Testosterone production by the fetal testes of silver foxes selected for their domesticated behavior]

Testosterone content was determined by radioimmune assay in the testes of silver foxes between the 31st and 50th days of gestation. Small quantities of testosterone were found in the testes already at the 31st day of prenatal life, gradually increasing up to a maximum value at the 50th day. No significant difference in testosterone content was found between domesticated and undomesticated silver foxes during prenatal life. It is suggested that selection for domesticated behaviour affects rather central control of endocrine functions than steroid biosynthesis in the testes.     

Relative hypertrophy of the right ventricle in silver foxes selected for domestication behavior

Silver foxes selected for domestication behavior were found to have relative hypertrophy of the right heart ventricle, which was 21% as enlarged in males and 18% as enlarged in females as compared with non-domesticated animals. It was established by stereological methods that hypertrophy occurs mainly at the expense of an increase in the absolute and relative content of myofibrils in cardiomyocytes, with the absolute total volume of the mitochondria being equal both in domesticated and non-domesticated animals. It was shown by means of dissociated cell counts that in both animal groups, the absolute number of cardiomyocytes and their nuclei in the right ventricles is approximately similar. It is suggested that there is a relationship between right heart ventricle hypertrophy in domesticated foxes and variation in the balance of sympathetic and parasympathetic effects.     

Reproductive potential of male silver foxes Vulpes vulpes after long selection for the domesticated behavior type

Age peculiarities of reproductive activity and fertility during the reproductive season as well as the hormonal and the testis spermatogenic function were studied in silver foxes Vulpes vulpes from the population selected for a long time for domesticated behavior. It has been established that during the first reproductive season the male sexually activity (the coupled females’ number) was reduced as compared with subsequent seasons, and about one third of the males were not coupled at all with females; however, the young (one-year old) males did not differ from adults by the testis weight, ejaculate spermatozoid number, and the fraction of abnormal spermatozoids. Comparison of indexes of spermatogenesis and the testicular hormonal function in the young males with a decreased and “normal” sexual activity did not reveal any statistically significant differences. It is suggested that the decreased (or absent) sexual activity in young silver fox males can be a response of the population to the human-created environment.     

Interhemisphere neurochemical differences in the brain of silver foxes selected for behavior and the problem of directed asymmetry

The study deals with the mechanisms that bring about a directional asymmetry in the expression of some morphological traits observed in some animals subjected to experimental domestication. The key role in the integration of development is attributed to the genetic systems controlling the activity of brain neurotransmitter systems. Therefore, the investigation of directional asymmetry of morphological traits began with the analysis of interhemispheral differences in neurotransmitter activity in animal lines selected for domestic and aggressive behavior. Experiments on silver foxes reveal interhemispheral differences in the dopaminergic system emerging in the striatum. An increased dopamine level is observed in the right half of the striatum of aggressive foxes and in both right and left halves of the striata of domestic foxes. On the basis of the literature data, it is suggested that the considerable increase in the dopamine level in the right halves of the striata of both aggressive and domestic animals is related to a genetic increase in the manifestation of emotional response in both lines, whereas its increase in the left half of the striatum of domesticated foxes may be related to a correlated deterioration of the function of the pituitary-adrenal system.     

Expression of the DNA methyltransferase genes in silver foxes experimentally selected for domestication

Animal domestication is a model of a rapid evolutionary process. In experimental fox domestication, the time required for the emergence and fixation of specific evolutionary changes was reduced from thousands to tens of years, compared to historical domestication. Tame foxes were obtained by extreme selective breeding for emotionally positive response toward humans. Unselected foxes, as well as foxes bred for enhanced aggressiveness to humans were used as control. Epigenetic regulation of gene expression is considered as one of the possible mechanisms of rapid evolution. In this study, expression of DNA methyltransferase genes, DNMT1 and DNMT3A, was investigated. It was demonstrated that, in tame foxes, the level of DNMT3A gene expression in the prefrontal cortex, hippocampus, and spleen was increased by more than 2 times in comparison with aggressive and unselected foxes. At the same time, the DNMT1 expression level did not differ among the studied groups of animals. A possible reason for the differences found in the DNMT3A expression could be the changes in the level and metabolism of methionine, which serves as a donor of methyl groups during DNA methylation. However, this study showed that there were no differences in the serum methionine levels between tame, unselected, and aggressive foxes. Thus, the data support the hypothesis that selection for positive emotional response toward humans affected the DNA methylation machinery.     

Role of the amygdala in the behavior of domesticated and aggressive foxes

Influence was studied of the bilateral electric coagulation of the basolateral part of the amygdala on the behaviour in unknown environment and on the attitude to the man of domesticated and aggressive foxes. Increase of exploratory behaviour with enhancing of motor activity was observed in all foxes after amygdala ablation. At the same time a decrease of positive emotional reactions to man was observed in the domesticated animals, while in the aggressive foxes defensive reactions to man and in unknown environment were weakened.     

Vocalization toward conspecifics in silver foxes (Vulpes vulpes) selected for tame or aggressive behavior toward humans

We examined the production of different vocalizations in three strains of silver fox (unselected, aggressive, and tame) attending three kinds of behavior (aggressive, affiliative, and neutral) in response to their same-strain conspecifics. This is a follow-up to previous experiments which demonstrated that in the presence of humans, tame foxes produced cackles and pants but never coughed or snorted, whilst aggressive foxes produced coughs and snorts but never cackled or panted. Thus, cackle/pant and cough/snort were indicative of the tame and aggressive fox strains respectively toward humans. Wild-type unselected foxes produced cough and snort toward humans similarly to aggressive foxes. Here, we found that vocal responses to conspecifics were similar in tame, aggressive and unselected fox strains. Both cackle/pant and cough/snort occurred in foxes of all strains. The difference in the use of cackle/pant and cough/snort among these strains toward humans and toward conspecifics suggest that silver foxes do not perceive humans as their conspecifics. We speculate that these vocalizations are produced in response to a triggering internal state, affiliative or aggressive, that is suppressed by default in these fox strains toward humans as a result of their strict selection for tame or aggressive behavior, whilst still remaining flexible toward conspecifics.     

Morphofunctional state of hypothalamic and epiphyseal structures in relatively wild and domesticated silver foxes during estrus

Histochemical and cytometric analyses of the hypothalamo-hypophysial neurosecretory system (HHNS), suprachiasmatic and arcuate nuclei, as well as the pineal gland in mature female silver foxes during oestrus revealed activation of peptidergic and monoaminergic neurosecretory units alongside with activation of the glial and vascular components. The increase of the secretory activity of the HHNS and pineal gland was more evident in domesticated animals. The data obtained show more intense production and excretion of neurohormones which are involved in regulation of reproduction, in domesticated foxes. These peculiarities may account for changes in the oestral cycle in domesticated animals.     

Role of corticosteroids in the ontogenetic development of behavior in silver foxes

In domesticated at the age from 30-th to 60-th days foxes an intensification of orienting-investigating behaviour takes place in unknown situation. Cortisol level is not changed. In foxes, selected for aggressive behaviour towards man, at 30 days age the orienting-investigating behaviour is also observed, and hormone concentration does not differ from that in domesticated fox-cubs. In aggressive cubs of 45 days a defensive behaviour develops and cortisol content in blood significantly increases. Participation is discussed of adrenal cortex hormones in manifestation of defensive reactions and mechanisms of restriction of sensitive period of socialization.     

Serotonin receptors in the brain of animals selected for their domesticated type of behavior

Participation was studied of central serotonin receptors of the first and second types in behaviour change of animals selected by the character of defensive reaction to man. Serotonin receptors were determined by radioligand method by binding of the brain preparations 3H-serotonin and 3H-spiperone. An increase of C2 receptors number was found in the frontal brain cortex of the tame brown rats in comparison with the aggressive ones. Differences were not found in specific C1-receptor binding in the frontal brain cortex of tame and aggressive brown rats, silver foxes and American minks in various relatively early selection stages. It is supposed that disappearance of aggressive reaction to man at domestication is connected with an increase of C2 receptors number.     

Gonadal and adrenal endocrine functions in female minks of 2 genotypes in postnatal ontogeny

Radioimmune assay has been made of the content of estradiol and progesterone in the blood (from July to March including the 7th day after mating), as well as on the level of estradiol and progesterone production in young females of the standard and mutant sapphire minks in November. It was shown that within certain periods, estradiol and progesterone content of the blood was significantly higher in the standard animals. Gonadal production of estradiol, as well as progesterone production both by the gonads and adrenals in November, were similar in females of both genotypes. It is suggested that sapphire minks have another pattern of correlation between estradiol content of the blood and gonadal production of estradiol as compared to that in standard animals.     

The role of brain noradrenaline in regulating the endocrine function of the gonads in male brown rats selected for behavior

In two groups of gray rats--nonaggressive ones, selected by the lack of aggression towards investigator, and aggressive animals--studies have been made on the relationship between noradrenaline system of the brain and the activity of pituitary-testicular system. In tame rats, less evident dependence of the gonadal activity on activating effect of noradrenaline was noted. After injection of norepinephrine, dopamine, and serotonin in the same brain region. *****were less significant in tame rats as compared to those in aggressive ones. The disturbed relationship between noradrenaline system of the brain and hypothalamo-pituitary-testicular complex is presumably one of the causes of associate changes in the reproductive system during selection for the domestic type of behaviour.     

Photoperiodic control of gonadal endocrine function in silver-gray foxes and its alteration during domestication

Studies have been made on the level of estradiol and progesterone in domesticated and nondomesticated female silver foxes under different photoperiodic regime. The duration of the light--darkness cycle was equal to 24 h. Total duration of the light period was equal to 9 1/2 h; illumination was natural for 7 h, being artificial for the rest 2 1/2 h in a dark part of a day. It was demonstrated that additional illumination affects the hormonal level and sexual activity, the effect being dependent on the particular time of illumination. Nocturnal illumination significantly increased the level of sex hormones in the blood, domesticated animals being more sensitive as compared to nondomesticated ones.     

Brain tyrosine hydroxylase activity in behavior-selected silver foxes

In two groups of silver foxes--i.e. selected by the domestic type of behaviour and aggressive ones--studies have been made on the activity of the key enzyme in biosynthesis of catecholamines--i.e. tyrosine hydroxylase from the brain. Domesticated animals exhibited higher enzymic activity in the locus coeruleus, hypothalamus and cortex. Animals from both groups did not differ with respect to the level of tyrosine hydroxylase activity in the corpus striatum. The enzymic reactions of homogenates from locus coeruleus region of the brain in both groups of animals, as well as homogenates from the corpus striatum of the brain of aggressive animals exhibited low and approximately equal values of Michaelis constant for tyrosine. The value of KM was 3 times higher in the hypothalamus in both groups of foxes and in the corpus striatum of tame animals. Presumably, selection of silver foxes for the domestic type of behaviour resulted in the increase of biosynthesis of catecholamines in the brain due to the increase in the number of enzyme molecules. The increase in the activity of tyrosine hydroxylase in noradrenaline system of the brain may be associated with changes in the behavioural pattern of animals resulting from selection.     

Gonadal hormones modulate the display of submissive behavior in socially defeated female Syrian hamsters

There are striking differences in the behavioral response to social defeat between male and female Syrian hamsters. Whereas males exhibit a prolonged behavioral response to defeat (i.e., conditioned defeat), many females remain aggressive or show only a transient submissive response following defeat. The current study tested the hypothesis that sex steroids underlie this differential behavioral responsivity to social defeat. Female hamsters were ovariectomized and implanted with Silastic capsules containing estradiol (E(2)), testosterone (T), progesterone (P), dihydrotestosterone (DHT), or a blank capsule (no hormone replacement). After a 3-week recovery period, each subject was placed inside the home cage of a larger, more aggressive female for four 5-min defeat trials. The following day, each animal was tested for conditioned defeat by testing it in its own home cage in the presence of a smaller, non-aggressive intruder. Submissive, aggressive, social, and nonsocial behaviors were subsequently scored. Hamsters receiving E(2) or T displayed significantly lower levels of submissive behavior than did animals receiving P, DHT, or no hormone replacement. There were no significant differences in aggressive behavior among groups. These data suggest that gonadal hormones can influence submissive behavior in female hamsters. Collectively, these results suggest that the sex differences observed in conditioned defeat may, in part, be explained by sex differences in circulating gonadal hormones.     

Morphology of hair pigmentation in wild red foxes,silver foxes,and their hybrids

The effects of dominant allele Ar of locus Agouti on the morphology of hair pigmentation were described in foxes. The Ar allele was shown to determine the type of melanin and its content in hair with no effect on the morphology of pigment granules and their distribution throughout a hair. Using the method of electron spin resonance (ESR), the types of melanin (eumelanin and pheomelanin) and their content in the hair of red (ArArEE) and silver (aaEE) foxes and their hybrids (AraEE) were determined. In silver foxes, only one type of melanin (eumelanin) was found. In red foxes and their hybrids (which are phenotypically similar but darker than red foxes), both types of melanin (eu- and pheomelanin) were found. The highest melanin content was detected in the coat of silver foxes. In the hybrids, the total melanin content was lower than in silver foxes, but significantly higher than in red foxes. In red foxes, the contribution of pheomelanin to the total hair melanin content was twice as large as in the hybrids.     

Adrenal function in different phases of the estrous cycle in the domesticated silver fox, Vulpes fulvus

Experiments were made on silver foxes from a population which had been selected for 10 to 15 generations for the domestic behaviour and on animals from a control, unselected population. In females from both populations, studies were made of the level of 11-OHCS in the blood serum, in vitro production of 11-OHCS, the size of fascicular zone in the adrenal cortex, the volume of cellular nuclei and nucleoli, as well as the reaction of the adrenals in vitro to 2 doses of ACTH (1 and 5 units/g of the adrenals) during anestrus, proestrus and estrus. In control females, all the characters investigated significantly increased from anestrus to proestrus. In domesticated females, no changes in the production of 11-OHCS in vitro or changes in morphological features of the fascicular zone were observed in the course of estrous cycle. During proestrus, the adrenals of the domesticated animals were not able to increase the production of 11-OHCS in vitro after application of ACTH. The decrease in the reactivity of the adrenals to the effect of ACTH is presumably the main cause why in unselected females the adrenal are not activated during proestrus. Therefore, in the course of selection for the domestication type of behaviour, species specific dynamics of activation of the adrenals during estrous cycle is lost.     

Morphofunctional status of the adrenal cortex in female silver foxes and the effect of domestication and photoperiodic conditions

Studies on 7-day-old and 8-month-old female silver foxes indicate that additional photostimulation affects to a various extent morphological and functional condition of the adrenal cortex in animals selected and unselected for the domestication type of behaviour. In relatively wild foxes, additional illumination stimulated the activity of the reticular zone, whereas in domesticated animals the same photoperiod increased the activity in the fascicular zone. It is suggested that selection of silver foxes for the domestication type of behaviour affects the sensitivity of the adrenals to external illumination.