Predation drives interpopulation differences in parental care expression

Authors


Correspondence author. E-mail: wshuang@mail.nmns.edu.tw

Summary

  1. Expressing parental care after oviposition or parturition is usually an obligate (evolved) trait within a species, despite evolutionary theory predicting that widespread species should vary in whether or not they express parental care according to local selection pressures. The lizard Eutropis longicaudata expresses maternal care only in a single population throughout its large geographical range, but why this pattern occurs is unknown.
  2. We used reciprocal translocation and predator exclusion experiments to test whether this intraspecific variation is a fixed trait within populations and whether predator abundance explains this perplexing pattern.
  3. Wild-caught female lizards that were reciprocally translocated consistently guarded or abandoned eggs in line with their population of origin. By contrast, most lizards raised in a common garden environment and subsequently released as adults adopted the maternal care strategy of the recipient population, even when the parents originated from a population lacking maternal care.
  4. Egg predation represents a significant fitness cost in the populations where females display egg-guarding behaviour, but guarding eggs outweighs this potential cost by increasing hatching success.
  5. These results imply that predators can be a driving force in the expression of parental care in instances where it is normally absent and that local selection pressure is sufficient to cause behavioural divergence in whether or not parental care is expressed.

Introduction

Animals have evolved a wide range of parental care strategies to increase offspring survival, ranging from nest-site-guarding to much more complex provisioning behaviours (Clutton-Brock 1991; Wesołowski 1994; Klug & Bonsall 2009). These strategies have evolved independently numerous times in disparate animal lineages, providing strong evidence of their success as an evolutionary strategy for increasing fitness (Tallamy 1984; Shine 1988; Clutton-Brock 1991; Gross 2005). Evolutionary theory predicts that parental care should evolve from a nonparental caring ancestral state when the fitness benefits accrued by protecting offspring outweigh the costs of providing care (Tallamy 1984; Clutton-Brock 1991; Wesołowski 1994). The general life-history circumstances favouring parental care thus include low egg or juvenile survival without care, relatively long egg incubation periods and relatively short juvenile stages (Clutton-Brock 1991; Klug & Bonsall 2009), while the general ecological circumstances favouring parental care include food availability and predation risk (Lack 1968; Tallamy & Wood 1986). Overall, species are more likely to evolve parental care when the risk of predation on eggs or offspring is high (Lack 1968; Tallamy & Wood 1986).

Parental care is any action of the parent after oviposition or parturition that increases the chances of survival of the offspring (Shine 1988; Greene et al. 2002). For example, within single populations of cichlid fish (Sarotherodon galilaeus Linnaeus 1758) and birds (Remiz pendulinus Linnaeus 1758), parental care can be provided by both parents or by either one of the sexes alone (Persson & Öhrström 1989; Balshine-Earn & Earn 1998). Furthermore, the type and intensity of parental care behaviours expressed can vary with offspring development (reviewed by Magnhagen 1992) or local environmental conditions (Dunn & Robertson 1992; Wheelwright, Schultz & Hodum 1992; Székely & Cuthill 1999). Thus, many widespread species show differences within and among populations in the duration, type or intensity of parental care (e.g. Dunn & Robertson 1992; Wheelwright, Schultz & Hodum 1992; Székely & Cuthill 1999; Webb et al. 1999), but almost all species expressing parental care are known to do so throughout their entire geographical range. By contrast, examples of parental care expression in some populations but not others are extremely rare; known examples include a frog (Martins, Pombal & Haddad 1999) and, if we consider maternal care to occur throughout the entire duration of pregnancy, lizard species with both oviparous and viviparous populations (e.g. Heulin et al. 1999). Other rare cases include nest abandonment in species which provision their offspring, which almost always results in offspring death (e.g. Burley & Johnson 2002; Reynolds, Goodwin & Freckleton 2002). Parental care is thus almost always expressed throughout the entire distribution of most species. This evolutionary pattern limits our capacity to unravel the selective pressures leading to the evolution of parental care (Clutton-Brock 1991; Wesołowski 1994; Gross 2005). It also seems to contradict the notion that because widespread species are under different selection pressures throughout their geographical range, they should express parental care in line with predation risk, so as to maximize offspring survival. Why, then, do most widespread species show obligate parental care expression, rather than facultative care, where adults adjust whether they provide or do not provide parental care according to the threat of predation of the offspring? This situation would only persist in species where the offspring can physically survive without relying on either of the parents (e.g. many reptiles, amphibians and invertebrates; Shine 1988).

Long-tailed skinks (Eutropis longicaudata Hallowell 1857) are widely distributed throughout South-East Asia and only express parental care in a single population on Orchid Island, located off the south-eastern coast of Taiwan (Huang 2004). In this population, mothers nesting within concrete retaining walls remain at the nest during incubation and actively deter egg-eating snakes (Oligodon formosanus Günther 1872) from entering the nest and consuming the eggs (Huang 2006a; Supplementary Information S1). Antisnake behaviour is only expressed while gravid, immediately prior to oviposition (Huang & Pike 2011a). By contrast, females nesting in natural habitats nearby (beneath rocks and logs) never guard their eggs during incubation (Huang & Pike 2011b). Females nesting in the retaining wall gain a fitness advantage by exposing their eggs to higher incubation temperatures, which decreases incubation duration and increases the proportion of eggs that hatch, offspring growth rates, and survival (Huang & Pike 2011b). However, this behaviour is costly because eggs laid inside the retaining wall are exposed to predators throughout incubation, as compared with eggs buried beneath cover objects in natural habitat (Huang & Pike 2011b). Despite the increased risk of predators locating eggs laid inside of these retaining walls, female long-tailed skinks from 12 other populations (11 from the island of Taiwan and one from Green Island, all nesting in similar retaining walls) abandon their eggs immediately after oviposition (W.-S. Huang, unpublished). Thus, the expression of parental care is not a fixed trait shared among populations (i.e. obligate; as it appears to be in all other species) because only the females nesting inside the retaining wall on Orchid Island express parental care. As in many other taxa, not all females remain with the eggs for the entire incubation period (but they still initially express, and thus provide, maternal care; Huang & Wang 2009). The duration of maternal nest-guarding depends on the risk of egg predation: when egg-eating snakes are rare, females leave the nest early in incubation, but when snakes are abundant, females continue defending the nest until the eggs hatch (Huang & Wang 2009). This suggests that local predation pressure plays an important role in the evolution of maternal care. We can test this hypothesis using reciprocal translocation experiments to test directly whether exposing female lizards to local ecological conditions they normally would not experience will alter the expression of parental care behaviour from that of the source population.

An additional hypothesis for the facultative parental care expression in long-tailed skinks is that geographical isolation has resulted in substantial genetic divergence, where the diverged population now provides maternal care. This would mean that other populations do not have the capacity to express maternal care, and thus would not do so, even under the ecological circumstances in which other (genetically diverged) individuals do provide care. We would then expect the long-tailed skink population on Orchid Island to represent a highly divergent lineage (or possibly even a separate, but morphologically similar, species). The straight separating the island of Taiwan from Orchid Island is sufficiently deep (>1 km) that Orchid Island has never been connected with other islands, even during low sea levels of the last glacial maximum (LGM). Consequently, we might expect an absence of shared haplotypes between Orchid Island and the other populations and perhaps the presence of distinct genetic lineages there. Parental care can have an underlying genetic component; for example, mice lacking the fosB gene will not nurture their offspring (Brown et al. 1996). Although our understanding of the genetic mechanisms underlying parental care is preliminary and limited to mammals (a group almost exclusively comprised of species that always express parental care), other genes and experiential learning also play important roles (Rosenblatt 1994; Brown et al. 1996). Because parental care initially evolves from ancestors that do not provide care (Tallamy 1984; Shine 1988; Clutton-Brock 1991), all populations of long-tailed skinks could have genes coding for the expression of parental care, but there may be some ecological trigger for this behaviour.

These two pathways – local adaptation via behavioural plasticity vs. genetic divergence – are not mutually exclusive, in that both ecological factors and genetic history could play a role in the facultative expression of parental care. We predicted that (i) parental care expression is a behaviourally plastic trait that depends on the local environment; consequently, females translocated among populations should alter their behaviour to conform to that of the recipient population; (ii) because the function of nest-guarding is to protect eggs from predation (Huang 2006a), predator abundances should be markedly higher on Orchid Island than in other populations; (iii) the benefit of increased nest survival because of maternal care should be higher in the population that expresses care and (iv) there will be little to no genetic divergence between the population expressing parental care and other populations not expressing care. If our predictions hold true, this will provide compelling evidence that predation plays a major role in the plasticity of parental care expression, and thus that local selective pressures can influence this behavioural innovation.

Materials and methods

Reciprocal Translocation Experiments

In our first reciprocal translocation experiment (2007), we moved 20 individually marked (using microchips) wild-caught adult females with a history of nesting within the concrete retaining wall and guarding eggs from Orchid Island to a concrete wall at SanDiMen on the island of Taiwan and moved a further 20 females from SanDiMen to Orchid Island. In September 2010, we repeated this experiment by translocating 40 additional females from each population to the other population. Following translocation, we surveyed all potential nesting sites within the retaining walls at six-hour intervals (4 surveys day−1) from June to August 2007–2011 and recorded whether recaptured females remained with their eggs following oviposition. We classified females as not having laid eggs when we saw no evidence of them being gravid and did not observe them with a clutch; gravid females had visibly distended abdomens and eggs that we could feel by gently palpating the abdomen. In the maternal-caring population, females remain with the nest for at least the first week of incubation (and can stay up to 35 days, that is, for the entire incubation duration), but in the noncaring populations, females leave the nest immediately after laying eggs (typically within a few hours; Huang 2006a, 2007). We defined maternal care as remaining with the eggs for >24 h after laying; these females generally stayed at the nest for several days, but noncaring females had abandoned their nest within 6–12 h of laying eggs.

Following this initial translocation, we were unable to locate most of the females that were moved from Orchid Island to SanDiMen; this is likely due to migration outside of the study area or predation. Orchid Island contains only one lizard predator (the colubrid snake Elaphe carinata Günther 1864), but SanDiMen has many additional predators (including the saurophagous snakes, Viperidae: Protobothrops mucrosquamatus Cantor 1839, Trimeresurus stejnegeri Schmidt 1925; Elapidae: Bungarus multicinctus multicinctus Blyth 1861, Naja naja atra Cantor 1862; Colubridae: Ptyas mucosus Linnaeus 1758). Females from Orchid Island are naïve to these predators and may suffer higher predation rates on SanDiMen. We thus conducted a second reciprocal translocation experiment by releasing lizards raised in a common garden environment. In June 2009, we brought 32 gravid females from Orchid Island and 28 females from SanDiMen into the laboratory. These females were unrelated to the ones used in our other translocation experiments, and thus, the wild-caught and captive-reared females are independent of one another. Females laid eggs over the next 3 months, which we incubated in individual jars containing moist vermiculite at 25 °C. Hatchling lizards were placed in cages with their clutch mates and raised until August 2010, when the lizards had grown large enough to reach maturity (>98·1 mm snout-vent length; Huang 2006b) but had not yet reproduced for the first time. We released progeny in a fully factorial design by releasing part of each clutch at the initial capture population of the mother (SanDiMen or Orchid Island, serving as controls) or at the opposite population (i.e. offspring whose mothers were collected from Orchid Island were moved to SanDiMen and vice versa). This design allowed us to test whether individuals from different populations, but raised under identical laboratory conditions, can alter their expression of parental care in line with the recipient population.

Predator Abundances and Hatching Success Rates

To test our hypothesis that egg-eating snake predation rates on lizard eggs are higher in the insular population showing maternal care than elsewhere, we surveyed four long-tailed skink nesting sites (SanDiMen on mainland Taiwan and Green Island, which both lack lizard maternal care, and Little Paiday Bay and Tungching on Orchid Island, which both express care) to quantify snake numbers and the proportion of lizard nests preyed upon by egg-eating snakes. At each site, we surveyed a 2-km stretch of retaining wall (containing 1200–1500 holes used by skinks as nesting sites) at 6-h intervals daily (i.e. 4 samples day−1) from May to October 2001–2004 and individually marked all egg-eating snakes encountered and recorded snake predation rates on eggs. We inferred predation by egg-eating snakes when the entire clutch was consumed; the only other egg predators at these sites are ants, which leave behind empty eggshells (Huang 2010).

Efficacy of Parental Care Among Populations

We used a manipulative experiment to determine the relative effectiveness of maternal care in each population. We experimentally excluded vertebrates from accessing some nests while leaving others exposed to predators at Little Paiday Bay on Orchid Island, SanDiMen on the island of Taiwan and on Green Island. To exclude vertebrate predators (and thus document hatching rates in the absence of snake predation), we covered the entrance holes leading to freshly laid lizard nests with plastic mesh (1 mm diameter, too small for any vertebrate to enter). The mesh was large enough to allow invertebrate predators (ants) to enter the nest and fungus to infect the eggs (Huang 2006a) and thus allowed us to document rates of egg hatching in the absence of both egg-eating snake predation and the mother lizard. We did not cover the entrance to control nests with any form of mesh, and thus, these nests were left completely exposed to both vertebrate and ant predators. Because female lizards at Little Paiday Bay on Orchid Island guard their eggs upon hatching, we removed the female from nests in both treatments to match the absence of nest-guarding at the other two populations. Comparing the difference in hatching success of nests covered with mesh (= 30, 15 and 22 clutches from Little Paiday Bay, Green Island and SanDiMen, respectively) to a different set of control nests at the same site that were not covered by mesh (= 142, 52 and 156 clutches, respectively) allowed us to determine empirically the increase in hatching success that would be derived from guarding a nest in each population (note that other associated costs may be minimal; Huang 2007).

Genetic Diversity

We collected 76 Eutropis longicaudata tail muscle tissue samples (stored in 95% ethanol before DNA extraction) from 14 insular populations: 12 populations from the island of Taiwan plus one population each from nearby Green and Orchid Islands (Fig. 3). We isolated total genomic DNA using Qiagen DNeasy Blood & Tissue Kits (Qiagen, Taipei, Taiwan). DNA was suspended in 1× TE buffer and stored at −20 °C. We amplified the complete sequence of the mitochondrial cytochrome b gene (1137 bp) using polymerase chain reaction (PCR). The primer pair PL (5′-aaccaagacctgtgayaygaa-3)/PH (5′-ggcttacaagaccarkgcttt-3′) was designed from the consensus sequences of several lizard species in the family Scincidae.

Reactions were conducted in a 20 μL reaction volume containing 1× PCR buffer (10 mm Tris–HCl, pH 9·0; 50 mm KCl, 0·01% (w/v) elatine and 0·1% Triton X-100), 0·8 U Taq DNA polymerase (Amersham Pharmacia Biotech Inc., Piscataway, NJ, USA), 0·2 μm each primer, 0·5 mm dNTP and 50 ng template DNA. The PCR condition was set to denaturation at 94 °C for 3 min, then 35 cycles at 94 °C for 30 s, 52 °C for 40 s and 72 °C for 90 s, followed by a final extension at 72 °C for 10 min using iCycler Thermal Cycler (Bio-Rad Laboratories Taiwan Ltd., Taipei, Taiwan). PCR products were purified with a PCR Product Pre-sequencing Kit (USB Molecular Biology Reagents and Biochemicals, Santa Clara, CA, USA) and used as the template for direct DNA sequencing reactions with a DYEnamic ET Dye Terminator Cycle Sequencing Kit (Amersham Pharmacia Biotech). We sequenced products on a MegaBACE 1000 automated DNA sequencer (Amersham Biosciences). Sequences were determined in both directions, and the original signals were proofread using sequencher version 4.7 (Gene Codes Corporation, Ann Arbor, MI, USA). We compared our sequences to other published skink sequences to confirm PCR amplification accuracy.

We evaluated genetic diversity of the 14 skink populations using haplotype (h) (Nei & Tajima 1983) and nucleotide diversity (π) (Nei 1987) using dnasp 5.0 (Librado & Rozas 2009). We used statistical parsimony (Templeton, Crandall & Sing 1992) implemented in tcs Version 1.21 (Clement, Posada & Crandall 2000) to define the primary uncorrected distance (p) above which there is a >5% probability that the parsimony criterion is violated. All connections were established among haplotypes, starting with the smallest distances and ending when all haplotypes were connected or when the distance that corresponded to the parsimony limit was reached (Clement, Posada & Crandall 2000).

We used φST to assess the genetic structure between populations, which takes into account the genetic distance between haplotypes and the haplotype frequencies [using arlequin (Excoffier & Lischer 2010)]. We used the Kimura two-parameter genetic distance as the genetic model (K2P; Kimura 1980). We calculated population structure using F-statistics (FST; Wright 1965) using strictly haplotype frequencies. Significance values were obtained after 1000 permutations and tested whether FST values differed significantly from 0. We used amova (Excoffier & Lischer 2010) to compare the proportion of contribution at among-island and within-island levels. Finally, we used beast ver. 1.6 (Drummond & Rambaut 2007) to estimate the time of most recent common ancestor (tMRCA) for all populations.

Results

Reciprocal Translocation Experiments

All of the wild-caught adult females released from SanDiMen onto Orchid Island abandoned their nests immediately after laying (two individual females did so in each of 2008, 2009 and 2010; Table 1). Egg-eating snakes consumed all of these clutches. Of the females known to have expressed maternal care that were released onto SanDiMen, only two were found in 2008 and both females were guarding their eggs (Table 1). We found a similar pattern after repeating this reciprocal translocation with larger sample sizes: all encountered nesting females expressed or did not express maternal care in line with their population of origin, rather than changing their behaviour to match that of the recipient population (Table 1). Rates of encountering nesting females, non-nesting females (i.e. those that were not gravid and who thus did not lay eggs) or not finding females were similar between release locations (χ2 = 1·54, d.f. = 2, = 0·46; Table 1). Adult females originating from Orchid Island were significantly more likely to express parental care than were females originating from SanDiMen, even when in a novel environment (100% vs. 0%, respectively; χ2 = 32·0, d.f. = 1, < 0·00001; Table 1). Orchid Island females remained with their egg clutch on SanDiMen for 1–9 days (mean = 3·5 days). This strongly suggests that wild-caught adult females with prior reproductive experience make consistent decisions about whether or not they express maternal care, regardless of the environmental conditions experienced at the time of nesting.

Table 1. Results from reciprocal translocation experiments, moving lizards from the population expressing maternal care (Orchid Island) to a population not expressing maternal care (SanDiMen, located on the main island of Taiwan) and vice versa. We performed two experiments, the first of which moved gravid wild-caught adult females from one population to the other and the second of which captured a different set of gravid females from the wild and raised the progeny to maturity in captivity. We ran the first experiment twice (in both 2007 and 2010) because of low recapture rates. The last column summarizes the percentage of females known to have expressed maternal care, given as the percentage of females that laid eggs and expressed care and then as the percentage of all females observed expressing care (i.e. including females who were not observed to be gravid and thus were not known to lay eggs). We could not assess the reproductive condition (gravid and not gravid) of females that were not recaptured
PopulationNumber of lizardsRate of parental care expression (including females that did not lay eggs)
OriginRecipientExpressing maternal careNot expressing maternal care (not gravid)Not recapturedTotal
Wild-caught females translocated in 2007 and recaptured from 2007 to 2011
Orchid IslandSanDiMen201820100%
SanDiMenOrchid Island014 (10)6200%
Wild-caught females translocated in 2010 and recaptured in 2011
Orchid IslandSanDiMen1810 (10)1240100%
SanDiMenOrchid Island025 (15)15400%
Progeny raised in captivity from wild-caught females, released in 2010
Orchid IslandOrchid Island285 (5)1245100% (87·5%)
SanDiMenSanDiMen015 (12)17320%
Orchid IslandSanDiMen610 (10)3046100% (37·5%)
SanDiMenOrchid Island1310 (10)932100% (56·5%)

For the lizards born and raised to maturity in captivity, both experimental groups significantly altered their maternal care expression relative to the control groups (comparing lizards from Orchid Island released there vs. moved to SanDiMen; Fisher's exact test, < 0·001; comparing lizards from SanDiMen released there vs. moved to Orchid Island, χ2 = 12·89, d.f. = 1, < 0·0001; Table 1). All female lizards born to mothers collected from Orchid Island and released there as adults expressed maternal care, as did the individuals moved to SanDiMen (Table 1). By contrast, female lizards originating from SanDiMen and released there were never observed to express maternal care, but 100% of individuals moved to Orchid Island displayed maternal care (Fisher's exact test, = 0·002; Table 1). This clearly suggests that females from both populations have the capacity to express parental care when raised in a common garden environment, strong evidence that the expression of parental care can be a behaviourally plastic trait.

For the lizards born and raised to maturity in captivity, the duration of maternal care varied significantly among recipient populations (F2,44 = 12·12, < 0·0001). Captive-raised females originating from Orchid Island and released there, and females originating from SanDiMen and released to Orchid Island, stayed at the nest significantly longer (mean durations of 17·4 and 20·3 days and ranges of 8–31 and 8–33 days, respectively; Tukey's post hoc test, < 0·0001 in both cases) than those originating from Orchid Island and released onto SanDiMen (mean duration = 3·2 days, range: 1–5 days).

By randomly allocating individual lizards to each treatment with respect to clutch of origin, we avoided confounding effects of initial body size (comparing hatchling body size among the four translocation treatments using clutch as a random effect: snout-vent length (SVL) F3,137 = 1·69, = 0·17; mass F3,137 = 1·07, = 0·36). However, at the time of release, there was significant variation in female body size among treatments (comparing body size among the four translocation treatments using clutch as a random effect: SVL at release F3,150 = 1·69, = 0·002; mass at release F3,151 = 1·07, = 0·008). Despite variation in body size at release among treatments, body size did not play a significant role in whether or not maternal care was expressed (comparing body size at release among the four translocation treatments and parental care expression using clutch as a random effect: SVL: release treatment, F3,68 = 2·38, = 0·08; maternal care expression F1,68 = 2·39, = 0·13; mass: release treatment, F3,68 = 8·36, < 0·0001; maternal care expression F1,68 = 0·68, = 0·41).

Egg-Eating Snake Abundances and Hatching Success Rates

Predation is the plausible trigger that has caused female lizards to guard their nests during incubation on Orchid Island. If so, long-tailed skink populations lacking maternal care should have lower snake abundances and thus higher rates of egg hatching success than Orchid Island. This is exactly what we found; egg-eating snake abundances were substantially higher at the two Orchid Island sites than at two sites lacking lizard maternal care (χ2 = 156·73, d.f. = 3, < 0·0001; Fig. 1). The Orchid Island sites had snake abundances 9–20 times higher than the two sites lacking maternal care. Snake numbers corresponded with egg-eating snake predation rates on lizard eggs; significantly higher proportions of lizard clutches were preyed upon by snakes on the Orchid Island sites than at the other two sites (χ2 = 69·15, d.f. = 3, < 0·0001; Fig. 1). The two Orchid Island sites had similar rates of lizard egg predation by snakes (χ2 = 1·09, d.f. = 1, < 0·296; Fig. 1), but despite snake abundances being 9–20 times higher at these sites, predation rates were only twice as high as those documented from the two populations not expressing parental care (Fig. 1). Snake abundances were unrelated to lizard nest availability: Little Paiday Bay on Orchid Island (= 173) and SanDiMen (= 156) had large numbers of lizard nests, while Green Island (= 54) and Tungching on Orchid Island (= 37) had substantially fewer lizard nests.

Figure 1.

Egg-eating snake (Oligodon formosanus) numbers and the associated percentage of lizard nests preyed upon by snakes from two insular lizard populations not expressing parental care (SanDiMen on the main island of Taiwan and Green Island) and two insular lizard populations from Orchid Island expressing parental care (Little Paiday Bay and Tungching). Snake numbers are the total number of snake encounters along a 2-km stretch of concrete retaining wall from 2001 to 2004, and the total number of lizard nests is as follows: SanDiMen = 156, Green Island = 54, Little Paiday Bay = 173 and Tungching = 37.

Efficacy of Parental Care Among Populations

In the absence of maternal care, hatching success was lowest on Orchid Island and much higher in the two populations lacking maternal care (comparing hatched/unhatched eggs among the mainland, Green Island and Orchid Island, = 108, 60 and 450 eggs, respectively; χ2 = 106·74, d.f. = 2, < 0·0001; Fig. 2). The two populations lacking maternal care had similar hatching success rates (χ2 = ·08, d.f. = 1, = 0·08; Fig. 2). Nest temperatures and relative humidity were similar among these populations and thus cannot explain this pattern [mean ± SEM: SanDiMen = 30·8 ± 0·21 °C; 91·5 ± 2·4% (= 248 nests), Green Island = 30·3 ± 0·30 °C; 91·0 ±1·9% (= 125 nests) and Little Paiday Bay on Orchid Island = 30·0 ± 0·30 °C; 90·9 ± 2·6% (= 45 nests); F2,415 = 1·57, = 0·21 for temperature; F2,415 = 0·89, = 0·76 for relative humidity].

Figure 2.

Experimental demonstration of the fitness benefits derived from maternal care in three different populations. Predation rates of lizard nests are expressed as the proportion of long-tailed skink eggs hatching from the three populations without any form of nest protection (i.e. without a nest-guarding female present) for populations on Taiwan (SanDiMen), Green Island and Little Paiday Bay on Orchid Island, and when vertebrate predators were excluded from accessing eggs using mesh netting. The only population in which maternal care would result in a fitness benefit is from Orchid Island, where females naturally express maternal care.

Experimentally excluding vertebrate predators from accessing lizard eggs (a proxy for the provision of maternal care) substantially increased hatching success rates on Orchid Island above that of nests without an attending female (= 1000 eggs, χ2 = 200·89, d.f. = 1, < 0·0001; Fig. 2), but had no significant effect at the other two sites (mainland: = 70 eggs, χ2 = 0·31, d.f. = 1, = 0·58; Green Island: = 156 eggs, χ2 = 0·17, d.f. = 1, = 0·68; Fig. 2). This confirms that Orchid Island is the only population for which maternal nest-guarding would provide a direct fitness benefit via an increase in egg hatching success.

Genetic Diversity

The 76 long-tailed skink sequences revealed five different haplotypes of 1137 bp with four segregating sites (GenBank accession numbers: JF689928JF689932; Table 2; Fig. 3). Most individuals (63/76 or 83%) belonged to the most frequent haplotype H1. This haplotype appeared in all sampling sites, including Green and Orchid Islands (Table 2; Fig. 3). The other haplotypes (H2–H5; all with a single mutation separating them from H1) were rare and private and had a very restricted distribution (Fig. 3). Pairwise FST and corrected φST values between Taiwan vs. Green Island, Taiwan vs. Orchid Island and Green Island vs. Orchid Island were 0·02142, −0·00092, and 0·00000 and 0·02139, −0·00085, and 0·00000, respectively, (all pairwise P-values > 0·05). amova indicated that most genetic variation was within populations (73·91%), while variation among islands was much lower (Table 3). We estimated the tMRCA to be between 49 750 and 18 020 years ago (using divergence rates ranging from 1% to 2·85% per MY; Gubitz, Thorpe & Malhotra 2000; Paulo et al. 2001), indicating that these island populations were differentiated roughly after the LGM.

Figure 3.

Sampled populations of Eutropis longicaudata, showing FST values between the island of Taiwan and Green and Orchid Islands. Also shown are haplotype frequencies within populations and a statistical parsimony network of haplotypes (H1–H5). Populations are numbered as in Table 1: 1 = Orchid Island, 2 = Green Island and 3–14 = Island of Taiwan.

Table 2. Long-tailed skink (Eutropis longicaudata) molecular data, showing sample sizes (n), number of haplotypes (Nhap), haplotype diversity (h) and nucleotide diversity (π)
Sample localityPopulation n N hap h π
Orchid Island11010·00000·0000
Green Island21020·20000·0002
Island of Taiwan 5640·36950·0004
BeiNan3510·00000·0000
TaiMaLi4310·00000·0000
DaWu5110·00000·0000
LaiYi61020·55560·0005
TaiWu71020·53330·0005
WanAn8210·00000·0000
SanDiMen9310·00000·0000
TianLiao101020·33330·0003
ShanLin11110·00000·0000
HsinHua12510·00000·0000
DaNei13410·00000·0000
SanJiePu14610·00000·0000
Total 7650·30600·0003
Table 3. Analysis of molecular variance results
Genetic variationSum of squares% of VariationFixation indicesSignificance tests
Among groups0·378−29·46ΦCT = −0·29458< 0·001, d.f. = 2
Among populations within groups5·16055·54ΦSC = 0·42905< 0·001, d.f. = 11
Within populations6·63373·91ΦST = 0·26086= 0·49, d.f. = 62

Discussion

We sought to understand why long-tailed skinks show facultative maternal care expression throughout their geographical range, which sharply contrasts the obligate expression of parental care throughout the entire geographical range of most other species (see Martins, Pombal & Haddad 1999 for an exception). We emphasize that many animal populations vary in the type of parental care expressed or in the duration of parental care, and yet nearly all of these populations still express parental care to some degree. Our reciprocal translocation experiments revealed two exciting patterns: (i) adult females living in the wild consistently displayed the same behaviour as their natal population, even after reproducing in a new population up to three consecutive years following translocation (= 2 females), but that (ii) individuals born and raised to maturity in a common garden environment have the capacity to express parental care, regardless of their population of origin (Table 1). Consequently, the low levels of genetic divergence between Orchid Island and other populations (Table 3, Fig. 3) do not adequately explain underlying geographical differences in maternal care expression (see Heulin et al. 1999 for a similar outcome comparing viviparous and oviparous populations of Lacerta vivipara Lichtenstein 1823). Given the lack of genetic divergence between populations (Table 3; Fig. 3), we might expect the ecological factors on Orchid Island (e.g. elevated predation pressure on lizard eggs) to elicit otherwise noncaring females to remain with their eggs during incubation. This is exactly what we found; females with prior nesting experience consistently expressed or did not express parental care depending on their population of origin, whereas captive-raised females lacking prior nesting experience from both populations of origin expressed maternal care on Orchid Island. Thus, once a female expresses parental care initially, it is possible that she continues doing so during future reproductive events, despite any change in selection pressure to do so.

How do nesting females ‘decide’ whether or not to guard their eggs during incubation? Interactions with predators during ontogeny cannot play a role – our lizards raised in captivity never came in contact with any potential predators until after we released them as adults. Because some of these naïve individuals expressed maternal care, individual experience with predators immediately before nesting (combined with the propensity for individuals to learn) is a likely mechanism. However, we cannot discount the fascinating possibility that lizards are eavesdropping on the nest-guarding of conspecifics and using the social behaviour of others as a cue to their own behaviour. Further experiments are necessary to disentangle these hypotheses.

The Orchid Island females that expressed maternal care on SanDiMen remained only briefly at the nest-site (1–5 days) as compared with those located on Orchid Island (8–35 days). We suspect that this (and our decreased ability to locate these females; Table 1) is because these females are unable to recognize many of the new predators they could be encountering there (e.g. Downes & Adams 2001). The only lizard predator on Orchid Island is the snake Elaphe carinana, which long-tailed skinks readily recognize and flee from (Huang & Wang 2009). By contrast, SanDiMen supports five additional saurophagous snakes in three families (Colubridae, Elapidae and Viperidae). Thus, we suspect that females attempting to stay at the nest-site to provide maternal care at SanDiMen either abandon their nest after encounters with these predators (sensu Huang & Wang 2009) or become depredated.

Over a 4-year period, we found substantially more egg-eating snakes on Orchid Island than elsewhere, and these snakes were responsible for higher egg predation rates (Fig. 1). One of the main ecological characteristics associated with parental care is the low survival rates of eggs in the absence of care (Clutton-Brock 1991; Klug & Bonsall 2009), which is in line with the interpopulation differences we observed. When female long-tailed skinks guard their nests on Orchid Island, they are extremely effective at deterring egg-eating snakes (which pose no threat to the mother lizard; Huang 2006a; Supporting Information S1). This seems to contradict the notion that egg-eating snakes are responsible for the seemingly high predation levels on Orchid Island, where females should be able to protect their eggs from predation. However, on Orchid Island, snake abundances were 9–20 times higher than elsewhere, but egg predation was only twice that of other populations (Fig. 1). Hence, nest-guarding females are quite effective at deterring these snakes, in addition to the snakes relying on additional food sources (e.g. sea turtle eggs; Huang et al. 2011). Most instances of egg-eating snakes consuming clutches on Orchid Island can be narrowed down to (i) snakes predating eggs when females are foraging away from the nest (Huang 2007), (ii) lizard predators (e.g. the snake Elaphe carinata) scaring the female away (Huang 2006a) or (iii) the female abandoning the nest prior to the eggs hatching (Huang 2007).

We found low levels of genetic diversity among populations, with an absence of any defined structure between Orchid Island and the other populations (Fig. 3). Clearly, this indicates that the observed phylogeographical pattern of long-tailed skinks on these islands is very recent and that there are no distinct genetic lineages present on the different islands. The lack of phylogeographical structure could be due to the recent colonization of these islands after the LGM, which substantially altered regional fauna and flora distributions in the region (Dynesius & Jansson 2000; Harrison et al. 2001). Global cooling during the LGM could have eliminated long-tailed skinks from the region, which subsequently recolonized the area following warming after the LGM. Reptiles can travel long distances across oceans by rafting on floating debris, which may have aided recolonization (Calsbeek & Smith 2003; Vidal et al. 2008). This has been reported in other Taiwanese reptiles (Lin et al. 2008) and is consistent with long-tailed skinks reaching their northernmost range limit in southern Taiwan (Shang, Yang & Li 2009). Fast, adaptive genetic changes have been documented in the wild (Hairston et al. 2005), and thus, we cannot discount the possibility that the differences in parental care expression among populations has a genetic component. Additional population genetic studies are needed to infer migration and colonization scenarios. According to current knowledge of this lizard and available genetic evidence, this behaviour must have evolved within a relatively short time span. The results from our reciprocal translocation of females raised in captivity, along with predator abundances, are consistent with the hypothesis that predators play a major role in the expression of parental care.

Our results – the lack of any discernable genetic differences among populations, wild-caught female lizards expressing parental care in line with their population of origin, some captive-reared lizards adopting parental care expression of the recipient population, and predators and predation rates being higher in the population with maternal care – provide compelling evidence that predation has played a major role in the evolution of parental care expression in a single population of this widespread species. Why there are substantially more egg-eating snakes on Orchid Island as compared with other populations is not entirely clear, but further ecological studies may help elucidate this. This is one of only a few species known to contradict the general paradigm that parental care is an evolved response consistently expressed throughout the entire range of a species (even though the type and duration of parental care can vary geographically) and shows that parental care can be a behaviourally plastic trait among individuals and populations. Although this pattern may be more common than reflected by the literature, virtually all endotherms provide parental care (Clutton-Brock 1991; Klug & Bonsall 2009), so ectotherms are most likely to display this pattern (e.g. our study; Martins, Pombal & Haddad 1999). Given the overall paucity of research on parental care in ectotherms (especially terrestrial ectotherms; Tallamy 1984; Shine 1988; Clutton-Brock 1991; Stahlschmidt 2011), the lack of additional reports is unsurprising.

Acknowledgements

We thank C.H. Chang, R.K. Lee, Y.B. Lin and E.A. Roznik for assistance. The Kuo Wu Hsiu Luan Culture and Education Foundation and the National Science Council of Taiwan (NSC 99-2621-B-178-001-MY3) provided funding. Animal ethics permits were issued by the Taiwanese National Museum of Natural Science (NMNSHP02-002).

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