Activity Budget Hypothesis Research Articles

Size dimorphism and sexual segregation in pheasants: tests of three competing hypotheses.

Fine scale sexual segregation outside of the mating season is common in sexually dimorphic and polygamous species, particularly in ungulates. A number of hypotheses predict sexual segregation but these are often contradictory with no agreement as to a common cause, perhaps because they are species specific. We explicitly tested three of these hypotheses which are commonly linked by a dependence on sexual dimorphism for animals which exhibit fine-scale sexual segregation; the Predation Risk Hypothesis, the Forage Selection Hypothesis, and the Activity Budget Hypothesis, in a single system the pheasant, Phasianus colchicus; a large, sedentary bird that is predominantly terrestrial and therefore analogous to ungulates rather than many avian species which sexually segregate. Over four years we reared 2,400 individually tagged pheasants from one day old and after a period of 8–10 weeks we released them into the wild. We then followed the birds for 7 months, during the period that they sexually segregate, determined their fate and collected behavioural and morphological measures pertinent to the hypotheses. Pheasants are sexually dimorphic during the entire period that they sexually segregate in the wild; males are larger than females in both body size and gut measurements. However, this did not influence predation risk and predation rates (as predicted by the Predation Risk Hypothesis), diet choice (as predicted by the Forage Selection Hypothesis), or the amount of time spent foraging, resting or walking (as predicted by the Activity Budget Hypothesis). We conclude that adult sexual size dimorphism is not responsible for sexual segregation in the pheasant in the wild. Instead, we consider that segregation may be mediated by other, perhaps social, factors. We highlight the importance of studies on a wide range of taxa to help further the knowledge of sexual segregation.

Sexual segregation in the Darwin's wild sheep,Ovis ammon darwini, (Bovidae, Artiodactyla), in the Mengluoke Mountains of Xinjiang, China

Sexual segregation has been found among many vertebrate species. Argali, like other sexually dimorphic Capridae, form sexually segregated groups outside the breeding season. The degree of sexual segregation and its seasonal changes have never been examined and quantified in Asiatic species of wild sheep. In this paper we are considering seasonal fluctuation in the degree of sexual segregation and check the activity budget hypothesis to explain this phenomenon in the Darwin's wild sheep (Ovis ammon darwini), a subspecies of argali sheep (Ovis ammon). The activity budget hypothesis states that sexually size-dimorphic males and females segregate into different groups due to incompatibilities in activity budgets and movement rates. We collected data on activity budgets in the argali sheep from 2015 to 2016, in the Mengluoke Mountains of Xinjiang Province, and used Conradt's segregation coefficient (SC) to measure the degree of sexual segregation outside and during the rutting seasons. Our results showed that the SC value was highest outside the rutting season (0.98), when argalis were almost completely segregated. During the rutting period, the segregation coefficient dropped more than twofold (0.43) compared to the non-rutting season. In addition, our data supported the activity budget hypothesis: female and male argali had different behavioural activity patterns during the non-rutting season. Female argali spent more time feeding, followed by resting, standing, moving, and other behaviours, while males spent most of their time resting, followed by feeding, standing, other behaviours, and moving. Female argali spent significantly more time feeding than males, while males spent significantly more time resting and in other behaviours than females. Activity synchronization indices for both female groups and male groups were significantly higher than in mixed-sex groups. Sexual segregation is the best behavioural strategy for sexually dimorphic ungulates and the activity budget hypothesis explained this phenomenon well in Darwin's wild sheep.

A Field Test of the Activity Budget Hypothesis: The Case of Blue Sheep in Helan Mountains of Ningxia, China

A Field Test of the Activity Budget Hypothesis: The Case of Blue Sheep in Helan Mountains of Ningxia, China

Test of the activity budget hypothesis on Asiatic ibex in Tian Shan Mountains of Xinjiang, China

Asiatic ibex (Capra sibirica) is a threatened species in China and is distributed in the mountains of Central Asia. It is a sexually dimorphic ungulate. The males are much larger than females, and except for the breeding season, both sexes live in separate groups. Many hypotheses have been developed to explain the sexual segregation in sexually dimorphic ungulates. These hypotheses are not mutually exclusive, and in recent years, the activity budget hypothesis has received special attention. To test this hypothesis, we studied the activity budget of Asiatic ibex in the autumn of 2005. According to the activity budget hypothesis, females should spend more time feeding than the males, and the degree of activity synchronization should be higher in same-sex groups than in mixed-sex groups. Our results are consistent with the hypothesis. Both sexes of the Asiatic ibex spent most of the time feeding, and females spent a significantly longer time feeding than males, and males spent a significantly longer time standing. Activity synchronization indexes of both the female groups and males groups were significantly higher than mixed-sex groups. These results indicated that in Asiatic ibex, the activity budget hypothesis about sexual segregation is supported.

Mixed-sex group formation by bighorn sheep in winter: trading costs of synchrony for benefits of group living

Animals commonly form groups and most sexually dimorphic ungulates form sex-specific groups outside the breeding season. Differences in diet, in vulnerability to predation, and in behaviour between the sexes are hypothesized to drive sexual segregation; however, low-density conditions sometimes favour aggregation. This may occur because larger groups provide greater antipredator benefits that overcome the cost of association between the sexes. In this study we sought to determine whether bighorn rams and ewes (Ovis canadensis) aggregate during winter as an antipredator strategy and whether they alter their behaviour to maintain mixed-sex groups. We compared the occurrence of mixed-sex groups during winter against the population size and sex ratio, and assessed vigilance as an indication of predation risk. We also analysed activity budgets to determine whether group cohesion required departure from sex-specific time budgets. We observed a low proportion of mixed-sex groups in all years of our study except one, in which the population size was very small and the proportion of females was the smallest on record. Mixed-sex groups were larger than single-sex groups, and vigilance was lower in larger groups, indicating that mixed-sex groups provided greater antipredator benefits. However, there was evidence of the oddity effect for older females, which displayed greater vigilance in groups composed primarily of males. Females modified their activity budgets in mixed-sex groups by spending less time foraging and travelling and more time bedding, providing support for the activity budget hypothesis. The costs of mixed-sex group formation limit the circumstances under which aggregation of the sexes is favourable.

Activity budgets and sociality in a monomorphic ungulate: the African oryx (Oryx gazella)

Sexual size dimorphism in ruminants is associated with social and (or) habitat segregation, while monomorphic species often live alone, in monogamous pairs, or in mixed-sex groups. Most research on sexual differences in behaviour has been done on sexually size-dimorphic ruminants, whereas little is known about monomorphic species. We tested the activity budget hypothesis which predicts that similar-sized individuals should have similar activity budgets, while dissimilarly sized individuals should differ in their behaviours. We studied African oryx ( Oryx gazella (L., 1758)), which are size-monomorphic, to investigate how age, sex, or reproductive state affect their foraging behaviour. We expected that males and females have similar activity budgets, except for lactating females who were expected to spend slightly more time foraging. Younger, smaller animals were predicted to differ in their activity budgets from adults, owing to high energy demands. We predicted that air temperature, vegetation height, and group sizes would be more important in shaping activity budgets and feeding rates than sex or reproductive differences. The sexes did not differ in their behaviour, while subadults spent less time active than adults. Temperature and vegetation height significantly affected their behaviour. We discuss the importance of environmental, as well as intrinsic, factors when studying sexual differences in behaviour and habitat use.

Trade‐offs of predation and foraging explain sexual segregation in African buffalo

1. Many studies have investigated why males and females segregate spatially in sexually dimorphic species. These studies have focused primarily on temperate zone ungulates in areas lacking intact predator communities, and few have directly assessed predation rates in different social environments. 2. Data on the movement, social affiliation, mortality and foraging of radio-collared African buffalo (Syncerus caffer) were collected from 2001-06 in the Kruger National Park, South Africa. 3. The vast majority of mortality events were due to lion (Panthera leo) predation, and the mortality hazard associated with being an adult male buffalo in a male-only 'bachelor' group was almost four times higher than for adult females in mixed herds. The mortality rates of adult males and females within mixed herds were not statistically different. Mortality sites of male and female buffalo were in areas of low visibility similar to those used by bachelor groups, while mixed herds tended to use more open habitats. 4. Males in bachelor groups ate similar or higher quality food (as indexed by percentage faecal nitrogen), and moved almost a third less distance per day compared with mixed herds. As a result, males in bachelor groups gained more body condition than did males in breeding herds. 5. Recent comparative analyses suggest the activity-budget hypothesis as a common underlying cause of social segregation. However, our intensive study, in an area with an intact predator community showed that male and female buffalo segregated by habitat and supported the predation-risk hypothesis. Male African buffalo appear to trade increased predation risk for additional energy gains in bachelor groups, which presumably leads to increased reproductive success.

Does the Jarman–Bell principle at intra-specific level explain sexual segregation in polygynous ungulates? Sex differences in forage digestibility in Soay sheep

The Jarman-Bell principle states that large-bodied mammalian herbivores can subsist on lower quality diets because of their lower metabolism requirement/gut capacity ratio. Two major hypotheses for sexual segregation (the behaviour in which animals of the same species aggregate by sex) base their foundations on extending this principle to the intraspecific level, despite the lack of experimental evidence to support this. The first proposes that the larger males can process fibre (low-quality diet) more efficiently than the smaller females, leading to sexual segregation by habitat partitioning due to selection of different food quality and/or quantity (sexual dimorphism-body size hypothesis). The second suggests that the longer time and extra rumination required to digest low-quality food will cause asynchrony of behaviour between males and females, which then leads to sexual segregation (activity budget hypothesis). To provide experimental evidence for the Jarman-Bell principle at the intraspecific level we carried out a set of digestibility trials in Soay sheep (Ovis aries) using grass hay as the diet to test whether sexual dimorphism in body mass can produce significant sexual differences in the efficiency of food digestion. Males were slightly more efficient in digesting forage than females that were at least 30% smaller than the males. Overall, there was a decrease in faecal output of 1 g/kg body mass in favour of males. These differences were not due to differences in food selection, passage rates or faecal particle size and it was not clear why males were more efficient in digesting forage. Although these results do not directly support arguments for either the sexual dimorphism-body size or activity budget hypotheses, they do indicate that the physiological argument upon which the Jarman-Bell principle is founded also operates at the intraspecific level and may be an important factor influencing sexual segregation.

Activity Budgets and Sexual Segregation in African Elephants (Loxodonta africana)

Abstract The activity budget hypothesis is 1 of 4 main hypotheses proposed to explain sexual segregation by large herbivores. Because of their smaller body size, females are predicted to have higher mass-specific energy requirements and lower digestive efficiency than males. As a result, females are expected to forage longer to satisfy their nutritional demands. Maintaining the cohesion of a mixed-sex group with differing activity budgets and asynchronous behavioral patterns is increasingly difficult, ultimately leading to spatial segregation of males and females. We tested this hypothesis using data (2002–2005) from 3 distinct populations of African elephants (Loxodonta africana), a species that exhibits marked sexual segregation. Group and individual behaviors were assessed at discrete points in time throughout the day, with a minimum of 10 min between consecutive records. Focal samples of individual male and female elephants also were recorded, with behavioral data logged every minute for 15 min. Data ...

TESTING SEXUAL SEGREGATION AND AGGREGATION: OLD WAYS ARE BEST

The study of sexual segregation has received increasing attention over the last two decades. Several hypotheses have been proposed to explain the existence of sexual segregation, such as the "predation risk hypothesis," the "forage selection hypothesis," and the "activity budget hypothesis." Testing which hypothesis drives sexual segregation is hampered, however, by the lack of consensus regarding a formal measurement of sexual segregation. By using a derivation of the well-known chi-square (here called the sexual segregation and aggregation statistic [SSAS]) instead of existent segregation coefficients, we offer a reliable way to test for temporal variation in the occurrence of sexual segregation and aggregation, even in cases where a large proportion of animals are observed alone. A randomization procedure provides a test for the null hypothesis of independence of the distributions of males and females among the groups. The usefulness of SSAS in the study of sexual segregation is demonstrated with three case studies on ungulate populations belonging to species with contrasting life histories and annual grouping patterns (isard, red deer, and roe deer). The existent segregation coefficients were unreliable since, for a given value, sexual segregation could or could not occur. Similarly, the existent segregation coefficients performed badly when males and females aggregated. The new SSAS was not prone to such limitations and allowed clear conclusions regarding whether males and females segregate, aggregate, or simply mix at random applicable to all species.

Sexual segregation in Tibetan gazelle: a test of the activity budget hypothesis

AbstractTibetan gazelle Procapra picticaudata, a threatened endemic species on the Qinghai‐Tibet Plateau, is a sexually dimorphic ungulate. Males are larger than females and both sexes live in different groups, except during the rutting season. In order to test the activity budget hypothesis, which was proposed to explain social segregation in ungulates, we studied the activity budget of male and female Tibetan gazelles during the summers of 2005 and 2006. The activity budget hypothesis predicts that females spend more time feeding and group members synchronize their activities more in the same‐sex groups than in the mixed‐sex groups. We found that females and males of Tibetan gazelle had different activity budgets; females spent significantly longer time feeding and comparably less time on other activities. Activity synchronization indexes between female groups and male groups were similar (female groups: 0.81±0.17, male groups: 0.80±0.19), whereas both of them were significantly higher than that of mixed‐sex groups (0.58±0.29). These results suggest that although sexual segregation might be caused by multiple mechanisms, the activity budget hypothesis about sexual segregation is supported by our studies of Tibetan gazelle on the Qinghai‐Tibet Plateau.

WHY DO POLYGYNOUS UNGULATES SEGREGATE IN SPACE? TESTING THE ACTIVITY-BUDGET HYPOTHESIS IN SOAY SHEEP

Sexual segregation is the behavior in which animals of different sex in a species live in separate groups outside the mating season. Recently a new concept, namely, the “activity-budget hypothesis,” has claimed to be the ultimate explanation of this behavior. The new hypothesis explains not only sexual segregation, but also segregation between animals of different size within sex (i.e., social segregation). The hypothesis states that the activity patterns of animals will differ when big differences in body mass exist between them, because of the associated difficulties of the synchronization in behavior making it costly to form groups, leading to segregation by size. Here we tested the assumptions and predictions of the activity-budget hypothesis using 40 Soay sheep (Ovis aries) as the model species in a 2.3-ha experimental arena. Sheep were divided into treatment groups to test the effect of sex, body mass, and food supplementation in their activity budgets, behavioral synchronization, diet composition, intake, food digestibility, and spatial segregation. Our animals segregated by sex but not by size, and food supplementation did not affect the spatial distribution of any sex, which is all against the predictions of the hypothesis. We also found sexual differences in dry-matter digestibility independent of body mass, which questions the Jarman-Bell principle at the intra-specific level, which is the foundation for some other hypotheses of sexual segregation. Increasing behavioral synchronization led to segregation, but at the same time forming groups facilitates synchronization, so it is unclear which (i.e., synchronization or segregation) is the cause and which the consequence. Our results do not support the activity-budget hypothesis and clearly indicate that there is no strong association between behavioral synchronization and segregation.

Sexual segregation in vertebrates: proximate and ultimate causes

Sexual segregation is very common in vertebrates that live in groups. In this article, I will review proximate and ultimate causes of sexual segregation in social species and in particular in ungulates in which the bulk of research on the topic has been carried out. In most social ungulate species, males and females live in separate groups outside the breeding season, sometimes using different home ranges and types of habitat. In most of these species, males are larger than females. Dimorphism in body size can lead to sexual differences in ecology and behavior making it difficult for the two sexes to stay in the same group. It is important for our better understanding of the evolution of sociality, sexual dimorphism and different mating systems to determine why sexual segregation is so widespread not only in ungulates but also in other vertebrates. In this article, I discuss the ecology of the two sexes by reviewing proximate and ultimate causes of sexual segregation. To do this, I compare a range of studies of ruminants and include explanations for social segregation as well as for habitat segregation by gender. This leads into a review and updates current knowledge of the phenomenon. Although I present a number of different hypotheses, I focus in particular on predation risk, forage selection and activity budget and discuss the social-factors hypothesis. I stress that the key in solving the enigma of sexual segregation lies in clearly separating hypotheses that try to explain social segregation and habitat segregation, as well as in including experiments or model systems. To that end, I present a preliminary study on a test of the activity-budget hypothesis in three-spine sticklebacks and explain why I believe that shoaling fish are useful for analysing the underlying processes and mechanisms that lead to sexual segregation in animals. Lastly, I argue that it is unlikely that a single factor can explain social segregation or habitat segregation but that a model integrating different factors and different levels of segregation might succeed in describing proximate and ultimate causes of sexual segregation.

Sexual segregation among feral goats: testing between alternative hypotheses

Sexual segregation occurs widely in ungulates, and four principal hypotheses (predation, forage quality, social preferences and activity patterning) have been suggested as possible explanations. However, no single explanation has so far received widespread support. We used data on the grouping behaviour of feral goats, Capra hircus, on the Isle of Rum, Scotland, U.K. to test between competing hypotheses. We first used Conradt's (1999, Animal Behaviour, 57, 1151–1157) test to show that sexual segregation is not a consequence of sex differences in habitat preference. Since the predation risk hypothesis can be ruled out by both the absence of important predators on Rum and the tendency for those individuals most sensitive to predation (mother–offspring pairs) to be the furthest away from their neighbours, any explanation for sexual segregation must either lie in differences in activity patterning or be the consequence of a social preference for associating with same-sex individuals. We analysed data on the patterns of fission, behavioural synchrony and neighbourhood in relation to party size and composition to show that the activity budget hypothesis is the more likely. However, the data also suggest that the social preferences hypothesis none the less has a residual influence on segregation tendencies of the Rum goats. We conclude that one likely reason for the conflicting results in the literature may be that all four hypotheses in fact apply simultaneously, but that their relative weightings may depend on habitat- and species-specific characteristics.

Testing five hypotheses of sexual segregation in an arctic ungulate

1. Sexual segregation occurs in most species of sexually dimorphic ungulates. At least five not mutually exclusive hypotheses have been formulated to explain patterns of social, habitat and spatial segregation; the indirect competition hypothesis (H1), the nutritional needs hypothesis (H2), the activity budget hypothesis (H3), the weather sensitivity hypothesis (H4), and the predation risk hypothesis (H5). 2. Each hypothesis has a unique set of predictions with respect to the occurrence of segregation, and how seasonality, density dependence and reproductive status affect sexual segregation. 3. We tested this set of predictions in order to separate the hypotheses H1-H5 for patterns of sexual segregation of the Svalbard reindeer based on 9 years data on seasonal estimates of spatial, habitat and social (i.e. grouping with their own sex) segregation in combination with resource selection functions. 4. Our results do not support that one single mechanism causes segregation. The activity budget hypothesis, the nutritional needs hypothesis and the weather sensitivity hypothesis were all partially supported, while the predation risk hypothesis was discarded for Svalbard reindeer because predators have been absent for at least 5000 years. Several mechanisms are thus interacting to explain the full-year pattern of sexual segregation and the combination of mechanisms varies among species and populations.

Sexual dimorphism, activity budget and synchrony in groups of sheep

The activity budget hypothesis has been proposed to explain the social segregation commonly observed in ungulate populations. This hypothesis suggests that differences in body size--i.e. between dimorphic males and females--may account for differences in activity budget. In particular, if females spend more time grazing and less time resting than males, activity synchrony would be reduced. Increased costs of maintaining synchrony despite differences in activity budget would facilitate group fragmentation and instability of mixed-sex groups. In this paper two prerequisites of the activity budget hypothesis were tested: (1) that males should spend less time feeding and more time resting than females in single-sex groups and (2) that lower activity synchrony should be observed in mixed-sex compared to single-sex groups. The activity budget and synchrony in mixed and single-sex groups of merino sheep (Ovis aries) of different sizes (2, 4, 6, 8 individuals) were measured in three contiguous 491-m2 arenas located in a natural pasture. Three same-size groups, one of each category, were observed simultaneously. We found no sexual differences in the time spent inactive and active (i.e. grazing, standing, moving, interacting). Males spent significantly more time grazing and less time standing than females. These differences disappeared when yearling males were omitted from the group. Males and females had similar bite and step rates. Sheep of both sexes spent less time resting and more time grazing and moving and had lower bite rates when in mixed-sex groups than when in single-sex groups. The synchrony among visually isolated groups was near zero, indicating that they changed activities independently. On the contrary, within-group synchrony was high; however it was higher in single-sex groups, in particular for males, than in mixed-sex groups. Our results suggest that differences in activity budget and synchrony alone are insufficient to explain social segregation.

Can the activity budget hypothesis explain sexual segregation in western grey kangaroos?

In many sexually dimorphic and polygynous species, individuals exhibit social segregation by grouping with others of their own sex. I tested one hypothesis proposed to explain this form of sexual segregation. The activity budget hypothesis proposes that segregation evolved because sex differences in body size induce asynchrony in activity between the sexes, leading to lower stability of mixed-sex than same-sex groups. Specifically, I examined two assumptions of this hypothesis in western grey kangaroos, Macropus fuliginosus, which show strong social segregation: (1) that differences in body size result in activity budget differences, and consequently activity asynchrony, and (2) that activity asynchrony causes mixed-body size groups to be less cohesive than same-body size groups. Definitive tests of the first assumption are lacking in ungulates, because body size is confounded with sex in most ungulates. However, kangaroo populations are composed of a spectrum of body sizes: young adult males are the same size as adult females, and older males may be more than double that size. By exploiting this heteromorphism, I found that differences in body size do not result in activity budget differences, and consequently activity asynchrony in kangaroos: individuals of different body size did not have different transition times between feeding and resting, sex-size classes were not least synchronized when in mixed-body size groups, and synchrony was not lower among mixed-body size than same-body size groups. Activity asynchrony did not cause mixed-body size groups to be less cohesive than same-body size groups in kangaroos. Although mixed-body size groups were fused for less time than same-body size groups, synchrony was not correlated with group stability. These results question the validity of the activity budget hypothesis as a universal cause for social segregation.

Alternating sexual segregation during the mating season by male African buffalo (Syncerus caffer)

AbstractPatterns of association in many ungulate species vary throughout the year, with males periodically segregating from females. Sexual segregation can occur if males and females use different habitats (habitat segregation), occupy exclusive ranges (spatial segregation) or occur in separate social groups (social segregation). Previous studies on social segregation in ungulates have focused mostly on non‐reproductive periods and species in temperate systems with short mating seasons. This study examined social segregation in African buffalo Syncerus caffer at Hluhluwe‐iMfolozi Park, South Africa, during their 6‐month mating season. During this period, mature male buffalo alternated between joining mixed‐sex herds which comprise females and young, and small all‐male groups. Activity patterns of male buffalo in both group types were examined to determine if differences in time allocation or activity asynchrony could explain the observed segregation. Mature males foraged for less time than both adult females and younger males when they were with mixed herds. Mature male buffalo differed in how their active time was apportioned to foraging and reproductive activities between the two group types. Females maintained group cohesion despite only low synchrony in activity, therefore segregation of male buffalo could not be explained by the activity budget hypothesis. For males in mixed‐sex herds, reproductive activities were apparently at the expense of foraging time. The high time and energy costs of reproductive activities were probably responsible for the periodic separation of male buffalo from mixed herds during the extended mating period.

Sexual segregation in bison: a test of multiple hypotheses

AbstractSexual segregation, in which males and females form separate groups for most of the year, is common in sexually dimorphic ungulates. We tested multiple hypotheses to explain sexual segregation in bison (Bison bison) at National Bison Range, Montana and Fort Niobrara National Wildlife Refuge, Nebraska during June-August of 2002-2003. Fieldwork involved use of GPS to record space use by segregated groups, vegetation transects to measure forage availability, fecal analyses to document diet composition and quality, and behavioural observations to characterize activity budgets. During sexual segregation, males in bull groups used areas with greater per capita abundance of forage, higher proportion of weeds, and less nutritious grasses (as indicated by lower % fecal nitrogen) compared with females in cow or mixed groups. However, there was no difference between the sexes in activity budgets, predation risk factors, or distance to water. Single-sex bull groups were no more synchronized in activity than mixed groups. These results support the 'sexual dimorphism-body size hypothesis', which proposes that males segregate from females because their larger body size requires more abundant forage, while longer ruminal retention permits efficient use of lower-quality forage. The gastrocentric model, based on the digestive physiology and foraging requirements of dimorphic ungulates, supplies the most likely proximate mechanism for bison sexual segregation. Our results would also partly support the 'reproductive strategy-predation risk hypothesis' if females form large groups to reduce predation risk. The predictions of the 'activity budget hypothesis' were not supported for bison.

An experimental test of hypotheses explaining social segregation in dimorphic ungulates

In many sexually dimorphic ungulates, adults spend most of their time in single-sex groups outside the mating season. We tested two hypotheses proposed to explain this social segregation. The activity budget hypothesis predicts that females spend more time grazing than males, and that activity synchrony is higher within single-sex than mixed-sex groups. The social affinity hypothesis states that higher spatial proximity and aggregation within the sexes results from sexual differences in motivation to interact socially. Fifteen merino sheep, Ovis aries , 2 or more years old, of each sex were penned together in three 1-ha paddocks in succession during 7 winter weeks. Despite the high sexual dimorphism in body weight, both sexes were found together, and did not differ substantially in activity budget. Males grazed less than females but the difference was small. The degree of activity synchrony was similar within and between the sexes. When grazing, males walked more rapidly and tended to be at the front of the group more often than females. Ewes were rarely seen interacting with other sheep, in contrast to males, which interacted twice as often with females as with males. However, same-sex pairs of nearest neighbours were found more frequently than expected, whether or not the males were at the front of the group. These results indicate a social segregation at a small spatial scale, which can be explained by differences in speed of movement between males and females and/or higher social affinity between same-sex than opposite-sex peers. The lack of single-sex groups can be explained by the strong gregariousness of merino sheep, and possibly the young age of the adult males.