Mothers Frequent Caves: Lactation Affects Chimpanzee (Pan troglodytes verus) Cave Use in Southeastern Senegal

Drinking frequency in wild lactating chimpanzees (Pan troglodytes schweinfurthii) and their offspring

Maintaining water balance is essential for organismal health, and lactating females must balance individual needs with milk production and offspring hydration. Primate milk is dilute and presumed to be the primary source for infant hydration for a considerable time period. Few studies have investigated the hydration burden that lactation may place on female primates. In this study, we investigated sources of variation in female and offspring drinking frequency among wild chimpanzees (Pan troglodytes). We hypothesized females would experience seasonal and lactation hydration burdens and adjust their drinking behavior to accommodate these, but this hydration burden would vary between females of different dominance ranks. We also predicted that parity would relate to maternal drinking frequency since primiparous females are still investing in their own growth. Finally, we predicted that offspring would drink more in the dry season and as they aged and lost milk as a water source, but that offspring of high-ranking females would be buffered from these effects. Using 41 years of long-term data on the behavior of mothers and offspring of Gombe National Park, we found that mothers drank more in the dry season, but there was no significant difference between mothers of different ranks during this period. Low-ranking females drank significantly more than mid- and high-ranking females during late lactation. Offspring also drank more in the dry season and as they aged, but there was no evidence of buffering for those with high-ranking mothers. While chimpanzees in our study population drank infrequently, they do demonstrate noticeable shifts in drinking behavior that suggests seasonal and reproductive hydration burdens.

Mechanical loading of primate fingers on vertical rock surfaces

Mechanical loading of finger bones (phalanges) can induce angular curvature, which benefits arboreal primates by dissipating forces and economising the recruitment of muscles during climbing. The recent discovery of extremely curved phalanges in a hominin, Homo naledi, is puzzling, for it suggests life in an arboreal milieu, or, alternatively, habitual climbing on vertical rock surfaces. The importance of climbing rock walls is attested by several populations of baboons, one of which uses a 7-m vertical surface to enter and exit Dronkvlei Cave, De Hoop Nature Reserve, South Africa. This rock surface is an attractive model for estimating the probability of extreme mechanical loading on the phalanges of rock-climbing primates. Here we use three-dimensional photogrammetry to show that 82–91% of the climbable surface would generate high forces on the flexor tendon pulley system and severely load the phalanges of baboons and H. naledi. If such proportions are representative of vertical rock surfaces elsewhere, it may be sufficient to induce stress-mitigating curvature in the phalanges of primates.

Chimpanzees (Pan troglodytes) in savanna landscapes

Chimpanzees (Pan troglodytes) are the only great apes that inhabit hot, dry, and open savannas. We review the environmental pressures of savannas on chimpanzees, such as food and water scarcity, and the evidence for chimpanzees' behavioral responses to these landscapes. In our analysis, savannas were generally associated with low chimpanzee population densities and large home ranges. In addition, thermoregulatory behaviors that likely reduce hyperthermia risk, such as cave use, were frequently observed in the hottest and driest savanna landscapes. We hypothesize that such responses are evidence of a "savanna landscape effect" in chimpanzees and offer pathways for future research to understand its evolutionary processes and mechanisms. We conclude by discussing the significance of research on savanna chimpanzees to modeling the evolution of early hominin traits and informing conservation programs for these endangered apes.