Female Intruders Research Articles

Age and Gender-Related Differences in the Social Behavior and Mating Success of Free-Living Collared Lizards, Crotaphytus collaris

Field studies on collared lizards at Lake Arcadia Dam, Edmond, Oklahoma, revealed that the social and spatial behavior of individuals differed depending on gender and male age. Body size measurements of known lizards over a threeyear period revealed that females > 70 mm SVL at the beginning of the activity season in April become reproductively active in their first year, males > 100 mm in April are two years and older (adults), whereas smaller males (60-90 mm) are in their first full year (yearlings). Experimental introduction of female intruders onto occupied female home ranges elicited aggression in four of 17 trials. However, female home ranges overlapped one another greatly, and intrasexual female aggression occurred infrequently. These results suggest that female collared lizards at Arcadia Lake do not defend exclusive territories but that occasional aggression may play some other role in the social relationships among adjacent females. Areas occupied by adjacent adult males overlapped little and were defended against other adult males primarily by high rates of travel punctuated with frequent display. Yearling males occupied nondefended home ranges within adult male territories by adopting subordinate tactics characterized by low rates of display and travel and rapid retreat from adult males. The home ranges of approximately 60% of the females were overlapped by the territory of a single adult male. In cases of multiple adult male overlap, one of the males overlapped a larger percentage of the female home range and interacted more frequently with these females than did the other adult males. Most female home ranges were also overlapped partially by as many as four yearling males. Adult males initiated encounters with females much more frequently than did yearlings, and females were almost always receptive to adult males whereas they more often withdrew from yearling males. Estimates based on the relative frequencies of interactions of females with each overlapping male indicated that annual male mating success was correlated with age class, snout-vent length, hourly frequency of social acts initiated, and rate of travel, such that mating success of adult males far exceeded that of yearlings. Among adult males alone, one estimate of mating success was weakly correlated only with the number of females on territories. Our results show that, even though male mating success is low during their first year, yearlings remain on adult male territories by adopting subordinate behavior patterns, perhaps because this tactic enhances future opportunities to inherit territories and mates.

Effects of chlordiazepoxide on maternal aggression in mice depend on experience of resident and sex of intruder

Lactating mice respond differentially to intruders of differing sex, displaying defensive attack against the male and offensive attack against the female. Such a phenotypic dichotomy appears to have adaptive value in that unfamiliar males pose a much greater threat to the offspring than do females. The present study examined the effects of the benzodiazepine anxiolytic chlordiazepoxide (CDP) (2.5–10.0 mg/kg) on this differential response pattern in aggression-naive (nonscreened) (NS) and aggression-experienced (screened) (S) lactating female mice ( Mus musculus domesticus) confronting intruders of either sex in a 10-min test. This procedure was used to evaluate the influence of both the type of opponent and previous aggressive experience on basal behavioural profiles and drug action. Results showed that both intruder sex and prior screening for attack modulated the behaviour of lactating females toward intruders. In turn, both variables strongly influenced CDP effects on maternal aggression. In particular, in S dams, CDP dose-dependently increased maternal attack against males but decreased attack against female intruders. Conversely, in NS dams, CDP decreased attack (and fear) against males but did not affect it against females. In both S and NS conditions, CDP modified the attack strategy of lactating females against the male, switching it from a defensive to an offensive pattern. Exploration, social investigation, eating, and immobility were differentially affected by the drug treatment, depending on screening and/or intruder sex condition. These differential effects of CDP between S and NS conditions, toward either male or female intruders, cannot be fully explained by differences in the baseline levels of these behaviours. Alternative hypotheses are discussed. These findings demonstrate that the effects of CDP on maternal attack behaviour depend on not only the drug but also the object of attack, and hence the function of attack and the prior experience of the attacker.

Serum NGF levels increase during lactation and following maternal aggression in mice

In the present study, serum levels of nerve growth factor (NGF) were assessed in virgin and in lactating female CD-1 mice. In the case of the lactating females, NGF levels were assessed both under basal conditions and 60 and 180 min following a 10 min encounter with a male or a nonlactating female mouse. Basal serum NGF levels of lactating females were higher than those of virgin females but did not increase significantly above base after an aggressive encounter with a male or a female conspecific. Female intruders were attacked in a ritualized manner. In contrast, males received numerous bites to vulnerable regions of their body. A positive correlation was found between serum NGF levels and pattern of aggression in females confronting male conspecifics. Thus, in lactating mice, serum NGF levels following an aggressive encounter relate to the specific pattern of behavior the female uses to defend the offspring.

Neurosteroids in the mouse brain: Behavioral and pharmacological effects of a 3β-hydroxysteroid dehydrogenase inhibitor

Inhibition of the aggressive behavior of castrated male mice toward lactating female intruders by dehydroepiandrosterone (DHEA) is correlated with a decrease of pregnenolone sulfate (PREG S) concentrations in brain. We attempted to establish a cause to effect relationship by preventing the decrease of PREG S with trilostane (TRIL), a competitive inhibitor of Δ5-3β-hydroxysteroid dehydrogenase Δ5 → 4 isomerase enzyme. Indeed, TRIL elicited a large increase of PREG levels in brain. Those of PREG S were, however, unchanged, and TRIL unexpectedly decreased the aggressive behavior of control castrated males and did not counteract the inhibition elicited by DHEA. The neurosedative progesterone (PROG) metabolite, 3α-hydroxy-5α-pregnan-20-one (TH PROG), undetectable in the brain of control mice, reached nanomolar concentration range in TRIL-treated ones. However, injection of appropriate amounts of PROG, producing an even larger increase of brain TH PROG, had no antiaggressive effect. Finally, the latter was attributed to the large (up to 80 nM) TRIL-induced increase of brain 3α,21-dihydroxy-5α-pregnan-20-one, which like TH PROG potentiates inhibitory γ-aminobutyric acid (GABA)ergic neurotransmission.

Male and Female Competitive Strategies of Wild House Mice Pairs (Mus Musculus Domesticus) Confronted With Intruders of Different Sex and Age in Artificial Territories

Male and female aggression at different reproductive stages was investigated in pairs of wild mice. Fourteen pairs of laboratory-outbred wild mice were established, each pair living in a multiple set of cages, connected by runaways. Intruder tests were carried out at different stages of the reproductive cycle, i.e. 48 h after introduction, during pregnancy and lactation. In these stages, a female, a male and two pups were consecutively introduced in each territory (24 h separating each intrusion). Male residents were highly aggressive towards (and always intolerant of) male but not female intruders. Conversely, resident females preferentially attacked same sex intruders after colony establishment and during pregnancy, but they attacked either sex of intruder when nursing young. Seven out of 14 female intruders were tolerated 48 h after introduction of residents but tolerance of females decreased during pregnancy and lactation. Male and female residents were essentially responsible for the intolerance of same-sex intruders. Both males and females exhibited infanticide, but sex differences in the timing of attack on alien pups were observed. In the 7 colonies where the intruder female was tolerated (since that two females were present) only one female reproduced successfully. This suggests that, as in males, females of this stock compete for the opportunity to reproduce; they can be exclusively territorial or form a dominance hierarchy which probably determines reproductive success. While male competitive aggression appears to be mostly directed to other males, females seem largely responsible of the regulation of the reproductive potential of a deme unit throughout intrasexual aggression (intolerance towards other females), and possibly also inhibition of subordinate reproduction and killing of unrelated pups.

Urine marking and maternal aggression of wild female mice in relation to anogenital distance at birth

A series of experiments were conducted with wild house mice to verify the effect of intrauterine position on females' anogenital distance at birth (AGD) and to examine the relationships between a female's AGD, used as a bioassay of androgen exposure during fetal life, and her social behavior and reproductive success in adulthood. Experiment 1 showed that cesarean-delivered females that developed in utero between two males (2 M females) have significantly longer AGD's than females positioned between two females (0 M females). We then categorized naturally delivered females shortly after birth as having a long, medium or short AGD. In adulthood, these females were tested for their behavior towards unfamiliar pups, their rate of urine-marking in response to a variety of social stimuli, postpartum aggression and success in protecting their litters in response to male and female intruders. Adult females with different AGD's at birth did not differ either in their behavior toward pups or in their rate of urine marking. Conversely, males housed across a wire mesh partition from a long-AGD female deposited a higher number of urine marks than those exposed to a short-AGE) female. When tested after delivering a litter, long-AGD females displayed more tail-rattling (a component of agonistic behavior) towards intruders of both sexes in comparison to short-AGD females. These results are consistent with the hypothesis that females with a long AGE) are exposed to higher levels of Testosterone during fetal life than females with a short AGD. Although not related to AGD, other measures of maternal aggression were affected by postpartum day, sex of intruders and a female's infanticidal potential while a virgin.

Nest defense and survival of offspring in highly aggressive wild Canadian female house mice

Nest defense behavior was examined in wild female house mice ( Mus domesticus) that were derived from a stock initially trapped in Alberta, Canada. The first objective was to determine whether behavior toward pups prior to mating was related to the intensity of postpartum aggression in a variety of social situations. Therefore, prior to the experiments we screened virgin females for their behavior toward a newborn pup [60% of the females exhibited infanticide and 40% were noninfanticidal: 7% were parental (retrieved and hovered over the pup) and 33% ignored the pup]. Infanticidal and noninfanticidal females were then mated with males and used in four experiments. In Experiment 1 the females were housed individually prior to delivery, while in Experiment 2 the females were allowed to remain with their mates; in both situations all females successfully reared litters of similar sizes. Male and female intruders (that had all exhibited infanticide when previously tested with a pup) were placed separately into a test cage containing a lactating female during the first four days after delivery. Regardless of the presence of the stud male, previously infanticidal females were more aggressive (exhibited more attacks per min) toward both male and female intruders than were previously noninfanticidal females; infanticidal females also exhibited more of both forms of attack (offensive and defensive) and also attacked with greater intensity than did noninfanticidal females. The number of attacks toward intruders of both sexes increased for both infanticidal and noninfanticidal females between Day 1–4 postpartum, but very high rates of attack were observed on all days by the lactating females, including the day of delivery. In Experiments 3 and 4 only the most aggressive (previously infanticidal) females were tested. In Experiment 3, two unrelated, unfamiliar females were mated separately and then were housed together just prior to delivery, which was planned to occur 3–4 days apart. In 5 of the 15 cages, all pups disappeared on the day of delivery of the second female to deliver her litter. In the remaining 10 cages, it appeared that none of the pups produced by the 20 females were killed. Thus, in this experiment, 66% of pups survived to Day 4 postpartum. In Experiment 4, two previously infanticidal female siblings, which had been housed together since birth, were placed together with a stud male. In all 9 cages only one female became pregnant and delivered pups, but only 3 litters survived to Day 4 (no litters were observed being attacked during intruder tests). In contrast to kin-selection theory and our expectation that a sibling would contribute to communal rearing and nest defense, housing siblings together thus resulted in the lowest overall reproductive success ( 3 18 ; 17% of the females produced litters that survived to Day 4 postpartum) of any of the social conditions examined. We discuss the implications for social structure and population dynamics of the extremely high aggresiveness and infanticidal tendency, regardless of kinship, of females from this stock of wild mice.

Biosynthesis and assay of neurosteroids in rats and mice: Functional correlates

Pregnenolone (PREG), synthesized de novo in rodent brain, is the precursor of PREG sulfate (S) and progesterone (PROG). PROG is further converted to 5 alpha-pregnane 3, 20-dione (DH PROG) and to 3 alpha-hydroxy-5 alpha-pregnan-20-one (TH PROG). PROG, DH PROG and TH PROG have been measured in the brain of male and female rats. Neither PROG nor DH PROG disappeared from brain, contrary to plasma, after combined adrenalectomy (ADX) and gonadectomy (CX). Trilostane decreased PROG and increased PREG in the brain of CX+ADX rats and mice, in accordance with a precursor to product relationship. As previously described in CX male mice, the neurosteroid DHEA and its analog 3 beta-methyl-androst-5-en-17-one (CH3-DHEA) inhibited the aggressive behavior of female mice towards lactating female intruders. The decrease of biting attacks by DHEA was definitely more prominent in females neonatally imprinted with testosterone. The degree of inhibition of aggressive behavior was related to the decrease of PREG S concentrations in brain. The memory-enhancing effects of DHEA S and PREG S in male mice have been previously documented. Infusion of PREG S (12 fmol) into the nucleus basalis magnocellularis (NBM) of the rat after the acquisition trial enhanced memory performance in a two-trial recognition task (TTRT). Conversely, TH PROG (6 fmol), which potentiates GABAergic neurotransmission, disrupted performance when injected before the acquisition trial. Accordingly, we have found a positive correlation between the performances of 2-year-old rats in the TTRT and the concentrations of PREG S in the hippocampus, namely animals which performed best had the highest steroid levels.

Polygyny in the blue tit: intra- and inter-sexual conflicts

The occurrence and importance of intra- and inter-sexual conflicts were studied in a blue tit,Parus caeruleus, population with a high level of polygyny. Males could increase their chances of becoming polygynous by actively attracting extra females or by displaying towards visiting floater females. Polygynous males allocated their parental care in relation to the hatching interval between the two broods. If the hatching interval was small, males divided their effort between the primary and the secondary nest. If the hatching interval was large, males fed only the nestlings of the primary female. Primary females that had to share male help did not fledge fewer young, but survived less well. Primary females tried to prevent or delay the settlement of secondary females by behaving aggressively towards all intruder females and by guarding their mates. Secondary females fledged more young if they received more help, that is, if the hatching interval was small, but there was no effect on female survival. Secondary females tried to settle early during the breeding season despite severe aggression of primary females. Secondary females needed less time for nest building and incubation and laid a smaller clutch, which increased the probability of hatching their young early relative to the primary brood. It is concluded that sexual conflicts play an important role in shaping the mating system of the blue tit and that parental care is the key factor in these conflicts.

Familiarity with Intruders Modulates Agonism towards Outgroup Conspecifics in Wied's Black‐tufted‐ear Marmoset (Callithrix kuhli: Primates, Callitrichidae)

AbstractEncounters between individuals in established groups have important implications for demographic processes. While most intergroup encounters in callitrichid primates (marmosets and tamarins) are characterized by intense agonistic exchanges, some interactions occur in the absence of aggression. This paper investigates the role of familiarity among partners in determining the quality and intensity of aggression during stranger encounters in Wied's black‐tufted‐ear marmoset (Callithrix kuhli). Resident male and female pairs were presented on several occasions with a single male or female intruder. Half of the intruders were housed in cages in colony rooms distant from the residents, and the other half of the intruders resided in cages in the same room as the residents. Aggression was higher in male residents than in females, and males showed significantly higher rates of agonistic interactions with unfamiliar intruders of both sexes. The behaviour of the intruder did not differ according to its degree of familiarity to the residents, although aggression by resident males and submission by intruders were highly correlated in encounters with unfamiliar, but not familiar, intruders. Resident pairs approached each other more often after intruder trials than before. The results of this experiment suggest that familiarity among individuals in neighbouring groups is sufficient to modulate aggression during encounters. Since home‐range overlap among free‐ranging black‐tuftedear marmosets is extensive, differential familiarity among groups may have important implications for dispersal‐related processes.

Prior Residence Effects in Shelter Defense in Adult Signal Crayfish (Pacifastacus Leniusculus (Dana)): Results in Same- and Mixed-Sex Dyads

In a variety of taxa, the initial resident in a spatial area enjoys a dominance advantage over a subsequent intruder. In decapod crustaceans this residence has been demonstrated in lobsters, prawns, and crabs. However, there is scant evidence for the effect in crayfish. In the present series of experiments, individual signal crayfish (Pacifastacus leniusculus) were introduced into one side of divided aquaria, which contained a shelter. Another crayfish was put into the opposite side. Twenty-four hours later the divider was removed. In same-sex encounters 76% of female residents retained shelter possession, as did approximately 70% of male residents, both sexes demonstrating a significant territorial prior residence effect. In mixed-sex encounters, female residents retained shelter possession against male intruders in 80% of them. However, male residents retained shelter occupancy in only 33% of the encounters with female intruders. These findings demonstrate the existence of a prior resident effect in P. leniusculus. The contest advantage of females over males is consistent with previous research showing female dominance over males in social dominance hierarchies in this species.

Maternal Aggression in Red Swamp Crayfish (Procambarus Clarkii, Girard): the Relation Between Reproductive Status and Outcome of Aggressive Encounters With Male and Female Conspecifics

Abstract A series of experiments investigated intraspecific aggression by maternal (carrying eggs and/or hatched young) and non-maternal female red swamp crayfish (Procambarus clarkii) residents against intrusions by non-maternal female and Form I (reproductively active) male conspecifics. Each resident occupied an aquarium containing a shelter for 24 hours prior to the initial intrusion. The winner of each encounter was determined, as well as the pair member that initiated the aggressive interaction, and the relation between the initiation of aggression and contest outcome. The results showed that: (1) Maternal female residents were winners in 92% of the encounters with male intruders, and initiated aggression in a significantly higher proportion of encounters than the male intruders. (2) Maternal female residents were the winners in 75% of the encounters with non-maternal female intruders. The maternal residents also initiated aggression in a significantly higher proportion of the encounters than the non-maternal intruders. (3) Non-maternal female residents lost 77% of the encounters with non-maternal intruders. Also, the non-maternal intruders initiated aggression in a significantly higher proportion of the encounters than did the non-maternal residents. (4) Non-maternal female residents lost all of their encounters with male intruders. However, there was no significant difference in the proportion of the encounters in which aggression was initiated by residents or intruders. (5) For all experiments combined, maternal residents won a significantly higher proportion of their encounters than did non-maternal residents, regardless of whether the intruders were males or non-maternal females. (6) For the three experiments combined, the initiation of aggression reliably predicted contest outcome (i.e. the initiator of aggression ultimately won). The present results provide the first empirical demonstration, with appropriate non-maternal controls, of maternal aggression in decapod crustaceans. Also, the direct relation between reproductive status and contest outcome in both ovigerous and post-hatch P. clarkii are the first such data reported in crustaceans, in general. Finally, the findings of these experiments bear notable similarities to the results of maternal aggression research in other taxonomic groups.

Aggression in female mice: Contrasting effects of amiflamine (FLA 336), a selective and reversible MAO-type A inhibitor

The effects of FLA 336 on the attack by resident groups of female mice on strange female intruders were assessed. This MAO-type A inhibitor clearly suppressed attack in treated subjects without producing major alterations in other activities but there was no clear dose-response relationship. The substance also increased attack on treated animals, an effect largely mediated via changes in the odor characteristics of the urine. The data emphasize the need for appropriate controls in studies with psychoactive compounds to distinguish direct (CNS-mediated) from indirect (mediated via changed signalling or perception) actions.

Why Does the Typically Monogamous Oystercatcher (Haematopus Ostralegus) Engage in Extra-Pair Copulations?

(1) We attempted to identify the possible costs and benefits of Extra-Pair Copulations (EPCs) from field observations on a colour-marked population of Oystercatchers, a long lived monogamous species, where male and female share parental care and territory defence. (2) Despite our broad definition of EPCs, only 7.0% of successful copulations by male breeders and 5.1 % of successful copulations by female breeders were classified as EPC's. (3) Many pairs first copulated more than 2 months before egg-laying. The rate at which breeders engaged in Within-Pair Copulations (WPCs) was highest in the month preceding egg-laying: almost 1 copulation per hour during low tide. Males were increasingly likely to initiate WPCs close to the period of egg-laying. Once the clutch was complete copulation rates dropped to near zero. (4) Most EPCs were observed well before egg-laying. In the month preceding and including egg-laying only 3.3% of all successful copulations by females were EPCs. DNA-fingerprinting confirmed that of 65 chicks (from 26 clutches), only 1 was not fathered by the male partner, but by a neighbouring male, which was seen to copulate with the female before egg-laying. Thus, extra-pair paternity was extremely rare, comprising 1.5% of all chicks. Fingerprinting provided no evidence for intraspecific brood parasitism or quasi-parasitism. (5) Members of new pairs (a minority in the population) were observed to copulate with more mates than were members of old pairs, for the same number of copulations observed. (6) Two case studies suggested that EPCs by males and females of old pairs may be attempts to change mate. One female switched to a new mate after 2 years of EPCs with this bird, while the other female is expected to switch to a neighbouring male in 1992, after 3 years of EPCs with this neighbour. (7) The majority of EPCs by male breeders were in their own territory, while female breeders more often moved to the territory of the male, often the neighbour. This sex difference resembles the sex difference in breeding dispersal: female breeders are more likely to switch territory when switching mate (ENS et al., 1993). (8) Male breeders whose mate was absent sometimes evicted soliciting female intruders instantly. This suggest that EPCs were not necessarily beneficial, even when there was no apparent risk of a penalty by the mate. (9) IfEPCs are primarily attempts to change mate, we predict an increase in the probability of mate change when an individual has engaged in EPCs in the previous year. We surmise that we failed to establish this relationship, because our record of EPCs was incomplete and because attempts at mate change apparently often failed due to intra-sexual competition.

Changes in Female Breeding Behaviour Across the Ovulatory Cycle in the Brook Stickleback, Culaea Inconstans (Kirtland)

In this paper I investigate the changes in female Culaea inconstans behaviour at different stages of the ovulatory cycle in response to the presentation of a courting male and an ovulated female intruder. These experiments reveal that female brook sticklebacks display a complex repertoire of behaviours including all of the threat displays commonly described for male gasterosteids. On spawning days, females direct more nudges, bites, broadside displays, head down displays and redirected bites towards an intruding female, and spend more time engaged in hovering, ground head-up courtship, and near, an intruding male. The interspawning interval in females is accompanied by a surge of aggression towards males. In essence, females show a fluctuation in behavioural intensities that mirrors their ovulatory cycle when interacting with males, but show no such cycling during interactions with other females. At the population level female behaviour is thus more predictable when the intruder is a male than when the intruder is another female. At the individual level, however, a female's responses to another female are tightly correlated across her breeding cycle. Aggressive fish spend more time near the intruding female and less time in the plants on all days of their spawning cycle than their less aggressive counterparts, who displayed a strong tendency to flee, S-displaying either during the retreat or from the safety of the plants. Correlations between behaviours are more susceptible to underlying ovulatorty cycle when the intruder is a male. Aggressive fish spend more time near the intruding male only during the interspawning interval.

A fostering study of the effects of prenatal cocaine exposure: I. Maternal behaviors

The effect of rearing condition and prenatal exposure to cocaine on maternal behaviors was examined. Sprague-Dawley dams were given SC injections of 40 mg/kg/3cc cocaine HCl (C40) or saline (LC) daily from gestational days 8–20. Maternal behavior was assessed in treated dams rearing their biological pups (LC/LC; C40/C40), treated dams rearing untreated pups (LC/FOS; C40/FOS), and foster dams rearing treated pups (FOS/LC; FOS/C40). All dams were monitored for home cage behavior (time eating, drinking,and with pups) for 2 h during both the light and dark cycle on postnatal day 4 (P4), pup retrieval on P5–P9, and maternal aggression to a female intruder (latency to the first attack, number of attacks, boxing, pins, intruder time spent submissive and motionless) on P10. No differences were observed in nest behavior or in tests of pup retrieval among the six groups. Dams rearing their biological litter (LC/LC and C40/C40) were significantly quicker to initiate the first attack when compared to all other groups. This increased aggression was maintained throughout the test session in the C40/C40 dams who made significantly more intruder attacks than all other groups, with the intruder spending significantly more time in a submissive posture (lying on back). In contrast, LC/LC dams did not exhibit an increased number of attacks during the test, apparently responding to an increased freezing in their intruders with a reduction in aggressive behavior. Taken together these findings suggest that prior cocaine exposure results in alterations in maternal aggression that is evident when these dams rear their own pups.

Offensive and defensive bite‐target topographies in attacks by lactating rats

The attacks by resident lactating Wistar rats on sexually naive conspecifics of both sexes were examined. Male and female intruders were equally attacked in terms of frequency and number of bites, but the topographies of biting seen in these encounters were different. Similarly to male-male agonistic interactions, females were attacked in a fashion which avoided bites to the head and snout (“offensive” attack), whereas males were frequently bitten on such vulnerable regions (“defensive” attack). This dichotomy in bite pattern suggests that different motivations and functions underlay maternal aggression in these situations. The defensive attack on males may be a deterrent to infanticide since only male intruders counterattack lactating females and kill their pups. The attack on females may be concerned with resource competition.

The adaptive significance of male subadult plumage in purple martins: plumage dyeing experiments

I tested two hypotheses for the adaptive significance of subadult plumage in male purple martins (Progne subis) : the female mimicry and subordinance signaling hypotheses. Subadult males were at a competitive disadvantage in obtaining territories, as they arrived later in the spring than adult males. Contrary to the predictions of both hypotheses, adult male territory owners were not less aggressive toward subadult male than adult male intruders. The subadult plumage was not effective in mimicking females, as adult male owners were significantly more aggressive toward subadult male than female intruders. Summer adaptation hypotheses predict that young males in subadult plumage are more successful in acquiring territories and mates than they would be with an adult plumage. I tested this prediction by dyeing the plumage of “floater” subadult males to mimic the appearance of adult males. In 13/17 paired experiments, dyed subadults obtained territories before control subadults. There was no difference in the time it took dyed and control males to attract a mate after they obtained a territory. These results suggest that the subadult plumage is not an advantage to young males in competing with adult males for breeding resources. In late winter, subadult males were growing mostly femalelike feathers on their underside, suggesting that the subadult plumage is not the result of a molt constraint. The subadult plumage could enhance survival of yearlings in winter roosts if it improves access to good roost sites or reduces the risk of predation.

Aggression by a female rat cohabitating with a sterile male: Termination of pseudopregnancy does not abolish aggression

Female rats each cohabited with a sterile male for a 6-week period. At the end of that time, each female was assessed for aggressiveness toward an unfamiliar female intruder once each week for 3 weeks. Those females displaying a high level of aggression had their male cagemate changed. For half of the females, the new male cagemate was a castrated male with a testosterone implant. For the other half, the new cagemate was a castrated male without a testosterone implant. Replacement males had been subjected to surgery 9 weeks previously. There were no differences in the aggressiveness of females of the two groups on any of 3 subsequent weekly tests of aggression. In a 3-h evaluation of male sexual behavior, none of the 9 castrated males without testosterone replacement displayed sexual activity with an estrogen/progesterone primed ovariectomized female, but 6 of 9 males with testosterone replacement did. Reanalysis of the aggression data comparing the females whose males had no testosterone replacement and females housed with the 6 males that were sexually active also revealed no differences in aggression over the 21-day test period. Since pseudopregnancy is known to last 13 days, these results indicate that the continuous presence of pseudopregnancy is not required for maintenance of aggression by a female cohabiting with a sterile male.

Response to intruders in female rabbit colonies is related to sex of intruder and rank of residents

Behavioural reactions to unfamiliar conspecifics of both sexes were studied in female domestic rabbits, living in stable unisexual groups. Intrusion caused an abrupt increase in the frequency of social investigation and agonistic behaviours directed to both intruders and group-mates. Reactions depended on the rank of resident females and the sex of the intruder, and were generally more marked in the presence of the male than the female intruder. Dominant and sub-dominant females investigated the male more than the female intruder; the opposite held for subordinate females. In the presence of the female intruder, only dominant and subdominant females were aggressive towards the intruder and group-mates. In the presence of the male intruder, aggression was directed to the intruder and group-mates by dominant females only. They tended to frequently attack sub-dominants, which in turn fled away from them more often than they did from other group-mates.