1. The cost of reproduction due to limiting of the reproductive female’s locomotion capability has been suggested many times, but has rarely been directly examined, especially in fishes. Here, the effect of pregnancy on swimming performance in the viviparous Mosquitofish, Gambusia affinis, was studied.
2. Eight females of G. affinis were isolated, each in a separate aquarium, and critical swimming speed (Ucrit), body mass (BM) and cross-section area were measured every 5 days from the beginning of the pregnancy until 2–4 days after parturition.
3. Swimming kinematics (tail beat frequency and amplitude) was measured in non-pregnant and pregnant females at different swimming speeds.
4. BM increased during pregnancy from 0·47 ± 0·13 g to 0·72 ± 0·19 g, and the cross-section area also increased during pregnancy from 0·21 ± 0·06 cm2 to 0·32 ± 0·07 cm2. Ucrit decreased from 25·0 ± 1·3 cm s−1 before pregnancy to 20·1 ± 1·5 cm s−1 just before parturition, and returned to 24·7 ± 1·4 cm s−1 2–4 days after parturition. Interindividual variation was repeatable and reflects real differences among individuals.
5. Swimming kinematics was not affected by pregnancy.
6. The results suggest that reductions in Ucrit are probably because of aerobic constraints and not necessarily due to hydrodynamic changes resulting from changing in body form or plasticity. Moreover, the reduction in Ucrit is, potentially, a ‘cost of reproduction’ owing to decrease in the ability to gain food during pregnancy in G. affinis females.