Abstract

Previous studies evidenced that major environmental factors for mammalian reproduction are food availability, ambient temperature, rainfall, the day/night cycle and a variety of social cues, of which, photoperiod is believed to be the most important factor that could impose significant effect on seasonal rhythms.^1,2 Photoperiod triggers synchronization of the annual reproductive cycles of experimental,³ domestic,⁴ and some exotic species.⁵ However, scarce systematic study on reproductive response to photoperiod has been conducted in aquatic mammals,^6-9 especially on males. This study was undertaken to investigate the effects of extended photoperiod (Light/Dark: 14 h/10 h) during the nonreproductive season on the testicular morphology and sexual behavior frequency of male Yangtze finless porpoises (YFPs).

Two mature male animals were included and were exposed to the natural photoperiod of the facility. During the experiment the subjects were exposed to artificial long-day conditions (light:dark (LD) cycle of 14:10; 0830–2230 day length) in the natural short-day period (natural photoperiod: 10–11.5 h/day), which was controlled by 24 ceiling-mounted fluorescent tubes of 1149-mm 6500k T5 tubes (Philips, Royal Dutch Philips Electronics Ltd., Shanghai) evenly distributed on the ceiling to simulate the natural daylight. The lamps were constantly switched on during the light phases of the experiment. Testicular ultrasound was performed, and sexual behavior frequency was observed, twice a week for each animal. Data collected in these 2 animals in the next year under natural photoperiod condition was used as a control.

Results showed that during the extended photoperiod, the testicular parenchyma intensity (PI) and cross-sectional area (p < 0.005) were increased significantly compared with the control year under natural photoperiod regime. Sexual behavior frequency was also thought to be activated by the extended photoperiod, and strongly correlated to the testicular PI changes (p < 0.05, r = 0.61–0.68).

The present study is the first of this kind to investigate the possible influence of extended photoperiod on the seasonal rut pattern of testicular development and regression longitudinally in YFPs, suggested that extended photoperiod results in an elevation in reproductive physiological activities in captive YFPs. Therefore, we hypothesize that extended photoperiod in spring, may act as triggers to "switch on" the testicular activity, thereby playing a role in the synchronization of reproductive cycles in male YFPs. Captive breeding with assisted reproductive technologies has gradually been seen as one of the key countermeasures to protect this endangered subspecies of finless porpoise by the Chinese government and conservationists.¹⁰ The results of the present study add knowledge to the reproductive seasonality in male YFPs, and the technique used may enhance the captive breeding programs for the conservation of this endangered freshwater cetacean species.

Acknowledgments

The authors would like to thank the training staff at Baiji dolphinarium for their assistance during the course of the experiment and support in data collection. The study was funded by the Knowledge Innovation Program of the Chinese Academy of Sciences (No. KSCX2-EW-Z-4).

* Presenting author

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