Cool Sex? Hibernation and Reproduction Overlap in the Echidna

Citation: Morrow G, Nicol SC ( Cool Sex? Hibernation and Reproduction Overlap in the Echidna Gemma Morrow 0 Stewart C. Nicol 0 Stuart Humphries, University of Hull, United Kingdom 0 School of Zoology, University of Tasmania , Hobart, Tasmania , Australia During hibernation there is a slowing of all metabolic processes, and thus it is normally considered to be incompatible with reproduction. In Tasmania the egg-laying mammal, the echidna (Tachyglossus aculeatus) hibernates for several months before mating in mid-winter, and in previous studies we observed males with females that were still hibernating. We monitored the reproductive activity of radio-tracked echidnas by swabbing the reproductive tract for sperm while external temperature loggers provided information on the timing of hibernation. Additional information was provided by camera traps and ultrasound imaging. More than a third of the females found in mating groups were torpid, and the majority of these had mated. Some females re-entered deep torpor for extended periods after mating. Ultrasound examination showed a developing egg in the uterus of a female that had repeatedly re-entered torpor. The presence of fresh sperm in cloacal swabs taken from this female on three occasions after her presumed date of fertilization indicated she mated several times after being fertilized. The mating of males with torpid females is the result of extreme competition between promiscuous males, while re-entry into hibernation by pregnant females could improve the possibility of mating with a better quality male. - Funding: This work was supported by grants from the the University of Tasmania Instititutional Research Grants Scheme (grant N0016573, http://www.research. utas.edu.au/, the National Geographic Committee for Research and Exploration (grant 49286, http://www.nationalgeographic.com/field/grants-programs/cre. html, and the Holsworth Wildlife Research Endowment (grant M0017093, http://www.anz.com/documents/au/aboutanz/HolsworthWildlifeGuidelines2007.pdf). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Hibernation has been documented in species from a wide range of mammalian orders [1], and although originally thought to be an adaptation to the cold, hibernation is now considered to be an energy conserving strategy which different species employ in a range of ecological circumstances [2]. Hibernation is characterized by a reduction in body temperature (Tb) which typically falls to within 1uC of ambient, and a very substantial, but size dependent, reduction in metabolic rate [1]. Because metabolic processes are slowed during hibernation it is generally considered to be incompatible with reproduction: hibernation prevents spermatogenesis in males [3], slows fetal development, delays parturition [4] and inhibits lactation [5]. Among Australian mammals many dasyurid marsupials enter daily torpor during pregnancy [6], but bats are the only mammalian group in which reproduction and deep, extended torpor (i.e. hibernation) are known to overlap. In temperate zone bats, which show an extensive period of winter torpor, the reproductive cycle is interrupted by hibernation [7]. A number of strategies, including sperm storage and delayed ovulation, allow gestation to be initiated on arousal from hibernation in spring although gametogenesis occurs in summer [7,8]. However, the only species known to enter deep, prolonged torpor while pregnant is the North American hoary bat (Lasiurus cinereus) - in extreme spring weather conditions pregnant females showed bouts of deep torpor lasting up to 5.6 days [9]. The short-beaked echidna Tachyglossus aculeatus is distributed throughout southern and eastern New Guinea, mainland Australia, Tasmania, Kangaroo Island, and smaller offshore islands. It is the most common of the egg-lying mammals and is in fact the most widespread native Australian mammal [10]. Throughout their range echidnas show some degree of seasonal inactivity. In Tasmania (subspecies T. a. setosus) reproductively active males hibernate from mid February to mid June, while reproductively active females hibernate from early March until mid July [11,12]. By contrast reproductively active adults of the Kangaroo Island subspecies (T. a. multiaculeatus) show reduced activity and only intermittent bouts of torpor between April and June [13]. Courtship behaviour also appears to differ between the two areas [14]. Kangaroo Island echidnas have been described as forming mating trains of up to 11 individuals with a period of competition between males and courtship lasting between 1444 days [15]. After mating there is a gestation period of 2224 days, after which the female normally lays a single egg [12,15]. Since 1996 we have been studying a population of echidnas in the Tasmanian southern midlands, and on several occasions during the course of this study we found males with females which were torpid, or which subsequently re-entered hibernation. In order to examine more closely the relationship between hibernation and reproduction in Tasmanian echidnas we conducted a detailed investigation of radio-tracked echidnas during the 2007 and 2008 mating seasons. Results and Discussion Over the two mating seasons we found 26 mating groups. The most common number of males in a group was one (15 mating groups), but on three occasions in 2008 females were found with four males. In ten of the mating groups the female was torpid and reacted slowly to stimuli; body temperatures of these torpid females ranged from 10 to 29uC. All males in mating groups were active and had normal (euthermic) body temperatures (ca 32uC [16]). Of five torpid females checked, four had sperm in their reproductive tract. Four of the females found torpid in mating groups were radio tracked and were observed to re-enter hibernation; three of these were later found in mating groups while euthermic and had fresh sperm recovered from their tracts. One female (echidna 5D5E) was studied intensively in 2008, using a combination of field observations (including cloacal swabbing), camera traps, an external temperature logger and ultrasound. This showed that while hibernating she was visited by a male at least twice without mating occurring. In one of these events (July 6) a camera trap showed that a male was with her in her hibernaculum for 13 hours, while the temperature logger showed that she did not rewarm significantly. She subsequently mated five times between July 11 and 28 before entering a nursery burrow on August 7. During the period of mating she repeatedly re-entered torpor with a minimum Tb of about 10uC. Maximum length of these torpor bouts was only about 12 hours, but she was being frequently disturbed by us. On July 15 female 5D5E was with male 5036, had fresh sperm in her tract, and had a temperature o (...truncated)