Tuatara: Volume 31, Issue 1, July 1991
Nesting and Clutch Size of Tuatara (Sphenodon Guntheri) on North Brother Island, Cook Strait
Nesting and Clutch Size of Tuatara (Sphenodon Guntheri) on North Brother Island, Cook Strait
Nesting activity of a rare species of tuatara (Sphenodon guntheri) was observed on North Brother Island, Cook Strait, New Zealand, during 9-11 November 1989. Thirty-three nest-diggings were observed on sunny, unvegetated sites, especially on walking tracks. Three females were observed digging nests at night, but no completed nests or nest-guarding females were seen. Forty-one adult female tuatara were given an injection of oxytocin (1 IU/100 g body weight) to induce oviposition of eggs for captive incubation. Eight females (19.5%) laid eggs in response, suggesting that as in the more common species S. punctatus, individual female S. guntheri do not nest each year. Adult female S. guntheri are smaller than are female S. punctatus on nearby Stephens Island and have a smaller mean clutch size (X ± 1 SE = 6.5 ±0.6 eggs). However, the mean weight of freshly oviposited eggs of S. guntheri (4.9 ± 0.2 g) is similar to that of S. punctatus on Stephens Island.
Key words: Tuatara, Sphenodon guntheri, nesting, North Brother Island.
Tuatara survive on about 30 islands off the New Zealand coast (Daugherty et al., 1990), and until recently all populations were regarded as belonging to the species Sphenodon punctatus. The population on tiny (4 ha), windswept North Brother Island in Cook Strait was once recognised on morphological grounds as a separate species, Sphenodon guntheri, by Buller (1877). This and other described species of tuatara were later reduced to sub-specific status by Wermuth and Mertens (1977), although Dabwin (1982) regarded even these sub-specific rankings as of no biological significance. M.B. Thompson et al. (unpubl. obs.) surveyed the North Brother Island population in 1988 and estimated that about 300 adults were present; seven juveniles were also observed, indicating that successful breeding was occurring.
Recently, we examined the variation in blood allozymes among 24 populations of tuatara, and found the population on North Brother Island to be sufficiently genetically distinct from all others that we recommended it be reinstated as the species S. guntheri (Daugherty et al., 1990). The presence of lighthouse keepers has deterred illegal landings and possible tuatara poaching or rodent introduction on North Brother Island since 1877, but automation of the lighthouse in mid–1990 makes this species more vulnerable to possible extinction. We therefore embarked on a captive egg incubation programme that aims to raise juveniles of this species in captivity in order to found new wild and captive populations. While collecting eggs for this programme, we made the following observations on nesting activity and oxytocin-induced oviposition on North Brother Island. These observations increase our knowledge of the natural history of S. guntheri and are useful for future management.
North Brother Island (174° 27′ E, 41° 07′ S; Fig. 1) lies in eastern Cook Strait about 35 km northwest of Wellington and 43 km northeast of Picton. It is bounded page 10 by precipitous cliffs on the western, southern and eastern flanks and rises to a height of 79 m. The northern face, although steeply-sloping, is the most accessible on the island. It consists of bare rock and low coastal vegetation (primarily taupata = Coprosma repens, and iceplant = Disphyma australe). Tuatara are most abundant on this face. The southwestern face is also steep but accessible, but is heavily burrowed by nesting seabirds and few tuatara are found in this area. The island has been a manned lighthouse station since 1877, and in addition there are walking tracks, a railway track, a helicopter pad and several buildings on the island.
Observations on nesting activity
We visited the island from 9-12 November 1989. Nesting of S. punctatus on Stephens Island (about 62 km to the northwest) is concentrated between early November and mid-December (A. Cree, L.J. Guillette Jr and M.B. Thompson, unpubl. obs.), and nesting has been observed during the same time on North Brother Island (M.B. Thompson et al., unpubl. obs.; B. Blanchard, pers. comm.). Air temperature at 2100 h NZ Standard Time and total rainfall during our visit were obtained from the resident lighthouse keepers and the New Zealand Meteorological Service.
The distribution of tuatara nesting sites on all accessible parts of the island above the cliff edges was noted by one of us (AC) on 9-10 November 1989. On Stephens Island, nesting S. punctatus spend several nights or weeks digging a nest chamber in the soil. Nest diggings can be recognised as distinct from seabird or tuatara burrows by their shallow width (about 8-10 cm) and depth (often 10 cm or less in the early phases of construction), by the freshly piled dirt outside and by the absence of bird excrement or feathers at the entrance. Only excavations that we confidently considered tuatara nest diggings were recorded.
On the nights of 9-11 November 1989, we searched between 2000-2400 h New Zealand Standard Time (NZST) for female tuatara on accessible parts of the island between the lighthouse and the northern end of the island. The southern face was not searched, as few tuatara live in this area (M.B. Thompson et al., unpubl. obs.). Observations on activity of nesting females were made at this time. In addition, all adult females (see below) that we could capture were returned to the visitors' dormitory on the island, measured (snout-vent length), weighed, and given an injection of oxytocin to induce egg-laying. Oxytocin is a mammalian neurohypophysial hormone that stimululates uterine contractions; it readily induces oviposition in gravid S. punctatus on Stephens Island (Thompson et al., 1990) and is more readily available and less expensive than the natural reptilian equivalent (arginine vasotocin; AVT).
On Stephens Island, we have observed that female S. punctatus of 172 mm SVL or greater may be gravid and that adult females typically have a more pear-shaped abdomen than juveniles. S. guntheri on North Brother Island are smaller in general than Stephens Island S. punctatus, and appear to mature at a smaller size. Based on abdomen shape, we considered female S. guntheri of 163 mm SVL and above as likely to be mature and therefore worth treating with oxytocin. However, we cannot rule out the possibility that a few untreated females smaller than this might also have been mature.
The oxytocin (Oxytocin-S, 10 IU/ml, Batch 28103, Intervet International, Boxmeer, Holland) was injected intraperitoneally at a dose (1 IU/100 g body mass) that usually induces oviposition of complete clutches from gravid S. punctatus on Stephens Island during October–December (Thompson et al., 1990). On Stephens Island only about 8-28% of adult female tuatara are gravid each year (Newman and Watson. 1985; Cree et al., 1991), but unless nesting activity is observed or page 11 females are examined by techniques such as X-raying (Newman and Watson, 1985) or laparoscopy (Cree et al., 1991), it is impossible to tell with certainty which are gravid or not prior to treatment.
Treated S. guntheri were given the injection of oxytocin within 24 h of capture. They were then placed individually in cardboard boxes (approximately 38 × 53 × 23 cm deep) at 18-19°C, and checked every 15 min for 3 h for evidence of oviposition. Females that laid eggs began to do so within this time. All females were checked at least hourly for a further 3 h. The eggs were removed, weighed to the nearest 0.5 g, and half-buried in moist vermiculite (medium grade, Revertex Industries, Auckland, NZ) for return to Victoria University of Wellington. The vermiculite was prepared with 80 g distilled water per 100 g of vermiculite, and had a water potential of −430 kPa (determined using a Wescor HR-33T dew point microvoltmeter, Wescor, Logan, Utah). All females were returned unharmed to their capture site within 43 h of collection.
The relationship between female SVL and clutch size of S. guntheri was also compared with that previously observed for S. punctatus on Stephens Island. Data for S. punctatus were obtained by X-raying 62 gravid females during October–November 1987 (A. Cree, L.J. Guillette Jr, D.G. Newman and P.R. Watson, unpubl. obs.). A regression line for the Stephens Island data was fitted using a STATGRAPHICS® package (Statistical Graphics Corporation, Rockville, MD) on a personal computer (Commodore PC III-40, Commodore Business Machines, Auckland). The variances of SVL and clutch size were compared between the two species using an F-test for homogeneity, and then an appropriate form of Student's t-test was used to compare mean values (Sokal and Rohlf, 1981). Differences were considered significant when P<0.05).
Mean egg weights were also compared between the two species. Mean egg weights for S. punctatus from Stephens Island were obtained by injecting AVT (2-20 ng/g body weight) into eight of the females X-rayed in November 1987 (L.J. Guillette Jr, A. Cree, and M.B. Thompson, unpubl. obs.). Results are reported as mean ±1 SE.
Thirty-three tuatara nest-diggings were noted on North Brother Island (Fig. 2). All were located on the eastern, northern or western faces and none was seen on the southern. Nest-diggings were always in sunny, open (unvegetated) patches of soil or soil/gravel. Such areas are in short supply on this rocky island, and all but seven of the nest-diggings were on cleared areas associated with lighthouse operations (e.g., walking tracks, and the verges of the railway track and helicopter pad).
Most nest-diggings were in the early stages of excavation (less than 10 cm deep, Fig. 2; fully excavated nests on Stephens Island are 10-30 cm or more; A. Cree, unpubl. obs.). Several diggings on North Brother Island appeared to have been abondoned at shallow depths (5-10 cm) when rock had been encountered. One nest digging had a desiccated egg outside the entrance. This egg had probably been laid the previous year, and its presence implies that the same nest site had been used two years in a row.
Three female S. guntheri were observed at night digging their nest chambers. On Stephens Island, female S. punctatus return to their completed nests at night for up to a week following oviposition (Cree and Thompson, 1988; Guillette et al., 1990), but we observed no ovipositing females, nest-attending females or completed nests on North Brother Island. Air temperatures at 2100 h NZST varied between 15-16°C, and no significant rain (<0.5 mm) fell during our visit.
Gravidity rate, clutch size and egg weight
Eight of 41 female S. guntheri from North Brother Island laid eggs in response to an injection of oxytocin to induce oviposition (Table 1). This indicates a gravidity rate among the female population of at least 19.5%. Females that oviposited showed no obvious pelvic contractions in response to oxytocin. Oviposition itself was accomplished with the tail lifted at the base and with the hindlimbs usually directed posteriorly. Females paid no obvious attention to oviposited eggs. Oviposition began 30-90 min after treatment (mean time to laying of the first egg was 57±7 min) and had ceased by 5.75 h after treatment. The smallest female to oviposit was 185 mm SVL. In total, 52 eggs were laid: the eggs were ovoid, white and soft-shelled, and most (45/52 = 86.5%) appeared completely calcified. Eggs ranged in mass from 3.0-5.8 g (mean 4.9±0.2 g) and clutch sizes ranged from 4-8 (mean 6.5±0.6).
On Stephens Island, 62 gravid S. punctatus showed a positive correlation between SVL and clutch size (Fig. 2; clutch size = −12.39 + 0.11SVL (in mm), r = 0.55, P<0.01). Only eight gravid S. guntheri of a narrow size range were collected on North Brother Island, and thus we had insufficient data to calculate a regression between SVL and clutch size for this population. However, the mean SVL of the eight gravid S. guntheri from North Brother Island (192±2 mm) differed significantly from that for the 62 female S. punctatus from Stephens Island (204±2 mm; t = 4.757, 7 d.f., P<0.001), and mean clutch size of S. guntheri (6.5±0.6 g) also differed significantly from that for S. punctatus (10.2±0.3 g: t = 5.663, 68 d.f., P<0.001). Nevertheless, although gravid S. guntheri are small compared with many gravid S. punctatus on Stephens Island, their clutch sizes are within the 95% prediction limits for Stephens Island S. punctatus of the same SVLs (Fig. 3).
Mean egg weight for the eight female S. guntheri was compared with that for a group of eight Stephens Island S. punctatus that were induced to oviposit (a sub-sample of the X-rayed females referred to in Fig. 3; see Table 1). As expected, female S. guntheri were significantly smaller and had a smaller mean clutch size than did S. punctatus, but mean egg weights did not differ significantly between the two species (t = 1.951, 14 d.f., P>0.05).
Nesting of S. guntheri on North Brother Island (present study; M.B. Thompson et al., unpubl. obs.) shows many similarities with previous observations of nesting of S. punctatus on Stephens Island (Cree and Thompson, 1988; Guillette et al., 1990). In both species, nest-digging is evident in early-mid November, females sometimes re-use the same sites from year to year, and nesting is concentrated in open, sunny locations.
Habitat modification associated with lighthouse operations appears to have provided new nesting habitats on both islands. Stephens Island was apparently forested above the clifftops until the establishment of a lighthouse in 1894, and in the late 1890s nesting was observed on the open clifftops and in clearings and tracks associated with lighthouse operations (Schauinsland, 1898; Thilenius, 1899). We have since observed females aggregating in large numbers to nest in sheep pasture on Stephens Island (Cree and Thompson, 1988). Warm soil temperatures are probably an important factor explaining the choice of open, sunny sites on both Stephens and North Brother Islands. On Stephens Island, eggs placed in artificial nests in the pasture develop more rapidly than those in artificial nests in the remnant forest, where soil temperatures are cooler (Cree et al., 1989).
Tuatara have a long lifespan on both Stephens Island (at least 60 y; Castanet et al., 1988) and on North Brother Island (where adults may live without growing page 13 for at least 30 y; M.B. Thompson et al., unpubl. obs.). Females of both species also have a low rate of reproduction. For S. punctatus, between 8-28% were gravid each year between 1982-1988 on Stephens Island (Newman and Watson, 1985; Cree et al., 1991) and a similar gravidity rate (23.3%) was observed in 1983 on Lady Alice Island in the Hauraki Gulf (Newman and Watson, 1985). Our gravidity rate estimated for S. guntheri in 1989 from the success rate with oxytocin injections (19.5%) is similar, although we cannot rule out the possibility that it is slightly lower than the true gravidity rate because we did not confirm that all females not responding to oxytocin treatment were non-gravid. However, on Stephens Island clutch sizes estimated from oxytocin injection experiments do not differ significantly from those obtained using radiography, suggesting that most gravid S. punctatus lay complete clutches in response to this dose of oxytocin (Thompson et al., 1990). In any case it is clear that, like other female tuatara, female S. guntheri do not nest each year.
Female S. punctatus on Stephens Island showed a significant positive correlation between SVL and clutch size, as previously reported for this population (Newman and Watson, 1985). Correlations between female SVL (or weight) and clutch size are common in reptiles, having been reported for crocodilians (Ferguson, 1985), turtles (Wilbur and Morin, 1988) and squamates (Case, 1982). Comparisons between mean clutch sizes of S. guntheri and Stephens Island S. punctatus are consistent with this trend: female S. guntheri lay smaller clutches, but are also smaller in SVL. Whether the generally smaller body size of female S. guntheri is related to genetic or environmental differences is unknown. Interestingly, mean egg weights did not differ significantly between the two species, suggesting that egg weight may not vary with SVL to the same extent that clutch size does.
When the lighthouse on North Brother Island was automated in 1990, the island became uninhabited. This increases the vulnerability of S. guntheri to illegal collection and rodent introduction. Will automation also have specific effects on tuatara nesting? In the event that walking tracks and trolley tracks are not maintained and become overgrown with vegetation, the availability of nesting sites may decrease. This may lead to a gradual, long-term reduction in numbers of S. guntheri. We recommend that long-term monitoring of nest sites and population size be carried out on North Brother Island. Given the small population size of this species, artificial maintenance of open nesting areas may be a possible future management strategy to maintain current numbers.
We thank lighthouse keepers G. Bohm, I. Hargreaves, B. Meginnis, C. Wallace and T. Watson for cordial assistance, D. Williamson and the crew of H.M.V. “Enterprise” for seeing us safely ashore, F. Cook and G. Lilley for assistance with measurements of water potential, and the Department of Conservation (especially K. Johnson) and the Ministry of Transport (especially B. Rees) for permission to carry out this study. AC thanks her colleagues L. Guillette, D. Newman, M. Thompson and P. Watson for comments on the ms and permission to cite unpublished observations from work on S. punctatus on Stephens Island. This study was supported by grants from the NZ Department of Conservation, NZ Lottery Board, Victoria University of Wellington, WWF-NZ and the Zoological Society of San Diego.
Buller, W.L. 1877: Notes on the tuatara lizard (Sphenodon punctatum), with a description of a supposed new species. Transactions and Proceedings of the New Zealand Institute 9: 317-325.
Case, T.J. 1982: Ecology and evolution of the insular gigantic chuckawallas, Sauromalus hispidis and Sauromalus varius. Pp. 184-212. In Burghardt, G.M. and Rand, A.S. (eds.). Iguanas of the World. Noyes, Park Ridge, New Jersey.
Castanet, J., Newman, D.G. and Saint-Girons, H. 1988: Skeletochronological data on the growth, age, and population structure of the tuatara, Sphenodon punctatus, on Stephens and Lady Alice Islands, New Zealand. Herpetologica 44: 25-37.
Cree, A. Thompson, M.B., Guillette, L.J. Jr, Hay, J.M. and McIntyre, M.E. 1989: Embryonic development of tuatara in forested and open habitats on Stephens Island, New Zealand. New Zealand Journal of Zoology 16: 270 (abstract).
Cree, A., Cockrem, J.F., Brown, M.A., Watson, P.R., Guillette, L.J. Jr, Newman, D.G. and Chambers, G.K. 1991: Laparoscopy, radiography and blood analyses as techniques for identifying the reproductive condition of female tuatara. Herpetologica 47: 238-249.
Dawbin, W.H. 1982: The tuatara Sphenodon punctatus (Reptilia: Rhynchocephalia): a review. Pp 149-181. In Newman, D.G. (ed.). New Zealand Herpetology. New Zealand Wildlife Service Occasional Publication No. 2.
Ferguson, M.W.J. 1985: Reproductive biology and embryology of the crocodilians. Pp. 329-491. In Gans, C., Billett, F. and Maderson, P.F.A. (eds.). Biology of the Reptilia Vol. 14. Wiley, New York.
Guillette, L.J. Jr, Cree, A. and Gross, T.S. 1990: Endocrinology of oviposition in the tuatara (Sphenodon punctatus): I. Plasma steroids and prostaglandins during natural nesting. Biology of Reproduction 43: 285-289.
Newman, D.G. and Watson, P.R. 1985: The contribution of radiography to the study of the reproductive ecology of the tuatara, Sphenodon punctatus. Pp. 7-10. In Grigg, G., Shine, R. and Ehmann, H. (eds.). Biology of Australasian Frogs and Reptiles. Surrey Beatty and Sons, Sydney.
Schauinsland, H. 1898: Beiträge zur Biologie der Hatteria. Sitzungsberichte der Akademie der Wissenschaften zu Berlin 1898: 701-704.
Sokal, R.R. and Rohlf, F.J. 1981: Biometry (2nd edn). W.H. Freeman, New York.
Thilenius, G. 1899: Vorläufiger Bericht über die Eiablage und erste Entwickelung der Hatteria punctata. Sitzungsberichte der königlich Preussischen Akademie der Wissenschaften zu Berlin 1: 247-256.
Thompson, M.B., Newman, D.G. and Watson, P.R. 1990: Use of oxytocin in obtaining eggs from tuatara, Sphenodon punctatus. Journal of Herpetology. In press.
Wermuth, H. and Mertens, R. 1977: Liste der rezenten Amphibien und Reptilien. Testudines, Crocodylia, Rhynchocephalia. Das Tierreich 100: 1-174.
Wilbur, H.M. and Morin, P.J. 1988: Life history evolution in turtles. Pp. 387-439. In Gans, C. and Huey, R.B. (eds.). Biology of the Reptilia Vol. 16. Alan R. Liss, New York.
Table 1. Mean (± SE) snout-vent length (SVL), mean clutch size (number of eggs per clutch) and mean egg weight in female tuatara induced to oviposit following treatment with neurophypophysial hormones in November. Female S. guntheri were treated in 1989 with oxytocin (1 IU/100 g body mass) and female S. punctatus from Stephens Island were treated in 1987 with arginine vasotocin (2-20 ng/g body mass).
|S. guntheri (North Brother I.) (n = 8)||192 ± 2a||6.5 ± 0.6a||4.9 ± 0.2a|
|S. punctatus (Stephens I.) (n = 8)||206 ± 3b||9.9 ± 1.2b||4.4 ± 0.2a|
a,b Mean values in each column, with different superscript letters, differ significantly (P<0.05).
Fig. 3. Relationship between snout-vent length (SVL) and clutch size for 62 female S. punctatus from Stephens Island (open or closed circles) and eight female S. guntheri from North Brother Island (open triangles). Data for the eight S. punctatus that were treated with AVT (Table 1) are shown with open circles. Solid line = regression line fitted for S. punctatus data; dashed line = 95% prediction limits.
1 School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand.
4 Present address: Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand.
2 Zoological Society of San Diego, PO Box 551, San Diego, CA 92112-0551, U.S.A.
3 Nelson-Marlborough Conservancy, Department of Conservation, Private Bag, Picton, New Zealand.