Ageing and Growth of the Endangered Kaiser's Mountain Newt, Neurergus kaiseri (Caudata: Salamandridae), in the Southern Zagros Range, Iran
Abstract
The development of appropriate management and recovery plans requires a better understanding of the demography of endangered amphibians. In this study longevity, age at maturation, age structure, growth rates, and growth patterns of a local population of the endangered Kaiser's Mountain Newt, Neurergus kaiseri, in southwestern Iran were studied using skeletochronology. To estimate the age of individuals, numbers of lines of arrested growth was used in the periosteal bone obtained from cross-sections of 73 live newt toes. The maximum observed longevity was 14 yr in males and 12 yr in females. Mean ± SE longevity recorded for males (6.81 ± 0.58 yr) and females (7.74 ± 0.34 yr) was not significantly different (P = 0.14, t-test, n = 73). The minimum age at sexual maturity in both sexes was 4 yr. Mean ± SE snout–vent length (SVL) in females (63.31 ± 4.58 mm) was higher than in males (54.68 ± 3.74 mm) (P = 0.007, t-test, n = 73). The growth curve for SVL was best described by the von Bertalanffy model compared with Gompertz and logistic growth models (on the basis of Akaike information criterion and coefficient of determination).
One effective method to understand the conservation status of a species is to provide data on population size and population trends for the whole species or a representative population of that species. Provision of this data requires the knowledge of individual ages and population age structure, which may be useful for understanding the causes of population instability and for developing appropriate management and conservation plans of populations in their natural environments (Driscoll, 1999). Data obtained for the age of individuals also is critical to understand the ecology and construct life tables for populations (Krebs, 2001; Gerber and Heppell, 2004). Various products of a life table, including survival curve, life expectancy, age-specific mortality and fecundity, innate rate of natural increase, and net reproductive rate are dependent on the data collected on individuals present in different age classes (Gabre et al., 2005).
Mark-and-recapture methods provide the most accurate and reliable data for age determination, because the age of known individuals is measured directly. Such methods, however, require long-term studies that might slow the development of a conservation management action (Chinsamy and Valenzuela, 2008). Mark-and-recapture methods also require much time and effort in tagging individuals and may be especially difficult in small larvae and pre- or postmetamorphic juveniles (Smirina, 1994; Smyth and Nebel, 2013). Many herpetologists have used indirect age estimation by correlating age and body size (Liao, 2011). An alternative way to estimate the age of individuals is through skeletochronology, a histological analysis of growth marks in the skeleton (Oromi et al., 2012; Baskale et al., 2013). Because seasonal variation in growth rates creates recognizable marks in the skeleton, skeletochronology is especially effective for ectothermic vertebrates that live in highly seasonal environments, such as amphibians that inhabit temperate climate regions (Castanet et al., 1993). This method also has been used to estimate growth rates, the age at sexual maturity, the annual life cycle, and provide data on geographic differences in demographic characteristics of various species of amphibians (Morrison et al., 2004).
Neurergus kaiseri is a species endemic to Iran and one of three species of the genus Neurergus reported from the Iranian Plateau in the southern Zagros Mountains in Lorestan and Khuzestan provinces. Neurergus kaiseri is easily distinguishable from the other two newts of the genus Neurergus in Iran because of a distinctive long, narrow band of orange with several round areas of white. Neurergus kaiseri is a critically endangered species by International Union for Conservation of Nature criteria (Sharifi et al., 2009), and has been amended to Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora. This newt is reported from 13 first-order highland streams in the southern Zagros Range (Sharifi et al., 2013). These streams are separated from one another by steep, rocky terrain with vegetation cover of thin oak–pistachio woodland to mature open oak woodland. In a recent study Sharifi et al. (2013) surveyed 12 of the 13 localities and counted a total of 1,277 adults, postmetamorphic subadults, and larvae, from which 86% were found in just two of the localities (Bozorgab and Kerser). The present study was conducted in Bozorgab Stream at an altitude of 1,330 m above sea level (Figs. 1 and 2), one of the two localities with the highest visual count reported by Sharifi et al. (2013). Terrestrial habitats around Bozorab streams where N. kaiseri is seen include diverse community types known as oak–pistachio open woodlands dominated by Quercus brantti, Pistachio spp., and Salix alba.
Citation: Journal of Herpetology 50, 1; 10.1670/14-142
Citation: Journal of Herpetology 50, 1; 10.1670/14-142
The aim of this study was to compile and analyze aspects of life-history traits, growth, and age structure of a population of the endangered Kaiser's Mountain Newt inhabiting Bozorgab Stream. This study emphasizes the skeletochronological estimation of 1) age at attainment of sexual maturity, 2) longevity, 3) age structure, and 4) growth pattern of the newt.
Materials and Methods
Sampling
Newts used in this study included 12 larvae, 27 adult males, and 46 adult females, all caught in the daytime on 18 June 2011 and 8 April 2012. Newts were caught by hand and quickly placed in wet cotton pouches. Coordinates for Bozorgab Stream (32°56′N 48°28′E) were recorded using a global positioning system (Garmin 60CSx, Garmin International, Inc., New York, NY, USA). The sex of each mature individual was determined according to sexual dimorphism reported for this species: males have a fleshy protuberance at the base of the tail, whereas females have a prominent cloaca but without the protuberance (Sharifi et al., 2012). Juveniles have a smaller body length than adults and lack both the protuberance and a prominent cloaca. Newts were measured from the tip of the snout to the posterior margin of the vent (SVL) using a dial caliper with an accuracy of 0.02 mm.
Skeletochronological Analysis
The longest finger of the forelimb (i.e., second or third) was cut and preserved in 70% ethanol. Newts were kept in small (30 cm × 30 cm) pools created with stones at the sampling site and monitored for approximately 2 h to see if toe amputation caused any visible side effects, such as bleeding; they then were released at their collection site. Age was determined using skeletochronological analysis previously described (Tomasevic et al., 2010): the phalange was washed in running water for about 24 h, then decalcified in a solution of ethylenediaminetetra-acetic acid for 24 h. Each phalange was sectioned at 8–10 μm using a rotary microtome. Ten slides were made from each phalange, at mid-diaphyseal level, with the smallest marrow cavity and four ribbons were placed on each slide. The resulting sections were stained in Harris hematoxylin, mounted on slides, and observed with a light microscope (Leica, Galen III, Leica Microsystems, Wetzlar, Germany). Bone sections from each individual were photographed using a Dine capture; all were photographed at the same magnification, allowing for simultaneous comparison and facilitating the analysis of the bone growth pattern.
Because phalangeal diaphyses are composed of two compact and concentric bone layers (an outer and broader layer of periosteal bone [PB] and an inner and narrow layer of endosteal bone [EB]), newts were aged by counting the lines of arrested growth (LAGs) that appeared in PB in the phalange sections of individuals (see Buhlmann and Mitchell, 2000; Kumbar and Pancharatna, 2004; Uzum, 2009; Baskale et al., 2013). LAGs appeared as colored stripes of various widths (Fig. 3). Counting the number of LAGs in cross-sections of the phalanges revealed the number of arrested growth periods experienced by each individual.
Citation: Journal of Herpetology 50, 1; 10.1670/14-142
Statistical Analysis
To compare ages between the two sexes, we used nonparametric Mann–Whitney U-test. We used Pearson's correlation to examine relationships between SVL and age in males and females. Growth data were fitted to the logistic model as: SVLt = a/(1 + b × exp[−kt]); Gompertz model as: SVLt = a × exp(−b × exp[−kt]); and von Bertalanffy model as SVLt = a × (1 − b × exp[−k × t]); all were models previously used in several other amphibian studies (e.g., Guarino et al., 2003; Uzum, 2009; Kolarov et al., 2010; Kutrup et al., 2011). In these models, SVLt is the average SVL (mm) at age t (years), a is the upper asymptotic or maximum SVL, b is the growth rate, m is the slope of growth, and k is the growth coefficient (shape of the growth curve). To evaluate the degree of fitness of these models to the growth values we use both coefficient of determination and the Akaike information criterion (AIC; Burnham and Anderson, 2002). AIC is calculated as: AIC = N Ln (SSE/N) + 2 (p+1), where N* = total sample size, SSE is the error (residual sums of squares), and p = the number of parameters estimated “3 in all models”). We calculated ΔAIC to choose the best model. All tests were processed with SPSS v.15.0 (SPSS Inc., Chicago, IL, USA) and Microsoft Excel. All descriptive statistics are expressed as means (±SE).
Results
Bone Histology and Skeletochronological Analysis
Seventy-three bones (27 males and 46 female adult newts) were scored and an estimation of the individual's age was calculated. Phalangeal cross-sections (Fig. 1) indicated that the diaphysis of the phalanges was composed of two compact and concentric bone layers: the PB and the EB. The two layers were separated by irregular and strongly hematoxylinophilic lines (Fig. 3). EB and PB did not always differ in histological organization. In the phalanges of most adults, periosteal lines that were strongly hematoxylinophilic, circular, and more or less complete (Fig. 3) were interpreted as LAGs. We considered individual age to be equivalent to the number of visible LAGs in phalange bone cross-section as reported by many authors (e.g., Buhlmann and Mitchell, 2000; Kumbar and Pancharatna, 2004; Baskale et al., 2013).
Body Size
We compared mean SVL of N. kaiseri sampled during two principal periods of the annual cycle of the species: in the early reproductive period (March–April) and the period of metamorphosis and immediate postmetamorphic growth (May–November). A majority (97.26%) of adult individuals was collected during the reproductive period in April (Table 1). Twelve juveniles with SVL range 20.7–33.2 mm were sampled in June. Among adults, there was a significant difference in SVL between males and females (t84 = −2.74, P = 0.007), with females tending to be the larger (Table 1).
Age Structure, Age at Sexual Maturity, and Growth
The minimum age group for adult salamanders sampled (Table 2) for skeletochronology was 4 yr for both males and females. The maximum observed longevity was 14 yr in males, whereas it was 12 yr for females. The average ages of the males and females were calculated as 7.74 ± 0.30 and 6.81 ± 0.58 yr, respectively (Table 1). No significant differences were found between the age distributions of the two sexes (Mann–Whitney U-test = 90.50, Z = −0.95, P = 0.34). Because the youngest male and female in our samples were 4 yr old, we estimated minimum age at maturity to be 4 yr old (Fig. 3). SVL and age were positively correlated in both males (Pearson's correlation, r = 0.86, P < 0.001, n = 39) and females (r = 0.91, P < 0.001, n = 58). Parameters calculated for growth curves for 12 larvae, 27 males, and 46 females of N. kaiseri with logistic, Gompertz, and von Bertalanffy growth models are shown in Table 3. The SVL growth curve fitted by the von Bertalanffy growth model provided the highest coefficient of determination (r2 = 0.98). The von Bertalanffy model (the lowest AIC score; ΔAIC = 0) was the best, given the set of three candidate models for males and females respectively (Table 3); the other models in the set of candidate models had considerably less support. The sexes were significantly different in SVL at 4 yr (independent-samples t-test: t9 = −3.92, P = 0.003) and females tended to be the larger (male: 51.88 ± 0.7, female: 56.2 ± 0.76). Beyond this age, growth curves differed between the sexes. Males and females differ in growth parameters (Table 3). The maximum length (SVLmax) was 58.77 ± 1.09 mm for males and 68.93 ± 1.34 mm for females.
Discussion
Our results for N. kaiseri in the population we studied show that this species has an SVL range of 48.8–73.8 mm (n = 73) and is a long-living newt. The maximum life span for N. kaiseri was 14 yr in males and 12 yr in females; similar life spans have been reported in other salamander species, such as Ambystoma tigrinum nebulosum (SVL: 140–220 mm), with a range of 15 yr (Eden et al., 2007) and Neurergus microspilotus (SVL: 42.24–75.32 mm) with a longevity of 14 yr (Sharifi et al., unpubl. data). The life span in still other species is ≤10 yr (e.g., Mertensiella caucasica [SVL: 51.28–76.60 mm, longevity: 10 yr; Uzum, 2009]; Triturus karelinii (SVL: 55.44–83.18 mm, longevity: 9 yr; Uzum and Olgun, 2009); Tylototriton verrucosus (SVL: 58.0–80.1 mm, longevity: 8 yr; Wichase et al., 2010)]. Many amphibians are longer lived with increasing latitude or altitude (Leclair et al., 2000).
Citation: Journal of Herpetology 50, 1; 10.1670/14-142
Citation: Journal of Herpetology 50, 1; 10.1670/14-142
The time required from birth to reach sexual maturity is an important life-history trait for any species, because it can influence the reproductive output by affecting the generation time. The age at maturity also is associated with factors such as growth, level of metabolism, and difference in size between adults and juveniles (Morrison and Hero, 2003). As reproductive maturity is usually determined by the size at maturation and juvenile growth, we expect species with faster growth rates to reach the minimum size for reproduction sooner than those with slower growth rates (Morrison et al., 2004). The results of the present study show that both sexes in N. kaiseri are potentially able to reproduce in their third breeding season (after their metamorphosis) when they are 4 yr old. Consistent with our results, Uzum (2009) reports the age of sexual maturity for M. caucasica to be 4 yr, whereas in some populations of Triturus karelinii (Olgun et al., 2005) the age of maturation is 3 yr.
Sexual size dimorphism is observed in many amphibians and in many salamanders and newts, females often are larger than males (e.g., Olgun et al., 2005). The SVL of adult males varied between 48.8 and 60.1 mm (n = 27) and 54.1 and 73.8 mm (n = 46) in females. SVLmax was higher for females than for males. Sexual dimorphism in N. kaiseri also has been documented by Sharifi et al. (2012); sexual dimorphism in this species was attributed to females showing large values for dimensions such as SVL. The present skeletochronological study also showed females to be larger than males, and can be interpreted as primarily concordant with the fecundity model, in that fecundity increases with increasing female body size (Janiga and Mlichova, 2004).
This study provides important information on age, growth, and longevity of an endangered species. The current age structure of the Kaiser's Mountain Newt in the Bozorgab Stream (Fig. 3) showed a pattern very similar to other similar studies on newts (Miaud et al. 2000; Ento and Matsui, 2002; Uzum, 2009). Neurergus kaiseri is a small newt with long life span and potentially delayed maturity in a fluctuating environment of highland streams in an arid region (Bozorab Stream) with chronic reduction in water quality; these life-history traits suggest that environmental conditions could play an important role on the population structures of this species. On the other hand, illegal collection for national and international trade (Sharifi et al., 2013) may have serious effects on the future trend of the population of this species in Bozorgab Stream. Therefore, more studies are needed on the proximate effects of environmental fluctuations on this newt's life-history traits for consistent conservation measures.
On the basis of limited data obtained from field studies, the annual reproductive cycle for N. kaiseri is characterized by movements of adult male and female newts from their terrestrial habitats into aquatic breeding streams in late winter; eggs are laid between May and June. Several female newts kept in a captive facility showed that individuals may produce over 100 individual eggs. Males and females leave the water at the end of the breeding season. The egg stage lasts 2–3 wk; the larval period 6–8 mo, reaching metamorphosis (loss of gills) with SVL approximately 31 ± 0.58 mm and weight 1.54 ± 0.05 g; at this stage young postmetamorphs leave the water. We found no information on the growth and development of N. kaiseri during the period between the end of metamorphosis and the attainment of sexual maturity. In the field during growth to maturity, subadults of the Kaiser's Mountain Newts occasionally return to aquatic life (pers. observ.).
This study demonstrates the average life span of the Kaiser's Mountain Newt in Bozorgab Stream to be 7.28 yr and survival for up to 14 yr is possible. This information, however, cannot provide any estimates of vital population rates such as mortality and fertility, innate rates for natural increase and survival (Caughley, 1994). Obtaining such information requires constructing a life table, normally obtained by collecting information about age-specific mortality rates in a cohort population (Krebs, 2001). Such tables also can provide life expectancy and population growth rates. Making such information available is time consuming for any species and may be impossible for taxa that have long life spans with complex life cycles. Current conservation efforts may increase the possibility of supplying more reliable information on population dynamics of endangered species by maintaining a higher number of individuals of such species (Heppell et al., 2000).
Geographic position of Bozorgab stream in the southern Zagros Range, Iran.
Bozorgab Stream contains a substantial number of Neurergus kaiseri. (Photo by Hossein Farasat).
Phalange bone cross-section of an 11-yr-old female Neurergus kaiseri, from Bozorgab Stream. Arrows = lines of arrested growth, endosteal bone, and periosteal bone.
Age class distribution of adult males and females of Neurergus kaiseri.
Growth curves in male (solid squares) and female (solid circles) Kaiser's Mountain Newt, Neurergus kaiseri, in the southern Zagros Mountains, Iran. Growth curves were fitted to von Bertalanffy's growth equation.
Contributor Notes
