Abstract We studied a population of large varanid lizards (yellow-spotted monitors Varanus panoptes) on a floodplain in tropical Australia. Growth records from radio-tracked lizards show that despite their large adult body sizes (to > 7 kg in males), these lizards attained sexual maturity at less than one year of age, and rarely lived for more than two years (females) or four years (males), even before mortality increased due to the arrival of toxic cane toads. This is a “faster” life-history than has been reported for other species of large monitors. Growth was especially rapid in males during the wet-season. The low survivorship prior to toad invasion was due to predation by pythons; communal nesting by female varanids may render them especially vulnerable. The life history of yellow-spotted monitors requires high feeding rates, favouring the evolution of “risky” tactics such as consuming novel prey items (such as cane toads); and the combination of high abundance (> 20 adult lizards per square kilometre) and high feeding rates (> 9.9 kg of prey per lizard per annum) means that these giant lizards play a critical role in energy and nutrient flow within the floodplain ecosystem. As a result, foodwebs with the yellow-spotted monitor as an apex predator are more vulnerable to disruption by cane toads than is the case in other parts of the toad’s invasive range, where the varanid species affected by toads have “slower” life histories.

Data collection:

Radio telemetry

Between November 2013 and January 2016 we radio‐tracked 110 yellow‐spotted monitors (Female lizards n= 52; Male lizards n= 58). During 15 three-week-long field trips, we searched for monitors between 0500 h and 1100 h each day; hence, new individuals were recruited to the study through time. By collaborating with indigenous rangers, we were able to collect lizards exhibiting a wide array of behavioural phenotypes (Ward-Fear et al. 2018, 2019).

Monitors were captured by hand and transported back to the research station where we recorded snout‐to‐vent length (SVL) and body mass and took tissue samples from the tail tip for genetic sex determination (see Appendix for methodology). We attached a Very High Frequency (VHF) radio transmitter to the tail of each monitor (Holohil RI‐2B, 15 g, < 5% total body mass) following the methods of Madsen and Ujvari (2009) and released the lizard back into the field at its point of capture within 6 h; telemetry began three days post‐release. We tracked monitors at least twice per field trip (but also opportunistically) for as long as they were alive and could be located (mean number of observations per individual 12.2, range 5–40 observations). Throughout the study we opportunistically recorded information on behaviour, ecology and life-history attributes of the animals.

Calculation of growth rates and ages

We calculated growth rates using repeated measurements of snout-to-vent length (SVL) and body mass (g) taken on 17 males, 11 females and 4 hatchlings (total N = 32). Larger individuals were measured every three to six months, whereas juveniles were measured as often as possible. From the SVL measurements we calculated a von Bertalanffy (1957) curve to estimate growth rates (size vs age)

Reproduction and mortality

Over the course of the study we made 1,172 observations of radio-tracked monitors, only 7 of which were opportunistic encounters (i.e., a telemetered lizard was rarely sighted except by following the signal from its transmitter) plus 256 sightings of non-telemetered animals. We inferred sexual maturity for lizards seen mating or engaged in courtship, or seen at nesting warrens (GWF, unpubl. data). Gravid females were readily identifiable by their distended abdomens. We witnessed 27 instances of radio-telemetered individuals with mates, one full sequence of courtship then mating (lasting at least 4 days), 13 instances of females nesting and an additional 18 instances of individuals thought to be engaging in mating and/or nesting in ‘warren’ burrow systems. We also recorded 25 predation events on yellow-spotted monitors by pythons.

Energy requirements and rates of prey consumption

To estimate the annual rate of prey consumption by varanid lizards at Oombulgurri, we created energy budgets both on a per-individual basis and also for the annual offtake of prey by yellow-spotted monitors at our study site. Overall energy expenditure by an individual goanna during its annual period of activity (kJ per kg of goanna per day/month) was estimated as the sum of:

(1) metabolic expenditure, from data on body mass (current study) combined with previous studies on mass-specific metabolic rates of active and inactive V. panoptes in a climatically similar site (Christian et al. 1995);

(2) allocation of energy to biomass growth, from growth rates in mass (calculated in the current study) combined with data on the energetic content of lizard tissue (Peterson et al. 1999); and

(3) (for females only) allocation of energy to a clutch of eggs, based on mean clutch sizes and egg masses (from 6-14 eggs at 30-80gm each) and caloric content of eggs (5.8cal/mg – 7.2 cal/mg Tinkle 1975; Angilleta et al. 2001).

Statistical Analysis:

Normality and homogeneity of variance were confirmed for all variables. All continuous variables (age, number of days tracked, SVL, mass etc.) were transformed with natural log (Ln) prior to analyses.

Seasonal growth rates - we ran full factorial ANOVAs with growth rates (i.e., the increase in mass between successive captures, standardised as gm/day) as our dependent variable, and independent variables of sex, season (wet = November to April; dry = May to October) and the interaction between the two.

Reproductive age and survivorship - we analysed data on the age of all telemetered individuals that were recorded as mating or nesting during the study. The dependent variable in our ANOVA was the inferred age of the animal at the time of mating (obtained from the von Bertalanffy growth curve at initial capture and adding number of days tracked to that point) and the independent variable was sex (male/female). To explore sex differences in survivorship, we conducted ANOVAs to compare the mean number of days animals were tracked alive and the inferred ages of those animal at their time of death.

We then created a population profile by calculating probable age at first capture from body size (using the von Bertalanffy growth curve then adding the number of days that the animal was tracked until it died). We used ANOVA to compare the average ‘ages’ of males and females within the population, as well as the distribution of age classes between the sexes.