A further potential direct beneﬁt of group living, that goes beyond the limits of subgroups, is information transfer. The amount of information that can be retained and preserved depends on number of individuals com- posing a group, but not necessarily on number of in- dividuals roosting together (even if transmission of this information at a given time is more likely to occur between individuals that are roosting together). Social animals, from insects to birds, might gain information from other colony members about their environment, speciﬁcally about the location of food patches (Ward & Zahavi 1973; Danchin & Wagner 1997; Chittka & Lead- beater 2005). These mechanisms have been poorly studied in bats, but information transfer both about foraging areas (Wilkinson 1992) and about new roosts (idem; Kerth & Reckardt 2003) have been shown for some species.
Finally, large groups spread across several roosts might be a consequence of female philopatry in forest bats. If roosts are a limited resource, territory inheritance would be beneﬁcial for female offspring, which are unlikely to ﬁnd better roosts away from ‘home’, and for their mothers, that by allowing daughters to remain in high- quality roost areas, increase the probability that their genes will be passed on (Lindstro¨ m 1986). Giant noctules, like all other temperate bats, have low reproductive rates, giving birth to one or two pups per year. However, bats are long lived and thus many generations overlap. This can eventually lead to the formation of larger groups than can ﬁt into the limited volume of a tree cavity. Sharing multiple roosts might be more beneﬁcial than splitting colonies whose members are familiar to each other and would thus be more likely to perform cooperative behaviours.
If Larger Groups are Advantageous, Why do not Cryptic Social Groups Mix?
The fact that giant noctules, like other forest bats, form ﬁssionefusion societies suggests that living in larger
groups among several roosts provides ﬁtness beneﬁts. In this case, we could also expect social groups living in adjacent roosting areas to mix, eventually forming one single group, but the three distinct giant noctule groups were stable at least over the medium term (5 and possibly up to 14 years). This is surprising given that no physical or geographical barriers exist between roosting areas, which even overlapped to some extent, and giant noctule bats foraged up to 40 km away from their roosting areas (A. Popa-Lisseanu, F. Bontadina & C. Ib‘an˜ ez, unpublished data). The overall foraging areas of the three social groups overlapped almost completely (idem); moreover, visits of bats to roosts of neighbouring social groups took place oc- casionally. Persistence of this social separation could be explained if the advantages of group living for giant noc- tules depend on associating with kin, and if female philo- patry generates signiﬁcant levels of relatedness among group members. However, colony members of other forest bat species, including those showing ﬁssionefusion be- haviour, differ greatly in their levels of relatedness and do not associate preferentially with kin. Average related- ness within colonies is low as a result of male dispersal, suggesting that kin selection does not explain group living (Burland et al. 2001; Kerth et al. 2002b; Metheny 2006). This remains to be tested in giant noctules. Alternatively, the costs of group living, in particular resource competi- tion, could limit the size of the larger group. If roosts are a limited resource, and in addition, if keeping track of roosts and transferring information between group mem- bers that cooperate is costly, xenophobic behaviours could be expected (Kerth et al. 2002b), contributing to the main- tenance of the observed high structuring in the giant noctule population.
Deﬁning population boundaries in a metapopulation system is a basic step for developing sound conservation plans. The consequences of loss of resources may be very different if distributed between all population subgroups versus the impact falling on one group. Giant noctules living in Mar´ıa Luisa Park are at risk because of regular felling of trees in response to incidents of limbs falling on tracks where people walk. The population will be more likely to recover if tree losses are partitioned between the subgroups than if all removed trees fall inside the roosting area of one colony.
Our results conﬁrm that a large number of roosts are needed to preserve colonies of forest bats (cf. O’Donnell
2000; Willis & Brigham 2004; Russo et al. 2005). Based
on our results, we suggest that a minimum number of
30 roost trees per colony of w100 adult females should be preserved to enhance the chances of subpopulation survival. We detected roosts which are exceptionally im- portant for colonies, deﬁned by their location in core roosting areas. Colonies of other forest bat species might also make preferential use of speciﬁc roosts within their roosting areas. For example, common noctules N. noctula in Germany used many trees but spent almost half of their time in the centre of their roosting areas (Kronwitter
1988). We recommend that for tree-dwelling bat species living in managed woodlands, special care is given to identify core roosts and to determine if cryptic social groups exist, to ensure that core roosts and a minimum number of roosts per social group (estimated from moni- toring roost-switching behaviour) are preserved.
We thank C. Ruiz for his help with the ﬁeld work, the Servicio de Parques y Jardines de Sevilla for permitting nocturnal access to the ML Park, and J. Juste, C. O’Donnell, R. Arlettaz, M. Brigham and an anonymous referee for valuable comments on the manuscript. Research was funded by the Organismo Auto´ nomo de Parques Nacio- nales of Spain, project 021/2002. Ministerio de Educacio´ n y Ciencia (MEC) of Spain supported A.G.P.-L. with a pre- doctoral grant of the programme Formacio´ n de Personal Universitario (FPU).
Supplemen tary Material
Supplementary data can be found, in the online version, at doi:10.1016/j.anbehav.2007.05.011.
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