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1. Comparing single- and mixed-species groups in fruit flies: differences in group dynamics, but not group formation

   https://doi.org/10.1093/jhered/esab041  

   Girardeau, Anna R; Foley, Brad R; Saltz, Julia B (August 2021, Journal of Heredity)

   Hughes, Kim (Ed.)

   Abstract Mixed-species groups describe active associations among individuals of 2 or more species at the same trophic level. Mixed-species groups are important to key ecological and evolutionary processes such as competition and predation, and research that ignores the presence of other species risks ignoring a key aspect of the environment in which social behavior is expressed and selected. Despite the defining emphasis of active formation for mixed-species groups, surprisingly little is known about the mechanisms by which mixed-species groups form. Furthermore, insects have been almost completely ignored in the study of mixed-species groups, despite their taxonomic importance and relative prominence in the study of single-species groups. Here, we measured group formation processes in Drosophila melanogaster and its sister species, Drosophila simulans. Each species was studied alone, and together, and one population of D. melanogaster was also studied both alone and with another, phenotypically distinct D. melanogaster population, in a nested-factorial design. This approach differs from typical methods of studying mixed-species groups in that we could quantitatively compare group formation between single-population, mixed-population, and mixed-species treatments. Surprisingly, we found no differences between treatments in the number, size, or composition of groups that formed, suggesting that single- and mixed-species groups form through similar mechanisms of active attraction. However, we found that mixed-species groups showed elevated interspecies male–male interactions, relative to interpopulation or intergenotype interactions in single-species groups. Our findings expand the conceptual and taxonomic study of mixed-species groups while raising new questions about the mechanisms of group formation broadly. 

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2. Migratory behavior in the enigmatic Late Pleistocene bovid Rusingoryx atopocranion

   https://doi.org/10.3389/fearc.2023.1237714  

   O'Brien, Kaedan; Podkovyroff, Katya; Fernandez, Diego P.; Tryon, Christian A.; Ashioya, Lilian; Faith, J. Tyler (August 2023, Frontiers in Environmental Archaeology)

   Reynolds, Sally (Ed.)

   For many animals, migration is an important strategy for navigating seasonal bottlenecks in resource availability. In the savannas of eastern Africa, herds of grazing animals, including blue wildebeest (Connochaetes taurinus), Thomson's gazelle (Eudorcas thomsonii), and plains zebra (Equus quagga), travel hundreds of kilometers annually tracking suitable forage and water. However, we know nearly nothing about migration among the extinct species that often dominated Late Pleistocene communities. Using serially sampled 87Sr/86Sr and δ13C, we characterize the prehistoric movement and diet of the enigmatic wildebeest Rusingoryx atopocranion from two localities (Karungu and Rusinga Island) in the Lake Victoria Basin of western Kenya. We find clear evidence for migration in all four individuals studied, with three 87Sr/86Sr series demonstrating high-amplitude fluctuations and all falling outside the modeled isoscape 87Sr/86Sr ranges of the fossil localities from which they were recovered. This suggests that R. atopocranion exhibited migratory behavior comparable to that of its closest living relatives in the genus Connochaetes. Additionally, individuals show seasonally-variable δ13C, with a higher browse intake than modern and fossil eastern African alcelaphins indicating behavioral differences among extinct taxa otherwise unrecognized by comparison with extant related species. That this species was highly migratory aligns with its morphology matching that of an open grassland migrant: it had open-adapted postcranial morphology along with a unique cranial structure convergent with lambeosaurine dinosaurs for calling long distances. We further hypothesize that its migratory behavior may be linked to its extinction, as R. atopocranion disappears from the Lake Victoria Basin fossil sequence coincident with the refilling of Lake Victoria sometime after 36 ka, potentially impeding its past migratory routes. This study characterizes migration in an extinct eastern African species for the first time and shapes our ecological understanding of this unique bovid and the ecosystems in which Middle Stone Age humans lived. 

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3. Interplay of competition and facilitation in grazing succession by migrant Serengeti herbivores

   https://doi.org/10.1126/science.adg0744  

   Anderson, T. Michael; Hepler, Staci A.; Holdo, Ricardo M.; Donaldson, Jason E.; Erhardt, Robert J.; Hopcraft, J. Grant; Hutchinson, Matthew C.; Huebner, Sarah E.; Morrison, Thomas A.; Muday, Jeffry; et al (February 2024, Science)

   Competition, facilitation, and predation offer alternative explanations for successional patterns of migratory herbivores. However, these interactions are difficult to measure, leaving uncertainty about the mechanisms underlying body-size-dependent grazing—and even whether succession occurs at all. We used data from an 8-year camera-trap survey, GPS-collared herbivores, and fecal DNA metabarcoding to analyze the timing, arrival order, and interactions among migratory grazers in Serengeti National Park. Temporal grazing succession is characterized by a “push-pull” dynamic: Competitive grazing nudges zebra ahead of co-migrating wildebeest, whereas grass consumption by these large-bodied migrants attracts trailing, small-bodied gazelle that benefit from facilitation. “Natural experiments” involving intense wildfires and rainfall respectively disrupted and strengthened these effects. Our results highlight a balance between facilitative and competitive forces in co-regulating large-scale ungulate migrations. 

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4. Reactive anti-predator behavioral strategy shaped by predator characteristics

   https://doi.org/10.1371/journal.pone.0256147  

   Palmer, Meredith S.; Packer, Craig (August 2021, PLOS ONE)

   Yue, Bi-Song (Ed.)

   Large mammalian herbivores use a diverse array of strategies to survive predator encounters including flight, grouping, vigilance, warning signals, and fitness indicators. While anti-predator strategies appear to be driven by specific predator traits, no prior studies have rigorously evaluated whether predator hunting characteristics predict reactive anti-predator responses. We experimentally investigated behavioral decisions made by free-ranging impala, wildebeest, and zebra during encounters with model predators with different functional traits. We hypothesized that the choice of response would be driven by a predator’s hunting style (i.e., ambush vs. coursing) while the intensity at which the behavior was performed would correlate with predator traits that contribute to the prey’s relative risk (i.e., each predator’s prey preference, prey-specific capture success, and local predator density). We found that the choice and intensity of anti-predator behaviors were both shaped by hunting style and relative risk factors. All prey species directed longer periods of vigilance towards predators with higher capture success. The decision to flee was the only behavior choice driven by predator characteristics (capture success and hunting style) while intensity of vigilance, frequency of alarm-calling, and flight latency were modulated based on predator hunting strategy and relative risk level. Impala regulated only the intensity of their behaviors, while zebra and wildebeest changed both type and intensity of response based on predator traits. Zebra and impala reacted to multiple components of predation threat, while wildebeest responded solely to capture success. Overall, our findings suggest that certain behaviors potentially facilitate survival under specific contexts and that prey responses may reflect the perceived level of predation risk, suggesting that adaptive functions to reactive anti-predator behaviors may reflect potential trade-offs to their use. The strong influence of prey species identity and social and environmental context suggest that these factors may interact with predator traits to determine the optimal response to immediate predation threat. 

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5. Mixed‐species groups of Serengeti grazers: a test of the stress gradient hypothesis

   https://doi.org/10.1002/ecy.3163  

   Beaudrot, Lydia; Palmer, Meredith S.; Anderson, T. Michael; Packer, Craig (August 2020, Ecology)

   Understanding the role of species interactions within communities is a central focus of ecology. A key challenge is to understand variation in species interactions along environmental gradients. The stress gradient hypothesis posits that positive interactions increase and competitive interactions decrease with increasing consumer pressure or environmental stress. This hypothesis has received extensive attention in plant community ecology, but only a handful of tests in animals. Furthermore, few empirical studies have examined multiple co‐occurring stressors. Here we test predictions of the stress gradient hypothesis using the occurrence of mixed‐species groups in six common grazing ungulate species within the Serengeti‐Mara ecosystem. We use mixed‐species groups as a proxy for potential positive interactions because they may enhance protection from predators or increase access to high‐quality forage. Alternatively, competition for resources may limit the formation of mixed‐species groups. Using more than 115,000 camera trap observations collected over 5 yr, we found that mixed‐species groups were more likely to occur in risky areas (i.e., areas closer to lion vantage points and in woodland habitat where lions hunt preferentially) and during time periods when resource levels were high. These results are consistent with the interpretation that stress from high predation risk may contribute to the formation of mixed‐species groups, but that competition for resources may prevent their formation when food availability is low. Our results are consistent with support for the stress gradient hypothesis in animals along a consumer pressure gradient while identifying the potential influence of a co‐occurring stressor, thus providing a link between research in plant community ecology on the stress gradient hypothesis, and research in animal ecology on trade‐offs between foraging and risk in landscapes of fear. 

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