Discussion And Conclusions

Severe drought years have been shown to impact dry, western forests in Madagascar (Gould et al., 1999). But droughts also affect the rainforest by extending the dry season and reducing annual rainfall (Wright, 1999). Drought in rainforests has resulted in high canopy tree mortality (Milton et al., 1994; Condit et al., 1995), reproductive failure (Struhsaker, 1997), fruit crop failure (Foster, 1980; Leighton and Wirawan, 1986), and decrease in young leaf abundance (Leighton and Wirawan, 1986; Struhsaker, 1997). No long-term studies have been conducted yet on the effects of natural disasters such as drought and cyclones on fauna and flora in the Manombo region. However, direct cyclone hits can result in complete defoliation, blowdowns of all canopy trees, landslides, and flooding (Ganzhorn, 1995a,b). As a consequence, neither fruits nor leaves may be available for consumption until the following spring, leaving only crisis foods, such as epiphytes, for the lemurs to eat (Ratsimbazafy, 1999).

In the pristine forests of Madagascar, the mean tree crown diameter (patch size) of fruit trees fed in by lemurs is 12 m (Overdorff, 1996; Balko 1998; Hemingway, 1998). In Manombo, cyclone Gretelle had two effects on the food supply of Varecia. First, surviving native trees suffered an average crown loss of 85%, and broad crowned trees are generally toppled first by the high winds generated by a cyclone (Wright, 1999). Six months after the cyclone of 1997, the mean crown diameter of trees from the seven sample plots was 3 m (range: 0.5-9 m; n = 439). These losses represent a greater than 70% reduction in the fruit-producing vegetation of the forest. Ninety percent of the few remaining trees forming the upper layers of the forest are still producing new leaves and new branches. Moreover, after the cyclone, there were significant losses of flowers and fruits among trees, and shrubs. The recovery of endemic species after this natural disaster is still very slow, but Varecia are able to survive in this highly disturbed forest by adjusting their feeding strategies.

It has been well-demonstrated that seasonal peaks in the abundance and scarcity of food used by lemurs are variable from year to year in Malagasy rainforests, as many canopy species produce flowers on prolonged, irregular, and asynchronous or alternate year cycles (Morland, 1991a, 1993; Hemingway, 1995; Powyzk, 1997). In the rainforest at Manombo, a study of the phenological sequence of 120 trees of 24 plant species consumed by Varecia over almost 2 years demonstrated that this irregularity of fruiting pattern is reflected in variable annual food availability, i.e., more species produced fruits in May 1999 than in May 2000. As noted by Wright (1999), this pattern of unpredictability requires lemurs to be able to adapt to energetic constraints at almost any time of the year. Manombo presents an extreme case of the effects of habitat disturbance; therefore, it is important to determine how an obligate frugivore like V. variegata copes in such a highly disturbed habitat. Did the ruffed lemurs at Manombo exhibit diet similarities with their conspecifics living in undisturbed forests?

Comparisons with undisturbed forests indicate that there are general similarities in the ruffed lemurs' diets in different habitats with regards to the overall food categories eaten. Field research on the diets of Varecia at different sites have shown that this species is highly frugivorous (see Table 2). When the data for both sexes are combined from studies on wild V. rubra, diets are similar from season to season, comprised mainly of fruits (Vasey, 1997). In the present study, it was expected that Varecia living in highly disturbed habitats would have general dietary patterns different from those living in undisturbed habitats. My results demonstrated that they remain predominantly frugivorous regardless of habitat type. This confirms that Varecia is an obligate frugivore (Balko, 1998). While dietary diversity fluctuated monthly, the percentage of fruit in the diet always outnumbered the percentage of nonfruit items, except during the month of November 1999 (the second driest month of the study). Varecia could feed heavily on young leaves when they were available. In other words, it is not necessary that Clidemia hirta be consumed when other foods are available. As fewer trees bore fruits (due to serious cyclone damage), Varecia at Manombo devoted much of their feeding time on fruits from shrubs and vines. For instance, the exotic shrub species C. hirta can provide a fairly constant source of fruits throughout the year. This became a staple food source and an essential fruit for 3 years after cyclone Gretelle hit Manombo. Ruffed lemurs living in undisturbed sites have not been reported to feed on C. hirta, although it is eaten by Hapalemur griseus in Valohoaka/Ranomafana (Grassi, 2001). As noted by Vasey (1997), during the transitional cold season V. rubra feeds exclusively on fruit.

Results from this study indicate that Varecia are able to diversify their diets in terms of the number of fruit species consumed, but the ability to digest and detoxify leafy material and other nonfruit items is limited. A comparative study of the gastrointestinal tract of five lemur species (Propithecus tattersalli, Propithecus verreauxi coquereli, Varecia variegata, HHapalemur griseus, and Lemur catta)

Table 2. Diets of Varecia variegata at different sites

Study Site1

Species

Seasons observed

Fruit

Nectar

Flowers

Leaves

Young Leaves

Other

Nosy Mangabe

V. v. rarkjjata

all

74%

21%

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