Complex effects of testosterone level on ectoparasite load in a ground squirrel: an experimental test for the immunocompetence handicap hypothesis.

BACKGROUND: The immunocompetence handicap hypothesis suggests that males with a higher testosterone level should be better at developing male secondary traits, but at a cost of suppressed immune performance. As a result, we should expect that males with an increased testosterone level also possess a higher parasite load. However, previous empirical studies aimed to test this prediction have generated mixed results. Meanwhile, the effect of testosterone level on parasite load in female hosts remains poorly known. METHODS: In this study, we tested this prediction by manipulating testosterone level in Daurian ground squirrels (Spermophilus dauricus), a medium-sized rodent widely distributed in northeast Asia. S. dauricus is an important host of ticks and fleas and often viewed as a considerable reservoir of plague. Live-trapped S. dauricus were injected with either tea oil (control group) or testosterone (treatment group) and then released. A total of 10 days later, the rodents were recaptured and checked for ectoparasites. Fecal samples were also collected to measure testosterone level of each individual. RESULTS: We found that testosterone manipulation and sex of hosts interacted to affect tick load. At the end of the experiment, male squirrels subjected to testosterone implantation had an averagely higher tick load than males from the control group. However, this pattern was not found in females. Moreover, testosterone manipulation did not significantly affect flea load in S. dauricus. CONCLUSIONS: Our results only lent limited support for the immunocompetence handicap hypothesis, suggesting that the role of testosterone on regulating parasite load is relatively complex, and may largely depend on parasite type and gender of hosts.

Combined effects of light pollution and vegetation height on behavior and body weight in a nocturnal rodent.

At a global scale, organisms are under threat due to various kinds of environmental changes, such as artificial light at night (ALAN), noise, climatic change and vegetation destruction. Usually, these changes co-vary in time and space and may take effect simultaneously. Although impacts of ALAN on biological processes have been well documented, our knowledge on the combined effects of ALAN and other environmental changes on animals remains limited. In this study, we conducted field experiments in semi-natural enclosures to explore the combined effects of ALAN and vegetation height on foraging behavior, vigilance, activity patterns and body weight in dwarf striped hamsters (Cricetulus barabensis), a nocturnal rodent widely distributed in East Asia. We find that ALAN and vegetation height affected different aspects of behavior. ALAN negatively affected search speed and positively affected handling speed, while vegetation height negatively affected giving-up density and positively affected body weight. ALAN and vegetation height also additively shaped total time spent in a food patch. No significant interactive effect of ALAN and vegetation height was detected. C. barabensis exposed to ALAN and short vegetation suffered a significant loss in body weight, and possessed a much narrower temporal niche (i.e. initiated activity later but became inactive earlier) than those under other combinations of treatments. The observed behavioral responses to ALAN and changes in vegetation height may bring fitness consequences, as well as further changes in structure and functioning of local ecosystems.

Ecological drivers of avian diversity in a subtropical landscape: Effects of habitat diversity, primary productivity and anthropogenic disturbance.

Understanding the roles of ecological drivers in shaping biodiversity is fundamental for conservation practice. In this study, we explored the effects of elevation, conservation status, primary productivity, habitat diversity and anthropogenic disturbance (represented by human population density and birding history) on taxonomic, phylogenetic and functional avian diversity in a subtropical landscape in southeastern China. We conducted bird surveys using 1-km transects across a total of 30 sites, of which 10 sites were located within a natural reserve. Metrics of functional diversity were calculated based on six functional traits (body mass, clutch size, dispersal ratio, sociality, diet and foraging stratum). We built simultaneous autoregression models to assess the association between the ecological factors and diversity of the local avian communities. Local avian diversity generally increased with increasing habitat diversity, human population density and primary productivity. We also detected phylogenetic and functional clustering in these communities, suggesting that the avian assemblages were structured mainly by environmental filtering, rather than interspecific competition. Compared with sites outside the natural reserve, sites within the natural reserve had relatively lower avian diversity but a higher level of phylogenetic heterogeneity.

Ecological correlates of ectoparasite load in a rodent: Complex roles of seasonality.

Understanding the mechanisms driving parasite distributions is not only important for understanding ecosystem functioning, but also crucial for disease control. Previous studies have documented the important roles of host sex, host body size, host behavioral trait (such as boldness and trappability), and seasonality in shaping parasite load. However, few studies have simultaneously assessed the roles of these factors, as well as their interactions. In spring and summer of 2021, we conducted live trapping in Hohhot, China, to collect ectoparasites on Daurian ground squirrel (Spermophilus dauricus), a small rodent widely distributed in East Asian grassland. We then used generalized linear models to explore the effects of several biological factors (sex, body weight, trappability, and reproductive status) and seasonality on the abundance of ticks and fleas in S. dauricus. Significant but inconsistent seasonal effects were observed: tick load was significantly greater in summer than in spring, while flea load was greater in spring than in summer. Seasons also significantly interacted with host trappability and body weight to affect tick abundance. Our results highlight the importance of considering seasonal changes in parasitism, as well as interactions between season and host biological traits in shaping parasite distributions.

Effects of artificial light at night on foraging behavior and vigilance in a nocturnal rodent.

Artificial light at night has greatly changed the physical environment for many organisms on a global scale. As an energy efficient light resource, light emitting diodes (LEDs) have been widely used in recent years. As LEDs often have a broad spectrum, many biological processes may be potentially affected. In this study, we conducted manipulated experiments in rat-proof enclosures to explore the effects of LED night lighting on behavior of a nocturnal rodent, the Mongolian five-toed jerboa (Allactaga sibirica). We adopted the giving-up density (GUD) method and camera video trapping to study behavioral responses in terms of patch use, searching efficiency and vigilance. With the presence of white LED lighting, jerboas spent less time in patches, foraged less intensively (with higher GUDs) and became vigilant more frequently, while their searching efficiency was higher than under dark treatment. Although both positive and negative effects of LEDs on foraging were detected, the net effect of LEDs on jerboas is negative, which may further translate into changes in population dynamics, inter-specific interaction and community structure. To our knowledge, this is the first field study to explore how LED lighting affect foraging behavior and searching efficiency in rodents. Our results may have potential implications for practices such as pest control.

Different elevational patterns of rodent species richness between the southern and northern slopes of a mountain.

Studies on elevational gradients in biodiversity have accumulated in recent decades. However, few studies have compared the elevational patterns of diversity between the different slopes of a single mountain. We investigated the elevational distribution of rodent diversity (alpha and beta diversity) and its underlying mechanisms along the southern and northern slopes of Mt. Taibai, the highest mountain in the Qinling Mountains, China. The species richness of rodents on the two slopes showed distinct distribution patterns, with a monotonically decreasing pattern found along the southern slope and a hump-shaped elevational pattern evident along the northern slope. Multi-model inference suggested that temperature was an important explanatory factor for the richness pattern along the southern slope, and the mid-domain effect (MDE) was important in explaining the richness pattern along the northern slope. The two slopes also greatly differed in the elevational patterns of species turnover, with the southern slope demonstrating a U-shaped curve and the northern slope possessing a roughly hump-shaped pattern. Our results suggest that even within the same mountain, organisms inhabiting different slopes may possess distinct diversity patterns, and the underlying mechanisms may also differ. The potential role of the factors associated with slope aspect in shaping diversity, therefore, cannot be ignored.

Effects of cattle grazing on small mammal communities in the Hulunber meadow steppe.

Small mammals play important roles in many ecosystems, and understanding their response to disturbances such as cattle grazing is fundamental for developing sustainable land use strategies. However, how small mammals respond to cattle grazing remains controversial. A potential cause is that most of previous studies adopt rather simple experimental designs based solely on the presence/absence of grazing, and are thus unable to detect any complex relationships between diversity and grazing intensity. In this study, we conducted manipulated experiments in the Hulunber meadow steppe to survey small mammal community structures under four levels of grazing intensities. We found dramatic changes in species composition in native small mammal communities when grazing intensity reached intermediate levels (0.46 animal unit/ha). As grazing intensity increased, Spermophilus dauricus gradually became the single dominant species. Species richness and diversity of small mammals in ungrazed and lightly grazed (0.23 animal unit/ha) area were much higher than in intermediately and heavily grazed area. We did not detect a humped relationship between small mammal diversity and disturbance levels predicted by the intermediate disturbance hypothesis (IDH). Our study highlighted the necessity of conducting manipulated experiments under multiple grazing intensities.