D. D. Thiessen

Androgen Control of Territorial Marking in the Mongolian Gerbil

Gerbils mark objects with the secretion of a midventral sebaceous gland. Both the behavior and the gland integrity are under androgen control, as indicated by castration and replacement with testosterone propionate. The integrity of the gland seems less important for marking than an influence on the central nervous system, although the gland can be used as an external measure of androgen levels. It is possible that the secretion acts as a pheromone to signal territorial possession.

Spontaneous seizures in the Mongolian gerbil (Meriones unguiculatus)

Approximately 50% of the Mongolian gerbils in our colony were observed to display an epileptiform seizure when placed in a novel area. Forty-six percent of the susceptible animals were descended from two pairs of animals. Stimulus quality seemed to have little bearing on the response, but stimulus intensity was important. Daily exposure to novelty resulted in a progressive loss of seizure activity until no seizures were observed by the fifth day. The peak seizure time was at the height of diurnal behavioral activity. Dilantin, a common anticonvulsant drug used in human epilepsy completely eliminated the seizure as did reserpine. Brain histologies revealed nothing peculiar about the seizing animals. Neither dietary deficiencies nor infections seemed to be involved. The seizure pattern and the conditions under which it occurred suggest a variable genetic threshold response contingent on central nervous system excitation.

A scent marking response associated with the ventral sebaceous gland of the Mongolian gerbil (Meriones unguiculatus)

Abstract The Mongolian gerbil regularly marks objects in an open field by rubbing them with a midventral sebaceous scent gland. A method and apparatus useful for the study of this response are described. The marking and gland size are sexually dimorphic, with the male marking about twice as frequently as the female and possessing a gland roughly twice the size. Ventral marking in males is positively correlated with urination and defecation, and it is suggested that all three of these activities may in some situations be territorial scent marking responses.

A move toward species-specific analyses in behavior genetics

It is essential that behavior genetics move toward a closer association with other biological disciplines and cast its experiments and interpretations within an evolutionary context. In my opinion, behavior genetics has been too preoccupied with the extent of genetic variability and may, in many cases of high heritability, be dealing with genetic junk. The species as a unit of behavioral response, and as the outcome of genetic polishing, deserves more consideration. Adaptation is always the crux of natural selection and offers the best hope of understanding the evolution of behavior and the restriction of genetic variability. Moreover, it is essential to understand the overwhelming significance of regulatory mechanisms of gene action in natural selection and to relate these to behavioral speciation. Examples for these arguments are discussed here.

The effect of different amounts of androgen on scent marking in the male Mongolian gerbil

Abstract The scent marking response associated with the ventral gland of the Mongolian gerbil declined in frequency following castration, and was restored by testosterone propionate (TP) in proportion to the amount of hormone administered. Subcutaneous injections of 0, 32, 64, 128, and 256 μg of TP in oil were given to 5 groups of 14 castrates once a week 3 days prior to each of 7 weekly behavioral tests. The average marking scores over the last 3 tests were 7.9, 9.3, 10.4, 11.4, and 18.2, respectively, and the corresponding score for sham operated gerbils receiving oil injections without hormone was 14.2. In a second experiment the marking frequency of 28 gonadally intact adults was found to correlate positively ( r = .54) with seminal vesicle weight, a variable which probably reflects androgen titer. The results of the two experiments suggested that individual differences in androgen titer are a cause of individual differences in marking frequency.

Social inhibition of territorial marking in the Mongolian gerbil (Meriones unguiculatus)

Male gerbils were tested for marking frequency in a residents’ territory over a number of days. Experimental animals were allowed contact with residents immediately after each marking trial; control animals were not. Animals exposed to residents eventually decreased their marking to about 25% of control males; however, marking was reestablished in novel territories. Olfactory cues from residents or other intruding males depressed marking. Visual and auditory cues were ineffective. Moreover, odors from the residents’ colony were avoided in a Y-tube preference test. Autopsies revealed no evidence for androgen inhibition or stress effects. Evidently, gerbils are territorial and defend their claims primarily by olfactory intimidation.

Social interactions and scent marking in the Mongolian gerbil (Meriones unguiculatus)

Abstract Scent marking is common for many species, and affords olfactory cues for a variety of social reactions. The Mongolian gerbil ( Meriones unguiculatus ) marks objects with a ventral sebaceous scent gland. In both paired and triplet male encounters, animals marking most frequently generally become dominant. Grouping, however, can have the additional effect of depressing marking and morphological indices of androgen activity. In related studies it was demonstrated that males are attracted to the sebum coming from the scent glands of other males but not to sebum from females. Females show no obvious reference. These studies add support to the notion that gerbil marking has territorial significance and serves recognition functions.

The effects of olfactory deprivation and hormones on territorial marking in the male mongolian gerbil (Meriones unguiculatus)

Mongolian gerbils (Meriones unguiculatus) mark objects in their environment with a midventral sebaceous scent gland. In the male the response is androgen dependent. The first study shows that olfactory bulb aspiration almost completely eliminates marking behavior. The reduction is apparently independent of any hormone dysfunction, as indicated by weights of seminal vesicles, testes, and adrenals. Marking, however, can be at least partially reinstated with massive doses of testosterone propionate (640–1280 μg every 5 days), indicating a hormone compensation for a neurological defect and demonstrating that smell is not absolutely necessary for marking. The second study shows that 80 μg of estradiol benzoate but not 800 μg progesterone administered to males twice a week for 6 weeks significantly depresses marking frequency, causes atrophy of the testes and seminal vesicles, and stops spermatogenesis. It would appear that estrogen acts by antagonizing the secretion of gonadotropins. Animals were also assessed for preference or aversion of premarked pegs during this study, but no differential marking could be demonstrated. It is concluded that territorial marking is normally controlled by androgen and that the disruption of the response can occur either because of a lack of sensory input or an antagonism of the gonadotropic system.

Territorial exclusion and reproductive isolation

Three experiments demonstrate that Mongolian gerbils exclude intruders from home territories and force them to migrate across a water barrier. Of those animals that did migrate in the third experiment, the majority avoided olfactory cues from the territory from which they were excluded.

Territorial marking in the female mongolian gerbil: Short-term reactions to hormones

Abstract Gerbils mark territories with a ventrally located sebaceous scent gland. For the male the response and gland morphology are strictly androgen dependent. In this study of the female, supplemental injections of testosterone elevated marking beyond that of either normal males or females and increased gland size. Estrogen, progesterone, and growth hormone were without effect. Regardless of hormone state there appeared a strong relation between marking, gland size, and gland secretion (gland blot).