Andrea Perna is a biologist interested to the common properties observed at different levels of biological organization. He graduated in molecular biology at the University of Pisa and at the Scuola Normale Superiore of Pisa, Italy, and received a PhD in neurosciences again from Scuola Normale Superiore with Concetta Morrone, working on the mechanisms of visual perception in human brain.
He has been post doc at the Research Centre on Animal Cognition in Toulouse, France, and in the Laboratory of Informatics of Nantes, France. He currently holds a research position at ISC-PIF.
His current research mainly focuses on the formation of spatio-temporal patterns as a result of collective behaviour of animals.

 

Form and morphogenesis of termite nests

Social insect societies have attracted much attention because of the complex level of coordination and organization of their collective activities. These allow them to perform complex tasks, such as finding the shortest path to a food source and building elaborate nests.
Termites in particular are known for building the most amazing nests observed in the animal world. The mounds built by some species can reach up to 6 meters of height against a size of the individual insect of the order of the millimeter. These nests do not result from the simple repetition of local patterns, but present a coherent global organization which enables them to maintain a stable internal environment against external fluctuations in temperature and humidity.
Such complicated structures are not the result of a complexity at the level of individual insects; on the opposite, the behavioural mechanisms underlying their formation can be incredibly simple, and the complexity of the nests emerges from the interactions between insects. Studying the formation of these patterns in ant and termite is important for our understanding of insect biology. Insect colonies also are a good experimental testbed for the study of complex morphogenesis phenomena, because the spatio-temporal scales of nest formation are such that both the exact behaviour of all individuals in a colony and the growth of the global pattern can be observed and altered experimentally.

Topology of spider mites silk networks

A large number of arthropods build silk threads that they use in different ways (by far not limited to the webs of spiders or the cocoons of silkworms). One such species producing silk is the red spider mite Tetranychus urticae, an acarian (not a spider as the common name might suggest) of economical importance because of the damages it causes to agriculture. Tetranychus are found in large groups on the leaves of plants, that they envelop with large webs of fine silk.
Silk is likely to promote the formation of aggregates of mites, because individual mites show a tendency to follow the threads let by other individuals, it forms a physical shelter which protects the mites from rain and pesticides. Silk is also responsible for the dispersion of mites over long distances. This happens because in some conditions the Tetranychus form balloons of silk on the tips of branches. These balloons are eventually swept away by wind, carrying with them acarians and eggs to new plants.
Still very little is known about the mechanisms of formation of silk networks. Only very recently Gwendoline Clotuche in Louvain (Belgique) set up a method to reliably visualize the individual threads, opening the possibility to study the phenomenon in controlled experimental conditions. The first experimental data show that asperities in the substrate, such as mite eggs or dust, may quickly become hubs in the silk network. When two such asperities are present, often they end up being connected by a large number of direct threads. This is interestingly, as sometimes the distance between the two nodes is much bigger than the size and the perceptual range of an acarian.

From spatial interactions to collective decision

Social animals often have to make decisions as a group, for example about which activity to perform and where to go. Large flocks of birds and schools of fish can move in perfect synchrony and when the group is attacked by a predator the school contract, expand or split, but the individuals seldom collide and soon the group reforms as a whole. This is astonishing if we consider that in general only a small fraction of individuals can directly detect the presence and the position of the predator.
Many of the central questions in collective animal behaviour concern the transfer of information between group members. Some recent work (e.g. Sumpter et al. Consesus decision-making by fish. Current Biology, 2008) has shown that fish make more accurate decisions as their group size increases. Fish are better able to detect predators, locate food and detect phenotypic differences when they follow others.
While these experiments and modeling work capture vital aspects of the decision-making process they do not capture the spatial structure of the group. Nor do they capture the detail of how information is transferred between the animals.

Other studies of decision-making and collective motion have emphasised a class of models known as self-propelled particle (SPP) models. This class of models are largely inspired by statistical physics and describe animals as particles that interact locally with their neighbours with simple local rules. Usually, these rules are in terms only of repulsion, attraction and alignment between individuals. Often, a small set of simple rules of local interaction is sufficient to maintain group cohesion and generate complex collective patterns.
SPP models help us understand the "mechanical" aspects of inter-individual communication, but can we go one step further and express problem of collective decision in term of force models? This is the aim of a project I am carrying in collaboration with David Sumpter and Jens Krause.

Selected publications

Perna et al. "The structure of gallery networks in the nests of termite Cubitermes spp. revealed by X-ray tomography". Naturwissenschaften (2008) (PDF Web)  -  This article has been featured in le Figaro (PDF Web)

Perna et al. "Topological efficiency in three-dimensional gallery networks of termite nests". Physica A (2008) (PDF Web)

Perna et al. "The topological fortress of termites". LNCS (2008) (PDF Web)

Perna et al. "BOLD response to spatial phase congruency in human brain". Journal of Vision (2008) (PDF Web)

Perna and Morrone "The lowest spatial frequency channel determines brightness perception". Vision Research (2007) (PDF Web)

Perna et al. "Neuronal mechanisms for illusory brightness perception in humans". Neuron (2005) (PDF Web)


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