Contributions to Zoology, 82 (2) – 2013E. Johanna Rode; K. Anne-Isola Nekaris; Matthias Markolf; Susanne Schliehe-Diecks; Melanie Seiler; Ute Radespiel; Christoph Schwitzer: Social organisation of the northern giant mouse lemur Mirza zaza in Sahamalaza, north western Madagascar, inferred from nest group composition and genetic relatedness

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Discussion

Nests and nest sites

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Often predation is suggested to be of high importance in the selection of sleeping sites (Hamilton, 1982; Fan and Jiang, 2008). Goodman et al. (1993) reported an individual of M. coquereli caught by a Madagascar buzzard (Buteo brachypterus Hartlaub, 1860) and several individuals found with scars, indicating an attack by the raptor. Since Buteo, as well as other raptors like the Madagascar harrier hawk (Polyboroides radiatus (Scopoli, 1786)), is diurnal, Goodman and colleagues suggested that the lemurs were caught from their nests. Furthermore, remains of Mirza have been found in scats of Cryptoprocta ferox Bennett, 1833 (Rasoloarison et al., 1995), which hunts during day and night (Karpanty and Wright, 2007). In the sub-humid forests of Sahamalaza National Park most of the trees keep their foliage even during the six-month dry season (Schwitzer, 2005), which provides basic cover from a predator’s view. Additionally, selecting nest sites in dense vegetation provides good camouflage and improves concealment (Bearder et al., 2003). Accordingly, due to dense foliage, only one nest of M. zaza could be seen directly in our study. Mirza zaza preferred tall and large sleeping trees with many lianas. Pages (1980) reported that sleeping trees of M. coquereli were usually covered in lianas. The high number of lianas covering the sleeping trees might decrease the risk of being detected by predators (Garcia and Braza, 1993; Rendigs et al., 2003). The nests were located a few meters below the top of sleeping trees, which was also reported for M. coquereli (Sarikaya and Kappeler, 1997). The hidden and high position suggests good protection against both aerial and ground predators (Rasoloarison et al., 1995). Mirza zaza used one to three different routes to leave or access the nests. Reuse of such routes was especially evident for the single used tree of Group 1 where three of four animals always used exactly the same branches of the nest and neighbouring tree to leave the site. Similar behaviour was observed for owl monkeys (Garcia and Braza, 1993) and slender lorises (Nekaris, 2006). Knowing escape routes in case of a predator attack should be advantageous (Aquino and Encarnación, 1986; Wells et al., 2006).

Environmental factors may also influence sleeping site choice (Aquino and Encarnación, 1986), which has been suspected for orang-utans (Ancrenaz et al., 2004). Strong winds such as the Varatraza or the Talio can occur in the region during the dry period (Schwitzer et al., 2007), and severe tree fall was common during several weeks of the study period. Selecting robust trees and a nest site near the tree trunk may be related to the importance of solid support.

Nest utilisation

Nest groups of M. zaza were stable in composition, with one exception where the inclusivity of one male was unclear, and did not change during the study period. A third adult individual of Group 2 was occasionally observed but may have joined the group for only a few days. In other species sleeping groups were not only stable in dispersed pairs or families (Lepilemur edwardsi (Forbes, 1894) – Rasoloharijaona et al., 2003; Microcebus murinusRadespiel et al., 1998; Cheirogaleus medius É. Geoffroy, 1812 – Müller, 1999), but also in mixed-sex groups of Microcebus ravelobensis (Weidt et al., 2004).

Nests were group-exclusive and groups stayed in the same nest during long time periods of up to at least 44 days (Group 1). Only up to three nests were used by each group, resulting in high return rates. In contrast, Kappeler et al. (2005) found individuals of M. zaza using two to five different nests on the three to seven days they could be located. Kappeler et al. (2005) conducted their study in March, April and October, and nest use might change seasonally. Only in Group 3 animals sometimes slept in different nests. Lepilemur edwardsi showed similarly high nest site fidelity as M. zaza, with two to three close nest sites (Rasoloharijaona et al., 2003), while for instance female Microcebus murinus used three to seven sites (Radespiel et al., 1998). Weidt et al. (2004) reported M. ravelobensis staying in one nest for a maximum of 16 successive days. There may be two non-exclusive explanations for a small number of unique nest sites. The continuous use and reuse of certain trees may increase due to the loss of suitable trees in degraded or logged forests (Ancrenaz et al., 2004). Less frequent change of nest sites may therefore be a function of low habitat quality. Alternatively, observed nest sites of Mirza zaza may be very high in quality, which would decrease the necessity for changing the site. For example, males of Microcebus murinus change their low-quality sleeping sites frequently, probably in order to decrease predation risk (Radespiel et al., 1998). Both explanations would normally lead to intensive intraspecific competition between groups for this resource and animals trying to monopolize high quality nest sites, as suggested for Lepilemur edwardsi (Rasoloharijaona et al., 2003), M. ravelobensis (Braune et al., 2005) or M. murinus (Radespiel et al., 1998).

Groups returned to the nest with an observed maximum time lag of 26 minutes between the first and the last individual. Individuals engaged in grooming, playing and other activities in the sleeping tree before permanently occupying the nest. They were often seen on the sleeping tree or neighbouring trees, grooming or engaging in social activities, as observed for L. l. lydek­kerianus (Nekaris, 2003, 2006). Pages (1978) found that Mirza coquereli showed more social activities during the second half of the night compared to the first half where behaviour focused more on feeding.

Group 3 changed their nest gradually, with a single group member sleeping in the new tree at first, as also observed for L. l. lydekkerianus (Nekaris, 2003). In M. zaza every individual had slept in the new tree at least once before the entire group finally moved over as a unit. Similar patterns of nest changes were observed in Microcebus ravelobensis (Weidt et al., 2004) and Aotus (Aquino and Encarnación, 1986).

Mirza zaza differs from its sister species M. coque­reli in its diurnal gregarious nesting behaviour. While M. coquereli mostly sleeps in nests alone, M. zaza was found to share nests between two to eight individuals (Kappeler et al., 2005). We observed nest groups of two to four individuals. During the dry season Microcebus murinus can gather in sleeping groups of up to 15 animals but average sleeping group size is usually much smaller for Malagasy nocturnal primates (Eberle and Kappeler, 2006). Bearder et al. (2003) report that galagines may sleep in groups of up to ten individuals, whereas the Mysore slender loris (L. l. lydekkerianus) sleeps in groups of up to seven (Nekaris, 2003). Interestingly, two groups we observed contained one sub-adult female and multiple adult males with fully developed testes. Kappeler et al. (2005) found on average 0.77 adult females and 1.06 adult males with fully developed testes in a nest. High numbers of adult males were only reported for a few other species. In L. l. lydekkerianus, several adult males were observed to sleep in a group with females and young, perhaps as a strategy to rear twin offspring (Nekaris, 2003).

Social organisation of nest groups can be inferred using a combination of genetic and behavioural results. At least two of the four sleeping groups were not “rearing” or “family groups”, as several adult males were sharing the nest (Groups 1 and 3). Even though the results of the kinship analysis have to be treated with caution due to the low sample size, we are confident that two closely related males were sharing the nest in sleeping Group 1. This provides some support for the social groups hypothesis. The formation of social groups may be explained by environmental challenges (Radespiel et al., 2001). Sahamalaza has a pronounced seasonal climate. The study was conducted in the dry season. Minimum nightly temperatures dropped to 10 °C in July and behavioural thermoregulation may be necessary. Predation risk may be another reason for social nest groups as detectability of predators increases with number of animals being alert (Elgar, 1989). Weidt et al. (2004) reported that some sleeping associations of Microcebus ravelobensis contained several adult males. This behaviour was suggested to represent a (temporary) mate guarding strategy where males have direct control and access to the females in their group instead of having to search for them (Weidt et al., 2004; Radespiel et al., 2009). Finally, the high rates of forest fragmentation and deforestation in the study area may affect the social organisation, as resources like nests or also mates may be limited. One indication for this might be the low genetic diversity of microsatellite loci and in the sequence data in comparison to Mirza coquereli (Markolf et al., 2008). Although gregarious nest behaviour by M. zaza was also observed in Ambato (Kappeler et al., 2005), we cannot be sure if this reflects their natural behaviour pattern. This should be further examined by comparing groups in intact, large forests to fragmented forests. If fragmentation and limitation of crucial resources has an impact on the social organisation, this might have negative consequences such as increased inbreeding.