The development of the tetrapod appendicular skeleton follows a conserved sequence of chondrogenesis and endochondral ossification, yet the evolutionary-developmental mechanisms that have yielded the immense morphological diversity of the vertebrate skeleton remain incompletely understood. Mechanical stress from embryonic muscle activity is a key regulator of skeletal development, and its absence can lead to several osteological abnormalities. Although the effects of altered embryonic movement on skeletal rudiment morphology are well documented, the relationship between experimentally induced developmental shape changes and large-scale evolutionary variation in skeletal form remains comparatively unexplored. Here, we assess the influence of embryonic muscle activity on skeletal shape by inducing paralysis in developing chicken embryos (Gallus gallus). We used microCT imaging and geometric morphometrics to quantify differences in humeral size and shape between paralyzed and control embryos and to compare these patterns with humeral morphology across extant bird diversity. We also examined the spatial distribution of proteins associated with endochondral ossification and cell proliferation. Paralysis did not affect overall humeral size but did cause marked changes in humeral curvature and the growth pattern of the distal epiphysis. Paralyzed and control humeri followed distinct developmental trajectories, with the most pronounced shape differences concentrated in the proximal epiphysis. Notably, paralyzed embryos fell outside the range of all sampled adult birds along PC3 of the proximal epiphysis. These findings suggest that embryonic muscle forces may contribute to evolutionary variation in avian limb shape.
Anhydrobiosis is a reversible process that allows organisms to survive prolonged periods in a desiccated state. Very little is known about the costs of anhydrobiosis on reproduction beyond a hiatus in activity. Tardigrades are suited to investigate such effects on male gamete function, since the latter can be tracked non-invasively. We compared the effects of three anhydrobiotic state durations (from one day to one month) on male-stored and female-stored sperm function in Macrobiotus dolosus. Our study shows that in-testis sperm motility upon rehydration is considerably impaired after long anhydrobiosis periods, and that its recovery is significantly slowed (up to several days). In contrast, we found no evidence for a negative impact of anhydrobiotic state on the fertilisation ability of spermatheca-stored sperm. Therefore, tardigrade female sperm storage structure can protect stored sperm even during long-term desiccation. Ultimately, our results highlight the need to further study on tissue-specific physiological adaptations to anhydrobiosis.
Flight is aerobically demanding, yet some songbirds fly thousands of meters above sea level. Such altitudes should increase metabolic rate and acid-base stress, but this has been rarely tested. We measured venous blood gases, metabolites, and electrolytes in migratory hermit thrushes (Catharus guttatus) flown at ground level, sub-maximum altitude, and maximal high altitude. Birds reached ∼2,300 m during maximal high-altitude flights. Venous PO2 did not differ between rest and flight at high altitude, indicating no detectable oxygen debt, while PCO2 declined with increasing altitude. Venous pH remained stable, but decreases in bicarbonate and base excess with elevated plasma lactate at higher altitudes indicate a mild, compensated metabolic acidosis. These patterns show that high-altitude flight imposes greater metabolic cost than ground-level flight, yet hermit thrush maintain acid-base balance sufficiently to sustain flight even at their maximum tolerated altitude.
Birds and mammals are endotherms that maintain a high and relatively constant body temperature. During intense muscular activity, body temperature can rise significantly, reaching up to ∼44°C and ∼42°C in the skeletal muscle of birds and mammals, respectively. Locomotor skeletal muscles are therefore subject to substantial thermal fluctuations, which may affect numerous cellular processes, including mitochondrial oxidative phosphorylation. The present study investigated the thermal sensitivity of mitochondrial coupling efficiency in the skeletal muscles of pigeons (Columba livia) and rats (Rattus norvegicus). Mitochondrial function was assessed in vitro across specific resting temperatures (37°C for rats, 40°C for pigeons) and under a physiological hyperthermia condition mimicking exercise-induced muscle heating for both species (43°C). Our results reveal a greater temperature-dependent decline in maximum mitochondrial coupling efficiency in rats relative to pigeons, and further highlight distinct bioenergetics flexibility strategies between the two species. In rats, mitochondria adopt a more dissipative strategy, characterized by elevated proton leakage, sacrificing coupling efficiency in favour of a stable oxidative cost. In pigeon, mitochondria engage complex II as a compensatory mechanism, partially buffering coupling efficiency decline at the expense of a higher oxidative cost. Together, these results highlight the specific flexibility of mitochondrial responses to physiological hyperthermia in pigeon and rat. However, broader bioenergetics comparisons across a wider range of avian and mammal species will be necessary to uncover evolutionary patterns and discriminate mechanisms conserved in endotherms from those representing independent adaptations specific to each vertebrate group.
Interactions with predators shape the evolution of escape behaviors in prey. Prey may exhibit evasive movements enhancing the unpredictability in their fleeing directions and trajectories, referred to as protean motion. Such behaviors may enhance survival of flying prey in the wild but quantifying them under natural conditions remains challenging. Here we used stereoscopic high-speed videography to record the escape flight behavior of wild males of the butterfly species Morpho menelaus in the Amazonian rainforest, and reconstructed 3D flight trajectories using artificial-neural-network-based tracking. During the experiments, we used a blue iridescent lure to attract freely patrolling male butterflies and elicited escape flights by performing an upward-directed entomological net swing. We then compared escape flight kinematics with pre-attack approach flight, using a measure of erraticity to quantify escape unpredictability for the first time in wild flying insects. Attacks first induced a rapid upward flight maneuver, directly followed by a horizontal turn, leading to a large range of escape directions. The following escape flight trajectories showed increased horizontal erraticity as compared to the pre-attack flight. The mean speed decreased during the escape phase, notably in the horizontal plane: a significant negative association between horizontal trajectory complexity and flight speed was indeed detected, consistent with a speed-erraticity trade-off. Because these large and relatively slow-flying butterflies display bright iridescent blue coloration on their dorsal wing side, erraticity during escape flight might enhance the dynamic flash coloration, likely limiting accurate targeting by predators.
