Determination of critical reproductive parameters of live, wild dugongs in a subtropical environment

Burgess, Elizabeth A. (2012). Determination of critical reproductive parameters of live, wild dugongs in a subtropical environment PhD Thesis, School of Biological Sciences, The University of Queensland.

       
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Author Burgess, Elizabeth A.
Thesis Title Determination of critical reproductive parameters of live, wild dugongs in a subtropical environment
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 2012
Thesis type PhD Thesis
Supervisor Janet Lanyon
Hugh Possingham
Total pages 177
Total colour pages 5
Total black and white pages 172
Language eng
Subjects 0603 Evolutionary Biology
060308 Life Histories
0608 Zoology
Formatted abstract
Assessing reproductive status and activity in a wild population is important for the effective management of vulnerable species such as the dugong (Dugong dugon). Until now, knowledge on reproductive parameters (e.g., sexual maturation, pregnancy, seasonality, reproductive senescence) for dugongs has been restricted by a lack of non-lethal methods to assess reproductive status in this cryptic marine mammal. These parameters have not been quantified for dugong populations in subtropical regions (i.e., > 23° 26’ latitude), where seasonality has stronger influences than in tropical regions. This study was conducted to obtain critical information on reproductive parameters in live dugongs from a wild population in subtropical Moreton Bay, southeast Queensland, Australia, within a broader study of population dynamics. Since most reproductive processes are hormone-dependent, endocrine analysis was considered to be an effective and reliable non-lethal method for assessment of reproductive status. This thesis presents the results of novel investigations using faecal hormone metabolite analysis to assess reproductive physiological states of 322 individual dugongs (159 males and 163 females; all size classes, except neonates) at a population level (i.e., > 30 % of the population).

Diagnosing pregnancy in a species allows evaluation of critical associated life-history parameters, such as female age at breeding, inter-calving interval, frequency of pregnancy, seasonality of reproduction, population fecundity rate, and capacity, all of which influence population viability. I have developed a method to identify pregnant individuals in a dugong population, using faecal progesterone metabolite concentrations in combination with body morphometrics. Using a discriminant function model, I classified the pregnancy status of all female dugongs sampled and diagnosed a total of 30 females as pregnant and 133 females as non-pregnant from the sampled population of 163 females over the sample period (six years). In sub-tropical Moreton Bay, pregnant dugongs are characterised by faecal progesterone metabolite concentrations
> 1000 ng/g, body length ≥ 260 cm, maximum girth ≥ 215 cm, anal girth ≥ 126 cm, and teat length ≥ 5 cm long. These results provide vital information to monitor breeding rates of the dugong population in Moreton Bay, and a new approach for investigating such parameters in other populations of sirenians.

Male reproductive processes can be challenging to understand in testicond marine mammals that exhibit no obvious sexual dimorphism and whose behaviour is only glimpsed at the water’s surface. I examined the relationships between body length, tusk eruption (a secondary sexual characteristic), seasonality, group association and faecal testosterone metabolite concentrations. Male dugongs sampled over six years included immature (40% of sampled population) and mature (60%) individuals. Male dugongs in Moreton Bay have a relatively long pre-reproductive period with puberty occurring at 240 - 260 cm body length. However, social maturity may be delayed until male dugongs are longer than 260 cm, with well-developed tusks. In Moreton Bay, male dugongs were at 81% of full-grown asymptotic length on reaching maturity. This suggests that the acquisition of reproductive maturity in subtropical male dugongs may occur relatively late in life similar to other long-lived mammal species. Achieving maturity at a large body size may be particularly pronounced in Moreton Bay compared to  populations from northern tropical waters. Males in this subtropical population exhibited pronounced seasonality with the highest testosterone concentrations in September–October, coincident with an austral spring mating season. My results also provide new insights into dugong group associations and the potential role of androgens in mediating roving in male dugongs, which has not previously been described in this cryptic species. Male dugongs in Moreton Bay appear to be involved in scramble competition within herds as a mating strategy, which has previously been reported in West Indian manatees.

The ability to determine reproductive parameters in live individual dugongs has important applications for the management of this species in captivity. Long-term endocrine monitoring of one male and two female dugongs currently held in captivity provided important insights into the sexual maturity and husbandry of these individuals, using hormonal levels of wild dugongs as reference ranges. Captive dugongs had similar faecal progesterone and testosterone levels to animals in wild populations, but faecal oestrogens were found to be significantly higher in one captive female; with oestrogen production potentially increasing as a function of body weight. The onset of puberty in each of one female (at 5 y of age) and the male dugong (at 9 y of age) was demonstrated by a sustained elevation in hormone concentrations and wider variation in hormone production with age. This study has shown that male and female dugongs can take 2–3 years to complete their pubertal development, even under conditions of high nutritional availability. Furthermore, my results lend further evidence that nutritional plane may account for differences in age at sexual maturity in dugongs. Data from faecal hormone monitoring were helpful in guiding routine management decisions for captive dugongs, which are particularly beneficial for facilities housing wildlife species whose reproduction and associated behaviours are poorly understood.

Breeding interactions can be important sources of stress in an animal’s life history.  Moreover, dugongs are a globally vulnerable species, and are suspected of suffering from environmental stressors that may modulate glucocorticoid expression and result in changes in body condition and health. I investigated adrenocortical activity (as measured by faecal glucocorticoid metabolite concentration) in relation to sex, reproductive state (immature, sexually mature, pregnant) and season in the Moreton Bay population. I also determined associations between glucocorticoid production, body condition, aggressive conspecific interactions, and environmental temperature both within and outside the mating season. Faecal glucocorticoid metabolite concentration varied significantly with sex and reproductive state. These were highest in pregnant females, followed by mature males, and lower in non-pregnant mature females and immature females and males. Lower glucocorticoid levels were also recorded in both maternal females and their dependent calves. Glucocorticoid levels and body condition in wild dugongs varied seasonally, with individuals reserving energy (i.e., lower glucocorticoid levels and improving body condition) over summer and autumn. The best body condition was observed in winter, which may have assisted animal’s ability to cope with adverse ecological conditions (including ambient temperature declines and reduced nutrient availability), and helped sustain the costs of reproduction (including mating activity in spring). Immature dugongs may be particularly vulnerable during winter periods, as are young individuals recently estranged from their mothers. During the spring mating period in this population, mature and sexually active males had heightened stress levels and significant loss of body condition. This finding is consistent with high levels of agonistic behaviour resulting in body scarring (from tusk rakes) that occurs during the mating season. Strenuous and competitive breeding activity of dugongs appears to be occurring after winter at a time when seagrasses are recovering from seasonal dieback. The results of this study suggest that competitive reproduction inherent in a potentially polygamous mating system may have stressful consequences for male dugongs. Dugongs living in seasonal environments, such as subtropical Moreton Bay, exhibit physiological manifestations that may enable them to cope with temporal fluctuations in climate and nutrient availability at the cold latitudinal limit of their range. Furthermore, these results provide baseline data of normal baseline patterns in adrenal physiology against which the impacts of environmental stressors on dugong populations can be gauged. For example, the extreme flood event that occurred in Moreton Bay in 2011.

This study makes a significant contribution to understanding reproductive status and activity of wild dugongs, and presents the first assessment of dugong reproductive parameters in subtropical waters. Reproductive parameters of large terrestrial herbivores vary with extrinsic factors, such as temporal variability in food availability and climate. The results of this study lend further evidence to the variability in dugong reproductive parameters between populations, with the body size at maturity in dugongs in Moreton Bay being the most protracted for both sexes yet recorded. Dugongs forage almost exclusively on seagrasses, which is a plant known to exhibit seasonality in growth and nutrient availability, particularly at the higher latitudes of distribution (i.e., subtropical regions). Female and male dugongs in Moreton Bay may be nutritionally limited by the pronounced seasonal changes in food abundance and productivity in this subtropical region at the extreme iv southern limit of dugong distribution. This thesis on subtropical dugongs contributes to the increasing number of studies  demonstrating that extrinsic factors can influence populations of marine mammals and their life history characteristics. Future studies should investigate differences in the growth rate, body condition and health of dugongs among populations, and potentially among years. Given the importance of reproductive parameters for population dynamics, the protracted reproductive maturity and pronounced reproductive seasonality in dugongs have important implications for the management of this species in subtropical regions.
Keyword Dugong dugon
Wild population
Sexual maturity
Seasonality
Reproduction
Stress physiology
Fecal hormones
Enzyme-immunoassay

 
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Created: Wed, 05 Jun 2013, 08:20:51 EST by Elizabeth Burgess on behalf of Scholarly Communication and Digitisation Service