±0.2), and Cavalier King Charles Spaniel (4.1±0.1) breeds, and less than four puppies per litter was reported in West Highland White Terriers (3.7±0.3) and Yorkshire Terriers (3.5±0.3) (Borge et al., 2011). Kelley (2002) showed that the litter size in the Yorkshire Terrier ranged from 2 to 5 pups, with 4 puppies born in 80% of the litters. In the present study, the litter size in the Yorkshire Terriers was much more varied: eight was the maximum number of pups per litter; there were 6 or more puppies in 9.78% litters, 5 pups in 18.2%,, and 4 pups in 21.7% of litters.

Comparison of the prolificacy in the two FCI Groups of small breeds (III and IX) confirms the hypothesis that there is a relationship between mother’s body weight and the litter size. This is consistent with the results obtained by other researchers (Kelley, 2002; Feldman and Nelson, 2007; Johnson, 2008; Borge et al., 2011; Caffaratti et al., 2013). Among born pups, the frequency of males in our study was 50.04% and this was consistent with the studies conducted by Tedor and Reif (1978) on different dog breeds, where 50.6% of born males were reported. Similarly, the gender proportions in Dogo Argentino puppies reached 50.8% of born males (Caffaratti et al., 2013) and 50.5% in the Drever breed (Bobic Gavrilovic et al., 2008).

Table 1. Number of born litters and raised puppies in the selected small dog breeds in 2003-2014.

FCI group	Dog breed	n	Male	Female	Number of litters	M	SE	Range
III	Cairn Terrier	218	121	97	33	6.61^Footnotec	0.4	1-11
III	Jack Russell Terrier	269	141	128	66	4.08^Footnotea	0.2	1-7
III	West Highland White Terrier	56	30	26	16	3.5^Footnotea	0.4	1-6
III	Yorkshire Terrier	1137	547	590	318	3.58^Footnotea	0.1	1-8
IX	French Bulldog	375	191	184	90	4.17^Footnotea	0.2	1-10
IX	Cavalier King Charles Spaniel	97	41	56	25	3.88^Footnotea	0.4	1-7
IX	Havanese	72	34	38	13	5.54^Footnotebc	0.4	3-8
IX	Shi Tzu	354	185	169	78	4.54^Footnoteab	0.2	1-8
	Total	2578	1290	1288	639	4.034	0.1	1-11

FCI, Fédération Cynologique Internationale.

^a,b,cDifferent letters denote significant differences in pairs (Tukey’s post-hoc test); groups marked with different letters differ significantly (P≥0.05).

Conclusions

The study has confirmed the hypothesis that larger females produce more numerous litters, but there are large intra-individual variations in the number of pups born in individual breeds.

Medium-size litters were observed in the analysed small dog breeds, which undermines the general opinion of the poor economic aspect of breeding thereof. Additionally, the gender ratio in puppies born in the analysed breeds was equal, despite the fluctuations in the individual breeds. The Cairn Terrier exhibited highest prolificacy. The highest average litter size and the highest maximum number of pups were reported in this breed (in 11 litters, 8 or more pups were born, including 11 pups born in two litters).