Juvenile Intruders Research Articles

Stress enhances aggression in male rats with genetic stress hyper-reactivity.

The current study investigated stress-induced aggressive behavior in the resident-intruder test in males of the genetically stress hyper-reactive Wistar Kyoto More Immobile (WMI), and the nearly isogenic, control Wistar Kyoto Less Immobile (WLI) strains. Tests were carried out against same-age intruders during adolescence, and same-age and juvenile intruders in adulthood. In adolescence and adulthood, prior acute restraint stress decreased social interactions and decreased aggressive behaviors of adolescents and adult WLIs. However, prior stress precipitated aggression in the adult WMI males toward both same-age, and juvenile intruders compared with control WMIs and WLIs. Trunk blood levels of testosterone and androstenedione increased in stressed WLIs, but not in WMIs, suggesting no direct role of androgens in the increased aggression of WMIs. Expressions of aggression-relevant genes showed patterns commensurate with being causative in aggressive behavior. The methyl-CpG binding protein 2 was lower in the frontal cortex of control WMIs, and in the amygdala of stressed WMIs compared with their respective WLIs. Frontal cortex expression of vasopressin receptor 1a and serotonin transporter increased, solely in WMI males after stress. As behaviors were the same toward same-age and non-threatening juvenile intruders, the stress-induced increase in confrontational behavior of the adult WMI male was not because of enhanced fear or anxiety. These results suggest that genetic stress hyper-reactivity is a risk factor for stress-induced increases in aggression in males. Additionally, as known aggression-related genes showed expression patterns paralleling aggressive behavior, this model system could identify novel molecular pathways leading to stress-enhanced aggression.

Territorial song sparrows tolerate juveniles during the early song-learning phase

Avian song learning is an important model system for understanding vocal learning in humans and other animals. Laboratory studies indicate that social interactions are critical for song learning, but field observations show that territorial males are aggressive to intruders, raising the question of whether young birds are tolerated, much less tutored in the wild. We examined how adult song sparrows (Melospiza melodia) treat juvenile and adult intruders during different seasons important for song learning—summer, autumn, and spring—using taxidermic mounts and song playback. Territorial adults responded aggressively to adult intruders throughout the year. Adults were tolerant of juvenile intruders in the summer, displayed somewhat reduced aggression in the autumn, but treated juveniles like adult intruders by the spring. In the summer and autumn trials, wild juveniles approached our simulated ‘‘interactions’’ between subjects and adult mounts; these wild juveniles were also tolerated, even at close distances. That juveniles can and do closely approach adults during the early sensitive phase of song learning suggests that direct interactions with adults are possible and might be important for learning. In contrast, since young birds are treated aggressively in early spring, most late song learning likely happens through eavesdropping or long-distance singing interactions.

Territorial Aggression of a Tropical Passerine, Zonotrichia Capensis, in Response to a Variety of Conspecific Intruders

[The expression of territorial aggression by reproductively active, resident birds varies between the sexes and in response to different intruder types. Previous studies have predicted that individuals should be more aggressive towards conspecific intruders of the same sex and tolerate intruders of the opposite sex and immature individuals. In this study, we investigated the behavioural responses of a tropical population of rufous-collared sparrows (Zonotrichia capensis) to a variety of caged intruder types: Singing Males, Silent Males, Females, and Juveniles. In this species, territories are used by the resident male and female and their young, and are also used by floaters — mature individuals that do not hold territories. Resident males responded similarly and aggressively to all adult intruders in terms of song number, closest approach to the intruder, time within 5 m of the intruder, and a composite aggression score. There was no significant variation in the response of resident females to the different intruder types, although the strongest responses of the resident female were to female intruders. Neither resident males nor females behaved aggressively towards juvenile intruders. These results fail to support the observational predictions for males and females that individuals should be most aggressive towards members of the same sex, who pose the greatest threat in terms of cuckoldry and territorial takeover., The expression of territorial aggression by reproductively active, resident birds varies between the sexes and in response to different intruder types. Previous studies have predicted that individuals should be more aggressive towards conspecific intruders of the same sex and tolerate intruders of the opposite sex and immature individuals. In this study, we investigated the behavioural responses of a tropical population of rufous-collared sparrows (Zonotrichia capensis) to a variety of caged intruder types: Singing Males, Silent Males, Females, and Juveniles. In this species, territories are used by the resident male and female and their young, and are also used by floaters — mature individuals that do not hold territories. Resident males responded similarly and aggressively to all adult intruders in terms of song number, closest approach to the intruder, time within 5 m of the intruder, and a composite aggression score. There was no significant variation in the response of resident females to the different intruder types, although the strongest responses of the resident female were to female intruders. Neither resident males nor females behaved aggressively towards juvenile intruders. These results fail to support the observational predictions for males and females that individuals should be most aggressive towards members of the same sex, who pose the greatest threat in terms of cuckoldry and territorial takeover.]

Agonistic Behaviour and Development of Territoriality During Ontogeny of the Sea Anemone Dwelling Crab Allopetrolisthes Spinifrons (H. Milne Edwards, 1837) (Decapoda: Anomura: Porcellanidae)

Allopetrolisthes spinifrons is an ectosymbiotic crab of the sea anemone Phymactis clematis. As a consequence of low host abundance, these represent a scarce and limited resource for the crab. Additionally, the relatively small size of the sea anemone host suggests that few symbiotic crabs can cohabit on one host individual, forcing crabs to adopt a territorial behaviour. In order to examine the potential presence and ontogenetic development of territoriality, the agonistic behaviour between crabs of various ontogenetic stages (adults, juveniles, and recruits) was studied in the laboratory. Laboratory experiments demonstrated that adult or juvenile crabs aggressively defended their sea anemone hosts against adult or juvenile intruders, respectively, but both adult and juvenile crabs tolerated recruits. Adult crabs behaved indifferently towards juvenile crabs, sometimes tolerating them, sometimes expelling them. Recruits never showed agonistic behaviour among themselves. The agonistic interactions observed in the laboratory and the uniform population distribution pattern on sea anemones recently described for A. spinifrons indicate that this species exhibits territorial behaviour, which develops during ontogeny. Territoriality in this species and other symbiotic decapods may function as a density-dependent mechanism of population regulation, being mediated by the availability of hosts. Resource monopolisation behaviours may be common among other symbiotic and free-living marine invertebrates inhabiting discrete habitats that represent a limiting resource.

Prenatal Flutamide Treatment Eliminates the Adult Male Rat's Dependency upon Vasopressin When Forming Social–Olfactory Memories

The sexually dimorphic number of cells expressing arginine vasopressin (AVP) in the bed nucleus of the stria terminalis and the density of AVP fibers within the lateral septum appear to be organized by pre- and postnatal androgens. Social recognition behaviors are also sexually dimorphic and AVP-dependent. Whereas AVP antagonists prevent males from recognizing familiar intruders by olfactory investigation of the anal–genital area, they have no effect in females. To test the hypothesis that the male's dependency upon AVP to form social recognition memories begins prior to birth, we compared the effectiveness of an AVP antagonist to block social recognition in control males and females with that seen in male offspring whose mothers were treated prenatally with an androgen antagonist (flutamide). In an initial study we showed that while sexual experience may enhance social recognition in males, virgin males exhibit the ability to recognize conspecifics and are sensitive to the memory blocking actions of AVP antagonists. In a second experiment, pregnant rats were treated daily for the last 10 days of gestation with either flutamide (10 mg) or control vehicle. Within 12 h of birth, male offspring from flutamide litters were injected with either testosterone proprionate (50 μg TP) or vehicle control. AVP-antagonist treatment in adults eliminated the ability of control males to recognize familiar juvenile intruders, but had no effect on males exposed prenatally to flutamide, regardless of whether these males were treated with TP or vehicle on day 1 of life. These data support the hypothesis that the development of the male's dependency upon AVP to express social recognition memories begins with the organizational actions of prenatal androgens.

Rules of engagement for adult salamanders in territorial conflicts with heterospecific juveniles

Coexisting species within a guild have the potential for resource overlap and consequently for interspecific competition (e.g., interspecific territoriality). When the adults are of different sizes, which frequently is the case in terrestrial salamander communities in eastern North America, competition may occur between juveniles of the larger species and adults of the smaller species. Adults of the relatively small redbacked salamander (Plethodon einereus: up to 13 cm total length) defend intra- and intersexual territories on the forest floor, and they are broadly sympatric with the larger P. glutinosus (up to 21 cm total length). Although individuals of the two species share the same forest floor habitat, we found significantly fewer juveniles of P. glutinosus sharing territories with 336 same-size adults of P. cinereus than would be expected by random chance alone. In laboratory experiments, residential adults of P. cinereus were as aggressive toward juvenile intruders of P. glutinosus as they were toward adult conspecific intruders. Therefore, adults of P. cinereus appear to defend territories against juveniles of P. glutinosus, illustrating how interference competition may depend on the symmetry of sizes between the species.

Aggression in damselfish: habituation by adults of Pomacentous parritus to juvenile intruders

Successful recruitment of juvenile coral reef fishes may depend, in part, upon the aggressive behavior of adults already on the reef. In addition to initial levels of aggression, changes in aggressive behavior of adults, e.g., due to habituation, may have an even greater influence on recruiting juveniles. Adult males of the bicolor damselfish, Pomacentrus partitus, were used as subjects to study habituation of aggression toward conspecific and congeneric (P. variabilis) juveniles. Adults, held in a 1000 liter aquarium, habituated after 4 hours of constant exposure to juveniles restrained in a 1 liter ‘model bottle’. Stimulus strength of the juveniles depended on their species identity, size and proximity to the resident adult's shelter. There was a recovery of aggression with a change in stimulus location, but at the same location, a stimulus of greater strength was required to bring about recovery. The implications of these findings for coral reef fish community structure are discussed.

Aggressive Interactions and Chemical Communication between Adult and Juvenile Salamanders

Aggressive behavior and recognition of chemical cues play important roles in territoriality and spatial distributions of adult plethodontid salamanders. The role of these behaviors in adult-juvenile interactions is unknown. We assessed adult-juvenile interactions in the Rich Mountain salamander, Pleth- odon ouachitae. In experiment 1, we examined behavioral responses of adult male salamanders (residents) toward juvenile intruders and compared these responses with the responses of the same residents toward adult male intruders. In experiment 2, we examined the role of chemical communication in these inter- actions. Juvenile salamanders were given a choice between control substrates and substrates bearing odors of adult males. Resident adults of P. ouachitae exhibited aggressive behavior (biting and chasing) in adult- adult and adult-juvenile trials. When paired with juveniles, adult salamanders were significantly less aggressive but exhibited significantly more investigatory (nose-tapping) behavior than when paired with other adults. The rate of biting observed in adult-adult trials was extremely high, while the rate of biting in adult-juvenile trials was similar to rates reported for adult-adult pairings in other species of Plethodon. Juvenile salamanders did not avoid odors of adults, despite the aggressive nature of adult P. ouachitae.

Aggressive behaviour of female prairie deer mice in laboratory populations

Prairie deer mice ( Peromyscus maniculatus bairdii), living in asymptotic laboratory populations established two years earlier, were observed for agonistic responses to conspecific intruders. In the first experiment, intruders of six age-sex classes were placed into 10 of the populations for 10 min. The sex of the intruder did not influence the behaviour of the residents, but juveniles elicited more aggression than did adults. A second experiment revealed that female residents were responsible for almost all of the attacks upon juveniles. Experiment 3, in which the responses of pairs of deer mice to juvenile intruders were recorded, demonstrated that the aggressiveness of a female was enhanced by the presence of a male. In the final experiment, females were observed to be highly aggressive during the first few days after giving birth. The aggressive behaviour of the female deer mouse may have greater significance for population dynamics than that of the male.

The effects of the reproductive status and prior housing conditions on the aggressiveness of female mice

The present study was designed to determine the conditions under which aggressive behavior towards a juvenile intruder develops in female mice. Mothers were very aggressive towards juvenile intruders during the first 2 weeks but not during the third week postpartum. However, this aggressiveness was not restricted to early lactation as pregnant and virgin females also attacked a juvenile intruder. Housing virgin female mice in groups as compared to isolation enhanced their aggressiveness towards a juvenile. The effects of housing on aggressiveness were reversible. Females that were aggressive after being housed in groups became relatively nonaggressive following 30 days of isolation and females that were nonaggressive, having been housed in individual cages, became highly aggressive after 30 days of housing in groups. These results indicate that aggressive behavior is rather easily modified by “environmental” factors in ICR female mice.