Relationship between reversed sexual dimorphism,breeding investment and foraging ecology in a pelagic seabird,the masked booby

Reversed sexual dimorphism (RSD) may be related to different roles in breeding investment and/or foraging, but little information is available on foraging ecology. We studied the foraging behaviour and parental investment by male and female masked boobies, a species with RSD, by combining studies of foraging ecology using miniaturised activity and GPS data loggers of nest attendance, with an experimental study where flight costs were increased. Males attended the chick more often than females, but females provided more food to the chick than males. Males and females foraged during similar periods of the day, had similar prey types and sizes, diving depths, durations of foraging trips, foraging zones and ranges. Females spent a smaller proportion of the foraging trip sitting on the water and had higher diving rate than males, suggesting higher foraging effort by females. In females, trip duration correlated with mass at departure, suggesting a flexible investment through control by body mass. The experimental study showed that handicapped females and female partners of handicapped males lost mass compared to control birds, whereas there was no difference for males. These results indicate that the larger female is the main provisioner of the chick in the pair, and regulates breeding effort in relation to its own body mass, whereas males have a fixed investment. The different breeding investment between the sexes is associated with contrasting foraging strategies, but no clear niche differentiation was observed. The larger size of the females may be advantageous for provisioning the chick with large quantities of energy and for flexible breeding effort, while the smaller male invests in territory defence and nest guarding, a crucial task when breeding at high densities. In masked boobies, division of labour appears to be maximal during chick rearing—the most energy-demanding period—and may be related to evolution of RSD.     

The role of foraging behaviour in the sexual segregation of the African elephant

Elephants (Loxodonta africana) exhibit pronounced sexual dimorphism, and in this study we test the prediction that the differences in body size and sociality are significant enough to drive divergent foraging strategies and ultimately sexual segregation. Body size influences the foraging behaviour of herbivores through the differential scaling coefficients of metabolism and gut size, with larger bodied individuals being able to tolerate greater quantities of low-quality, fibrous vegetation, whilst having lower mass-specific energy requirements. We test two distinct theories: the scramble competition hypothesis (SCH) and the forage selection hypothesis (FSH). Comprehensive behavioural data were collected from the Pongola Game Reserve and the Phinda Private Game Reserve in South Africa over a 2.5-year period. The data were analysed using sex as the independent variable. Adult females targeted a wider range of species, adopted a more selective foraging approach and exhibited greater bite rates as predicted by the body size hypothesis and the increased demands of reproductive investment (lactation and pregnancy). Males had longer feeding bouts, displayed significantly more destructive behaviour (31% of observations, 11% for females) and ingested greater quantities of forage during each feeding bout. The independent ranging behaviour of adult males enables them to have longer foraging bouts as they experience fewer social constraints than females. The SCH was rejected as a cause of sexual segregation due to the relative abundance of low quality forage, and the fact that feeding heights were similar for both males and females. However, we conclude that the differences in the foraging strategies of the sexes are sufficient to cause spatial segregation as postulated by the FSH. Sexual dimorphism and the associated behavioural differences have important implications for the management and conservation of elephant and other dimorphic species, with the sexes effectively acting as distinct “ecological species”.     

Sex‐specific food provisioning in a monomorphic seabird,the common guillemot Uria aalge: nest defence,foraging efficiency or parental effort?

Sexual differences in food provisioning rates of monomorphic seabirds are well known but poorly understood. Here, we address three hypotheses that attempt to explain female-biased food provisioning in common guillemots Uria aalge : (1) males spend more time in nest defence, (2) females have greater foraging efficiency, and (3) males allocate a greater proportion of foraging effort to self-maintenance. We found that males spent no more time with chicks than females but made longer trips and travelled further from the colony. There was extensive overlap between sexes in core foraging areas, indicating that females were not excluding males from feeding opportunities close to the colony. However, as a result of their longer trips, the total foraging areas of males were much greater than those of females. There was no difference between sexes in overall dive rate per hour at sea, in behaviour during individual dives or in a number of other measures of foraging efficiency including the frequency, depth and duration of dives and the dive: pause ratio during the final dive bout of each trip, which was presumably used by both sexes to obtain prey for the chick. These data strongly suggest that sexes did not differ in their ability to locate and capture prey. Yet males made almost twice as many dives per trip as females, suggesting that males made more dives than females for their own benefit. These results support the hypothesis that female-biased food provisioning arose from a difference between sexes in the allocation of foraging effort between parents and offspring, in anticipation of a prolonged period of male-only post-fledging care of the chick, and not from differences in foraging efficiency or time spent in nest defence.     

The individual counts: within sex differences in foraging strategies are as important as sex‐specific differences in masked boobies Sula dactylatra

Understanding how animals allocate their foraging time is a central question in behavioural ecology. Intrinsic factors, such as body mass and size differences between sexes or species, influence animals’ foraging behaviour, but studies investigating the effects of individual differences in body mass and size within the same sex are scarce. We investigated this in chick‐rearing masked boobies Sula dactylatra, a species with reversed sexual dimorphism, through the simultaneous deployment of GPS and depth‐acceleration loggers to obtain information on foraging movements and activity patterns. Heavier females performed shorter trips closer to the colony than lighter females. During these shorter trips, heavier females spent higher proportions of their flight time flapping and less time resting on the water than lighter females did during longer trips. In contrast, body mass did not affect trip duration of males, however heavier males spent less time flapping and more time resting on the water than lighter males. This may occur as a result of higher flight costs associated with body mass and allow conservation of energy during locomotion. Body size (i.e. wing length) had no effect on any of the foraging parameters. Dive depths and dive rates (dives h^?1) were not affected by body mass, but females dived significantly deeper than males, suggesting that other factors are important. Other studies demonstrated that females are the parent in charge of provisioning the chick, and maintain a flexible investment under regulation of their own body mass. Variation in trip length therefore seems to be triggered by body condition in females, but not in males. Consequently, shorter trips are presumably used to provision the chick, while longer trips are for self‐maintenance. Our findings underline the importance of accounting for the effects of body mass differences within the same sex, if sex‐specific foraging parameters in dimorphic species are being investigated.     

Sex differences in Adélie penguin foraging strategies

Consistent sex differences in foraging trip duration, feeding locality and diet of breeding Adélie penguins (Pygoscelis adeliae) were demonstrated at two widely separated locations over several breeding seasons. Differences in foraging behaviour were most pronounced during the guard stage of chick rearing. Female penguins made on average longer foraging trips than males, ranged greater distances more frequently and consumed larger quantities of krill. In contrast, males made shorter journeys to closer foraging grounds during the guard period and fed more extensively on fish throughout chick rearing. Mean guard stage foraging trip durations over four seasons at Béchervaise Island, Eastern Antarctica and over two seasons at Edmonson Point, Ross Sea ranged between 31 and 73 h for females and 25 and 36 h for males. Ninety percent of males tracked from Béchervaise Island by satellite during the first 3 weeks post-hatch foraged within 20 km of the colony, while the majority (60%) of females travelled to the edge of the continental shelf (80–120 km from the colony) to feed during this period. Received: 10 December 1997 / Accepted: 10 April 1998     

The three-dimensional flight of red-footed boobies: adaptations to foraging in a tropical environment?

In seabirds a broad variety of morphologies, flight styles and feeding methods exist as an adaptation to optimal foraging in contrasted marine environments for a wide variety of prey types. Because of the low productivity of tropical waters it is expected that specific flight and foraging techniques have been selected there, but very few data are available. By using five different types of high-precision miniaturized logger (global positioning systems, accelerometers, time depth recorders, activity recorders, altimeters) we studied the way a seabird is foraging over tropical waters. Red-footed boobies are foraging in the day, never foraging at night, probably as a result of predation risks. They make extensive use of wind conditions, flying preferentially with crosswinds at median speed of 38 km h(-1), reaching highest speeds with tail winds. They spent 66% of the foraging trip in flight, using a flap-glide flight, and gliding 68% of the flight. Travelling at low costs was regularly interrupted by extremely active foraging periods where birds are very frequently touching water for landing, plunge diving or surface diving (30 landings h(-1)). Dives were shallow (maximum 2.4 m) but frequent (4.5 dives h(-1)), most being plunge dives. While chasing for very mobile prey like flying fishes, boobies have adopted a very active and specific hunting behaviour, but the use of wind allows them to reduce travelling cost by their extensive use of gliding. During the foraging and travelling phases birds climb regularly to altitudes of 20-50 m to spot prey or congeners. During the final phase of the flight, they climb to high altitudes, up to 500 m, probably to avoid attacks by frigatebirds along the coasts. This study demonstrates the use by boobies of a series of very specific flight and activity patterns that have probably been selected as adaptations to the conditions of tropical waters.     

BIOLOGY OF THE BANK CORMORANT,PART 3: FORAGING BEHAVIOUR

Cooper, J. 1985. Biology of the Bank Cormorant, Part 3: Foraging behaviour. Ostrich 56: 86–95.

The Bank Cormorant Phalacrocorax neglectus, endemic to southern Africa, is primarily a solitary inshore forager. Bank Cormorants forage Primarily on the bottom among kelp beds but also may forage over shingle or coarse sand substrates or in midwater. Breeding birds forage up to 9 km from their colony. Little is known of foraging depth but birds may dive as deep as 28 m. Mean dive duration was 44,9 s and ratio of dives to surface rests was 2,18. In most cases prey is swallowed under water, presumably to avoid kleptoparasitism. Bank Cormorants foraged during daylight hours from before sunrise to after sunset. Birds did not forage in exceptionally rough seas. Mean female foraging bout duration (84,3 min) was significantly longer than that of males (68,4 min) in breeding individuals. Breeding males undertook significantly more foraging bouts (3,47 boutdday) than did females (3,02 bouts/day). No significant differences were found between the sexes when total time spent foraging/day by breeding birds was compared. It is not clear why males foraged more often, but for shorter periods, than did females, but the differences may be related to sexual dimorphism, males being larger than females.     

Sex-specific foraging behaviour in a monomorphic seabird

Sexual differences in the foraging behaviour of parents have been observed in a number of sexually sizedimorphic birds, particularly seabirds, and the usual inference has been that these sex-specific differences are mediated primarily by differences in body size. To test this explanation, we compared the foraging behaviour of parents in a monomorphic seabird species, the northern gannet Morus bassanus. Using specially designed instruments and radio telemetry we found that individuals of both sexes were consistent in the directions and durations of their foraging trips. However, there were significant differences in the foraging behaviour of males and females. Female gannets were not only more selective than males in the areas where they foraged, but they also made longer, deeper dives and spent more time on the sea surface than males. As the sexes are morphologically similar in this species, then these differences are unlikely to have been mediated by body size. Our work highlights the need to investigate sexual differences in the foraging behaviour of seabirds and other species more closely, in order to test alternative theories that do not rely on differences in body size.     

THE EVOLUTION OF REVERSED SEXUAL DIMORPHISM IN SIZE IN MONOGAMOUS SPECIES OF BIRDS

Reversed sexual dimorphism in size (RSD) occurs in most species of several taxonomic groups of birds. The hypotheses proposed to explain this phenomenon are examined theoretically, using inequalities to state selection in the most rigorous possible terms. The most pertinent empirical evidence is also examined critically. Proponents of hypotheses on the evolution of RSD have failed to consider the genetic constraints on the evolution of dimorphism. Selection for dimorphism can act on only that small portion of the genetic determination of body size that is sex limited. In general, selection for body size is much more likely to lead to a similar change (e.g. larger) in both sexes than to dimorphism. The most popular hypotheses involve selection for size-related differences in foraging ability. It is unlikely that there is variation in size-related foraging differences available for selection in a monomorphic, ancestral population. Foraging differences between the sexes cannot lead to the evolution of RSD; evolution of large and small morphs of both sexes is a more likely outcome. Selection for sex-role differentiation factors (e.g. large females lay larger eggs, small males are more agile in flight) can lead to the evolution of RSD, but only if the magnitudes of opposing selection for small males and for large females are equal. Combining selection for size-related foraging differences with selection for sex-role differentiation factors hinders the evolution of RSD until the sexes differ in size by 3 s.d . Empirical evidence supports this assertion: statistically significant differences between the sexes in the size of prey taken are found only in highly dimorphic species. The sex-role differentiation factors that have been proposed appear unlikely to provide the equal selection necessary for the evolution of RSD. Several authors have proposed that small size in males is selected for foraging ability and large size in females for some sex-role differentiation factor. Males cannot be more efficient foragers without females being less efficient and efficiency cannot be a factor only when the male is feeding his family. RSD cannot evolve in monogamous species if large females survive less well than small males. RSD might evolve as the result of sexual selection for small size in males and constraints on the reduction of size in females because of some factor associated with reproduction. Examination of seven studies indicating a relationship between female size and reproductive success shows very little unequivocal evidence for small size in females allowing breeding earlier in the season. Large size in females allows females to breed at a younger age in the sparrowhawk and pairs to form more rapidly in three species of sandpipers. Both of these may be the result of sexual selection. There are fewer theoretical problems with sexual selection as a cause for the evolution of RSD than with the other hypotheses. Empirical evidence for sexual selection is scarce but better than that for the other hypotheses. Evidence is contradictory for the selection of small size in males for agility in aerial displays for courtship or defence of territory. Large size in females does not appear to be the result of selection for competitive ability to obtain mates. Facilitation of female dominance and hence of the formation and maintenance of a pair bond is the most viable explanation of the evolution of RSD. It is most likely that all dimorphism (normal or reversed) is the result of sexual selection. RSD is correlated with birds in the diet in the Falconiformes and this is a central theme in the foraging hypotheses. This correlation may be because birds are abundant and available in a continuum of sizes, thus permitting but not causing the evolution of RSD or because species that prey upon birds are better equipped physically (and perhaps more likely behaviourally) to inflict damaging attacks on conspecifics and the greater RSD increases female dominance and the ease of pair formation.     

Female and male Leach's Storm Petrels (Hydrobates leucorhous) pursue different foraging strategies during the incubation period

Reproduction in procellariiform birds is characterized by a single egg clutch, slow development, a long breeding season and obligate biparental care. Female Leach's Storm Petrels Hydrobates leucorhous, nearly monomorphic members of this order, produce eggs that are between 20 and 25% of adult bodyweight. We tested whether female foraging behaviour differs from male foraging behaviour during the ~ 44-day incubation period across seven breeding colonies in the Northwest Atlantic. Over six breeding seasons, we used a combination of Global Positioning System and Global Location Sensor devices to measure characteristics of individual foraging trips during the incubation period. Females travelled significantly greater distances and went farther from the breeding colony than did males on individual foraging trips. For both sexes, the longer the foraging trip, the greater the distance. Independent of trip duration, females travelled farther, and spent a greater proportion of their foraging trips prospecting widely, as defined by behavioural categories derived from a hidden Markov Model. For both sexes, trip duration decreased with date. Sex differences in these foraging metrics were apparently not a consequence of morphological differences or spatial segregation. Our data are consistent with the idea that female foraging strategies differed from male foraging strategies during incubation in ways that would be expected if females were still compensating for egg formation.     

Sex differences in diet and foraging behavior in white-faced capuchins (Cebus capucinus)

I examined sex differences in diet and foraging behavior in two groups of white-faced capuchin monkeys (Cebus capucinus)in a tropical dry forest at Santa Rosa National Park, Costa Rica. I tested three hypotheses: sex differences in diet and foraging behavior are best explained by (1) sexual dimorphism, (2) the energy demands of pregnancy and lactation for females;and (3) avoidance of competition between the sexes. Sexual dimorphism offered the best explanation of sex differences in the diet and foraging behavior of C. capucinus,accurately predicting that males do more strenuous foraging activity, make less use of small foraging supports, and spend more time on or near the ground. Females spent more time foraging than males did but probably obtained a lower protein yield per unit foraging time. Females exploited more small and embedded invertebrates, while males ate more large invertebrates and vertebrate prey. Pregnant and lactating females spent more time resting and less time foraging than other females did, increasing their foraging return by focusing on foods requiring little handling. There was little evidence of competition avoidance between the sexes.     

Sex‐specific foraging behaviour in tropical boobies: does size matter?

Sex differences in the foraging behaviour of adults have been observed in a number of sexually size-dimorphic birds, and the usual inference has been that these sex-specific differences are driven primarily by differences in body size. An alternative explanation is that foraging differences result from sex differences unrelated to size, such as sex-specific nutritional requirements. To examine these alternative hypotheses, the foraging behaviour of parents was compared between two sympatric and congeneric species of seabird, the Brown Booby Sula leucogaster , which is highly sexually size-dimorphic (females 38% larger) and the Red-footed Booby S. sula , in which sex differences in body size are less marked (females 15% larger). Using temperature and depth loggers, we found that there were highly significant differences in the foraging trip durations and diving behaviour of male and female Brown Boobies. These sex differences were less marked in Red-footed Boobies. Thus, our interspecies comparison revealed that the magnitude of the difference between the sexes matched the sexual size dimorphism of the species, providing support for the size hypothesis.     

Sex-specific chick provisioning and diving behaviour in the wedge-tailed shearwater Puffinus pacificus

Mechanisms that drive sex-specific foraging behaviour in seabirds are not fully understood. In some cases, sexual-size dimorphism has been implicated. However, recent empirical work indicates that foraging behaviour may also differ between sexes of monomorphic seabird taxa. We simultaneously examined sex-specific differences in adult foraging behaviour, chick provisioning rates and maximum dive-depths in a monomorphic seabird, the wedge-tailed shearwater Puffinus pacificus . We found significant divergence between sexes. Mean foraging trip length was longer, provisioning rate lower and mean maximum dive-depth shallower in females. We found no evidence of divergence in foraging behaviour due to condition-dependant increases in self-provisioning by females, or differences in the nest attendance patterns of each sex. In addition, chick body condition did not influence meal mass or trip length differently in one or other sex. Consistent with results obtained for dimorphic species we suggest that inter-sexual competition at the foraging grounds provides the most parsimonious explanation for the sex-specific differences observed in this monomorphic species. Based on our findings we believe this possibility warrants further critical investigation.     

Feeding time strategies of the fringe-toed lizard,Uma inornata,during breeding and non-breeding seasons

Summary I examined the foraging behavior during the breeding and non-breeding seasons, May and July 1986, of the fringe-toed lizard Uma inornata (Iguanidae). During the breeding season males differ from females in their diet and in their foraging time strategy, males exhibiting time minimization and females energy maximization. In May, plant associated foods were selectively eaten. Males concentrated on flowers, a readily available quick energy food, which reduced foraging time and increased time for reproductive activities. Time budgets indicate that males spend over twice as much time in the open and in movement in May than do females. Females at this time restrict their activities to the cover of perennial bushes, and feed primarily on plant foods (flowers and arthropods). Energy maximization appears to be maintained by both sexes in the non-breeding season when food resources diminished to one-half of those in the breeding season. The lizards were less selective in their July feeding habits, broadening their diets to include ground-dwelling arthropods and foliage. Predation by these lizards follows a wait-ambush mode of foraging.     

Sex and individual differences in foraging behavior of Japanese cormorants in years of different prey availability

Japanese cormorants, Phalacrocorax capillatus, are sexually dimorphic seabirds with males that are heavier and that dive deeper than females. Sex differences in prey composition and foraging behavior of those rearing chicks at Teuri Island, Hokkaido, were examined by collecting food-loads from parents in 1992–1998 and by radio-tracking ten birds each in 1997 and 1998 when prey availability was different. Males fed more on benthic and epibenthic fishes (82% mass) than did females (34%) while females fed more on epipelagic and coastal fishes (53%) than did males (18%). Males made longer dives (53 s) at feeding sites closer to the island (7 km) than females (39 s, 13 km) in 1997. In 1998 when the availability of epipelagic fish seemed to be higher, there were no sex differences in dive duration and distance to the feeding sites (35 s and 9 km for males, 36 s and 10 km for females). This sex difference in foraging behavior with a poor availability of epipelagic fish suggests that high diving ability possibly enables males to feed on demersal fish. Birds specializing in coastal shallow waters around the island made long dives; hence they were probably foraging in bottom layers. Those foraging in deeper shelf waters made short dives and they were thought to forage in surface layers. Electronic Publication     

Sexual dimorphism in relation to winter foraging in the white-backed woodpecker ( Dendrocopos leucotos)

Male white-backed woodpeckers (Dendrocopos leucotos) in a 250-km² study area in western Norway are significantly larger than females in bill length and depth, wing and tarsus lengths, and bodyweight. During the winters (October–March 1985–2002), most pairs were observed within their breeding territory where both sexes foraged mainly in grey alder and birch trees, and visited trees of the same tree height and stem width. However, males foraged more frequently on dead trees and on trees broken by storms. Males also used more trees with less bark cover, foraged nearer the ground and used foraging sites of larger diameter. Furthermore, males practised more deep wood-pecking and less bark-pecking than females. Unlike in other sexually dimorphic woodpecker species, the foraging niche breadth in wintering white-backed woodpeckers showed only minor sexual differences, and the sexes overlapped significantly in all parameters examined. Since previous studies in the area have shown that the sexes overlap considerably in use of their territory, it was expected, as found in other size dimorphic woodpeckers, that the larger male would displace the supposedly socially subordinate female to suboptimal feeding sites. In our area, the sexes were rarely seen together, and no sign of aggression between the sexes was observed. Despite the sex-specific differences found in the foraging behaviour of the birds, it is not obvious how the differences should be related to size dimorphism.Communicated by F. Bairlein     

Variance‐Sensitive Green Woodhoopoes: A New Explanation for Sex Differences in Foraging?

Studies of cooperatively breeding birds rarely benefit from the extensive research on adaptive foraging behaviour, despite the potential for concepts such as state‐dependent foraging to explain many aspects of behaviour in social groups. For example, sex differences in preferred foraging techniques used by green woodhoopoes, Phoeniculus purpureus, have previously been explained by sexual dimorphism in bill length and the benefits afforded by foraging specialization and niche differentiation within cooperative groups. Contrary to this argument, there were no sex differences in mean foraging success and/or prey size captured when males and females used the same foraging techniques. Subordinates of both sexes did experience lower and more varied foraging success compared with dominants, but probably only as a consequence of competition or inexperience. However, dominant males experienced greater variance in individual foraging success compared with dominant females, and dominant males also experienced greater variances in prey size when using their preferred foraging techniques. Dominant males therefore appeared to specialize in foraging techniques that provided more variable rewards, whilst dominant females consistently chose to minimize variation in reward. Dominant females also experienced less variance in foraging returns when using the same techniques as males, suggesting a possible link with sexual dimorphism in bill length. Partitioning of foraging niches in dominant green woodhoopoes therefore appears to be better explained by sex differences in variance (risk) sensitivity to foraging rewards. We suggest that this kind of detailed analysis of state‐dependent foraging has the potential to explain many of the crucial age and sex differences in behaviour within cooperative groups.     

Overnight foraging trips by chick‐rearing Nazca Boobies Sula granti and the risk of attack by predatory fish

Most tropical booby species complete breeding foraging trips within daylight hours, thus avoiding nights at sea. Nazca Boobies Sula granti are unusual in this respect, frequently spending one or more nights away from the nest. We used GPS dataloggers, time‐depth recorders, and changes in body weight to characterize foraging trips and to evaluate potential influences on the decisions of 64 adult Nazca Boobies to spend a night at sea, or to return to their chicks on Isla Española, Galápagos, in daylight hours. The tagged birds foraged east of Isla Española, undertaking both single‐day (2–15 h, 67% of trips) and overnight trips (28 h–7.2 days, 33%), and executing 1–19 foraging plunge‐dives per single‐day trip. Birds might forage longer if they are in nutritional stress when they depart, but body weight at departure was not correlated with trip length. Birds might be expected to return from longer trips with more prey for young, but they returned from single‐day and overnight trips with similar body weights, consistent with previous indications that Nazca Boobies forage until accumulating a target value of prey weight. Birds with a lower dive frequency during the first 5 h of a trip were more likely to spend the night at sea, suggesting that they might choose to spend the night at sea if prey capture success was low. At night, birds almost never dived and spent most of their time resting on the water’s surface (11.8–12.1 h, > 99% of the time between civil sunset and civil dawn). Thus, the night is an unproductive time spent among subsurface predators under low illumination. The birds’ webbed feet provided evidence of this risk: 24% of birds were missing > 25% of their foot tissue, probably due to attacks by predatory fish, and the amount of foot tissue lost increased with age, consistent with a cumulative risk across the lifespan. In contrast, other tropical boobies (Blue‐footed Sula nebouxii and Brown Boobies Sula leucogaster), which do not spend the night on the water, showed no such damage. These results suggest that chick‐rearing Nazca Boobies accept nocturnal predation risk on occasions of low prey encounter during a foraging trip’s first day.     

Blood metabolite levels in normal and handicapped pied flycatchers rearing broods of different sizes

We measured levels of select metabolites (glucose, triglycerides, free fatty acids, glycerol, uric acid) and corticosterone in the blood plasma of adult pied flycatchers Ficedula hypoleuca while they were rearing broods whose sizes were modified experimentally. We also made it more difficult than normal for some pairs of birds to forage by removing certain wing and tail feathers (handicapping them). Both procedures have been shown previously to change parental workload. We did this in order to determine if the birds alter their use of nutrients in response to differences in their workload. Metabolite levels were not influenced by handicapping or brood size. However, the concentration of free fatty acids in the plasma of females and of triglycerides in the plasma of males was directly related to the frequency with which the adults fed their nestlings. These findings suggest that the two sexes have different ways of coping with the work associated with rearing the brood: females apparently undergo brief daily fasts while feeding their chicks, whereas males take more time to feed themselves while providing food for their young, and spend more time doing so as their workload increases. The flycatchers exhibited high concentrations of uric acid and corticosterone in the blood plasma; corticosterone and glycerol were positively correlated in females; and corticosterone and triglyceride levels were negatively correlated in males; all of which suggest that gluconeogenesis provides some of the energy required for their parental activities.     

Body size dimorphism and sexual segregation in polygynous ungulates: an experimental test with Soay sheep

Sexual segregation in Soay sheep (Ovis aries) was investigated using an experimental approach in order to test the sexual dimorphism-body size hypothesis. Two corollaries of the sexual dimorphism-body size hypothesis were tested: (1) in dimorphic species males, the larger sex, have relatively smaller bite sizes on short swards because of the scaling of incisor arcade with body weight, and (2) they move off earlier to feed on taller but poorer-quality swards when such swards are patchily distributed on a scale which enables the spatial segregation of individuals. Patch choice between sexes was estimated using a matrix of grass patches which differed in both quality and biomass of grass on offer (HQ: high-quality-low-biomass; LQ: low-quality-high-biomass). Sex differences in patch choice and grazing behaviour were tested in short-term preference trials. Incisor breadth showed no significant difference between sexes. On the other hand, muzzle width was dimorphic, with females having a narrower muzzle than males. Bite size was significantly different between the sexes, being smaller in females than in males, although it was not significantly different between sward types. Females had a higher bite rate than males and the bite rate was higher in the HQ sward type than the LQ sward type. When the effect of body mass was removed, no sex differences in muzzle size, bite size or bite rate were found. The intake rate did not differ between the sexes or between sward types. Whilst both sexes preferred the HQ sward type, females spent a significantly longer time feeding on the LQ sward type than did males. The difference detected between the sexes in patch choice was not consistent directly with the sexual dimorphism-body size hypothesis. Alternative explanations based on sex differences in foraging behaviour in relation to body mass sexual dimorphism are discussed to explain the result. Received: 1 February 1999 / Accepted: 12 May 1999