I ran across this new paper by Jacinta Beehner and colleagues, which has a very intensive sampling of pregnancy outcomes in Amboseli baboons:
Environmental conditions are a key factor mediating reproductive success or failure. Consequently, many mammalian taxa have breeding seasons that coordinate critical reproductive stages with optimal environmental conditions. However, in contrast with most mammals, baboons (Papio cynocephalus) of Kenya reproduce throughout the year. Here we depart from the typical approach of evaluating seasonal effects on reproduction and engage in a more fine-grained analysis of the actual ecological conditions leading up to reproduction for females. Our aim was to determine how environmental conditions, in combination with social and demographic factors, might mediate baboon reproduction. The data set includes all female reproductive cycles from multiple baboon groups in the Amboseli basin between 1976 and 2004. Results indicate that after periods of drought or extreme heat, females were significantly less likely to cycle than expected. If females did cycle after these conditions, they were less likely to conceive; and if they did conceive after drought (heat effects were nonsignificant), they were less likely to have a successful pregnancy. Age also significantly predicted conceptive failure; conceptive probability was lowest among the youngest and oldest cycling females. There was also a trend for high ambient temperatures to contribute to fetal loss during the first trimester but not other trimesters. Finally, group size and drought conditions interacted in their effects on the probability of conception. Although females in all groups had equal conception probabilities during optimal conditions, females in large groups were less likely than those in small groups to conceive during periods of drought. These results indicate that in a highly variable environment, baboon reproductive success is mediated by the interaction between proximate ecological conditions and individual demographic factors (Beehner et al. 2006:741).
So even though baboons aren't seasonal, their reproductive success varies by season. And that last part, about group size depressing conception probability, is very interesting. The paper discusses it in terms of population density, and relate it also to the increase in interbirth interval exhibited among females in larger groups (Altmann and Alberts 2003):
Many mammalian studies have documented the costs of high density on female reproduction (voles, Microtus spp.: Agrell et al. 1995; European badgers, M. meles: Cresswell et al. 1992; Woodroffe and MacDonald 1995; deer mice, Peromyscus maniculatus: Eleftheriou et al. 1962; African mole rats, Cryptomys hottentotus: Jarvis 1969; house mice, Mus musculus: Ryan and Schwartz 1977). Additionally, previous work in Amboseli has shown that females living in larger groups had longer interbirth intervals (after a surviving offspring) than females in smaller groups (Altmann and Alberts 2003a). Consistent with these previous results, the current study revealed that conception rates were significantly altered by an interaction between the number of females in each group and periods of drought. Following "good" conditions (i.e., adequate rainfall for high primary plant productivity), large and small groups had almost identical rates of conception. In contrast, following "bad" conditions (i.e., drought), rates of conceptive failure increased for females in large groups (Figure 4). These results suggest that the costs of poor ecological conditions may be borne disproportionately by females living in large groups. The detrimental impact of large group size on reproduction, particularly during drought conditions, probably results from reduced foraging efficiency from scramble competition (Bronikowski and Altmann 1996; Altmann and Alberts 2003a), as found in several other cercopithecine populations (van Schaik and van Noordwijk 1988; Dunbar 1996). Increased within-group feeding competition is widely recognized as one of the main costs of group living among social mammals (Terborgh and Janson 1986; Wrangham et al. 1993; Janson and Goldsmith 1995). The ecological constraints model suggests that scramble competition limits group size because larger groups must forage further or more often to meet the energetic requirements of their members (Milton 1984; Janson 1988; Wrangham et al. 1993; Chapman et al. 1995), and previous studies on Amboseli groups have found that dry periods are associated with increased time spent foraging (Bronikowski and Altmann 1996) (.
The correlation between group size and conception probability during droughts was not super-strong (r was not reported, but there is much scatter). Still, the conception probability in the largest groups (between 22 and 25 females) was between 2 and 10 percent, while the smallest groups (between 6 and 14 females) had probabilities between 15 and 50 percent.
If we can believe that the effect was that large, it boggles the mind to think that some females tolerate it. Why don't they abandon the large groups? There are two basic alternatives -- either they can't abandon the large groups because of the local population densities, or staying in a large group has some compensatory advantages (such as decreased predation or increased mate quality). In that respect, living in small groups may be an adaptation to shortfalls. Paradoxically, the more marginal habitat may be safer in some respects during droughts. The smaller population local population density during the weter periods allows relatively higher reproductive output during the drier periods. Thus, population dynamics across time involves tradeoffs in habitat selection and group size.
Altmann J, Alberts SC. 2003a. Intraspecific variability in fertility and offspring survival in a non-human primate: behavioral control of ecological and social sources. In: Wachter KW, editor. Offspring: human fertility behavior in a biodemographic perspective. Washington, DC: National Academy Press. p 140-169.
Beehner JC, Onderdonk DA, Alberts SC, Altmann J. 2006. The ecology of conception and pregnancy failure in wild baboons. Behav Ecol 17:741-750. DOI link