High altitude imposes directional selection pressure from energetic constraints and other environmental challenges, driving population-level trait diversity along altitudinal gradients. Given energetic constraints are a major driver of intraspecific variation in vertebrate brain size, we tested this in a high-altitude system that is cognitively demanding and resource-scarce. Accordingly, we quantified brain size and structure of the Plateau Brown Frog (Rana kukunoris) across an altitudinal gradient to test the predictions of the cognitive buffer (CBH), expensive brain (EBH), developmental constraint (DCH), and functional constraint (FCH) hypotheses. We failed to find evidence for a general trend of reduced relative brain size at higher altitudes, or in relation to latitude. Notably, however, female cerebellum size decreased while optic tectum size increased with altitude, a pattern consistent with the FCH positing trade-offs between brain regions under environmental pressure. Additionally, an increased female optic tectum size supports the CBH, as the optic tectum’s relative size has also been shown to correlate positively with discrimination learning performance. However, no significant correlation was found between the relative size of any male brain structure and altitude or latitude. We therefore propose a sex-specific evolutionary strategy driven by altitudinal selection. For larger females, high energetic costs necessitate a trade-off that prioritizes investment in the optic tectum to maximize fitness components such as survival and fecundity, likely at the expense of investment in the cerebellum. Further investigation along altitudinal gradients is required to unravel the complex mechanisms driving adaptive brain evolution.
In symbiotic relationships, protection is believed to be the primary benefit that symbionts obtain from a host. Palaemonid shrimps are of notable interest when examining marine symbiosis, with over 500 described species forming symbiotic associations with other macrofauna. Instead of maintaining close contact with the host, several symbiotic palaemonid shrimps were found to leave their host temporarily when disturbed, suggesting that they may not completely rely on host protection and possess abilities to evade predators. This study examined protection obtained by symbiotic palaemonid shrimps from sea anemone host. It compared survival and behavioural responses of symbiotic shrimps with different levels of host association to free-living shrimps in the presence of a predator. A selected model predator, juvenile giant grouper, was introduced to the aquarium to investigate the protective benefit offered by the bubble-tip anemone. The survivorship of all three symbiotic shrimps examined, Ancylocaris brevicarpalis, Ancylomenes holthuisi, and Ancylomenes venustus, was significantly increased when associated with the anemone host, even though the latter two species form a loose association with their host. In the anemone treatment, most of the symbiotic shrimps showed a 100% survival rate, while all free-living shrimps were consumed. The stinging cells of the anemone serves as the major protective function rather than the physical complexity of its structure because the synthetic anemone did not provide detectable protection to the shrimps. Swaying motion and remaining motionless were the predominant behaviours for symbiotic cleaner shrimps and symbiotic non-cleaner shrimp, respectively, in responses to the predator. The symbiotic shrimps exhibited fewer tail-flipping responses as compared to the free-living shrimps, suggesting that the dependency on host protection reduced the self-protective capability of symbiotic shrimps. The present study highlights the importance of the protective benefit offered by sea anemone host to both closely and loosely associated symbiotic shrimps and reveals differences in predator-avoidance behaviours among palaemonid shrimps.
Jerboas (Dipus sagitta) are small bipedal desert rodents that evade predators through irregular jumping. This study investigates how the hindlimb joints of jerboas achieve jumping at specific speeds. We hypothesized that the power output patterns of hindlimb joints are synchronized across different jumping speeds. To test this hypothesis, inverse dynamics analysis was employed to quantify the mechanical output of jerboas' hindlimb joints at varying jumping speeds. The maximum jumping speed of jerboas in this study reached approximately 2.5 m/s. Normal force and forward force act synergistically to provide power for sagittal-plane jumping. During high-speed jumping, the peak moments of the hip, knee, ankle, and metatarsophalangeal (MTP) joints were 0.895, 0.292, 0.803, and 0.295 N·m, respectively, all exceeding those at low speeds. The temporal sequence of peak joint power exhibited a proximal-to-distal transmission pattern. The peak powers of the hip, knee, ankle, and MTP joints during high-speed jumping were 10.52, 5.02, 7.45, and 0.06 W, respectively, which were also higher than those at low speeds. Thus, the results of this study did not support our initial hypothesis. However, the locomotion mechanism that uses the MTP joint as the final energy release node effectively enhances jumping efficiency and explosiveness. This study reveals the complex processes of energy generation, transmission, and absorption in the hindlimbs of jerboas under different jumping speeds, providing a crucial basis for a deeper understanding of the energy metabolic efficiency and mechanical optimization mechanisms underlying their jumping locomotion.
Snakes exhibit extreme cranial kinesis that facilitates ingestion of prey with large cross-sectional area, but this ability is widely predicted to reduce bite performance due to decreased structural rigidity. Consequently, most large-bodied snakes rely on envenomation or constriction to subdue prey prior to ingestion. Species within the genus Drymarchon represent a notable exception: these large, non-venomous, non-constricting snakes routinely consume a wide range of prey, including large and potentially dangerous vertebrates, using only simple seizing and pinioning behaviors. Here, we quantify bite performance in three species of Drymarchon (D. corais, D. couperi, and D. melanurus), examine morphological predictors of biting performance, compare biting pressure to constriction pressure in similarly sized snakes, and synthesize dietary records across the genus. Our results showed that bite force scaled isometrically with head dimensions but showed negative allometry relative to body length, reflecting ontogenetic body elongation rather than diminished cranial performance. When expressed as pressure, biting by Drymarchon generated tissue-level stresses exceeding those produced by constriction in comparably sized snakes. Dietary synthesis revealed extreme trophic breadth dominated by non-avian reptiles, particularly snakes, with no relationship between predator size and prey size. Together, these results resolve a biomechanical paradox by demonstrating that high bite pressure, prolonged jaw engagement, and behavioral persistence can support lethal prey subjugation in the absence of venom or constriction. These findings highlight simple seizing as an effective and underappreciated prey-handling strategy, and underscore the importance of integrating biomechanics, behavior, and natural history to understand the evolution of snake feeding systems.
