Demography of Lemur catta at Berenty Reserve, Madagascar: Effects of troop size, habitat and rainfall

We censused Lemur catta within a 1 km² study area at Berenty Reserve, Madagascar, during the September-October birth season for 19 years between 1963 and 2000, a total of 290 troop counts (266 with age and sex). The non-infant population was 155 in 1972-5, fell to 105 in 1985, and rose to a maximum of 282 in 1997, while troops increased from 12 in 1972-1985 up to 25 in 1998-2000. Local density varies between habitat types from 1 per ha to ca. 6 per ha. Troops fission at ca. 15-25 individuals, or 6-10 females. Adult sex ratio has no apparent correlation with fissions, birth rate or survival. Birth rate falls steeply with number of adult females, from 80-100% in 2-female troops to about 50% in 8-10 female troops. The penalty for large troop size is greater in the dense, rich areas, but nonetheless troops there are also larger. One-year-survival does not vary with troop size, and is lower in the sparse, dry zone. Troop size is too large for optimal birth rate, but fissioning to much lower size might make troops too small for optimal adult survival, given the intense intertroop competition. This reflects Sibley's (1983) conjecture that troop sizes may not reach stable optima. Rainfall per lemur-year (beginning Oct 1) varied from 265 to 894 mm. Drought followed by rain can eliminate >90% of a cohort, especially in the dryest zone. Possibly this results from fruit failure in years following drought. It is unknown whether food supplementation of some Berenty troops is dangerous for the forest, or helpful for an isolated and vulnerable ring-tailed lemur population.