CHANGES IN TESTICULAR WEIGHT AND VOLUME OF AMERICAN MINK (NEOVISON VISON) BEFORE AND DURING MATING SEASON

Bogdan Lasota¹, Agata Jarczyńska¹, Lidia Felska-Błaszczyk², Maria Laszczyńska³, Beata Seremak¹, Agata Skuratko^1
1 Department of Biotechnology of Reproduction and Environmental Hygiene, West Pomeranian University of Technology in Szczecin, Poland
² Department of Animal Anatomy, West Pomeranian University of Technology in Szczecin, Poland
³ Department of Histology and Developmental Biology, Pomeranian Medical University, Poland

ABSTRACT

The aim of this study was to compare the size of testes in male mink of two color varieties, before and during the breeding season, with an intention of a possible application of any existing differences as one of the criteria for selection of males for mating.

Weight and volume measurements were performed on 246 testes collected from 123 males of two color types: Mahogany and Standard Black (Black Velvet). Testes were collected before the breeding season (December 8) and just on the end of the season (March 31). Comparison of weight and volume of testes collected in December and March showed a considerable and statistically significant increase in both parameters. Testicular weight increased by about 1.87-fold in Mahogany and 1.47-fold in Black Velvet (from 1.94 to 3.64 g and from 2.36 to 3.48 g, respectively), while the increase in volume was over 12-fold in Mahogany and 9-fold in Black Velvet (from 0.27 to 3.48 cm³ and from 0.34 to 3.07 cm³, respectively). Before the season the weight of about 57% of testes remained in the range 1.5–2.0 g, and during the season about 63% of samples were in the range of 3.5 to 4.0 g. Juxtaposition of the average weight and volume of the right and left testes in different color types before and during the mating season has shown that there was a significant increase in testicular weight as well as volume for both color types, in both right and left testes.

Key words: mink, testicle size, season.

INTRODUCTION

In most mammals living in the temperate, subpolar, and polar zones, the length of the day is a major determinant of their chronobiology [16, 17]. Among the many mammalian species showing changes in testicular activity during the year, the mink–as a short-day mammal–has a unique position. They are characterized by resumption of the testicular activity and seasonal gonadotropin stimulation during the autumn shortening days [23]. The development of the testes is regulated by hormones, but the stimulation is initiated by changes in the length of daytime [12]. Their activity begins when the daytime lasts less than 10 hours [4, 17]; however, the peak activity of testes is observed when daytime gains in length again [10]. Seasonal regulation of testicular activity has not yet been fully understood [22].

The circannual cycle in the American mink can be divided in three main phases in terms of changes in testicular function. The first phase is the resumption of activity (October), followed by maximum activity (March in most Northern Hemisphere regions), and ending with the resting period in testicular activity (May–October) [3]. Males reach sexual maturity at about 10 months of age and at the same time they can take part in their first breeding season [12]. The initial development of the testicles is caused by the increase of circulating testosterone produced by the Leydig cells. Its concentration increases in October or November triggering growth of testes, which begin to produce sperm in the convoluted tubules [12]. The maximum concentration of testosterone was found in January, when also the development of testes stops [7, 12]; however, one may encounter in the literature that the highest concentration of testosterone was observed in February [5, 13] or at the end of January and beginning of February [2].

Relatively small changes were found in the mass and functioning of the testes in adult mink in the period from June to October, whereas until the end of October we did not observe any changes in their cytology [23]. Boissin-Agasse et al. [5] observed in their research most rapid increase in testicular volume between December and January. At the end of March a gradual regression of spermatogenic tissue in the testes begins. No relationship has been found between the body weight and testicular weight [23]. In the same age group a significant difference is possible in the size of the gonads, though this does not affect fertility or libido [12]. Onstad [21] found evidence of significantly greater mass of the left testis. The testes that during sexual latency are generally invisible, increase in volume 7–8-fold and become easy to find [15].

Production and quality of sperm can be tested using a range of techniques [12], which are, however, difficult to use and do not guarantee absolute precision [6]. The simplest method consists in checking the testes, which should not be smaller than a half of the standard size for a sexually mature male. Testes are at this period of a pea-seed size, weighing less than 1 gram, do not contain developed spermatozoa, and will not produce semen during the mating season.

The aim of this study was to compare testicular volume in American mink males (color types: Mahogany and Standard Black, also referred to as Black Velvet) before and during the mating season with an intention of a possible application of existing differences as one of the criteria for selection of males for mating.

MATERIALS AND METHODS

The study was carried out on a mink farm located in West Pomerania, Poland. The testicles were collected during the slaughter, conforming to the current ethical regulations, from 123 males of American mink (testes were collected pairwise, the left testis separately from the right one). A total of 123 pairs of testes were collected, which makes together 246 testes. From the Mahagony males, 30 pairs (30 right and 30 left testes) before the season, and 32 pairs (32 left and 32 right testes) in the season were collected. From Black Velvet males we collected 30 pairs of testes (30 left and 30 right) before the season, and 31 pairs (31 right and 31 left) in the season. We collected the samples on two following dates: 8 Dec., 2010 (before season), and 31 March, 2011 (in season), at age about 7 month and 11 months, respectively.

None of the studied males showed any symptoms of disease or anatomical abnormalities. The glands were weighed using a digital scale (RADWAG, WPS 1100/C/10), whereas the length, width, and thickness of the testicle were measured using a caliper. Both weighing and measurements were carried out on testicles including epididymes.

Testicular volume (TV) was calculated from the following formula of Setchell and Waites, cited by Boissin-Agasse [5]:

where L – length, W – width, T – thickness.

Statistical analysis
The resulting data was processed statistically using Statistica 9.0 package (StatSoft® Polska). Means and standard deviations were calculated for the weight and volume of testes. Significance of differences were tested with Tukey's HSD test for equal and unequal samples.

RESULTS AND DISCUSSION

The first step of the statistical analysis was aimed at changes in the mean mass and volume of testes between the first (December) and the second (March) sampling, without differenciating among the color types. The increase in both weight and volume was considerable and statistically significant (p ≤ 0.01, Tables 1 and 2). Testicular weight increased nearly 1.66-fold, whereas in volume the growth was more than 10-fold. There were no significant differences between the weight of the left and right testes, either before or in the season. However, the volume of the left testis in Black Velvet males during the season was significantly greater than that of the right one (p ≤ 0.01). Onstad [21] found evidence on significantly greater mass of the left testis; however, this was not confirmed in later studies [23].

Table 1. Comparison of the average testicle weight in both color types prior to and in the mating season, disaggregated by the left and right side (Mean±SD)

Side	Weight [g]
	Mahogany				Black Velvet
	n	before season	n	in season	n	before season	n	in season
Left	30	2.02±0.52	32	3.70±0.65	30	2.44±0.65	31	3.58±0.53
Right	30	1.87±0.51	32	3.58±0.61	30	2.28±0.69	31	3.37±0.48
Mean	60	1.94±0.51	64	3.64±0.63	60	2.36±0.67	62	3.48±0.51
Explanation: n – number of samples; SD – standard deviation

Table 2. Comparison of average volume of testicles in both color types prior to and during the mating season, disaggregated by the left and right side (Mean±SD)

Side	Volume [cm3]
	Mahogany				Black Velvet
	n	before season	n	in season	n	before season	n	in season
Left	30	0.30±0.15	32	3.49±0.78	30	0.36±0.20	31	3.56±0.58
Right	30	0.25±0.13	32	3.44±0.81	30	0.32±0.17	31	2.58±0.49
Mean	60	0.27±0.14	64	3.47±0.79	60	0.34±0.19	62	3.07±0.72
Explanation: n – number of samples; SD – standard deviation

Onstad [21] and Hemmingsen [11] obtained results very similar to those presented in Table 1; the weight of testes was approximately 1.94 g  or 2.36 (in Mahohgany or Black Velvet males respectively) at the beginning of December and about 3.64  or 3.48 g (in Mahohgany or Black Velvet males respectively) at the end of March. In a study conducted by Basrur and Ramos [1], testicular weight in December was 1.25 g, and about 2.25 g in early March, decreasing towards the end of the month to about 1.75 g. According to Hunter [12], a proper weight of a testis before the start of mating should be in the range l to 3.3 grams.

The month when the testes reach the maximum size is debatable. According to some authors, testes are largest in February [5, 18], and according to others – in March [3, 23]. Krzywoszyński [14] states that mink testes reach their normal size, about 2.6 g, 2–3 weeks prior to mating season . Also Filistowicz and Kuźniewicz [8] give similar weight (2.5 g) in the period from February to early April. However, according to Lisiecki and Sławoń [15], testicular weight during the mating season remains in the range 2.5–3.0 g. Blottner et al. [2] in their study noted that an average testicular weight in March was about 2.6 g. In subsequent studies, Blottner et al. [3] found that the maximum testicular weight was in March (2.7 g), but it was only slightly higher than that at the beginning of February. In the present study, the average weight of testes in late March was greater than that reported by these authors – 3.64 g and 3.38 g in Mahogany and Black Velvet, respectively. To trace exact changes in the size of testes during their development would require more frequent collection of testes (every 2–3 weeks), which would be difficult without disturbing the production cycle on the farm.

The mean weight of the right and left testes by different types prior to and in the season (Table 1) in both color types significantly increased, from December 8 to March 31. As for the average weight of testes among color types, there were small differences in weight observed prior to and during the season in favor of the left-side testis, which, however, were statistically non-significant and ranged from approximately 1.03-fold to 1.08-fold.

If we look at the testicular volume (Table 2), there was a statistically significant difference (p≤0,01) of almost 38% between the right (lower) and left (higher) testes only in Black Velvet males and in the sample collected during the mating season. For comparison, at the same time both testes in Mahogeny males differed in volume by only about 2% in favor of the left-side one. In other cases, the left testis also had slightly larger volume. There is no available literature discussing the issues, though.

Before the season, the mean testis weight in Black Velvet males was significantly higher (p ≤ 0.01) than in Mahogany males, whereas in the season the difference was minor. Similar studies were conducted by Gulevich and Klochkov [9], who analyzed testicular weights and volumes in two color types – Standard and Sapphire – in December and March. Their experiment revealed differences between the color types. The average weight of the testes in both types studied by these authors in December (0.77 and 0.95 g in Standard and Sapphire, respectively) was much lower than that shown in this paper. In turn, March's average weight of the testes was 3.64 g in Standard males and 4.29 g in Sapphires, which are closer to our results.

Comparison of average weight and volume of the testes in both color types before and in the season, broken down into the left and right side (Tables 1 and 2) showed that before the season Black Velvet males had significantly more massive testes. The left testis was nearly 21% heavier than in Mahogany (p ≤ 0.01), and the right one by about 22% heavier (p ≤ 0.05). Statistically significant differences (p ≤ 0.01) in volume were found only in the right testis in the season, by about 1.33-fold larger in Mahogany, while the left testis in the same period was larger in the Black Velvet type of color.

The following graph (Fig. 1) shows the distribution of testicular weights measured before the season

Fig. 1. Distribution of weight of 120 testes in 60 subjects prior to mating season and of 126 testes in 63 subjects during mating season

Before the season, the weight of about 57% of testes remained in the range 1.5–2.0 g, and during the season, about 63% of samples were in the range of 3.5 to 4.0 g.

We have not found data on this type of analysis in the available literature; however, Nieschlag and Bieniek [20] observed maximum testicular weight (5–6 g) in the mating season, in March. As shown in Figure 1, the testes weighing more than 5 g were found only in three samples.

CONCLUSIONS

The average testicular weight and volume in all of the studied animals was significantly lower before than during the mating season. Determining the size of testes by palpation may therefore be useful in the selection of males for mating. The small differences in the size of the right and left testes and between varieties, however, should not be the base element of decision in this examination.

REFERENCES

1. Basrur P.K., Ramos A.S., 1973. Seasonal changes in accessory sex glands and gonaducts of małe mink. Canadian Journal of Zoology, 51, 1125–1132.
2. Blottner S., Roelants H., Wagener A., Wenzel U.D., 1999. Testicular mitosis, meiosis and apoptosis in mink (Mustela vison) during breeding and non-breeding seasons. Animal Reproduction Science, 15, 237–249.
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14. Krzywoszyński W., 1983. Disorders of the reproductive organs of male foxes and mink. [Schorzenia narządów rozrodczych u samców lisów i norek.] Hodowca Drobnego Inwentarza, 2, 17–18 [in Polish].
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18. Messager S., Caillol M., George D., Martinet L., 1997. Seasonal variation of melatonin binding sites in the pars tuberalis of the małe mink (Mustela vison). Journal of Neuroendocrinology, 9, 523–528.
19. Messager S., Caillol M., Martinet L., 1999. Long-term exposure of hypothalamic explants to melatonin alters the release of gonadotrophin releasing hormone and the density of melatonin binding sites in the pars tubelaris of the małe mink (Mustela vison). Journal of Pineal Research, 26, 17–27.
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22. Persson S., 2007. The Mink (Mustela visori) as an indicator of environmental reproductive toxicity. Swedish University of Agricultural Sciences, Faculty of Veterinary Medicine and Animal Sciences. Veterinary Medicine Programm, Uppsala.
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Accepted for print: 23.09.2013

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Bogdan Lasota
Department of Biotechnology of Reproduction and Environmental Hygiene, West Pomeranian University of Technology in Szczecin, Poland
Judyma 6, 71-466 Szczecin, Poland
Phone +48 91 4496741
email: Bogdan.lasota@zut.edu.pl

Agata Jarczyńska
Department of Biotechnology of Reproduction and Environmental Hygiene, West Pomeranian University of Technology in Szczecin, Poland
Judyma 6, 71-466 Szczecin, Poland
Phone +48 91 4496742
email: agata-jarczynska@o2.pl

Lidia Felska-Błaszczyk
Department of Animal Anatomy, West Pomeranian University of Technology in Szczecin, Poland
Judyma 14, 71-466 Szczecin, Poland
Phone: +48 91 4496761
email: lidia.felska-blaszczyk@zut.edu.pl

Maria Laszczyńska
Department of Histology and Developmental Biology, Pomeranian Medical University, Poland
Żołnierska 48, 71-210 Szczecin, Poland
Phone: +48 91 4800908
email: laszcz@sci.pam.szczecin.pl

Beata Seremak
Department of Biotechnology of Reproduction and Environmental Hygiene, West Pomeranian University of Technology in Szczecin, Poland
Judyma 6, 71-466 Szczecin, Poland
Phone: +48 91 4496742
email: beata.seremak@zut.edu.pl

Agata Skuratko
Department of Biotechnology of Reproduction and Environmental Hygiene, West Pomeranian University of Technology in Szczecin, Poland
Judyma 6, 71-466 Szczecin, Poland
Phone: +48 91 4496742
email: agata.skuratko@gmail.com

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Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.

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