Male Bank Voles Research Articles

A behavioral syndrome of competitiveness in a non-social rodent

Animals compete for limited resources such as food, mating partners, and territory. The outcome of this intraspecific competition should be determined by individual variation in behavioral traits, such as aggressiveness and dominance status. Consistent among-individual differences in behavior likely contribute to competitiveness and predispose individuals to acquire specific dominance ranks during parts of their adult life. Nevertheless, how dominance rank is correlated with animal personality traits remains largely unclear. In a first step towards better understanding these functional links, we studied trait integration into behavioral syndromes, using 26 captive male bank voles (Myodes glareolus). We repeatedly assessed boldness in an emergence test, exploration in an open-field test, aggressiveness in staged dyadic encounters, and the among-individual correlations between these behaviors. We further related these personality traits to dominance rank, from quantifying urine marking value (UMV), as marking in bank voles is related to dominance rank. We found repeatable variations in boldness, exploration, aggressiveness, and UMV, which were correlated at the among-individual level. Aggressiveness tended to be negatively correlated with body condition, a proxy for fitness. Thus, key personality traits and social rank are functionally integrated into a behavioral syndrome of intraspecific competitiveness. By joining social and non-social aspects of personality, our findings contribute to the ecological validation of personality traits and suggest how they contribute to higher-order component traits, such as dominance which directly affect fitness components.

Reproductive capacity of male bank voles (Myodes glareolus Schreber, 1780) – age-dependent changes in functional activity of epididymal sperm

The influence of age on male bank voles’ reproductive tract development, epididymal sperm quantity and functional activity was investigated. Experiments were carried out on male bank voles aged 1.5 to 15 months (n=10 each in 8 age groups). The developmental stage of the reproductive tract was assessed by the weight of testes, seminal vesicles and coagulation glands. In each age group the number of epididymal sperm and their functional activity were examined. Epididymal sperm functional activity was assessed by motility, viability, maturity, head morphology and integrity of the sperm tail membrane. Ageing males were heavier than pre-pubertal and mature ones. Male age also affected the testes, seminal vesicles and coagulation gland development. The heaviest accessory sex glands were noted in 3-month-old males and the lightest in pre-pubertal (1.5-month-old) and older (12- and 15-month-old) males. Sperm counts were significantly higher in 3-, 4- and 5-month-old males than in pre-pubertal and old males. Generally, adult males aged 3- and 4- months, produced sperm of better functional activity. In conclusion, the best male reproductive capacity is found in bank voles of 3 to 4 months of age.

The role of male scent in female attraction in the bank vole, Myodes glareolus.

Chemical signals are frequently utilised by male mammals for intersexual communication and females are often attracted to male scent. However, the mechanism underlying female attraction has only been identified in a small number of mammalian species. Mammalian scents contain airborne volatiles, that are detected by receivers at a distance from the scent source, as well as non-volatile molecules, such as proteins, that require physical contact for detection. Lipocalin proteins, produced within the scent secretions of many terrestrial mammals, are thought to be particularly important in chemical signalling. Here, we explore if the male-specific protein, glareosin, expressed by adult male bank voles, Myodes glareolus, stimulates female attraction to male scent. We show that female bank voles are more attracted to male compared to female scent, supporting the results of previous studies. Increased investigation and attraction to male scent occurred to both airborne volatiles and non-volatile proteins when they were presented separately. However, we found no evidence that attraction to male scent was driven by glareosin. Our results differ from those previously described in house mice, where a single protein induces female attraction to male scent, suggesting the mechanism underlying female attraction to male scent differs between species.

Seroprevalence of Tick-Borne Encephalitis (TBE) Virus Antibodies in Wild Rodents from Two Natural TBE Foci in Bavaria, Germany

Tick-borne encephalitis (TBE) is Eurasia's most important tick-borne viral disease. Rodents play an important role as natural hosts. Longitudinal studies on the dynamics of the seroprevalence rates in wild rodents in natural foci over the year are rare, and the dynamics of the transmission cycle still need to be understood. To better understand the infection dynamics, rodents were captured in a capture-mark-release-recapture-study in two natural foci in Bavaria, Germany, monthly from March 2019 to October 2022. Overall, 651 blood and thoracic lavage samples from 478 different wild rodents (Clethrionomys glareolus and Apodemus flavicollis) were analyzed for antibodies against tick-borne encephalitis virus (TBEV) by indirect immunofluorescence assay (IIFA) and confirmed using a serum neutralization test (SNT). Furthermore, a generalized linear mixed model (GLMM) analysis was performed to investigate ecological and individual factors for the probability of infection in rodents. Clethrionomys glareolus (19.4%) had a higher seroprevalence than A. flavicollis (10.5%). Within Cl. glareolus, more males (40.4%) than females (15.6%) were affected, and more adults (25.4%) than juveniles (9.8%). The probability of infection of rodents rather depends on factors such as species, sex, and age than on the study site of a natural focus, year, and season. The high incidence rates of rodents, particularly male adult bank voles, highlight their critical role in the transmission cycle of TBEV in a natural focus and demonstrate that serologically positive rodents can be reliably detected in a natural focus regardless of season or year. In addition, these data contribute to a better understanding of the TBEV cycle and thus could improve preventive strategies for human infections.

Chronic Background Radiation Correlates With Sperm Swimming Endurance in Bank Voles From Chernobyl

Sperm quantity and quality are key features explaining intra- and interspecific variation in male reproductive success. Spermatogenesis is sensitive to ionizing radiation and laboratory studies investigating acute effects of ionizing radiation have indeed found negative effects of radiation on sperm quantity and quality. In nature, levels of natural background radiation vary dramatically, and chronic effects of low-level background radiation exposure on spermatogenesis are poorly understood. The Chernobyl region offers a unique research opportunity for investigating effects of chronic low-level ionizing radiation on reproductive properties of wild organisms. We captured male bank voles (Myodes glareolus) from 24 locations in the Chernobyl exclusion zone in 2011 and 2015 and collected information on sperm morphology and kinetics. The dataset is limited in size and there overall was a relatively weak correlation between background radiation and sperm quality. Still, some correlations are worth discussing. First, mid-piece segments of spermatozoa tended to be smaller in bank vole males from areas with elevated background radiation levels. Second, we demonstrated a significant positive relationship between background radiation dose rates and the proportion of static spermatozoa among males within and among study locations after 10 as well as 60 min of incubation. Our results provide novel evidence of damaging effects of low dose ionizing radiation on sperm performance in wild rodent populations, and highlight that this topic requires further study across the natural gradients of background radiation that exist in nature.

Increased sperm production linked to competition in the maternal social environment.

Maternal or early life effects may prepare offspring for similar social conditions to those experienced by their mothers. For males, the ability to achieve mating and fertilization success is a key social challenge. Competitive conditions may therefore favour increased body size or ejaculate production in male offspring. We tested this experimentally by comparing reproductive traits of adult male bank voles (Myodes glareolus), whose mothers had experienced contrasting encounter regimes with female conspecifics while breeding. We found that daily sperm production rates and epididymis mass were significantly higher when dams had experienced more frequent encounters with female conspecifics. This response to maternal and early life experience was specific to sperm production and storage, with no evidence for effects on male body mass or the size of testes and accessory reproductive glands. Our findings reveal a potentially adaptive effect of maternal and early life experience on the development of sperm production, which is worthy of wider investigation.

Cocirculation of Two Orthohantavirus Species in Small Mammals of the Northwestern Ukraine.

Rodent-borne hantaviruses have been reported in many of the countries surrounding Ukraine; however, to date we have no knowledge of the viral strains circulating in Ukraine within reservoirs such as the striped field mouse (Apodemus agrarius), the yellow-necked field mouse (Apodemus flavicollis), and the bank vole (Myodes glareolus). To determine the prevalence of hantaviruses in Ukraine, we captured 1,261 mammals, of which 1,109 were rodents, in 58 field sites within the province of Volyn in western Ukraine. Foci of the striped field mouse tended to occur in the eastern and southern parts of the province, whereas the bank vole were clustered in western and northern regions. The striped field mouse and bank vole had detectable serum antibodies to Puumala virus (PUUV) or Dobrava virus (DOBV) antigens at 7% or 2%, respectively, using an indirect immunofluorescence assay. Antibody prevalence among the bank vole males and females was equivalent, whereas for the striped field mouse, the prevalence among males was 5% versus 1% for females. In two bank vole specimens, we were able to detect partial nucleotide sequences that showed identity to PUUV. In summary, this study suggests that two human pathogens, PUUV and DOBV, cocirculate in the bank vole and the striped field mouse, respectively, in Ukraine. Future studies will focus on new rodent collections that will enable obtaining the complete genome sequences of the PUUV and DOBV strains circulating in Ukraine to provide guidance on the design of optimal molecular diagnostics that can enable insight into the potential contribution of hantaviruses to human disease in Ukraine.

Inheritance of the acoustic signal parameters in interspecific hybrids of the bank (Myodes glareolus) and the Tien Shan (M. centralis) voles

BackgroundThe continuity of behavioral responses in the traits of offspring can be interpreted ambiguously because animal behavior can be transmitted from generation to generation genetically or can be trained. Inheritance of the sound signals characteristics in the absence of directional selection is of particular interest, and that was the purpose of the present work.ResultsComparisons of distress signals of hybrids of the Tien Shan vole females Myodes centralis (Kyrgyzstan) and the bank vole males M. glareolus suecicus (the Tver region), and the parent species were made. Acoustic signals of the hybrids and parent species were compared using the variance and discriminant analysis. The distress signals of the Tien Shan voles were shorter, had lower peak frequency than the signals of the bank voles. The peak frequency of hybrids signals was closer to that of the bank voles. However, the duration of the hybrids sounds, on the contrary, was closer to that of the Tien Shan voles. The expression of the noise component in the sounds of the hybrids occupied an intermediate position. The discriminant analysis of distress signals in the Tien Shan and bank voles showed 94% of correct attributions.ConclusionsOur results confirm that the acoustic signals parameters were inheritance intermediate. The paternal genome (the bank voles) had greater influence on parameters of the sound signals in hybrids.

Skin and gut microbiomes of a wild mammal respond to different environmental cues

BackgroundAnimal skin and gut microbiomes are important components of host fitness. However, the processes that shape the microbiomes of wildlife are poorly understood, particularly with regard to exposure to environmental contaminants. We used 16S rRNA amplicon sequencing to quantify how exposure to radionuclides impacts the skin and gut microbiota of a small mammal, the bank vole Myodes glareolus, inhabiting areas within and outside the Chernobyl Exclusion Zone (CEZ), Ukraine.ResultsSkin microbiomes of male bank voles were more diverse than females. However, the most pronounced differences in skin microbiomes occurred at a larger spatial scale, with higher alpha diversity in the skin microbiomes of bank voles from areas within the CEZ, whether contaminated by radionuclides or not, than in the skin microbiomes of animals from uncontaminated locations outside the CEZ, near Kyiv. Similarly, irrespective of the level of radionuclide contamination, skin microbiome communities (beta diversity) showed greater similarities within the CEZ, than to the areas near Kyiv. Hence, bank vole skin microbiome communities are structured more by geography than the level of soil radionuclides. This pattern presents a contrast with bank vole gut microbiota, where microbiomes could be strikingly similar among distant (~ 80 km of separation), uncontaminated locations, and where differences in microbiome community structure were associated with the level of radioactivity. We also found that the level of (dis)similarity between the skin and gut microbiome communities from the same individuals was contingent on the potential for exposure to radionuclides.ConclusionsBank vole skin and gut microbiomes have distinct responses to similar environmental cues and thus are structured at different spatial scales. Our study shows how exposure to environmental pollution can affect the relationship between a mammalian host’s skin and gut microbial communities, potentially homogenising the microbiomes in habitats affected by pollution.

Transcriptional Upregulation of DNA Damage Response Genes in Bank Voles (Myodes glareolus) Inhabiting the Chernobyl Exclusion Zone

Exposure to ionizing radiation (IR) from radionuclides released into the environment can damage DNA. An expected response to exposure to environmental radionuclides, therefore, is initiation of DNA damage response (DDR) pathways. Increased DNA damage is a characteristic of many organisms exposed to radionuclides but expression of DDR genes of wildlife inhabiting an area contaminated by radionuclides is poorly understood. We quantified expression of five central DDR genes Atm, Mre11, p53, Brca1 and p21 in the livers of the bank vole Myodes glareolus that inhabited areas within the Chernobyl Exclusion Zone (CEZ) that differed in levels of background radioactivity, and also from control areas outside the CEZ (i.e. sites with no detectable environmental radionuclides) in Ukraine. Expression of these DDR genes did not significantly differ between male and female bank voles, nor among sites within the CEZ. We found a near two-fold upregulation in the DDR initiators Mre11 and Atm in animals collected from the CEZ compared with samples from control sites. As Atm is an important regulator of oxidative stress, our data suggest that antioxidant activity may be a key component of the defense against exposure to environmental radioactivity.

Glareosin: a novel sexually dimorphic urinary lipocalin in the bank vole, Myodes glareolus.

The urine of bank voles (Myodes glareolus) contains substantial quantities of a small protein that is expressed at much higher levels in males than females, and at higher levels in males in the breeding season. This protein was purified and completely sequenced at the protein level by mass spectrometry. Leucine/isoleucine ambiguity was completely resolved by metabolic labelling, monitoring the incorporation of dietary deuterated leucine into specific sites in the protein. The predicted mass of the sequenced protein was exactly consonant with the mass of the protein measured in bank vole urine samples, correcting for the formation of two disulfide bonds. The sequence of the protein revealed that it was a lipocalin related to aphrodisin and other odorant-binding proteins (OBPs), but differed from all OBPs previously described. The pattern of secretion in urine used for scent marking by male bank voles, and the similarity to other lipocalins used as chemical signals in rodents, suggest that this protein plays a role in male sexual and/or competitive communication. We propose the name glareosin for this novel protein to reflect the origin of the protein and to emphasize the distinction from known OBPs.

How Do Infanticidal Male Bank Voles (Myodes glareolus) Find the Nest with Pups?

AbstractInfanticide, the killing of conspecific young, occurs in most mammal species, like in our study species, the bank vole (Myodes glareolus). Infanticide by adult males is regarded as a strong factor affecting recruitment of young into population. It is considered as an adaptive behaviour, which may increase male fitness via resource gain or an increased access to mates. When an intruder is approaching the nest, the mother should not be present, as her nest guarding is very aggressive and successful. Pups use ultrasonic vocalisation to call their mother when mother leaves nest for foraging but it is not know which cues do infanticidal males use to find the nest with vulnerable pups to commit infanticide? We studied whether the pups' sounds or the olfactory cues of the nest guide the males of known infanticidal behavioural trait towards the nest with vulnerable pups. Four nest boxes in a large indoor arena offered different nesting cues: nest odour, pup vocalisation, both odour and sound or control with no cue. The result showed that infanticidal males were more active in their searching behaviour than non‐infanticidal males and seemed to target the nest providing only acoustic cues. Four of the males, all infanticidal, intruded the nest box. Infanticidal males seem to actively search for nests with vulnerable pups by eavesdropping pup begging calls for absent mother. However, under natural conditions, mother presence and aggressive nest protection may be an effective counter strategy against strange male infanticide. When trapping study voles from the wild, we monitored occurrence of male infanticide across the breeding season from early to late summer. Proportion of infanticidal males was between 25 and 29% of all males tested along the breeding season. Our results suggest that male infanticide seems to cause a stable threat for pup mortality in increasing breeding season density.

ABorrelia afzeliiInfection Increases Larval Tick Burden onMyodes glareolus(Rodentia: Cricetidae) and Nymphal Body Weight ofIxodes ricinus(Acari: Ixodidae)

Several microorganisms have been shown to manipulate their host or vector to enhance their own transmission. Here we examined whether an infection with Borrelia afzelii affects its transmission between its bank vole (Myodes glareolus, Schreber, 1780) host and tick vector. Captive-bred bank voles were inoculated with B. afzelii or control medium, after which host preference of Ixodes ricinus L. nymphs was determined in a Y-tube olfactometer. Thereafter, infected and uninfected bank voles were placed in a semifield arena containing questing larvae to measure larval tick attachment. Engorged larvae were collected from these bank voles, molted into nymphs, weighed, and analyzed for infection by PCR.Nymphs were attracted to the odors of a bank vole compared to ambient air and preferred the odors of an infected bank vole over that of an uninfected bank vole. In the semifield arena, infected male bank voles had greater larval tick burdens then uninfected males, while similar larval tick burdens were observed on females regardless of infection status. Nymphal ticks that acquired a B. afzelii infection had higher body weight than nymphs that did not acquire an infection regardless of the infection status of the vole. These results show that a B. afzelii infection in bank voles increases larval tick burden and that a B. afzelii infection in larvae increases nymphal body weight. This finding provides novel ecological insights into the enzootic cycle of B. afzelii.

Ultrasonic Reaction of Bank Vole Males to the Presence of Females Varying in Hormonal Activity

AbstractRodents are known to emit ultrasounds during social interactions. Despite the evidence that ultrasonic vocalizations are emitted during sexual encounters and may play a certain role in sexual selection, only a few studies have investigated this phenomenon, mainly in laboratory rodents. We analysed the ultrasonic calls of bank vole (Myodes glareolus) males from an outbred colony in the presence of females differing in hormonal activity. Sexually experienced males were tested during interactions with naive, ovariectomized, pregnant and post‐partum oestrous females. We found that the males’ ultrasound vocalizations depend on the phase of the bank vole females’ reproductive cycle. During encounters with post‐partum oestrous females the males emitted significantly more ultrasounds, and the total duration of ultrasound vocalization was longer. The presence of post‐partum oestrous females also influenced the type of ultrasounds: the most typical constant‐frequency sounds were significantly shorter, and two additional types of ultrasounds were presented during interactions with females ready to mate: U‐shaped frequency‐modulated sounds and frequency‐modulated upsweep sounds. To our knowledge this is the first evidence that male voles of the subfamily Arvicolinae emit different types of ultrasounds in the presence of females depending on their reproductive stage. We suggest that these ultrasounds may be employed as an attractant during reproductive behaviour and that they are potentially an element of sexual selection.

Apoptosis, proliferation, and cell size in seasonal changes of body and organ weight in male bank voles Myodes glareolus

Adult male bank voles undergo the body and organ regression before winter, and in early spring, they resume the growth and reproductive processes. The aim of the present study was to determine whether the seasonal changes of body and organ weight in these animals depend on changes in the number of cells or their size. To study an autumnal regression, wild adult males captured in August were exposed to short photoperiod for 0, 4, and 8 weeks, while to study a spring resumption, overwintered males caught in March were exposed to long photoperiod for 0, 1, and 4 weeks. Apoptosis, proliferation, and cell size in the skeletal muscles, liver, and testes were examined. The study revealed that the seasonal changes of testes weight were associated with changes in the number of testicular cells. On the contrary, the changes in size of skeletal myocytes and hepatocytes appeared to be responsible for the seasonal changes of body (muscle) and liver weights in these rodents.

Infanticide effects on behavior of the bank vole (Myodes glareolus) in the fragmented breeding habitat

Habitat fragmentation affects individual movements between favorable resource patches. In many small mammal species, an important intrinsic factor affecting recruitment of young is infanticide, committed especially by males. We predict that habitat fragmentation hinders movements of males between patches due to predation risk in the open areas. Thus, fragmentation reduces the number of males to which young litters are exposed to and decreases risk of infanticide in isolated habitat patches. Nonfragmented habitat provides not only breeding possibilities for more females but also safe movements to mate or commit infanticide for males. In a replicated enclosure experiment, we tested how infanticidal status of male bank voles (Myodes glareolus) affects their movements in fragmented vs. nonfragmented habitats with same total area and how this affects female spacing and offspring recruitment into population. We found no difference in the number of offspring recruited per female between infanticidal and fragmentation treatments. Females in the fragmented enclosures had smaller movement areas and stayed closer to their nests, suggesting better protection of pups against intruders. Infanticidal males moved more in general but especially in fragmented enclosures, whilst noninfanticidal males were moving more in nonfragmented enclosures. Our results suggest that behavior of females is affected by the habitat fragmentation, as we expected, but males searching for mates move similarly in safe and risky habitats. Thus, the threat of infanticide was not reflected in the recruitment of young into the population as probably the male–female interactions and effective nest protection by the mothers remained similar in both habitats.

Puumala hantavirus infection alters the odour attractiveness of its reservoir host.

The random-mixing assumptions of many parasite-transmission models are challenged if healthy individuals can alter their behaviour to reduce their risk of infection. Some pathogens reduce the attractiveness of their hosts' excretions, for example, potentially altering contact rates and thus the predicted force of infection for pathogens transmissible by contact with excretions. For bank voles (Myodes glareolus), contact with contaminated urine is an important route of transmission for Puumala hantavirus (PUUV); however, it is not known whether PUUV infection changes the voles' urinary odours or their attractiveness. Here, we use a Y-maze to test whether PUUV infection alters the attractiveness of male bank voles' urine. We presented wild-caught PUUV-free male and female bank voles with PUUV-infected conspecific urine, uninfected urine and a water control, and measured the relative and absolute latency to first visit, number of visits, and total time bank voles spent investigating each treatment over 30 min. PUUV infection significantly altered the bank voles' initial response to conspecific urine, with fewer visits and less time spent close to infected urine relative to uninfected urine, and less total time spent near the infected urine than the uninfected urine or control. These strong preferences weakened over the 30-min trial, however, partly due to a general decline in male activity, and there were no absolute differences between the treatments overall. This suggests that PUUV infection does change the attractiveness of bank vole urine to conspecifics, and we discuss the implications of these results for random-mixing assumptions.

Seasonal modulation of sex-related differences in hippocampus size and spatial behavior in bank voles, Clethrionomys glareolus (Rodentia, Cricetidae)

Male and female bank voles (Clethrionomys glareolus) were compared with respect to seasonal variation in hippocampus size and dynamics of spatial behavior during the annual population cycle. The winter decrease in the size of the hippocampus (the brain region involved in the processing of spatial information about the environment) in young autumn-born voles coincided in time with reduction of their home ranges and did not differ significantly between males and females. The growth of the hippocampus in spring, after winter regression, was higher in males (19–28% in different populations) than in females (8–20%) and coincided with an increase in spatial activity, which was also more distinct in males. A significantly greater hippocampus size in males during the breeding period correlated with their higher mobility (reflected in capture rate) and greater size of home ranges. Thus, parallel seasonal modulation of sex-related differences in hippocampus size and spatial behavior was revealed: sexual dimorphism in these characters was insignificant among young voles in autumn; disappeared in winter; manifested itself again, synchronously in both characters, in early spring (with the onset of puberty); and reached a peak in summer.

Differential Susceptibility to Cadmium-Induced Liver and Kidney Injury in Wild and Laboratory-Bred Bank Voles Myodes glareolus

The objective of the study was to compare the sensitivity of wild and laboratory-bred bank voles to cadmium (Cd)-induced histopathological changes in the liver and kidneys. For 4 weeks, the male bank voles—both wild and laboratory-bred—were provided with diet containing Cd in quantities <0.1 (control), 30, and 60 μg/g dry weight. At the end of exposure period, histopathology and analyses of Cd, metallothionein (MT), glutathione (GSH), zinc (Zn), copper (Cu), iron (Fe), and lipid peroxidation—all considered to be critical factors during the development of Cd toxicity in the liver and kidneys—were carried out. Histopathological changes (focal hepatocyte swelling, vacuolation and inflammation [leukocyte infiltration] in the liver, and focal proximal tubule degeneration [including epithelial cell swelling] in the kidneys) occurred only in the wild bank voles fed a diet containing 60 μg Cd/g. There were no differences in concentrations of Cd, MT, GSH, Zn, and Cu in liver and kidney between the respective groups of wild and laboratory-bred animals. However, a decrease of hepatic Fe and lipid peroxidation was observed in the wild voles exhibiting histopathological changes. These data indicate the following: (1) wild bank voles are more susceptible to Cd-induced liver and kidney injury than those bred and raised in the laboratory; (2) the difference in sensitivity may be associated with a distinct decrease of hepatic Fe in response to Cd exposure between the two groups of bank voles; and (3) dietary Cd may produce histopathological changes indirectly through decreasing the hepatic Fe and Fe-dependent oxidative processes. These results also suggest that histopathology in the liver and kidney of wild bank voles living in a contaminated environment may occur at relatively low levels of tissue Cd.

Parental care and testosterone in males of the bank vole (Myodes glareolus): Sensitization and androgenic stimulation of paternal behavior

The relationship between parental responsiveness of bank vole males and their body weight, testes mass, and plasma and gonadal testosterone levels were examined. Two groups of voles were studied: Group I consisted of 14 breeding pairs where females gave birth to young within a month after formation of pairs, and Group II represented 13 pairs without offspring and signs of pregnancy in females. Males in Group I, unlike those in Group II, had contact with pups before the tests on parental responsiveness. In Group I, males were found to have larger testes and to exhibit a higher level of care-giving activity (pup retrieval) as compared to males in Group II. Both the plasma and gonadal testosterone levels in males exhibiting pup retrieval were revealed to be significantly higher than those for males exhibiting infanticide. Thus, promiscuity, competition for receptive females, and a higher level of testosterone secretion, which are characteristic of bank vole males during the breeding season, are not out of the realm of possibility of the males that care for young. Our findings suggest that factors promoting parental responsiveness in bank vole males are sensitization due to contact with pups and an increase in testosterone secretion. Obviously, there is a need to reexamine the role that testosterone plays in regulating rodent parental behavior.