The A. t. melanostictum pond populations in this region of the GYE are closely related to one another, exchange gene flow, and exhibit low overall genetic diversity [31–33]. Therefore, majority of the phenotypic variation encountered in this region is due to phenotypic plasticity rather than genetic differentiation and adaptation. Measuring 609 individuals collected from 25 ponds within the GYE, we documented substantial variation in the duration of larval periods and the sizes at metamorphosis, differences which correlated with differing pond hydroperiods.
Larvae in more permanent environments reached greater sizes than larvae in ponds with brief hydroperiods (Figures 1 and 3). While most larvae from ephemeral ponds had either metamorphosed or perished before reaching 60 mm SVL, larvae in permanent environments were able to remain under aquatic growth conditions up to nearly 80 mm SVL, after which they either became paedomorphic or metamorphic. The smallest metamorph collected from a permanent environment was 62 mm SVL, 17 mm larger than the smallest metamorph found in an ephemeral environment (45 mm). Indeed nearly half of metamorphs collected from ephemeral environments were smaller than the smallest metamorph collected from any permanent environment. Furthermore, while the first emerging metamorph was captured from an ephemeral location on 10 July (Figure 1: b), the first metamorph emerged from a permanent pond more than a week later (and this individual was more than 80 mm SVL, suggesting that it may have not been young-of-the-year; Figure 1: c).
Many permanent environments contain over-wintering larvae, which postpone metamorphosis until their second or third years. Individuals under this developmental regime grow as aquatic larvae for several years, and some eventually forego metamorphosis to become reproductively mature neotenic paedomorphs ("paedomorph advantage") [21]. Only the two most stable environments examined, Ice Lake and Rainbow Lake, contained paedomorphs. These were the only two YNP locations that never dried over the course of the three year study period. We did not identify any small progenic paedomorphs (i.e. "best of a bad lot" paedomorphs [21]), possibly because the resource-poor locations that might have promoted precocious paedomorphosis were ephemeral and would have destroyed lingering aquatic individuals.
Larval growth rate determines the amount of time required to reach SVLm, and therefore defines the minimum larval period required prior to metamorphosis. Environmental conditions greatly influence larval growth rates in controlled settings [12, 34]; however, we found average larval growth to be extremely similar between ponds and ponds of different hydroperiods. The earlier onset of metamorphosis observed in environments with shorter hydroperiods was not accompanied by accelerated larval growth rate. Between 16 June and 30 June, 2008 (during which all ponds were hydrated), individuals grew a linear average of more than 1 mm per day (Figure 2).
We used the early larval growth models to extrapolate dates of hatching in each of these ponds, all of which fell between 1 June and 14 June (Table 1). Assuming an incubation period of 55 days (the length of time between spawning and hatching in the permanent Ice Lake location [35]), we further extrapolated that spawning likely occurred in each of these ponds during the first two weeks of April, soon after snow melted from the region [36]. These inferred hatching dates are a month earlier than the hatching dates recorded at Ice Lake in 1993 [35]. Northern YNP has experienced warmer spring temperatures and earlier snowmelt dates in recent decades [28, 36], suggesting that long-term warming has contributed to phenological changes and earlier breeding of this amphibian population [see also [37–39]].
At metamorphosis, amphibians undergo major restructuring of internal and external organ systems, remodelling somatic tissues to adopt a terrestrial lifestyle. In addition to the loss of external gills, the lateral tail swim fin is lost, resulting in a decrease in tail height. Tail height dominated the second component (shape) of the principal components analysis (PCA; Table 2), and tail height compared to size reliably separated aquatic from terrestrial individuals (Figure 4). After loss of the swim fin, terrestrial individuals showed far narrower tails relative to their length. Metamorphosis furthermore modified the relationship between size and tail shape. For every centimetre SVL growth gained by an aquatic salamander, it gained approximately 30 mm of tail height; while every centimetre of snout-vent growth gained by a terrestrial salamander was accompanied by only 12 mm of tail height. Paedomorphs maintained the aquatic SVL/tail height relationship throughout their lives. We have developed these observations into a conceptual framework of allometric relationships though different developmental pathways (Figure 5). As shown, the metamorphosis of an individual at a small size (Figure 5: A), at a large size (B) or not at all (C) is functionally determined by the hydroperiod of the pond environment.
Individuals that are able to delay metamorphosis (Figure 5: B) may be more likely to survive and reproduce than individuals that metamorphose at or close to SVLm (Figure 5: A) [5, 18–20, 40]. Although early metamorphosis or progenic paedomorphosis allow immediate reproduction [21, 41], studies show that salamanders that delay metamorphosis to larger body sizes enjoy overall benefits in survival and fecundity [18, 40]. Larger individuals are at less risk of desiccation and predation in terrestrial environments [12], are able to more efficiently capture terrestrial food, have higher mating success, larger clutch sizes and generally higher fecundity than smaller individuals [24, 40]. As ponds in this region become increasingly ephemeral [28], resident amphibians may be forced to adopt ever more rapid and potentially less fecund developmental profiles.
Although differences in adult survival and fecundity have not yet been tested in this region, we determined that many ponds in the area are now entirely unsuitable for Ambystoma development, leaving the population vulnerable to continued warming. Larvae must reach a certain minimum size in order to undergo successful metamorphosis [19, 21, 24, 42], and based on field observations, the SVLm for this population is close to 45 mm. If the hydroperiod is shorter than the minimum larval period required to reach SVLm, larvae have no opportunity to escape a drying pond. Growing at the most rapid documented growth rate, the minimum larval period is approximately 33 days from hatching, and the incubation period of eggs is approximately 55 days. Thus, these salamanders need a minimum of 88 days of pond hydration achieve SVLm. Snowmelt and egg deposition occur in early April, and ponds must therefore contain water through approximately the first week in July to support populations through metamorphosis. As predicted by this model, the first young-of-the-year metamorph was captured on 11 July. Earlier snowmelt [36] could extend the functional hydroperiod, potentially buffering the effects of earlier drying, however, such changes in seasonality may negatively impact the population in other ways [37–39].
Over the course of three years, we observed six locations to dry during the month of June, and hundreds of bodies of young salamanders were found trapped in the drying wetlands [28]. These ponds with hydroperiods shorter than the required larval period serve as reproductive sinks, where few if any larvae survive to recruitment. Larvae in ephemeral environments do not appear to accelerate larval growth rate, and this species may thus be poorly equipped to thrive in newly ephemeral, rapidly drying environments.
A. t. melanostictum is a long-lived, opportunistically breeding organism, and populations are able to recover from dry years [43]. Indeed, although past droughts likely contributed to population bottlenecks and loss of genetic diversity [31], the GYE population has recovered from both ancient climatic fluctuations [27] and more recent dry periods during the late 1980s and the early 2000s [28]. Nonetheless, long term drought conditions can prevent breeding over the course of decades, exceeding both the lifespan of the organism and the capacity of populations to recover.