Long-term field studies are critical for our understanding of animal life history and the processes driving changes in demography. Here, we present long-term demographic data for the northernmost population of mantled howler monkeys (Alouatta palliata) residing in a highly anthropogenically fragmented landscape in Los Tuxtlas, Mexico. We carried out 454 monthly group visits to 10 groups of mantled howler monkeys between 2000 and 2011. The population remained relatively stable over the 11-year

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... udy period, with an overall increase in the total number of individuals. Birth rates and inter-birth intervals were comparable to those of howler monkeys at non-fragmented sites, suggesting that living in a fragmented landscape did not affect the reproductive output of our study population. Moreover, despite the landscape, dispersal events were commonplace, including many secondary dispersals (individuals emigrating from groups that they had previously immigrated into). Finally, we found a marked effect of seasonality on the dynamics of our study population. In particular, the period of lowest temperatures and resource scarcity between November and March was associated with higher mortality and reproductive inhibition, while the period of resource abundance between April and May was associated with the majority of conceptions and weaning of offspring. This, in turn, could be influencing dispersal patterns in our study area, as male howler monkeys seem to time some of their immigrations into new groups to coincide with the start of the period of higher fertility, while females preferentially joined new groups several months before the onset of this period. These data have important implications for the conservation and management of howler monkeys in fragmented landscapes, as well as for our understanding of the effect of seasonality over howler monkey dispersal, reproduction and survival. Manuscript to be reviewed 24 25 ABSTRACT 26 Long-term field studies are critical for our understanding of animal life history and the processes 27 driving changes in demography. Here, we present long-term demographic data for the 28 northernmost population of mantled howler monkeys (Alouatta palliata) residing in a highly 29 anthropogenically fragmented landscape in Los Tuxtlas, Mexico. We carried out 454 monthly 30 group visits to 10 groups of mantled howler monkeys between 2000 and 2011. The population 31 remained relatively stable over the 11-year study period, with an overall increase in the total 32 number of individuals. Birth rates and inter-birth intervals were comparable to those of howler 33 monkeys at non-fragmented sites, suggesting that living in a fragmented landscape did not affect 34 the reproductive output of our study population. Moreover, despite the landscape, dispersal 35 events were commonplace, including many secondary dispersals (individuals emigrating from 36 groups that they had previously immigrated into). Finally, we found a marked effect of 37 seasonality on the dynamics of our study population. In particular, the period of lowest 38 temperatures and resource scarcity between November and March was associated with higher 39 mortality and reproductive inhibition, while the period of resource abundance between April and 40 May was associated with the majority of conceptions and weaning of offspring. This, in turn, 41 could be influencing dispersal patterns in our study area, as male howler monkeys seem to time 42 some of their immigrations into new groups to coincide with the start of the period of higher 43 fertility, while females preferentially joined new groups several months before the onset of this 44 period. These data have important implications for the conservation and management of howler 45 monkeys in fragmented landscapes, as well as for our understanding of the effect of seasonality 46 over howler monkey dispersal, reproduction and survival.