Measuring mantled howler monkey (Alouatta palliata) testes via parallel laser photogrammetry: Expanding the use of noninvasive methods.

Parallel laser photogrammetry (PLP), which consists of attaching two or three parallel laser beams at a known inter-beam distance to a camera, can be used to collect morphological measurements of organisms noninvasively. The lasers project onto the photo being taken, and because the inter-beam distance is known, they act as a scale for image analysis programs like ImageJ. Traditionally, this method has been used to measure larger morphological traits (e.g., limb length, crown-rump length) to serve as proxies for overall body size, whereas applications to smaller anatomical features remain limited. To that end, we used PLP to measure the testes of 18 free-living mantled howler monkeys (Alouatta palliata) at La Selva Biological Station, Costa Rica. We tested whether this method could reliably measure this relatively small and globular morphology, and whether it could detect differences among individuals. We tested reliability in three ways: within-photo (coefficient of variation [CV] = 4.7%), between-photo (CV = 5.5%), and interobserver (intraclass correlation = 0.92). We found an average volume of 36.2 cm3 and a range of 16.4-54.4 cm3, indicating variation in testes size between individuals. Furthermore, these sizes are consistent with a previous study that collected measurements by hand, suggesting that PLP is a useful method for making noninvasive measurements of testes.

Small increases in group size improve small shoals' response to water flow in zebrafish.

Social context may influence the perception of sensory cues and the ability to display refined behavioral responses. Previous work suggests that effective responses to environmental cues can be contingent on having a sufficient number of individuals in a group. Thus, the changes in group size may have profound impacts, particularly on the behavior of small social groups. Using zebrafish (Danio rerio), here we examined how changes in group size influence the ability to respond to changes in water flow. We found that fish in relatively larger groups displayed stronger rheotaxis even when comparing pairs of fish with groups of four fish, indicating that a small increase in group size can enhance the responsiveness to environmental change. Individual fish in relatively larger groups also spent less time in the energetically costly leading position compared to individuals in pairs, indicating that even a small increase in group size may provide energetic benefits. We also found that the shoal cohesion was dependent on the size of the group but within a given group size, shoal cohesion did not vary with flow rate. Our study highlights that even a small change in group size could significantly affect the way social fish respond to the changes in water flow, which could be an important attribute that shapes the resilience of social animals in changing environments.

Primate microbial endocrinology: An uncharted frontier.

Gut microbial communities communicate bidirectionally with the brain through endocrine, immune, and neural signaling, influencing the physiology and behavior of hosts. The emerging field of microbial endocrinology offers innovative perspectives and methods to analyze host-microbe relationships with relevance to primate ecology, evolution, and conservation. Herein we briefly summarize key findings from microbial endocrinology and explore how applications of a similar framework could inform our understanding of primate stress and reproductive physiology and behavior. We conclude with three guiding hypotheses to further investigate endocrine signaling between gut microbes and the host: (a) host-microbe communication systems promote microbe-mediated stability, in which the microbes are using endocrine signaling from the host to maintain a functioning habitat for their own fitness, (b) host-microbe communication systems promote host-mediated stability, in which the host uses the endocrine system to monitor microbial communities and alter these communities to maintain stability, or (c) host-microbe systems are simply the product of coincidental cross-talk between the host and microbes due to similar molecules from shared ancestry. Utilizing theory and methodology for studying relationships between the microbiome, hormones, and behavior of wild primates is an uncharted frontier with many promising insights when applied to primatology.