Felicity A. Huntingford

The relationship between anti-predator behaviour and aggression among conspecifics in the three-spined stickleback, Gasterosteus aculeatus..

Abstract The response of individual breeding three-spined sticklebacks to territorial intruders of various species was compared with their response to a predator outside the breeding season. The nature of the territorial response to the different intruders, as revealed by factor analysis, was very similar and the level of aggression shown to the different species co-varied across individuals and during each breeding cycle. The level of territorial aggression of the individual fish was positively correlated with their ‘boldness’ towards a predator. These results provide tentative support for the idea that anti-predator behaviour and aggression towards conspecifics are linked in this species. Supporting evidence for and possible adaptive significance of these results are discussed.

Modelling the proximate basis of salmonid life-history variation, with application to Atlantic salmon, Salmo salar L.

Summary The great diversity of life-history patterns in the salmonids has stimulated many theoretical studies. However, virtually all studies are based on ultimate considerations, in which predictions are made by comparing the expected reproductive success of diAerent developmental or life-history pathways and choosing the one (or ones) with the highest fitness. Such models are post hoc because they attribute fitness to individuals at the completion of the particular phase of the life cycle and do not attempt to characterize the mechanisms that animals use to achieve the life-history pattern. We describe a model, based on proximate considerations, for salmonid life histories, focused on Atlantic salmon Salmo salar L. The model involves identification of the times at which developmental conversions are initiated or inhibited and the connection between physiological states and the thresholds for such conversions. Developmental paths are based on the comparison of the current physiological status of the fish (and its change of state) with a genetic threshold. The state of the fish and rate of change of state are determined by environmental opportunity, but the threshold is genetic. This approach therefore immediately generates a genotype‐environment interaction. We use expected reproductive success to determine the fitness of individuals with diAerent genetically determined thresholds. Instead of finding an optimal life history, our theory generates fitness surfaces for diAerent life histories, so that variation is inherent in this approach. We describe and explain the structure of the model and present evidence on which this structure is based, thus providing a framework within which one can understand how ecology relates to the physiological mechanisms leading to the developmental changes of smolt metamorphosis and maturation.

Early Social Status and the Development of Life-History Strategies in Atlantic Salmon

Atlantic salmon have a variable life cycle. In good growing conditions, underyearling fish may metamorphose into the migratory smolt phase during their second spring, or delay at least a further year. The strategy adopted by particular fish appears to become fixed during their first summer. This paper examines whether either feeding efficiency or dominance in mid-summer correlates with the life-history strategy adopted. Eighty fish were individually marked and their feeding efficiency ( = mean handling time for food items) and dominance rank measured under laboratory conditions in mid-July. Growth rates of the fish were then monitored over the next three months, until developmental strategies became apparent. Discriminant and logistic regression analyses revealed that both dominance rank and size attained by July were independent, significant predictors of future developmental pattern (the age at metamorphosis being correctly predicted on the basis of rank and size in 84% of cases) whereas feeding efficiency had no effect. Thus fish that were dominant or larger two months after first feeding or both had a greater probability of migrating after only one year in freshwater than those more subordinate or smaller or both.

The influence of predation risk on the feeding motivation and foraging strategy of juvenile Atlantic salmon

Juvenile Atlantic salmon, Salmo salar, changed their foraging strategy markedly after a brief exposure to a model trout predator, in ways that reduced their conspicuousness and hence risk of being preyed upon. After seeing the predator, the salmon were less likely to orientate to passing food particles, and having orientated, were less likely to attack them. They also reduced the extent of their movements, by only attacking those food particles that came close to them, and by delaying attacks until the food had reached its closest point. They were slower to orientate to approaching food items, possibly because more visual attention was switched to scanning for predators and less to feeding. However, there was no change in the outcome of attacks, indicating that the reduced food intake was not caused by a reduction in appetite. The relative priority given to foraging and predator avoidance varied with time elapsed since the predator was last sighted; as a consequence, intake rates in the 20 min following the predator presentation averaged only 33% of the pre-predator level, but had increased to 57% 20 min later, and had recovered completely within 2 h.

Indirect fitness consequences of mate choice in sticklebacks: offspring of brighter males grow slowly but resist parasitic infections

‘Good genes’ models of sexual selection suggest that elaborate male sexual ornaments have evolved as reliable signals of male quality because only males of high genetic viability are able to develop and maintain them. Females benefit from choosing such individuals if quality is heritable. A key prediction is that the offspring of males with elaborate mating displays will perform better than those of less elaborate males, but it has proved difficult to demonstrate such an effect independently of the effects of differences in parental investment. We tested for ‘good genes’ linked to male ornamentation in the three–spined stickleback Gasterosteus aculeatus using in vitro fertilization to generate maternal half–siblings, which were raised without parental care. Maternal half–siblings sired by brightly coloured males grew less quickly than half–siblings sired by dull males but were more resistant to a controlled disease challenge. Among the offspring that became infected, those with brighter fathers had higher white blood cell counts. This suggests that highly ornamented males confer disease resistance on their offspring. The association with reduced growth suggests a mechanism for the maintenance of heritable variation in both disease resistance and male sexual coloration.

Proximate costs of fighting in male cichlid fish : the role of injuries and energy metabolism

While much is known about the functional significance of strategic decisions in animal fighting, relatively little is understood about the mechanisms that underlie the making of those decisions. In mechanistic terms, strategic decisions, such as either escalating a fight or giving up, are made in relation to the proximate costs that opponents inflict (or can potentially inflict) upon one another. These costs include physical injury and also the physiological consequences of engaging in an energetically demanding activity. We studied the role of injury and energy metabolism during fights between male cichlid fish, Tilapia zillii. In relation to injuries incurred during fights, scale loss differed depending on whether the winner was smaller or larger than its opponent; smaller winners inflicted significantly more damage on their opponents than they received, whereas this difference was not apparent in those fights won by the larger fish. In relation to energy metabolism, escalated fighting resulted in a significant depletion of total sugar reserves in the muscle and the liver. It appears that the muscle energy reserves are respired anaerobically, as was evident from the accumulation of lactate in the muscle. Losers had significantly higher levels of muscle lactate than winners. Together, the injury data and the metabolic data suggest that escalated fighting is costly for both winners and losers, but especially so for losers. These data are discussed in relation to models of animal decision making and we conclude that the difference between opponents in the proximate costs incurred during fighting is likely to underlie the making of decisions such as continuing, giving up or escalating the fight. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.

Weapon size versus body size as a predictor of winning in fights between shore crabs, Carcinus maenas (L.)

Relative body size (carapace width) and weapon size (chela length) were used as indicators of resource holding potential (RHP) in the agonistic behaviour of male shore crabs, Carcinus maenas (L.). Weapon size was found to be a more reliable predictor of the outcome of pairwise fights than body size. Crabs with longer chelae than their opponents were more likely to win fights than crabs with relatively larger bodies. Body size had less influence on the outcome of fights. Relative body and weapon size did not influence initiation of contests but did affect the likelihood of winning; however, this was significant only for weapon size. Winning crabs had heavier claws with greater surface area than losing crabs. There was no relationship between relative size and fight duration. The frequency of cheliped display increased with chela length and win- ners performed significantly more displays than losers.

FEEDING INTENSITY, GROWTH RATES, AND THE ESTABLISHMENT OF LIFE-HISTORY PATTERNS IN JUVENILE ATLANTIC SALMON SALMO SALAR

SUMMARY (1) Under good growing conditions, populations of juvenile Atlantic salmon develop a bimodal size-frequency distribution by late autumn of their first year; the larger fish in the upper modal group (UMG) will metamorphose into the sea-going phase the following spring, a year ahead of fish in the lower modal group (LMG). The feeding behaviour and growth rates of individually-marked fish from a sibling laboratory population were studied during the period when these two distinct life-history patterns become apparent. (2) The length-frequency distribution of the laboratory population was unimodal in August, significantly skewed by September, and clearly bimodal by December. Bimodality was a result of fish which joined the UMG putting on a brief growth spurt in late September/early October, at a time when water temperatures were falling and growth rates of LMG fish were decreasing. (3) Studies of the feeding behaviour of single fish in laboratory flume tanks revealed that the growth rate changes could be linked to changes in appetite rather than any external factors. The appetites of the two types of fish were initially similar in August, but then diverged: UMG fish showed a marked increase in appetite through to October, whereas the appetite of LMG fish decreased, with a small peak in October. (4) The results indicate that the fast and slow developing life-history patterns become fixed prior to September of the first year, and the corresponding changes in appetite and growth rate are thereafter under internal control. The October peak in appetite does not appear to be related to natural food availability, and may instead allow fish to obtain nutritional reserves prior to the winter when feeding is constrained.

Predation risk and inter-population variation in antipredator behaviour in the three-spined stickleback, Gasterosteus aculeatus L.

Abstract Predation risk from fish and bird predators was assessed at seven Scottish Gasterosteus aculeatus L. sites. Samples of adult male and female sticklebacks and fry from each site were tested with either a model heron or a live pike to measure anti-predator responses. Principal Component Analyses were then used to describe the responses, the first factor to emerge from each multivariate analysis providing an index of overall level of response toward the predator. Significant sexual and inter-population differences in behaviour are described. Sticklebacks from populations at high risk from either predatory fish or birds showed higher fright response scores than fish from low risk sites. In general, adult male sticklebacks are bolder than adult females during interactions with predators.

A problem for game theory analysis: assessment and intention in male mouthbrooder contests

Abstract With conditions of residency and recent experience held constant, staged contests between males of the Mozambique mouthbrooder, Oreochromis mossambicus (Peters) were settled on the basis of size. Contest length was not correlated with the degree of size asymmetry, and giving-up time was apparently random, although the contests were not wars of attrition. Contest intensity was found to decrease as size disparity increased. Contrary to the predictions of game theory, eventual winners and losers of encounters were found to show significant differences in the frequencies of several behavioural measures, even in the early part of the contest. It is suggested that there are circumstances where there may be costs as well as benefits resulting from the concealing of intensions.