CHAPTER 4. TESTOSTERONE AND PROLACTIN LEVELS IN AN
4.4. Discussion
In this uncommon parental care system, male and female Mountain Plovers have equal responsibilities during incubation and similar incubation patterns [51] and both sexes develop a brood patch [33], so it is not surprising that there was no difference in the PRL levels of the two sexes. This is consistent with other studies in which both sexes incubate equally [10, 26] although this is the first case in which PRL was measured in a species in which each sex tends its own nest. While plasma PRL levels in some other birds with precocial young tend to increase at the start of incubation and gradually decline throughout the brood-rearing stage [42, 26], in Spotted Sandpipers PRL
concentrations remain elevated after hatching [44] and in Mallards PRL concentrations do not decrease until after the chicks are as large as the adults [5]. Although Mountain Plover chicks are precocial and leave the nest within 24 hours of hatch, the incubating parent remains with the brood until after the chicks have fledged at around 30 days,
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performing anti-predator distraction displays and brooding the chicks when temperatures are cool [28], similar to Spotted Sandpipers and Mallards.
Early studies of Wilson’s Phalaropes (Phalaropus tricolor) suggested that T concentrations were greater in females than in males in sex-role reversed species [31]. However, more recent studies have found that the levels of circulating T in sex-role reversed birds [Spotted Sandpipers, 21, Wilson’s Phalarope, 22, Red-necked Phalarope, Phalaropus lobatus, 26, African Black Coucals, Centropus grillii, 24] are not reversed, and are similar to those of males and females of socially monogamous bird species. In these species males have higher levels of T than females in the prelaying stage although T levels usually decrease significantly to similar levels as females depending on the degree of involvement of the male in incubation. Male Mountain Plovers in Montana establish territories and continue to compete for females, defending the area around their nests throughout incubation more aggressively than do females [PDBS, SJD, pers. obs.], which may be why males have persistently higher levels of T than females. Male
Mountain Plovers have a high degree of male-male aggression, with territorial disputes occurring until late in incubation but male and female incubating plovers maintain a high degree of nest-attentiveness throughout the entire incubation period [51]. Aggressive interactions with other males can cause increased levels of T at each “challenge” but this response wanes at the end of the breeding season [59], which could explain the high variation in T among males.
The lack of a clear pattern of changes in these hormones across the incubation stage may result from an individual’s responses to prolonged chronic stress [25, 15, 1]. One major source of stress to this population of Mountain Plovers comes from the wide range
of environmental conditions that occur during the breeding season. Throughout the season there are large fluctuations in ambient temperature, often ranging from below 10°C to above 40°C, as well as frequent drought conditions and locally concentrated rain events [16]. The three field seasons of this study all experienced unusually high
precipitation with 2011 being one of the wettest years on record [37, 38, 39]. In other species such weather extremes can lower baseline PRL concentrations across an entire breeding season [15] and prolong elevated levels of T [58].
Other factors can affect circulating hormone levels including prior breeding
experience [36, 2], physical condition [11, 12], and changes in clutch size [15]. Mountain Plovers usually have a clutch size of three eggs, so further research should determine how hormone concentrations are related to the physical condition of birds at the time of
sampling and if possible incorporate the age or prior breeding experience of the bird into the analysis. This is a relatively long-lived species with several individuals exceeding ten years of age [SJD, pers. obs.] and this could provide an opportunity to further examine the relationship between PRL and T and how they change with age. Only six known-age individuals were sampled: five females (of ages 1, 2, 2, 4, and 4 years) and one two-year old male and these were not enough data to test for a pattern. Further study of circulating hormones in this species should also include testing during the courtship, brood-rearing, and post-breeding stages to better understand seasonal changes in hormonal levels in this unusual parental care system.
Traditionally, studies of circulating avian plasma PRL have used radioimmunoassay (RIA) procedures that require costly antibodies and the necessity of working with radioactive ligands [9]. Commercial enzyme-linked immunosorbent assay kits based on
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mammal PRL were first used on House Wrens (Troglodytes aedon) nearly two decades ago [50]. Commercial kits are now available that utilize chicken PRL, as in this study. However, there is a need for comparative field studies of endocrinology that connect the ultimate and proximate causes of phenotypic variation in life-histories [57]. This method requires only a small volume of plasma (10 μl) and so it increases the possibilities of studying even very small birds with low blood volumes.
Acknowledgements
Iowa State University, the U.S. Bureau of Land Management (Phillips Resource Area, Montana, USA), and Montana Fish, Wildlife, and Parks provided financial support. The staff at Charles M. Russell National Wildlife Refuge supplied additional logistical support. J. J. Grensten and T. M. Harms assisted with nest searching and collecting samples. We thank B. Matovitch, D. Robinson, and J. Robinson for allowing us access to their lands and thank the F. and D. Veseth families for additional support. A. L.
Brockman provided invaluable assistance with the PRL ELISA and I. Alvarez Castro assisted with statistical analysis.
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69 Figures
Fig. 1. Standard curve of chicken prolactin concentrations produced by a commercial ELISA assay, performed 14 December 2011.
Fig. 2. Relationship between the ln plasma prolactin concentration (ng/ml) and ln plasma testosterone concentration (ng/ml). Plasma samples were collected from incubating male and female Mountain Plover (Charadrius montanus) in 2006, 2007, and 2011 in Phillips Co., MT, U.S.A.
Fig. 3. Relationships between day of incubation (5 day mean ±SE) and Julian day of nesting season (10 day mean ± SE; Day 1 = 23 May and Day 47 = 8 July) with plasma prolactin concentration (ng/ml). Plasma samples were collected from incubating male and female Mountain Plover (Charadrius montanus) in 2006, 2007, and 2011 in Phillips Co., MT, U.S.A.
Fig. 4. Relationships between day of incubation (5 day mean ±SE) and Julian day of nesting season (10 day mean ± SE; Day 1 = 23 May and Day 47 = 8 July) with plasma testosterone concentration (ng/ml). Plasma samples were collected from incubating male and female Mountain Plover (Charadrius montanus) in 2006, 2007, and 2011 in Phillips Co., MT, U.S.A.
Fig. 5. Relationships between condition index (mass/wing chord) and both plasma testosterone (ng/ml) and plasma prolactin (ng/ml) concentrations. Plasma samples were collected from incubating male and female Mountain Plover (Charadrius montanus) in 2006, 2006, and 2011 in Phillips Co., MT, U.S.A.
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