1. We assessed sex-specific seasonal changes in major energy storage compounds (triglycerides, glycogen) in Gammarus fossarum and Gammarus pulex collected from the field, with respect to their reproductive activity.
2. The dynamics of stored energy followed a seasonal pattern in both species and sexes. Moreover, over a 4-year period, these changes were independent of the year in which they were investigated. Stored energy reached a peak in late winter, but was depleted in late summer and early autumn, coinciding with the reproductive periods.
3. Triglyceride (annual mean ± SD) accounted for 79.7 ± 11.9% of the total stored energy and was responsible for the seasonal pattern. In contrast, glycogen contributed a lesser percentage (20.3 ± 11.9%). Over the study period, the amount of stored energy ranged between 0.39 and 4.08 kJ g−1 dry mass (triglyceride: 0.19–3.69 kJ g−1 dry mass; glycogen: 0.14–0.80 kJ g−1 dry mass).
4. In both species, the energy reserves of males were drastically depleted shortly before the cessation of precopulatory mate guarding in the field, thus offering a bioenergetic explanation for the reproductive period in these two widespread species.