Stefano Focardi

Social cohesion and foraging decrease with group size in fallow deer (Dama dama)

We studied the impact of group size on foraging behaviour and level of movement synchronisation among female herdmates of a fallow deer population in Central Italy. Both proportion of foraging events and movement synchronisation decreased with increasing group size. The proportion of foraging events was higher for animals on the edge of the group than for deer in the centre of the group; hence, there appears to be a trade-off between protection against predators and foraging interference, both of which decrease from the centre to the periphery of the group. This is the first time this type of behaviour has been recorded for wild ungulates. As expected, we also found that the movement of peripheral animals was less synchronised than that of central animals. Consequently, peripheral animals may lose contact with their herdmates and split off the group. We conclude that social inequalities may lead to conflicting requirements among group members and instability of large groups. Movement synchronisation (as a function of group size) appears to interact with habitat openness to produce variations of group size (which appear to be adaptive for individuals) as an emergent property of these aggregations.

A comparative study on the movement patterns of two sympatric tropical chitons (Mollusca: Polyplacophora)

The individual positions of 200 intertidal chitons [Acanthopleura brevispinosa (Sowerby) and A. gemmata (Blainville)] were recorded throughout 55 low tides on a Somalian shore, using a discrete sampling method. Both species feed during nocturnal low tide and rest during the day and high tide. Due to this activity rhythm, individual positions recorded during diurnal and nocturnal low tides give information on the rest habits of the chitons and on their movements, respectively. The resting and feeding fixes were analyzed with two new complementary methods. Both species show a distinct homing behaviour with periodical shift to new homes more frequent in A. brevispinosa than in A. gemmata. The latter species shows a stronger constancy to its feeding ground. The two species differ most in the orientation and length of their excursions. Due to their different movement pattern the two species minimize zonal overlapping during the night, which in turn reduces the interspecific competition for food.

Analysis of Movement Patterns and Orientation Mechanisms in Intertidal Chitons and Gastropods

Despite their different organization and biology, chitons (Mollusca, Polyplacophora) and gastropods (Mollusca, Gastropoda) share a large number of adaptations to intertidal life, including morpho-functional and behavioural traits. Communication, clustering, aggressiveness and even simple parental cares have been reported in both classes but, as in other animals, the basis of their behavioural adaptation to the intertidal environment is a proper organization of activity in space and time.

Investigating population dynamics in ungulates: Do hunting statistics make up a good index of population abundance?

Abstract Time series analysis plays an important role in the detection of mechanisms that drive population fluctuations. However, long time series are rare, with ungulate data sets usually not exceeding 50 years. In this article, we describe a long-term data set of population density indices of five ungulate species obtained from the analysis of bag records collected in the Castelporziano Preserve, Rome, Italy. Hunting statistics are often used as proxies for population density; however, in the case of long time series for large mammals, there are no comparative studies to assess the validity of such data. We evaluated the ungulate time series, using two different approaches: we 1) compared hunting statistics with independent animal counts, and 2) assessed whether or not habitat composition of the drive areas was representative of habitat availability in the whole estate. Regressions between bag data and animal counts gave significant results only for three species, whereas bag data corrected for hunted area were significantly correlated to animal counts for all five species. The results suggest that use of bag records not corrected for hunting effort and without any previous validation could lead to misleading estimates of abundance indices. Finally, our analysis showed that density indices of the five species were not significantly affected by the selection of habitats where hunting drives were organised. Our data set may contribute to the understanding of ungulate ecology in the Mediterranean environment.

Competition for the home and aggressive behaviour in the chiton Acanthopleura gemmata (Blainville) (Mollusca: Polyplacophora)

SummaryComputerized screening of all the positions recorded during a synodic month on 120 individually marked chitons (Acanthopleura gemmata) pinpointed their preferential resting points. Unlike the majority of intertidal chitons so far studied, A. gemmata rests in well-defined homes actively dug in the rock. Homes proved to be not strictly individual and periodically interchangeable. A complex aggressive behaviour was recorded in the field when two animals came in contact at home. When competing for ownership of a resting site rival chitons may suppress their nocturnal feeding activity. Despite its strong home-related territoriality A. gemmata showed no mutual exclusion on the feeding area. The highly specialized resting habits clearly protects A. gemmata from its most important predator, the toad fish Arothron immaculatus. The behaviour of A. gemmata is compared to that of other chitons and gastropods, and the current hypotheses concerning the adaptive value of the homing behaviour in littoral molluscs are discussed.

Pelagic seabird flight patterns are consistent with a reliance on olfactory maps for oceanic navigation

Homing studies have provided tantalizing evidence that the remarkable ability of shearwaters (Procellariiformes) to pinpoint their breeding colony after crossing vast expanses of featureless open ocean can be attributed to their assembling cognitive maps of wind-borne odours but crucially, it has not been tested whether olfactory cues are actually used as a system for navigation. Obtaining statistically important samples of wild birds for use in experimental approaches is, however, impossible because of invasive sensory manipulation. Using an innovative non-invasive approach, we provide strong evidence that shearwaters rely on olfactory cues for oceanic navigation. We tested for compliance with olfactory-cued navigation in the flight patterns of 210 shearwaters of three species (Corys shearwaters, Calonectris borealis, North Atlantic Ocean, Scopolis shearwaters, C. diomedea Mediterranean Sea, and Cape Verde shearwaters, C. edwardsii, Central Atlantic Ocean) tagged with high-resolution GPS loggers during both incubation and chick rearing. We found that most (69%) birds displayed exponentially truncated scale-free (Lévy-flight like) displacements, which we show are consistent with olfactory-cued navigation in the presence of atmospheric turbulence. Our analysis provides the strongest evidence yet for cognitive odour map navigation in wild birds. Thus, we may reconcile two highly disputed questions in movement ecology, by mechanistically connecting Lévy displacements and olfactory navigation. Our approach can be applied to any species which can be tracked at sufficient spatial resolution, using a GPS logger.

Adaptive Lévy Walks in Foraging Fallow Deer

Background Lévy flights are random walks, the step lengths of which come from probability distributions with heavy power-law tails, such that clusters of short steps are connected by rare long steps. Lévy walks maximise search efficiency of mobile foragers. Recently, several studies raised some concerns about the reliability of the statistical analysis used in previous analyses. Further, it is unclear whether Lévy walks represent adaptive strategies or emergent properties determined by the interaction between foragers and resource distribution. Thus two fundamental questions still need to be addressed: the presence of Lévy walks in the wild and whether or not they represent a form of adaptive behaviour. Methodology/Principal Findings We studied 235 paths of solitary and clustered (i.e. foraging in group) fallow deer (Dama dama), exploiting the same pasture. We used maximum likelihood estimation for discriminating between a power-tailed distribution and the exponential alternative and rank/frequency plots to discriminate between Lévy walks and composite Brownian walks. We showed that solitary deer perform Lévy searches, while clustered animals did not adopt that strategy. Conclusion/Significance Our demonstration of the presence of Lévy walks is, at our knowledge, the first available which adopts up-to-date statistical methodologies in a terrestrial mammal. Comparing solitary and clustered deer, we concluded that the Lévy walks of solitary deer represent an adaptation maximising encounter rates with forage resources and not an epiphenomenon induced by a peculiar food distribution.

How shore morphology and orientation mechanisms can affect the spatial organization of intertidal molluscs

Homing behaviour and clustering are very common in intertidal molluscs. A large number of observations stressed the importance of trial following in the orientation of each snail to its home and its cluster. The aim of this paper is to provide a mathematical model of the clustering and homing behaviours of Nerita textilis , adopting different hypotheses about the mechanisms employed by snails to find their way back home after each feeding excursion. Moreover, the model is able to simulate similar processes in other species on the basis of their specific behavioural ecology. The results given by the model confirm some hypotheses about the nature of the orientation cues and suggest some new aspects of the adaptive value of trail-following in molluscs.

The Lévy flight foraging hypothesis in a pelagic seabird

Lévy flight foraging represents an innovative paradigm for the analysis of animal random search by including models of heavy-tailed distribution of move length, which complements the correlated random walk paradigm that is founded on Brownian walks. Theory shows that the efficiency of the different foraging tactics is a function of prey abundance and dynamics with Lévy flight being especially efficient in poor prey fields. Lévy flights have been controversial in some quarters, because they previously have been wrongly ascribed to many species through the employment of inappropriate statistical techniques and by misunderstanding movement pattern data. More recent studies using state-of-the-art statistical tools have, however, provided seemingly compelling evidence for Lévy flights. In this study, we employ these maximum-likelihood methods and their Bayesian equivalents by analysing both turning angles and move length distributions. We tested, for compliance with Lévy flight foraging, a set of 77 independent foraging trajectories of Corys shearwaters Calonectris diomedea diomedea. Birds were tagged with high-resolution GPS loggers in two Mediterranean colonies (Linosa and Tremiti) during both incubation and chick rearing. We found that the behaviour of six birds was fitted by a correlated random walk; the movement of 32 birds was better represented by adaptive correlated random walks by switching from intensive to extensive searches; and the trajectories of 36 birds were fitted by a Lévy flight pattern of movement. The probability of performing Lévy flights was higher for trips during chick provisioning when shearwaters were forced to forage in suboptimal areas. This study supports Lévy flight foraging as an appropriate framework to analyse search tactics in this pelagic bird species and highlights that the adoption of a given search strategy is a function of biological and ecological constraints.

Cooperative Interactions and Environmental Control in the Intertidal Clustering of Nerita Textilis (Gastropoda; Prosobranchia)

1. Somalian populations of the intertidal gastropod Nerita textilis Gmelin show a vertical separation between resting (mid-upper eulittoral) and feeding (lower eulittoral) zone. During high tides and mostly diurnal low tides some snails remain scattered (SF), while other form aggregations (AF) at lower levels of the rocky shore. During the mostly nocturnal low tides both SF and AF migrate downward to feed on microalgae. 2. Lower and upper aggregations are mostly frequented around neap (NT) and spring tides (ST), respectively. Clustering significantly reduces the exposure to waves during high tide, and probably overheating and dehydration during diurnal low tide. 3. About 90 percent of the total population enters into the cluster system throughout each NT-ST-NT cycle. Clustering begins to increase at NT and falls at ST, reaching a maximum (52 percent of the population) between NT and ST. The relative frequency of transition between SF and AF shows a spike after NT and falls after ST. The inverse transition (AF to SF) is negatively correlated to the amount of AF. 4. Return to the original cluster after one feeding excursion is high (76 percent) and decreases slightly at increasing intervals (about 40 percent after 14 feeding migrations), but it becomes significantly lower after one or more rest phases spent in SF. 5. The field study suggests that the periodical (NT) increase in density at the lower rest-zone (tide-pools belt), long-lasting chemical marking of collective rest sites and releasing mucus trails during feeding excursions cause the clustering. According to this hypothesis, a primer group stops at each aggregation site at NT, mostly returning to it by self-trailing after each feeding migration, while the progressive recruitment of snails from SF is caused by the interindividual trail-following during the return migration from the feeding ground. 8. The same model can explain the clustering behaviour of other intertidal gastropods, though the importance of the different parameters (periodical variation in zonation, marking of rest places, trail-following) probably vary among the different species.