The Enemy Within: How Does a Bacterium Inhibit the Foraging Aptitude and Risk Management Behavior of Allenby's Gerbils?

AbstractMicrobes inhabiting multicellular organisms have complex, often subtle effects on their hosts. Gerbillus andersoni allenbyi are commonly infected with Mycoplasma haemomuris-like bacteria, which may cause mild nutrient (choline, arginine) deficiencies. However, are there more serious ecological consequences of infection, such as effects on foraging aptitudes and risk management? We tested two alternatives: the nutrient compensation hypothesis (does nutrient deficiency induce infected gerbils to make up for the shortfall by foraging more and taking greater risks?) and (2) the lethargy hypothesis (do sick gerbils forage less, and are they compromised in their ability to detect predators or risky microhabitats?). We compared the foraging and risk management behavior of infected and noninfected gerbils. We experimentally infected gerbils with the bacteria, which allowed us to compare between noninfected, acutely infected (peak infection loads), and chronically infected (low infection loads) individuals. Our findings supported the lethargy hypothesis over the nutrient compensation hypothesis. Infected individuals incurred dramatically elevated foraging costs, including less efficient foraging, diminished "quality" of time spent vigilant, and increased owl predation. Interestingly, gerbils that were chronically infected (lower bacteria load) experienced larger ecological costs than acutely infected individuals (i.e., peak infection loads). This suggests that the debilitating effects of infection occur gradually, with a progressive decline in the quality of time gerbils allocated to foraging and managing risk. These increased long-term costs of infection demonstrate how small direct physiological costs of infection can lead to large indirect ecological costs. The indirect ecological costs of this parasite appear to be much greater than the direct physiological costs.

Alarm calls or predator calls: which elicit stronger responses in ungulate communities living with and without lions?

Alarm calls and predator vocalizations convey information on predator presence and potential risk. Generally, prey employ anti-predator behaviours more in response to alarm calls. However, occasionally prey respond more to the vocalizations of specific predators. A key question is do prey still respond to alarm calls and predator vocalizations when a dangerous predator is absent? Additionally, would the prey species' response (e.g. vigilance) differ from prey already living with these predators? Using auditory playbacks, we tested whether four herbivore species living with lions responded more to alarm calls than lion vocalizations compared to a black cuckoo control call. Overall, red hartebeest, wildebeest and zebra had greater vigilance in response to the lion roars compared to the alarm calls. The differences in vigilance suggest that, despite the lion roars not being related to hunting, these herbivores perceived the predator vocalizations as a more immediate indicator of risk than the alarm calls. We then tested whether herbivores living with lions increased their vigilance more in response to the calls than conspecifics in a lion-free section. Despite greater overall vigilance in the lion section, gemsbok and zebra in the lion-free section significantly increased their vigilance in response to the lion roars. This indicates that species under the greatest threat from a predator (e.g. preferred prey) may maintain innate anti-predator responses to an absent but dangerous predator longer than less preferred prey. Ultimately, our results indicate that cues from dangerous predators can have greater effects on anti-predator behaviours than alarm calls for some prey species.

Does intraspecific competition among Allenby's gerbils lead to an Ideal Free Distribution across foraging patches?

Employing the Ideal Free Distribution (IFD) principle as a tool, we investigated how Allenby's gerbils (Gerbillus andersoni allenbyi) utilized food patches within and moved between connected quadrants (i.e., 'habitats') in a large outdoor semi-natural enclosure. These habitats differed in initial forager densities, but provided equal numbers of standardized food patches that provided equal rewards (i.e. food) and costs (i.e. predation risk, metabolic, and missed opportunity). We quantified the gerbils' giving-up-densities (GUDs) within foraging patches and recorded their daily distribution between habitats. Individual gerbils were tagged with unique bar-coded numbers to compare their locations within and across habitats. The mean number of gerbil foragers (9.1 and 8.9 individuals, respectively) and GUDs evened out across habitats over time. Despite this, the distribution of gerbils did not remain static within foraging patches; instead, gerbils altered their use of patches across and within habitats on a nightly basis. This may be due to a combination of factors including, high levels of interference competition between foragers at patches, a lag effect before the gerbils perceived changes in competition intensity with the arrival and departure of individuals, and gerbils having imperfect knowledge of their environment. Furthermore, the pattern of microhabitat (open vs bush patches) use by gerbils differed over time, indicating that despite the distribution of gerbils and their GUDs evening out between habitats, they still preferred foraging from safer bush patches over riskier open patches. This study provides insights into how under low predation risk, strong levels of intraspecific competition can shape the distribution of foragers across and within habitats.