On the move around the clock: correlates and determinants of cathemeral activity in wild redfronted lemurs (<Emphasis Type="Italic">Eulemur fulvus rufus</Emphasis>)

Whether animals are active at night or during the day has profound consequences for many aspects of their behavioral ecology. Because of ecological and physiological trade-offs, most animals, including primates, are either strictly nocturnal or diurnal. However, a few primate species exhibit cathemeral activity, i.e., their activity is irregularly distributed throughout the 24-h cycle. Details and determinants of this unusual activity pattern are poorly understood because long-term 24-h observations are not feasible in the field. We therefore used small data loggers to record the activity of cathemeral redfronted lemurs (Eulemur fulvus rufus) from several neighboring groups quantitatively and continuously over a complete annual cycle in order to evaluate various proposed proximate and ultimate determinants of cathemeral activity. Activity data were examined for variation as a function of ambient temperature, time of day, lunar phase, and season. We found that cathemeral activity occurred year-round and that, on average, 3.5 times more activity occurred during the day. Total and diurnal activity increased during the long days of the austral summer. Nocturnal activity increased during the longer nights of the cool dry season. Irrespective of season, lunar phase had a significant effect on the distribution of activity across the 24-h cycle, with most nocturnal activity recorded during parts of the night with greatest brightness. These data indicate that light availability is the primary proximate determinant for the patterning of cathemeral activity. Several lines of evidence suggest that cathemerality in lemurs has evolved from nocturnal ancestors and that it represents a transitory state on the way to the diurnal niche.Communicated by F. Trillmich     

Arrived in the light: diel and seasonal activity patterns in wild Verreaux’s sifakas (<Emphasis Type="Italic">Propithecus v. verreauxi</Emphasis>; Primates: Indriidae)

Madagascar is characterised by pronounced annual climatic and ecological seasonality and harbours a radiation of closely related sympatric primates (Lemuriformes) that exhibit diurnal, nocturnal and cathemeral activity patterns. We collected continuous activity data over a complete annual cycle from wild diurnal Verreauxs sifakas (Propithecus v. verreauxi) to contribute detailed and comparative data: (1) to characterise their diel and seasonal activity rhythms, (2) to identify factors shaping variation in activity rhythms, and (3) to help reconstruct the evolutionary transition from nocturnal to diurnal activity. We fitted eight adult sifakas from seven different groups living in Kirindy forest with an accelerometer/data logger device and recorded their activity in 5-min bins for a total of 12 months. We characterise P. verreauxi as a strictly diurnal species with a pronounced bimodal activity pattern that briefly changed to a more unimodal pattern during their annual mating season (January to March). We documented significant annual variation in total daily activity, activity time, and activity level, as well as in most parameters characterising their bimodal activity pattern. Despite a significant positive correlation of the animals activity time with day length, pronounced annual variation in the phase position of onset and end of activity in relation to sunrise and sunset times could also be discerned. Minor enhancing effects of moonlight on nocturnal activity were only found for the first 3 h of the inactivity period. Bimodality of the activity pattern and the additional reduction of activity time during the cold and dry winter months associated with reduced food availability can be interpreted as flexible behavioural adaptations to reduce energy expenditure. We therefore propose that energetic and thermoregulatory benefits are important factors shaping these primates activity pattern.Communicated by C. Nunn     

Feeding over the 24-h cycle: dietary flexibility of cathemeral collared lemurs (<Emphasis Type="Italic">Eulemur collaris</Emphasis>)

Animals show specific morphological, physiological and behavioural adaptations to diurnal or nocturnal activity. Cathemeral species, i.e. animals with activities distributed over the 24-h period, have to compromise between these specific adaptations. The driving evolutionary forces and the proximate costs and benefits of cathemerality are still poorly understood. Our goal was to evaluate the role of predator avoidance, food availability and diet quality in shaping cathemeral activity of arboreal mammals using a lemur species as an example. For this, two groups of collared lemurs, Eulemur collaris, were studied for 14 months in the littoral forest of southeastern Madagascar. Data on feeding behaviour were collected during all-day and all-night follows by direct observation. A phenological transect containing 78 plant species was established and monitored every 2 weeks to evaluate food availability during the study period. Characteristics of food items and animal nutritional intake were determined via biochemical analyses. The ratio of diurnal to nocturnal feeding was used as response variable in the analyses. The effects of abiotic environmental variables were removed statistically before the analyses of the biotic variables. We found that diurnal feeding lasted longer during the hot–wet season (December–February), whereas nocturnal feeding peaked during the hot–dry and cool–wet seasons (March–August). Although the lemurs foraged mostly in lower forest strata during daylight and used emergent trees preferably at night, the variables which measured animal exposure to birds of prey failed to predict the variation of the ratio of diurnal/nocturnal feeding. Ripe fruit availability and fiber intake are the two variables which best predicted the annual variation of the lemur diurnality. The data indicate that feeding over the whole 24-h cycle is advantageous during lean periods when animals have a fibre-rich, low-quality diet.     

A dusk chorus effect in a nocturnal bird: support for mate and rival assessment functions

The dawn chorus is a striking feature of spring mornings and a characteristic behaviour of many bird species, particularly the passerines. Dawn singing has been considered a reliable signal of male quality for mate and rival assessment. Singing is presumed to be relatively costly at dawn both because air temperatures are relatively low and because birds have not fed overnight. Models of optimal daily routine predict the existence of a “dusk chorus” in nocturnal birds, although this prediction has received little empirical attention. Nocturnal birds at dusk may be energy-limited because of a lack of daytime feeding, and singing at dusk may thus ensure signal reliability. Here, we used an observational and experimental approach to study vocal behaviour at dusk and dawn in a nocturnal raptor, the little owl Athene noctua. We assess whether male little owls adjust their vocal behaviour according to feeding stage (i.e. period of the night), ambient air temperature and territorial context (i.e. spontaneous calling behaviour vs elicited calling by intrusion). Across different temperatures, we find that both spontaneous vocal activity and inter-individual variability in call duration increased at dusk, clearly indicating a dusk chorus phenomenon. Results from playback presentations suggest that food, rather than air temperature, is likely to be more constraining at dusk. We discuss how comparing dusk and dawn choruses in nocturnal and diurnal species can provide insights into both mechanistic and functional aspects of signalling behaviour.     

Relative intensity of predation on the French grunt, Haemulon flavolineatum, during diurnal, dusk, and nocturnal periods on a coral reef

Established hypotheses state that the rate of predation on coral reef fish should be highest during crepuscular periods (dawn and dusk) intermediate diurnally, and lowest nocturnally. We examined the relative risk of predation on juvenile French grunts (Haemulon flavolineatum Desmarest) during diurnal, dusk, and nocturnal periods on the fore- and back-reef at Teague Bay, U.S. Virgin Islands in July and August 1996. Tethering-devices recorded the exact amount of time between attaching a prey fish to its tether and subsequent predation on the prey fish. As tethering of prey usually inflates the actual rate of predation, times from our tethering devices were used to establish only the relative predation risk among treatments. During 3-h diurnal and nocturnal tethering experiments, relative predation was significantly higher during the nocturnal period, and differences between side of reef were not significant. In 30-min tethering experiments, which included all three time periods, the relative predation risk was significantly higher during dusk and nocturnal periods than during the diurnal period. Relative predation was not significantly different between the dusk and nocturnal periods, or between side of reef during any time period. The unexpected finding that the diurnal period had the lowest relative risk of predation indicates that the timing of predation events on reefs, as well as the adaptive reasons for nocturnal larval settlement, may need to be re-examined. Received: 11 February 1997 / Accepted: 21 October 1998     

Voices in the dark: predation risk by owls influences dusk singing in a diurnal passerine

Predation is an important cost of communication in animals and thus a potent selection pressure on the evolution of signaling behavior. Heterospecific eavesdropping by predators may increase the vulnerability of vocalizing prey, particularly during low light, such as at dusk when nocturnal predators are actively hunting. Despite the risk it entails, dawn and dusk chorusing is common in passerines. However, the dusk chorus has not been studied much, neglecting the opportunity for understanding how eavesdropping between predators and prey may shape communication in birds. Here, we report the first demonstration of simulated predation risk (playback of owl vocalizations) altering the dusk chorus of a diurnal passerine, the veery (Catharus fuscescens). Veeries have a pronounced dusk chorus, singing well after sunset and potentially exposing themselves to predation by owls. In response to brief playbacks of owl calls (~30 s of calls presented three times over 25 min), veeries sang fewer songs post-sunset and stopped singing earlier relative to control trials. These changes in singing remained evident 30 min after the last owl stimulus. Although the avian dusk chorus has received relatively little attention to date, our results suggest that the dusk chorus may pose a higher predation risk to singing males that may influence the evolution of singing behavior in diurnal birds.     

Time stress,predation risk and diurnal–nocturnal foraging trade-offs in larval prey

Insect larvae increase in size with several orders of magnitude throughout development making them more conspicuous to visually hunting predators. This change in predation pressure is likely to impose selection on larval anti-predator behaviour and since the risk of detection is likely to decrease in darkness, the night may offer safer foraging opportunities to large individuals. However, forsaking day foraging reduces development rate and could be extra costly if prey are subjected to seasonal time stress. Here we test if size-dependent risk and time constraints on feeding affect the foraging–predation risk trade-off expressed by the use of the diurnal–nocturnal period. We exposed larvae of one seasonal and one non-seasonal butterfly to different levels of seasonal time stress and time for diurnal–nocturnal feeding by rearing them in two photoperiods. In both species, diurnal foraging ceased at large sizes while nocturnal foraging remained constant or increased, thus larvae showed ontogenetic shifts in behaviour. Short night lengths forced small individuals to take higher risks and forage more during daytime, postponing the shift to strict night foraging to later on in development. In the non-seasonal species, seasonal time stress had a small effect on development and the diurnal–nocturnal foraging mode. In contrast, in the seasonal species, time for pupation and the timing of the foraging shift were strongly affected. We argue that a large part of the observed variation in larval diurnal–nocturnal activity and resulting growth rates is explained by changes in the cost/benefit ratio of foraging mediated by size-dependent predation and time stress.     

Time-lapse cinematography of feather stars (Echinodermata: Crinoidea) on the Great Barrier Reef,Australia: demonstrations of posture changes,locomotion, spawning and possible predation by fish

Aspects of feather star behavior and ecology were recorded by time-lapse cinematography approximately 1 frame min^-1 on the Great Barrier Reef, Australia over a 1 mo period in 1983. The current regime influenced body postures of most species studied, whether nocturnal or not. Moreover, feather stars of several species crawled on the substratum with their arms; each crawling episode lasted roughly 10 min, and the maximum speed attained was about 1 arm length min^-1. Nocturnal feather stars crawled to their nighttime feeding perches around dusk and crawled back to their daytime hiding places around dawn. Surprisingly, some species of feather stars living on the reef surface both day and night also crawled around at dawn and dusk for reasons that are not known. In the time-lapse films, and individual of Comanthus bennetti (sex undetermined) spawned for about 2 min just after dark on 5 July 1983. Another film showed possible predation on a feather star (Himerometra robustipinna) by a saddled coralfish (Chaetodon ephippium).     

Diet composition and feeding intensity of horse mackerel,<Emphasis Type="Italic"> Trachurus trachurus</Emphasis> (Osteichthyes: Carangidae) in the eastern Adriatic

Diet composition and feeding intensity of the horse mackerel, Trachurus trachurus, collected in the eastern Adriatic Sea, were examined. Stomach contents of 1,200 specimens, taken at monthly intervals (January–December 1996), were analyzed. The stomachs contents consisted of five major prey groups: Crustacea (Euphausiacea, Mysidacea, Decapoda), Cephalopoda and Teleostei. Euphausiacea constituted the most important food resource by weight, number and frequency occurrence. Teleosts were the second most important food category, while mysids, decapods and cephalopods were occasional food. There was little seasonal variation in diet. Euphausiids were dominant prey during all seasons, and were especially abundant from spring and autumn. Feeding intensity of horse mackerel varied during the year. The lowest intensity of feeding was recorded in winter (February, March) and early spring (April) during the lower sea temperature as well as at the time of intensive spawning. Feeding activity increased upon spawning period (May and June) and was also higher during summer. Feeding intensity and diet composition changed during the diurnal cycle. The highest feeding intensity was recorded at night and during early morning hours. Euphausiids, mysids and a greater part of teleosts dominated night and morning diet, while decapods and cephalopods were most frequently in the daily and evening diet.Communicated by O. Kinne, Oldendorf/Luhe     

Response of a Group of Sichuan Snub-Nosed Monkeys to Commercial Logging in the Qinling Mountains, China

Abstract:  Habitat destruction is one of the greatest threats to primates worldwide. To understand the impact of forest logging on the habitat use of primates in temperate mixed forest, we compared the range, habitat used, population size, and diet of a troop (ERT) of Sichuan snub-nosed monkeys ( Rhinopithecus roxellana ) in the Qinling Mountains before (1989), during (1997), and after (2002–2003) commercial logging. Logging significantly changed the composition of the forest and the food supply for the troop. Some areas were heavily logged and formed patches in the forest that lacked canopy cover. The troop moved 7 km away from their original range when logging took place and returned to their original range after logging stopped, but they avoided heavily logged areas that lacked canopy cover. Their movement indicated some degree of site fidelity in this species. Diet and home range changed after logging, but the population size remained stable, which suggests that this species has some ability to adapt to habitat changes. Our results may reflect a natural flexibility in primates to adapt to the changing food resources in temperate areas with marked seasonal variations in food availability and distribution. This flexibility may have contributed to their higher degree of resilience to habitat alterations caused by human activities compared with tropical forest primates that have a more specialized diet. Our findings provide important baseline information that will help decision makers in their efforts to conserve primates, especially in temperate regions, and to sustainably manage primate habitat.     

Movement and activity patterns of the European lobster, Homarus gammarus, revealed by electromagnetic telemetry

European lobsters, Homarus gammarus (L.), were tracked on an artificial reef in Poole Bay on the south coast of England using an electromagnetic telemetry system which monitored movements between reef units and recorded body movements (pitching and rolling) detected with a tilt switch incorporated into the transmitting tag. Several environmental variables (water temperature, light, hydrostatic pressure, current velocity and direction) were recorded simultaneously by the telemetry system, which was self-contained on the seabed. Movements between units of the artificial reef (excursions outside shelter) were predominantly nocturnal, peaking 1.5 to 3 h after sunset and returning to low levels shortly before dawn. A marked decline in the number of inter-reef unit movements from late summer to winter was related to decreasing water temperature rather than to daytime light level, wave height or tidal range. Activity indicated by the tilt switch was also greater at night, but declined gradually from a peak early in the night to a minimum at around midday, on average, implying a degree of activity within reef units during daylight. As with movements between reef units, activity declined seasonally with decreasing water temperature; in addition, the diel pattern of activity disappeared in winter. Received: 9 February 1998 / Accepted: 24 July 1998     

Field observations on the emergence rhythm of the Norway lobsterNephrops norvegicus,using different methods

The emergence ofNephrops norvegicus (L.) from their burrows was studied on several fishing grounds around Scotland at 28 to 62 m depth. Various methods were used: sampling by trawl and baited creels, observations by television and time-lapse cameras. Peak emergence generally occurred around dusk and dawn. In shallow water (30 m) there was also a significant level of activity during the intervening night period. Day/night variations in the size composition of catches suggest that relatively more time is spent out of the burrow with increasing size. There was good agreement between the various methods employed, suggesting that trawl and camera avoidance has little influence on the pattern of emergence observed. The use of bait in some experiments confirmed that emergence is essentially a feeding activity.     

The relationship between lunar periodicity and activity of fish-parasitic gnathiid isopods in the Caribbean

Gnathiid isopod larvae are members of the marine demersal zooplankton community and are common ectoparasites of coral reef fishes, emerging from the substratum, mostly at night and crepuscular periods to feed on fish blood. Given that the activity of many marine organisms is often linked to changes in the phase of the moon, we examined the relationship between lunar phase and activity in gnathiid isopods on Caribbean reefs. We employed two sampling methods to quantify gnathiid activity: (1) Emergence traps set on reefs over a 24 h period; and (2) live fish placed in cages on reefs and retrieved during night and dawn peaks in gnathiid activity. These were compared during discrete phases as well as a continuous metric, the lunar “angle”, and an estimate of ambient light availability (luminance). Lunar phase and angle varied in their statistical effect on gnathiid activity patterns by sampling method and/or year. Luminance had a significant but inconsistent effect on measures of gnathiid activity. Our results suggest that changes in the lunar cycle are not a strong predictor of gnathiid activity at our shallow reef study sites.     

Feeding and burrowing ecology of two East African mangrove crabs

The behavior and ecology of two mangrove crabs,Sesarma meinerti De Man, 1887 andCardisoma carnifex (Herbst, 1794) were investigated at the beginning of the rainy season (October–November 1988) at Mida Creek, Kenya. Both species occupy upper intertidal levels, above mean high-water neap, and completely overlap in their zonation. Each burrow lasts ca. 3 wk, with no significant difference between the species. Both are more active around dusk and dawn and also follow a similar trend in their foraging activity, but differ in that burrowing is mainly diurnal inC. carnifex and nocturnal inS. meinerti. A hierarchy of food preference, established by offeringC. carnifex andS. meinerti leaves of five mangrove species, proved similar for both, withBruguiera gymnorhiza ranking first andAvicennia marina last, but significant only forC. carnifex. A rough estimate of the amount of litter consumed by these two species and of the soil mixed up by their burrowing activity indicates that they play a role of primary importance in the ecology of East African mangroves.     

Persistent bimodal activity patterns in wild and captive black-tailed godwit <Emphasis Type="Italic">Limosa limosa</Emphasis> under different environmental conditions

There is a large literature dealing with daily foraging routines of wild birds during the non-breeding season. While different laboratory studies have showed that some bird activity patterns are a persistent property of the circadian system, most of field studies preclude the potential role of an endogenous circadian rhythm in controlling bird’s foraging routines. In this study we compared the patterns of diurnal foraging activity and intake rates of migrating black-tailed godwits, Limosa limosa (radio-tagged and non-tagged individuals) at two stopover sites (habitats) with different environmental characteristics, aiming at identifying proximate factors of bird activity routines. To gain insights into the role of food availability in control of such foraging routines, we also estimated foraging activity patterns in captive godwits subjected to constant food availability. Captive and wild black-tailed godwits showed a persistent bimodal activity pattern through daylight period. Food availability had a significant effect on the intake rates, but had a subtler effect on foraging and intake rate rhythms. Temperature and wind speed (combined in a weather index) showed non-significant effects on both rhythms. Although we could not discard a role for natural diurnal changes in light intensity, an important timing cue, our findings support the idea that an endogenous circadian rhythm could be an important proximate factor regulating foraging activity and food items taken per unit time of wild black-tailed godwits during migration.     

Diurnal and nocturnal feeding rate in Kentish plovers <Emphasis Type="Italic">Charadrius alexandrinus</Emphasis> on an intertidal flat as recorded by telescopic video systems

To elucidate the effects of light, site, temperature, time after emersion, and wind speed on foraging attempt rate (FAR) (attempts/unit time) and feeding success (FS) (captures/attempts) in the obligate visual foraging shorebird, Kentish plovers Charadrius alexandrinus, field observations were performed at a sandflat in Tokyo Bay, using a visible-light telescope camcorder during the day and a thermal infrared telescope camcorder at night. The re-analysis capability and frame-step replay of highly magnified video-images can contribute to accurate measurement of feeding behaviour over conventional focal observation techniques. Kentish plovers increased both FAR and FS in areas of high prey (polychaetes and crabs) density, resulting in a synergistic increase in feeding rate (captures/unit time). In areas of high prey abundance, FAR was higher at lower wind speed. FS increased with increasing time after emersion. Increasing temperatures induced a positive effect on FAR but a negative effect on FS. The effect of light on FAR was not observed; however, time-to-defecation occurrence was 2.2-fold shorter at night than during the day, indicating that feeding rate and FS are higher at night. These results are attributable to an increase in availability of cues due to higher nocturnal activity in polychaetes. Since available foraging time (emersion time) at night was 1.7-fold longer than during the day in the present study period, Kentish plovers could capture 3.7-fold (2.2 × 1.7) more prey at night than during the day. Results from this study indicate that nocturnal feeding in overwintering plovers is not a compensation but a major foraging activity to meet their energy requirements. An erratum to this article can be found at     

Parasitic flies (Diptera: Phoridae) influence foraging rhythms and caste division of labor in the leaf-cutter ant,Atta cephalotes (Hymenoptera: Formicidae)

Summary Three lines of evidence, including interspecific comparisons, temporal division of foraging between size castes, and experimental manipulations, suggest that the diurnal parasitoid Neodohrniphora curvinervis (Diptera: Phoridae) influences both the caste sizes and numbers of leaf-cutter ants (Atta cephalotes) that leave their underground nests to collect leaves. At Parque Nacional Corcovado in Costa Rica, A. cephalotes was attacked by Neodohrniphora during the daytime, and foraged less during the day than at night; a closely related ant at the same site, A. colombica, had no phorid parasites and foraged exclusively during the day. Most daytime foragers of A. cephalotes were smaller than the lower size threshold for attack by Neodohrniphora, while nocturnal foragers, active when parasitoids were absent, were both larger than this threshold and within the energetically optimal size range for foraging. When I supplied artificial lighting to allow phorids to hunt at A. cephalotes colonies past dusk, ants foraged less than when light was provided but flies were removed. The influence of Neodohrniphora on the foraging activity of A. cephalotes may explain why investigations focusing on abiotic factors have largely failed to discover what drives this ant's daily foraging cycles, and suggests that forager sizes are influenced not only by energetic efficiency, but also by the threat of parasitism.     

Diurnal and nocturnal feeding rate in Kentish plovers <Emphasis Type="Italic">Charadrius alexandrinus</Emphasis> on an intertidal flat as recorded by telescopic video systems

To elucidate the effects of light, site, temperature, time after emersion, and wind speed on foraging attempt rate (FAR) (attempts/unit time) and feeding success (FS) (captures/attempts) in the obligate visual foraging shorebird, Kentish plovers Charadrius alexandrinus, field observations were performed at a sandflat in Tokyo Bay, using a visible-light telescope camcorder during the day and a thermal infrared telescope camcorder at night. The re-analysis capability and frame-step replay of highly magnified video-images can contribute to accurate measurement of feeding behaviour over conventional focal observation techniques. Kentish plovers increased both FAR and FS in areas of high prey (polychaetes and crabs) density, resulting in a synergistic increase in feeding rate (captures/unit time). In areas of high prey abundance, FAR was higher at lower wind speed. FS increased with increasing time after emersion. Increasing temperatures induced a positive effect on FAR but a negative effect on FS. The effect of light on FAR was not observed; however, time-to-defecation occurrence was 2.2-fold shorter at night than during the day, indicating that feeding rate and FS are higher at night. These results are attributable to an increase in availability of cues due to higher nocturnal activity in polychaetes. Since available foraging time (emersion time) at night was 1.7-fold longer than during the day in the present study period, Kentish plovers could capture 3.7-fold (2.2 × 1.7) more prey at night than during the day. Results from this study indicate that nocturnal feeding in overwintering plovers is not a compensation but a major foraging activity to meet their energy requirements.     

Interdemic variation in mixed-species association patterns: common diurnal primates of Kibale National Park, Uganda

We used interdemic variation in the tendency to form mixed-species groups to examine the costs and benefits of association among the primates of Kibale National Park, Uganda. A year-long survey of six sites revealed that the amount of time that the five common diurnal primates [red colobus (Procolobus tephrosceles), black-and-white colobus (Colobus guereza), redtail monkeys (Cercopithecus ascanius), blue monkeys (Cercopithecus mitis), and grey-cheeked mangabeys (Lophocebus albigena)] spent in mixed-species groups varied dramatically among sites. In many cases, the proportion of time that species associated was positively related to their densities. By using detailed behavioral observations of redtail monkeys and red colobus made over 4 years (2660 h) at four sites, we were able to reject the null hypothesis that associations occur by chance for only one of four sites. However, a correlative approach exploring the costs and benefits of association suggests that ecological variables do influence association patterns. We found that redtail monkeys and red colobus overlapped in diet (19.2% of their foraging effort) and traveled further when in mixed-species groups than when alone. Having demonstrated this, we examined the applicability of the ecological constraints model for predicting the proportion of the time spent in mixed-species groups based on food availability. For this analysis we concentrated on red colobus from the site with 35 months of observation and demonstrated that their tendency to be in mixed- species groups was related to food availability. We used two methods to examine if mixed-species associations function to decrease predation risk. First, chimpanzees are known to prey heavily on red colobus, but rarely kill other primates. The time red colobus spent in mixed-species groups was correlated to chimpanzee density, but it was not for the other monkey species, suggesting that mixed-species groups serve to decrease predation risk. Second, when red colobus groups contain more infants and are presumably at the greatest risk of predation, they form mixed-species groups most often. These results demonstrate that the costs and benefits of mixed-species associations vary dramatically over small spatial and temporal scales. If such variation is generally the case, then studies conducted at different locations or different times could easily highlight the importance of difference selective agents in favoring mixed-species associations. Received: 10 February 1999 / Received in revised form: 16 September 1999 / Accepted: 2 October 1999