Optimal diving behaviour for foraging in relation to body size

Overall, large animals dive longer and deeper than small animals; however, after the difference in body size is taken into account, smaller divers often tend to make relatively longer dives. Neither physiological nor theoretical explanations have been provided for this paradox. This paper develops an optimal foraging diving model to demonstrate the effect of body size on diving behaviour, and discusses optimal diving behaviour in relation to body size. The general features of the results are: (1) smaller divers should rely more heavily on anaerobic respiration, (2) larger divers should not always make longer dives than smaller divers, and (3) an optimal body size exists for each diving depth. These results explain the relatively greater diving ability observed in smaller divers, and suggest that if the vertical distribution of prey in the water column is patchy, there is opportunity for a population of diving animals to occupy habitat niches related to body size.     

Trade-offs between social behaviour and foraging by sheep in heterogeneous pastures

In heterogeneous pastures, groups of sheep may have to alter their social behaviour in order to graze patches of preferred vegetation. In this event, patch size, inter-patch distance and the contrast between patch and background vegetation are likely to affect behaviour. In this experiment, groups of five female Scottish Blackface sheep grazed for 2-h periods in 0.1ha grass plots containing seven 1.5mx1.5m patches of improved vegetation, with inter-patch distances of 1.5, 6 or 11.5m. Background vegetation was of either medium or poor quality. Control plots contained no patches. On average, sheep spent 44% of the time grazing patches, although patches comprised only 1.6% of the total plot area. Inter-patch distance did not affect accumulated time spent grazing patches during the first 30min, but patch residence time and the number of sheep on a patch increased with inter-patch distance. The distribution of nearest neighbour distances was altered when patches were 6 or 11.5m apart, compared to no patches. Accumulated time spent grazing patches and the number of sheep on a patch were greater with poor than medium backgrounds. Sheep visited patches frequently and for short periods and it is suggested that sheep often moved off patches as a result of competition. The results provide evidence that sheep make trade-offs between social and foraging behaviour and demonstrate the importance of interactions between social spacing and the size and spacing of vegetation patches, when sheep forage in heterogeneous pastures.     

Diving behaviour in relation to water temperature in the southern elephant seal: foraging implications

Summary A time-depth-temperature recorder provided a continuous record of diving by a female southern elephant seal in relation to water temperature for 27 days (1939 dives) after completion of moult. Mean maximum dive depth was 391±2.6 m and the overall maximum was 775 m. Dives lasted on average 17.5±0.09 min. Most dives showed a rapid descent to the discontinuity between the cold surface water and warmer deep water. Consequently the seal spent 57% of its time while diving at a depth of 200–400 m when it may have been foraging. This strongly suggests that the seal was exploiting a food source at the discontinuity between vertically stratified water masses. The water temperature data also indicated that the seal was diving in waters south of the Antarctic Polar Front and at some distance from the northern edge of the pack ice. The seal spent 88% of its time under water. Normal surface intervals between dives lasted an average of 2.1 ± 0.1 min whereas 16 extended surface intervals (>10 min duration) lasted 32.7±4.6 min. Dives were deeper during the day than at night and all but one extended surface interval occurred at night. The pattern of dives was similar to records from northern elephant seals but this is the first study to show how diving behaviour relates to water temperature.     

The relationship between foraging behaviour and energy expenditure in Antarctic fur seals

By using time-depth recorders to measure diving activity and the doubly-labelled water method to determine energy expenditure, the relationship between foraging behaviour and energy expenditure was investigated in nine Antarctic fur seal females rearing pups. At-sea metabolic rate (MR) (mean of 6.34 ± 0.4 W. kg^-1; 4.6 times predicted BMR) was positively correlated to foraging trip duration (mean of 4.21 ± 0.54 days; r²= 0.5, P < 0.04). There were no relationships between MR and the total number of dives, the total time spent diving or the total vertical distance travelled during the foraging trip. There was, however, a close negative sigmoidal relationship (r²= 0.93) between at-sea MR and the proportion of time at sea spent diving. This measure of diving behaviour may provide a useful, inexpensive means of estimating foraging energy expenditure in this species and possibly in other otariids. The rate of diving (m.h^-1) was also negatively related to at-sea MR (r²= 0.69, P < 0.005). Body mass gain during a foraging trip had a positive relationship to the time spent at sea (r²= 0.58, P < 0.02) and the total amount of energy expended while at sea (r²= 0.72, P < 0.004) such that, while females undertaking long trips have higher metabolic rates, the energetic efficiency with which females gain mass is independent of the time spent at sea. Therefore, within the range of conditions observed, there is no apparent energetic advantage for females in undertaking foraging trips of any particular duration.     

Food and foraging behaviour of the Snares fernbird

Abstract

The Snares fernbird (Bowdleria punctata caudata Buller, 1894) is an opportunistic insectivore, its prey ranging greatly in size and type. Birds forage over a wide variety of sites, including forest, tussock, boulder beaches, cliffs, penguin colonies, and floating kelp. Foraging methods vary with the prey sought, and at times birds adopt specific “search images” when hunting certain insects. Details of foods brought to nestlings are given. Four postures used by fernbirds when foraging actively are described.     

Interspecific dominance and the foraging behaviour of juncos

We observed mixed groups of dark-eyed juncos (Junco hyemalis) and grey-headed juncos (Junco caniceps) at baited sites in northern Arizona during the non-breeding season. In interspecific and inter-racial conflicts, J. caniceps dorsalis was dominant to J. caniceps caniceps and to two races of dark-eyed juncos. Junco caniceps dorsalis also fed significantly faster than any of the other juncos. For both species, feeding rates were approximately the same in large and small mixed-species groups, though in larger groups, individual grey-headed juncos won conflicts at a higher rate and individual dark-eyed juncos lost conflicts at a higher rate. Also, dark-eyed juncos fed at a significantly lower rate in groups comprised mostly of grey-headed juncos than in groups of similar size but composed mostly of conspecifics. Residency times and recapture probabilities were similar for the two species, suggesting little difference in over-winter survival.     

Optimizing foraging behaviour through learning

Manifestation of life-history strategy is through the allocation of resources acquired by foraging. Foraging efficiency can be improved by learning, as fishes adjust their behaviour to changing circumstances. We briefly review the influence of learning on the foraging behaviour of fishes and make recommendations for further research. We stress the importance of quantifying learning and memory in relation to ontogeny and life history.     

Risk-indifferent foraging behaviour in honeybees

We studied the influence of variance in reward volume on choice behaviour of honeybees, Apis mellifera carnica, by training bees to collect sucrose solution from four newly developed artificial feeders. The feeders were electromechanical devices, each controlled by a microprocessor, which monitored the experiments, controlled reward delivery and stored the data. The parameters that varied between the feeders were the amount and variance of reward. The four feeders were arranged in two pairs, with the two feeders in each pair set to the same reward parameters. Constant feeders offered a fixed amount of sucrose solution at each bee visit; variable feeders offered a normally distributed reward with a standard deviation equal to the mean. We tested three reward combinations under two variance conditions. The bees matched their choice frequencies to the mean amount of reward. This applied both to the constant and the variable feeders. Thus the bees were able to discriminate feeders by the amount of reward and were able to estimate the mean reward for the variable flowers. The proportion of immediate returns to the same feeder increased with the amount of sucrose solution imbibed at each visit, indicating that bees were able to perceive the amount of reward at each visit. However, there was no influence of variance on the choice behaviour of the bees, ruling out the possibility that bees are risk sensitive under these conditions. We discuss risk indifference in choice behaviour of bees in the context of several models of risk sensitivity. Copyright 2000 The Association for the Study of Animal Behaviour.     

Body temperature and foraging behaviour of the Eurasian otter (Lutra lutra), in relation to water temperature

The effects of water temperature (T_w) on core body temperature (T_b) and foraging behaviour were studied in four free-ranging and one captive otter Lutra lutra in NE Scotland. The free-ranging animals were observed in and around two freshwater lochs, and measurements were made of T_w (2–16°C), T_b at the beginning and end of swimming bouts, lengths of swimming bouts, and dive times and intervals. T_b ranged from 35.9 to 40.4°C (mean 38.1°C). At the beginning of a period of activity and of a swimming bout. T_b rose significantly; during swimming it fell at a rate of 2.3°C/h, independent of T_w. Dive intervals were longer in colder waters, but this was probably due to other, seasonal environmental changes (perhaps water level). T_w had no significant effect on length of swimming bouts or on T_b. Although otters entered water with increased T_b, they exited when T_b was not significantly different from the mean resting T_b. The data suggest that otters maintain T_b irrespective of T_w, by increasing it before entering water, then either exiting at a given T_b, or at an earlier time determined by other environmental conditions.     

Pandora neoaphidis transmission and aphid foraging behaviour

Pandora neoaphidis is an aphid-specific entomopathogen that produces infective conidia. As aphid movement increases, so does the likelihood of contact with conidia. Volatile distress signals released in response to aphid infestation as an indirect defence against herbivory may affect aphid foraging and, therefore, the fungus-aphid interaction. In this study, two different methods were used to investigate the effect of plant volatiles and P. neoaphidis-sporulating cadavers on (1) the colonisation of Vicia faba plants by Acyrthosiphon pisum and (2) P. neoaphidis transmission. This study indicates that A. pisum does not avoid bean plants containing P. neoaphidis and that transmission of conidia occurs during plant colonisation and, to a lesser extent, during in situ feeding. Although significantly more aphids were recovered from damaged plants compared to undamaged plants, the likelihood of infection was not affected by previous infestation by aphids.     

Ecomorphology and foraging behaviour of Pacific boobies

Wing size and shape, expressed as wing loading and aspect ratio respectively, together with bill morphology are parameters that can reveal differences related to the foraging ecology of seabirds. Six species of booby (Sulidae) that inhabit the Pacific are the focus of this study: four mainly pelagic species, Masked Booby Sula dactylatra, Nazca Booby Sula granti, Red‐footed Booby Sula sula and Brown Booby Sula leucogaster, and two coastal species, Blue‐footed Booby Sula nebouxii and Peruvian Booby Sula variegata. Pelagic boobies showed segregation among species in body mass and relative bill size, and they differed in wing morphology (wing loading and aspect ratio) from the coastal boobies. The coastal Peruvian and Blue‐footed Boobies are largely allopatric but overlap in northern Peru. In their area of sympatry, they showed evidence of character displacement in body size and in wing and bill morphology, which suggests that competition plays an important role in sympatry. This study improves our understanding of ecological interactions among Pacific boobies and of how selective pressures have shaped their ecomorphology and foraging behaviours.     

The relationships between soil factors,grass nutrients and the foraging behaviour of wildebeest and zebra

Summary We examined the relationships between soil factors, nutrients in grasses and foraging behaviour of wildebeest (Connochaetes taurinus) and zebra (Equus burchelli) in a semi-arid nature reserve in South Africa. We tested the hypotheses that: (1) Soil nutrient levels determine the abundance and distribution of grass species; (2) nutrient levels within grass species are correlated with soil nutrient levels; (3) the spatial distribution and diet composition of ungulates is influenced by the nutrient availability in grasses. The distribution of soil factors in upper ground levels did explain the differential abundance of grass species in the reserve. Ordination of nutrient levels in grass species showed high levels of particular nutrients in certain species, but no one species showed uniformly high levels of all nutrients. Moreover, grasses on fertile soils did not necessarily accumulate higher nutrient levels than grasses on poor soils. Thus, nutrient levels in grasses were not correlated with soil nutrient levels. Wildebeest and zebra responded to monthly variations in the levels of N and P in grasses by moving seasonally to habitat types characterized by grass communities containing a high proportion of nutritional species, rather than by selecting particularly nutritious species within communities. We suggest that within semiarid savannas, areas with a higher diversity of grass communities will be more likely to have some of these communities containing high nutrient levels at any given season, than a lower diversity area. Therefore, the higher-diversity area would be likely to support more herbivores, and thus diversity would control carrying capacity.     

Relationships between densities of previous and simultaneous foragers and the foraging behaviour of three bumblebee species

1. The local insect composition may be as important as the local floral composition for bumblebees' foraging behaviour. However, little is known about how the local abundance of insects affects the foraging patterns of individuals. 2. Using field observations, we studied the relationships between the local density of previous and simultaneous foragers and the local foraging behaviour of Bombus pascuorum, Bombus lucorum/B. terrestris and Bombus lapidarius while pollinating Centaurea jacea. 3. The number of bumblebees foraging in the plots was positively related to the number of new individuals arriving at these plots. The number of inflorescences contacted and the duration of visits were negatively related to the number of simultaneous foragers, but only in B. lucorum/B. terrestris. The effects of previous foragers on the behaviour of other bumblebees were species‐specific and variable in their direction. Such contrasting effects can be explained in terms of bumblebee species' abundances and functional similarity. In some cases, the effect of previous foragers increased with Centaurea density. 4. The local abundance of previous and simultaneous foragers affected the foraging behaviour of particular bumblebee individuals in complex ways. Future studies on local foraging behaviour might benefit from including the abundance of co‐foragers.     

Ocean acidification impairs crab foraging behaviour

Anthropogenic elevation of atmospheric CO₂ is driving global-scale ocean acidification, which consequently influences calcification rates of many marine invertebrates and potentially alters their susceptibility to predation. Ocean acidification may also impair an organism''s ability to process environmental and biological cues. These counteracting impacts make it challenging to predict how acidification will alter species interactions and community structure. To examine effects of acidification on consumptive and behavioural interactions between mud crabs (Panopeus herbstii) and oysters (Crassostrea virginica), oysters were reared with and without caged crabs for 71 days at three pCO₂ levels. During subsequent predation trials, acidification reduced prey consumption, handling time and duration of unsuccessful predation attempt. These negative effects of ocean acidification on crab foraging behaviour more than offset any benefit to crabs resulting from a reduction in the net rate of oyster calcification. These findings reveal that efforts to evaluate how acidification will alter marine food webs should include quantifying impacts on both calcification rates and animal behaviour.     

Irrational choices in hummingbird foraging behaviour

It is conventionally assumed that, when animals evaluate alternative options, the value assigned to an option is absolute and independent of the other options available. It follows that animal choices should exhibit the rational property of regularity whereby the proportion of choices for an option cannot be increased by the addition of further options to the choice set. However, violations of regularity occur in human decision making, suggesting that humans may use comparative evaluation mechanisms whereby the value of an option is computed relative to the other options available. For example, in the asymmetrically dominated decoy effect the preference for a target option over a competitor is altered by the addition of a decoy option that is inferior to the target and competitor on one attribute, but lies between them on a second. We tested whether foraging wild rufous hummingbirds, Selasphorus rufus, would demonstrate violations of regularity in response to an asymmetrically dominated decoy. Sixteen birds chose between three artificial flower types (Target: 15 μl, 40% sucrose; Competitor: 45 μl, 30%; Decoy: 10 μl, 35%) in Binary (Target versus Competitor) and Trinary (Target versus Competitor versus Decoy) treatments. We predicted higher preference for the Target in the Trinary treatment. The birds ranked the three options in the same order in the Binary and Trinary treatments (Competitor>Target>Decoy). Seven birds showed violations of regularity, six increasing their absolute preference for the Competitor in the Trinary treatment. Overall, relative preference for the Competitor over the Target was higher in the Trinary than in the Binary treatment. These changes in preference are incompatible with an absolute evaluation mechanism.     

A modular concept of plant foraging behaviour: the interplay between local responses and systemic control

In this paper we examined the notion that plant foraging for resources in heterogeneous environments must involve: (1) plasticity at the level of individual modules in reaction to localized environmental signals; and (2) the potential for modification of these responses either by the signals received from connected modules that may be exposed to different conditions, or by the signals reflecting the overall resource status of the plant. A conceptual model is presented to illustrate how plant foraging behaviour is achieved through these processes acting in concert, from the signal reception through signal transduction to morphological or physiological response. Evidence to support the concept is reviewed, using selective root placement under nutritionally heterogeneous conditions and elongation responses of stems and petioles to shade as examples. We discussed how the adoption of this model can promote understanding of the ecological significance of foraging behaviour. We also identified a need to widen the experimental repertoires of both molecular physiology and ecology in order to increase our insight into both the regulation and functioning of foraging responses, and their relationship with the patterns of environmental heterogeneity under which plants have evolved.