Males In Mixed-sex Groups Research Articles

The influence of physiological status on the reproductive behaviour of humpback whales (Megaptera novaeangliae)

For most cetacean species, there is little known about how an individual's physiology influences its behaviour. Humpback whales (Megaptera novaeangliae) are a good candidate to examine such links as they have a well-described distribution and behaviour, can be consistently sampled using remote biopsy systems, and have been the subject of several previous endocrine studies. The objective here was to examine whether a female humpback whale's social state (i.e. escorted by a male or not) is related to her endocrine condition, and whether male dominance ranking is related to testosterone levels. Skin and blubber biopsies were collected from the east and west Australian humpback whale populations in 2010–2016 (n = 252) at multiple times throughout the winter-spring breeding season. Steroid hormones were extracted from blubber and concentrations of progesterone (a marker for pregnancy), testosterone (a marker of male testicular activity) and oestradiol (a potential marker of ovarian activity) measured using enzyme-immunoassays. Principal escorts—the dominant males in mixed sex groups—had significantly higher blubber testosterone levels (mean ± SE; 1.43 ± 0.20 ng/g wet weight) than subordinate, secondary escorts (0.69 ± 0.06 ng/g wet weight). Females that were escorted by males typically possessed elevated blubber oestradiol levels (1.96 ± 0.25 ng/g wet weight; p = 0.014); few were considered to be pregnant (p = 0.083). ‘Unescorted’ females displayed characteristically lower blubber oestradiol levels (0.56 ± 0.06 ng/g wet weight). Together, these results are consistent with ‘challenge hypothesis’ theory and suggest the existence of associated reproductive patterns in humpback whales.

Do Males Form Social Associations Based on Sexual Attractiveness in a Fission-Fusion Fish Society?

Recent theory predicts that males should choose social environments that maximize their relative attractiveness to females by preferentially associating with less attractive rivals, so as to enhance their mating success. Using the Trinidadian guppy (Poecilia reticulata), a highly social species, we tested for non-random social associations among males in mixed-sex groups based on two phenotypic traits (body length and coloration) that predict relative sexual attractiveness to females and sexual (sperm) competitiveness. Based on a well-replicated laboratory dichotomous-choice test of social group preference, we could not reject the null hypothesis that focal males chose randomly between a mixed-sex group that comprised a female and a rival male that was less sexually attractive than themselves and another mixed-sex group containing a sexually more attractive male. The same conclusion was reached when females were absent from the two groups. As might be expected from these laboratory findings, free-ranging males in the field were not assorted by either body length or colour in mixed-sex shoals. The apparent lack of an evolved and expressed preference in wild male guppies from our study population to form social associations with other males based on their relative sexual attractiveness and competitiveness might be due to the fission-fusion dynamics of guppy shoals in nature. Such social dynamics likely places constraints on the formation of stable phenotype-based social associations among males. This possibility is supported by a simulation model which assumes group departure rules based on relative body size and coloration in males.

The effects of group type and young silverbacks on wounding rates in western lowland gorilla (Gorilla gorilla gorilla) groups in North American zoos.

In North American zoos, male gorillas are often housed in all-male (bachelor) groups to provide socialization for males not managed in breeding groups. These groups exhibit long-term cohesion and stability and males in bachelor groups are no more aggressive than males in mixed-sex groups. Previous studies have shown that aggression in male gorillas is more directly related to age rather than group type, with young silverbacks (YSB; males 14-20 years of age) having higher rates of aggressive behavior than males of other age classes. Despite this, anecdotal reports have persisted that bachelor groups have higher wounding rates than mixed-sex groups. To assess wounding in zoo-housed gorillas, all instances of wounding across 28 zoos (180 gorillas, 45 social groups) were recorded over a 26 months period via a standardized data sheet. Similar to previous reports, we found age to be an important determinant in wounding. Bachelor groups that contained YSB's had significantly more wounds than bachelor groups without YSB's (U = 14.0, z = -2.193, P = 0.029). There was no difference in wounding rates between mixed-sex and bachelor groups without YSB's (U = 69.5, z = -0.411, P = 0.689). These data further demonstrate the importance of behavioral management of YSB's in zoos and the viability of bachelor groups as a long-term housing solution for male gorillas.

Male-male mounting and the unreliability of body size as a character for mate choice in male Japanese beetles (Popillia japonica Newman)

Background: Same-sex pairing is common in many animal species. In many insects, same-sex pairing is often thought to be a result of poor sexual discrimination (i.e., a mistake), but few detailed studies of the mechanisms underlying the mistaken pairing have been conducted. Previous studies have found that in the field, a small proportion of Japanese beetle (Popillia japonica) mating pairs consist of two males instead of a male and a female. In the current study, we investigated the relationship between body size, the tendency to mount other males, and the duration of these mounts, in laboratory experiments on male Japanese beetles. Results: In the first experiment, we observed male-male mounting in all-male groups in which each male had been uniquely marked. Males of all sizes were likely to mount other males and extend their aedeagus (copulatory organ), but the mounts were longer, and aedeagus extension was more likely to occur, if the mounted beetle (in the ‘female’ position) was larger than the mounting beetle (in the ‘male’ position). In the second experiment, we observed male-female behavior in mixed-sex groups. Females did not immediately copulate with males that had mounted them. If copulation did occur, males tended to remain on the back of females for an extended period of time. Males that mounted other males in mixed-sex groups tended to mate subsequently with a female and then stay with her. Conclusions: We propose that the minimal physical difference between the sexes, in combination with benefits to the males of rapidly attempting to pair with any available female, explains the tendency for males to mount other males. Extended mounts may occur because larger individuals are more likely to be female and because of selection on males to persist in a copulation attempt when females do not immediately copulate with a male.

The influence of forage, protected areas, and mating prospects on grouping patterns of male elephants

Factors affecting social group size in mammals are relatively well studied for females, but less is known about determinants of group size for males, particularly in species that live in sexually segregated groups. Male grouping patterns are thought to be driven more by spatial and temporal dispersion of mating opportunities than by food resources or predation risk. We evaluated the influence of 3 factors on male group sizes and number of males in mixed-sex groups in African elephants; forage availability (using Normalized Difference Vegetation Index, a satellite-based indicator of primary productivity), anthropogenic mortality risk (using distance of elephants from a protected area center), and mating opportunities (using the number of males in mixed-sex groups with and without estrous females). Using zero-truncated negative binomial regressions and a model-selection approach, we found that male elephants occurred in larger groups where primary productivity was higher and where they were further from a protected area center. However, we found an interaction between primary productivity and anthropogenic mortality risk: at low primary productivity, elephants formed larger groups further away from a protected area center, but did less so at higher primary productivity. This pattern suggests that male elephants are sensitive to seasonal variation in potential anthropogenic mortality risk, by remaining in smaller groups when risk is low, but forming larger groups when risk is high. Mating opportunities also led to an increase in male numbers in mixed-sex groups, but its relative influence on male grouping was less important because mating opportunities were rare.

Gender effects on tail damage development in single- or mixed-sex groups of weaned piglets

While extensive research on tail biting among pigs has focused on external factors (e.g., enrichment material), limited research has been conducted on internal factors (e.g., gender, breed or age), which may affect the predisposition of piglets to start tail biting. Furthermore, to test internal or external factors, most previous research used end point observations (e.g., tail damage at abattoirs). However, the potential factors causing tail biting, and the expression of tail biting itself can change over time as pigs grow older. Tail damage development over time might provide more accurate information on external and internal factors affecting tail biting than end point observation. Using tail damage development, we studied the effect of gender in single-sex and mixed-sex groups on tail biting. Tail damage development was recorded two ways: a) number of days before 40% of the piglets was observed with tail damage (40% incident point) and b) number of days a piglet was observed with tail damage (tail damage duration). A 2 × 2 factorial design was used and this resulted in four treatment categories: (1) all-male groups, (2) all-female groups, (3) males in mixed-sex groups and (4) females in mixed-sex groups. During the observation period tail damage (no damage, bite marks or tail wound) of 700 weaned piglets was scored three times per week for 32 days. Following the onset of tail biting, all-female groups had a lower 40% tail damage incident point (10.9 days), compared to the other three treatment categories (average of 16 days; P < 0.05). In all-female groups, piglets also had a higher tail damage duration (20.2 days), compared to the other three treatment categories (average of 16 days; P < 0.05). Several interactions between gender and mixing were found ( P < 0.05); males in mixed-sex groups had a lower 40% tail damage incident point and a higher tail damage duration than females in mixed-sex groups. These results indicate that female piglets are more likely to tail bite compared to male piglets. Furthermore, at the end of the observation period, tail damage had developed to high levels in all groups and, at that point, differences between all-female groups and the other groups were absent. Tail damage development is therefore a better way to analyse the effects of external and internal factors that result in tail biting, compared to methods based on end point analyses.

Male–female interactions affect foraging behaviour within groups of small-spotted catshark, Scyliorhinus canicula

To examine potential effects of male–female interactions on the sexes' foraging behaviour, we studied equal-sized, single- and mixed-sex groups of a benthic elasmobranch, the small-spotted catshark, in response to electric stimuli. A total of 88 size-matched, adult individuals were repeatedly presented with a range of artificial, prey type electric fields (E fields) under laboratory conditions in groups of (1) four males, (2) four females and (3) two of each sex. Females in single-sex groups were more responsive towards E fields than both single- and mixed-sex group males, despite showing similar activity levels. Equally, females in mixed-sex groups were more responsive than males in mixed groups, but showed similar activity levels. Furthermore, females in mixed-sex groups were less responsive to E fields than females from single-sex groups, despite showing similar activity levels. Males in mixed-sex groups were less responsive than males in single-sex groups, but were more active. These results indicate more intense foraging behaviour among females than males and significant reductions in foraging behaviour when grouped with the opposite sex. Both findings are probably associated with consequences of differing reproductive strategies and resultant sexual conflict (specifically male harassment and female avoidance).

Sexual selection in harems: male competition plays a larger role than female choice in an amphipod

Spatial distribution can be affected by sexual selection, particularly in mating systems where males have exaggerated traits used to compete for access to females. Although it is rare to be able to disentangle selection arising from male competition versus female choice in such mating systems, we here report experiments in which we manipulated group size and sex ratio to determine the relative roles of males and females in a harem polygyny. Megalorchestia californiana is an amphipod (family Talitridae) that is sexually dimorphic—males are not only larger than females but also possess enlarged red antennae and gnathopods that are used to hold and guard females prior to mating inside their burrows. We conducted 2 main types of experiments: 1) we varied the mobility of 3 amphipods (2M/1F) to determine the relative roles of male competition and female choice and 2) we monitored groups of 9 or more individuals to determine how intersexual and intrasexual interactions affect overall spatial distribution. In our 3-amphipod tests, we found that male competition was more important than female choice in explaining burrow preferences. Our large group experiments, however, indicated that females also play an active role in mate selection, as females settled in random locations in single-sex groups, whereas females clustered around larger males in mixed-sex groups. Finally, we found that male body mass was correlated with the length and redness of the antennae, which suggests that males may signal their quality through visual communication. Key words: honest signaling, intrasexual competition, mate choice, Megalorchestia californiana, polygyny, sexual dimorphism. [Behav Ecol]

Intersexual social behavior of urban white-tailed deer and its evolutionary implications

White-tailed deer ( Odocoileus virginianus (Zimmermann, 1780)) are now common in many urban environments throughout their geographic range. Yet, how male and female deer in the urban environment associate, behave socially, and the evolutionary implications of that behavior remains unstudied. We examined predictions of the predation risk and the social factor hypotheses to explain intersexual grouping patterns and social behavior observed in white-tailed deer inhabiting the small city of San Marcos (population size ~45 000) in central Texas. Two routes were surveyed weekly from a vehicle at dawn and dusk for 1 year. Group size, composition, distance to vehicle, and alarm state of deer to the vehicle were recorded. Focal animal sampling was used to measure the time males and females spent and number of aggressions within one body length of each other when in groups. Female-only groups were most prevalent year round followed by mixed-sex groups, which increased in prevalence in summer and during the mating season. Alarm state was weakly related to group size but not to group composition. Males were farther apart and more aggressive than females in groups. Proportion of males in mixed-sex groups declined with increased group size. Intersexual patterns of grouping and social behavior of urban deer supported the social factor hypothesis but not the predation risk hypothesis.

Exogenous vasotocin alters aggression during agonistic exchanges in male Amargosa River pupfish ( Cyprinodon nevadensis amargosae)

Pupfishes in the Death Valley region have rapidly differentiated in social behaviors since their isolation in a series of desert streams, springs, and marshes less than 20,000 years ago. These habitats can show dramatic fluctuations in ecological conditions, and pupfish must cope with the changes by plastic physiological and behavioral responses. Recently, we showed differences among some Death Valley populations in brain expression of arginine vasotocin (AVT). As AVT regulates both hydromineral balance and social behaviors in other taxa, these population differences may indicate adaptive changes in osmoregulatory and/or behavioral processes. To test whether AVT is relevant for behavioral shifts in these fish, here we examined how manipulations to the AVT system affect agonistic and reproductive behaviors in Amargosa River pupfish ( Cyprinodon nevadensis amargosae). We administered exogenous AVT (0.1, 1, and 10 μg/g body weight) and an AVP V 1 receptor antagonist (Manning compound, 2.5 μg/g body weight) intraperitoneally to males in mixed-sex groups in the laboratory. We found that AVT reduced the initiation of aggressive social interactions with other pupfish but had no effect on courtship. The effects of AVT were confirmed in males in the wild where AVT (1 μg/g body weight) reduced the aggressive initiation of social interactions and decreased aggressive responses to the behavior of other males. Combined, these results show that AVT can modulate agonistic behaviors in male pupfish and support the idea that variation in AVT activity may underlie differences in aggression among Death Valley populations.

Roosevelt Elk Density and Social Segregation: Foraging Behavior and Females Avoiding Larger Groups of Males

Intersexual social segregation at small spatial scales is prevalent in ruminants that are sexually dimorphic in body size. Explaining social segregation, however, from hypotheses of how intersexual size differences affects the foraging process of males and females has had mixed results. We studied whether body size influences on forage behavior, intersexual social incompatibility or both might influence social segregation in a population of Roosevelt elk (Cervus elaphus roosevelti) that declined 40% over 5 y. Most males and females in the population occurred in the same forage patches, meadows, but occupied different parts of meadows and most groups were overwhelming comprised of one sex. The extent of segregation varied slightly with changing elk density. Cropping rate, our surrogate of forage ingestion, of males in mixed-sex groups differed from males in male-only groups at high, but not low, elk density. In a prior study of intersexual social interactions it was shown that females avoided groups containing ≥6 males. Therefore, we predicted that females should avoid parts of meadows where groups of males ≥6 were prevalent. Across the 5 y of study this prediction held because ≤5% of all females were found in parts of meadows where median aggregation sizes of males were ≥6. Social segregation was coupled to body size influences on forage ingestion at high density and social incompatibility was coupled to social segregation regardless of elk density.

Possible audience effect in Thomas langurs (Primates; Presbytis thomasi): an experimental study on male loud calls in response to a tiger model.

Recent research indicates that animal vocalizations can refer to objects in the outside world, and that call production may be controlled by the sender depending on the type of audience involved. Our research on wild male Thomas langurs (Primates: Presbytis thomasi) shows that these males call as a response to a tiger model when they are in a group, but do not call when they are solitary. This is the first experimental study on wild primates to demonstrate that the presence or absence of an audience influences calling behavior. Our results indicate that males in mixed-sex groups give more loud calls than solitary males when exposed to a predator model. This suggests that giving loud calls in response to predators is not purely a reflex and may be controlled in some way by the sender.

SEXUAL SEGREGATION IN ROOSEVELT ELK: CROPPING RATES AND AGGRESSION IN MIXED-SEX GROUPS

Few studies of sexual segregation in ruminants have tested widely invoked mechanisms of segregation in mixed-sex groups. In a sexually segregated population of Roosevelt elk (Cervus elaphus roosevelti), we examined if adult males had reduced intake of forage when in mixed-sex groups and if intersexual differences in aggression caused females to avoid males. Based on a mechanistic model of forage intake, animals with lower instantaneous feed intake should have higher cropping rates. Focal animal sampling indicated that adult males and females in summer and winter had similar cropping rates in mixed-sex groups, whereas males in male-only groups had lower rates of cropping than males in mixed-sex groups. Outside the mating season, males in male groups spent proportionally less time #1 body length of congenders than females in female groups, and the rate of aggression #1 body length was higher for males. Female‐female aggression was higher in mixed-sex groups that contained more males than the median proportion of males in mixed-sex groups. Female and mixed-sex groups walked away when groups of males numbering .6 were #50 m but did not walk away when male groups #50 m had #5 individuals. Sexual segregation was associated with behaviors of sexes in mixed-sex groups: reduced intake of forage by males and increased female‐female aggression with more males.

Ingestion of Dye by the Sexes of Trichostrongylus colubriformis

Trichostrongylus colubriformis ingested the fluorescent dye, Rhodamine B, while feeding in vivo and in vitro. Ingestion by females of T. colubriformis in vivo significantly exceeded that of males from 15 to 30 days of infection. However, the rate of ingestion declined linearly in both sexes as the helminths' age increased. Uptake of dye in vitro was dependent on time and concentration. Ingestion by the sexes of T. colubriformis in vitro increased significantly with elevated temperature, length of time that the worms were outside the host, decreasing concentrations of salt in the incubation medium and acidic pH's. In vitro ingestion was similar in light or dark conditions. Saturation of the medium with oxygen or nitrogen/carbon dioxide significantly increased or decreased, respectively, ingestion by helminths. Uptake of dye by T. colubriformis decreased significantly in groups when nematodes were increased from 250 to 1,000 worms of a single sex in 2 ml of medium. In females there was a similar decrease in single- or mixed-sex groups containing more than 500 worms while ingestion by males in mixed-sex groups was significantly less than in groups containing only males.