Lizard fertility and reproduction

By Paul Callister and Sahra Kress , August 2021

Over recent years Ngā Uruora has been trying to support lizard populations on the escarpment. Through our lizard protection trial, we have undertaken research on the best way of controlling pests, including mice. We have also endeavoured to support populations in the quarry site through creating lizard friendly habitat and establishing plants that we hope will provide food and shelter.

Pest control and habitat are clearly important. But for lizards to thrive, populations need to increase. This requires successful breeding. However, as a group we know very little about lizard reproduction. Increased knowledge is important for conservation efforts. If breeding is not successful translocations will not achieve the desired goal. The decline of a species could be the result of many factors, but if fertility and breeding is compromised this is important to know. For a variety of reasons, including their very cryptic behaviour, relatively little is known about New Zealand lizards. Clearly more research is needed.

Drawing on her expertise as a midwife, local volunteer Sahra Kress has been looking into this subject.

Here are some of the interesting facts Sahra found out:

Lizard Reproduction:

• Viviparity (giving birth to live young) applies to 99% of NZ lizard species.
• Pregnancies usually last about 3 months, but are temperature dependent and may reach as long as 14 months in some geckos.
• Each conceptus starts with a yolk mass complex, which then develops into a placenta.
• Both skinks and geckos significantly increase their body mass index during pregnancy, assumed largely to be associated with water uptake through the placenta.
• In the wild, there are high rates of failure in embryonic development, resulting in high numbers of abortions and ‘stillbirths’, as well as over-gestated foetuses. These result from various (speculated) causes.

Females:

• Mating usually occurs in late summer.
• Gestation length is reduced with increased basking opportunities during pregnancy.
• Duvaucelii pregnancies potentially last longer than a year, so have less-than-annual reproduction.
• Unlike geckos, no skinks are known to continue pregnancies over winter.
• Peri-ovulatory (fertile) females have much higher estrogen and progesterone levels than non-reproductive females.
• Some NZ geckos have the intriguing feature of being able to resume a substantial amount of vitellogenesis (yolk formation) before the end of pregnancy. This ability is rare, if not unique, among viviparous lizards (confirmed by dissection in captivity of Woodworthia females).
• Females may store sperm internally for about 6 months prior to ‘conception’.

Males:

• Levels of promiscuity may be lower for New Zealand geckos relative to overseas squamates.
• Peak mating season is during summer and autumn.
• No major seasonal changes.

Sexual Dimorphism:

• At birth, New Zealand lizards show no distinguishing differences between males and females.
• This lack of obvious dimorphism contrasts with many lizards overseas.
• Due to incomplete sexual differentiation at birth, sex is often incorrectly assigned.
• There is no major size difference between male and female adults.
• Sexual colour differences are generally modest or lacking.

Age at Maturity and Lifespan:

• Longevity is associated with infrequent reproduction, higher latitudes, cooler temperatures.
• Most New Zealand geckos take between 2-8 years to reach maturity.
• Duvaucelii take 7 years to mature, live up to 50 years.
• New Zealand lizards have unusual features of reproduction and lifespan when compared with lizards generally. Pregnancies last longer, maintenance of offspring in utero occurs over winter, and geckos ability to overlap reproductive cycles are unique features. This is likely due to our cooler climate.
• There is also increasing evidence that certain species of NZ lizards show some form of parental care, or at least tolerance of their young. Some live in family groups in the wild.

Some of the key references for this research are:

Cree A., Hare K.M. (2016) Reproduction and Life History of New Zealand Lizards. In: Chapple D. (eds) New Zealand Lizards. Springer, Cham. https://doi.org/10.1007/978-3-319-41674-8_7

Winkel, D. van, Baling, M. & Hitchmough, R. (2019). Reptiles and amphibians of New Zealand: a field guide. Auckland University Press.

Department of Conservation: https://www.doc.govt.nz/nature/native-animals/reptiles-and-frogs/lizards/

The New Zealand Herpetological Society: https://www.reptiles.org.nz/