Ultra-Slow Rhythmic Movement in the Elongating Tail of Xenopus Tadpoles

Since Darwin’s time, ultra-slow oscillatory movements have been recognized as a characteristic feature of axis elongation in plant organs such as shoots and tendrils, yet have not been reported in elongating animal body axes. Here, using long-term imaging of multiple anesthetized Xenopus laevis embryos with a modified document scanner, we identify a previously unrecognized ultra-slow rhythmic movement in the developing tadpole tail. This movement emerged during tailbud development, persisted once established, and occurred at an ultra-low frequency of approximately 10 ⁻⁴ Hz, several orders of magnitude slower than known active animal tail movements. Temperature-dependent imaging showed that its period varied with developmental rate and converged after time rescaling based on tail elongation rate, indicating a close link to morphogenetic elongation rather than conventional locomotor behavior. The timescale and physical context of this movement resemble plant circumnutation, in which slow organ-scale movement arises during constrained elongation. These findings suggest that slow rhythmic motion may represent a morphogenetic mode of animal movement and may be a recurrent dynamic feature of constrained elongation in living structures.