We studied the birth patterns of 8 groups of howler monkeys inhabit 7 patches of high evergreen rain forest in a fragmented landscape within the Los Tuxtlas Biosphere Reserve in southern Mexico. We used observed birth data (12-17 groupmo) as well as estimated data (4-yr period) from non consecutive census in a monthly basis. Births per female-mo averaged 0.059 mo being the mean interval between births when previous infant died 10.0 months (n=2) and when survived 16.33 months (n=3), shorter than other howler reports. Mean recent born infant mortality was 27.78%, similar to other locations. Births occurred throughout the year with peak in October-November (wetter months). We didn't find evidences of a reproductive depression in our population despite the apparent high density and degradation of the patches. We compare our results with a theoretical simple model of the immature dynamics.

Key words: Alouatta palliata mexicana, births, reproduction, fragmentation, model

Centre Especial de Recerca en Primats, Universitat de Barcelona. Passeig de la Vall d'Hebron 171, 08035 Barcelona, Spain. E-mail: cdbalcells@yahoo.com

INTRODUCTION

Most of data known about reproduction patterns of mantled howling monkeys came from Costa Rica (Glander, 1980; Clarke, 1990; Fedigan & Rose, 1995) and Panama (Carpenter, 1965; Milton, 1982). Just a few studies were conducted in Mexico, basically in the Agaltepec Island on Catemaco Lake, near of this study location (Cortes-Ortiz et al., 1994; Carrera-Sanchez et al., 2002), but never had it been reported before birth patterns of wild populations of Los Tuxtlas Reserve. Nevertheless, there are some differences in the latitude, the seasonality of rains, the kind of vegetation, and size of the monkeys (referencies?) between sites of study of Alouatta palliata, and these and others local ambient factors are likely to affect the reproductive patterns of each population. In general, despite certain doubts is admitted that howler monkeys from Mexico corresponds to a particular subspecies named A. palliata mexicana, which have their own differential characteristics and represents one of the most northern distributions of the genus (referencies, qui ha descrit aquesta subespecie?), but few attention has been paid to possible reproductive particularities.

Both male and female emigrated from their natal groups in howler monkeys (Glander, 1992). Males abandoned the group at 15-36 months old and females at 24-40 months old, reaching the sexual maturity (reproductive status into a social group) at 42 months for males and 36 months old for females when giving the first birth (Glander, 1980). The average interval between births (IBI) depends on survivorship of the infant. When survives was estimated in 22.5 months in Costa Rica (n=16, Glander, 1980; median of 19.9 mo reported by Fedigan & Rose, 1995), 17 months in Barro Colorado Island (BCI), Panama (n=3, Milton, 1982) and 15.79 months in Agaltepec Island, Mexico (n=19, Cortes-Ortiz et al., 1994). When infant dies into the postpartum period of acyclicity of the mother, she¡¯s giving birth 9 and 10 months after the birth of their previous infant (N=2, Glander, 1980), and in Agaltepec 9.5 and 10.5 months (n=2, Cortes-Ortiz et al., 1994). In a more extensive work in Agaltepec the mean interval between births including dead infants was of 20.4 months (n=20, range 8.3-50.1 months, Carrera-Sanchez et al., 2002). Glander (1980) determined gestation length in 186 days (range 180-194) by counting the number of days from birth back to the last observed double-plus swelling of the vulva of females. Vegas et al (in prep.) based on peaks of fecal progesterone estimated gestation in Los Tuxtlas in 4 months instead 6 months commonly taken following Glander. However there is a lack of studies about gestation duration using other more reliable methods. ¢¯There is more estimations, other methods, other howler species, other places? Infant mortality in Agaltepec was calculated as 24% without seasonality pattern, 35.3% corresponding to primiparous mothers (Carrera-Sanchez et al., 2002), values minor than those reported by Glander (1980) of 29.2% of infant mortality and 100% for infants of 31/2 to 4 year old primiparous females (the age at first birth) coinciding with their Alpha status. While most New World monkey populations show birth seasonality, howlers are less seasonal as they have a more folivorous diet than others (Di Bitetti & Janson, 2000). Since data reviewed by Di Bitetti & Janson (2000), three factors affect the degree of birth seasonality: diet, latitude, and body size. Births of Alouatta palliata are distributed along the year in BCI, Panama (Milton, 1982) and Agaltepec, Mexico (Carrera-Sanchez et al., 2002). There weren¡¯t differences in births by months in 14 years in Agaltepec, but there was a tendency of more births between September and February (wet season in Los Tuxtlas). In contrast, in Santa Rosa National Park, Costa Rica (Fedigan & Rose, 1995), there is a peak of births between January and April (dry season in Costa Rica). On the other hand, Glander (1980) found that births were scattered during some years and concentrated during others, suggesting facultative births spacing responding to food quality, although the author reported no significant seasonal differences in nutrients content. Other howler species, Alouatta seniculus in Venezuela (Crockett & Rudran, 1987a) reported that the time of year with fewer births is around the peak of new leaves and fruits, and peak of critical nutrient availability is coincident with weaning offspring. Crockett and Rudran (1987a), suggested that high temperatures associated with a low moisture content of foods when infants begin to become independent of milk could produce a low birth-rate early in the wet season, however the lack of strict seasonality may respond to the howlers ability to eat leaves or unripe fruits or because their large body size that makes them less sensitive to food shortages. In conclusion, the seasonality of births in Alouatta seems to be more related to local climatic seasonality and the related quality of available food.

We reported here information about natality, mortality and maturation of infants from 8 groups of mantled howling monkeys (Alouatta palliata mexicana), by demographic census and inferences over a 4-yr period. The study troops inhabit 7 fragments of forest into the buffer area of Reserve of Los Tuxtlas, in Mexico, which constitutes one of the most northern distributions of the species Alouatta palliata. We included additional information of interval between births, age at emigration and births seasonality. We compare finally our field data with simple model outputs, which is proposed as a useful tool of reproduction studies.