Isaac Planas-Sitjà



Université libre de Bruxelles, CP 231
Bd. du Triomphe, B-1050 Bruxelles

E-mail: iplanass [at]
Tel: 02 650 55 28 (from abroad +32 2 650 55 28)
Fax: 02 650 59 87 (from abroad +32 2 650 59 87)


Animal Behaviour: I am interested in animal behaviour in general but more precisely in collective dynamics. The ability of animals to reach complex decisions with simple rules and often poor information about the environment is fascinating. I aim to quantify the implications of personality at the collective level and better understand the existing synergies between two decision levels: individual and collective.

Conservation Biology: Based on my background (Biodiversity and Conservation Ecology) I use tools from the study of collective systems to better understand the biology of endangered social and gregarious species and therefore help to preserve their ecological niche.

Behavioural Ecology: I study the evolutionary constraints and the adaptive value of behavioural plasticity and personality variation. With the use of quantitative genetics, we are currently studing the origins of the behavioural variability in the American cockroach.

” While the individual man is an insoluble puzzle, in the aggregate he becomes a mathematical certainty ”
– W. Winwood Reade cited by Sherlock Holmes –

Main focus:

Recent studies allow us to understand the mechanisms linking individual and collective behaviour. My main focus is to improve our understanding of the mechanisms underlying the collective decision-making process.  For this purpose I study how consistent individual behavioural differences (behavioural syndromes or personalities) within a group of cockroaches (Periplaneta americana) can lead to different collective dynamics affecting the decision-making process as well as the exploitation of resources. Moreover, I study the benefits of heterogenous and homogenous groups and how this ‘personality’ can be integrated in mathematical models.



[1] Planas-Sitjà I, Nicolis SC, Sempo G, Deneubourg JL (2018). The interplay between personalities and social interactions affects the cohesion of the group and the speed of aggregation. PLOS One DOI: 10.1371/journal.pone.0201053

[2] Laurent Salazar MO, Planas-Sitjà I, Sempo G and Deneubourg JL. Individual thigmotactic preference affects the fleeing behavior of the American cockroach. Journal of Insect Science . DOI: 10.1093/jisesa/iex108.

[3] Laurent Salazar MO, Nicolis SC, Calvo Martín M, Sempo G, Deneubourg JL, and Planas-Sitjà I (2017). Group choices seemingly at odds with individual preferences. R. Soc. Open Sci., 4:170232. DOI: 10.1098/rsos.170232

[4] Planas-Sitjà I, Laurent Salazar MO, Sempo G, and Deneubourg JL (2017). Emigration dynamics of cockroaches under different disturbance regimes do not depend on individual personalities. Sci. Rep., 7:44528. DOI: 10.1038/srep44528

[5] Laurent Salazar MO, Planas-Sitjà I, Deneubourg JL, and Sempo G (2015). Collective resilience in a disturbed environment: stability of the activity rhythm and group personality in Periplaneta americana. Behav. Ecol. Sociobiol., 69:11, pp. 1879–1896. DOI: 10.1007/s00265-015-2000-3

[6] Planas-Sitjà I, Deneubourg JL, Gibon C, and Sempo G (2015). Group personality during collective decision-making: a multi-level approach. Proc. R. Soc. B, 282:20142515. DOI: 10.1098/rspb.2014.2515

[PhD Thesis] Quantification and determination of the interindividual variability observed in a group of cockroaches and its implications at a collective level (VIVA 10/04/2017)