Keeping the kids at home can limit the persistence of contagious pathogens in social animals

Lucile Marescot; Mathias Franz; Sarah Benhaiem; Heribert Hofer; Marion East; Stephanie Kramer-Schadt

ABSTRACT 23 In social species where offspring are reared together in communal burrows or similar 24 structures, young animals typically do not engage in between-group contact during their 25 developmenta behavioural trait we call 'offspring with restricted between-group contact' 26 (ORC). The impact of this trait on the persistence of contagious pathogens that generate 27 lifelong immunity in their hosts is currently unclear. We hypothesize that in populations with 28 ORC, the formation, in groups, of a 'protective barrier' of only recovered adults, prevents the 29 transmission of this type of pathogens to the new susceptible hosts (i.e. young animals), 30 thereby increasing the probability of epidemic fade-out. We implement a spatially implicit 31 individual-based Susceptible-Infected-Recovered (SIR) model for a large range of host and 32 pathogen traits and show that the epidemic fade-out probability is consistently higher in 33 populations with ORC, especially when disease spread is fast (high basic reproduction 34 number R0). We also show that ORC can counteract the cost of group-living in terms of 35 disease risk to a greater extent than variation in other traits. We discuss our findings in INTRODUCTION 42 Pathogens of infectious diseases that spread through direct contact or physical proximity can 43 imperil species that live in dense populations or large social groups, including humans, 44 livestock or wildlife (Altizer et al. 2003 . 53 A social network can further be characterized by its modularity, that is, its degree of division 54 into modules such as social groups (e.g. clans, prides or troops) or subgroups (e.g. age classes 55 in the case of eusocial insects, Stroeymeyt et al. 2018 , also see Griffin and Nunn 2012, Sah et 56 al. 2017). Generally, a social network is considered to be highly modular, or to have a high 57 'community structure' (White et al. 2017 ) when individuals within one module are more 58 connected to each other than they are to individuals in other modules (Newman 2006 ). It has 59 been hypothesized that a highly modular social network can generally reduce disease risk in 60 group-living animals (Griffin and Nunn 2012, Nunn et al. 2015 ). Yet this view has been 61 recently challenged by a study which found that the beneficial effects of modularity on 62 disease fade-out are restricted to extreme cases of exceptionally high modularity, which are 63 rarely observed in natural systems (Sah et al. 2017) . 64 Here we investigate how highly modular social networks characterized by age-dependent networks where young animals have no contact with members of other groups, and term this 67 life history trait 'offspring with restricted between-group contact' (ORC). 68 In contrast to most ungulate species for instance, in which precocial young follow their 69 mother shortly after birth (Lent 1974) , several group-living species rear their offspring in 70 communal burrows, nurseries, crèches or family groups within group territories or home 71 ranges for extended periods of time, during which time young rarely if ever contact 72 individuals of other groups. This includes for instance spotted hyenas Crocuta crocuta 73 (Kruuk 1972), African lions Panthera leo (Schaller 1972) or European rabbits Oryctolagus 74 cuniculus (Daly 1981). 75 Restricting contact of young animals with individuals of other groups may specifically limit 76 the persistence of pathogens that induce lifelong immunity following infection. Such Here we