C. Michael Bull

Network structure and parasite transmission in a group living lizard, the gidgee skink, Egernia stokesii

Gidgee skinks (Egernia stokesii) form large social aggregations in rocky outcrops across the Flinders Ranges in South Australia. Group members share refuges (rock crevices), which may promote parasite transmission. We measured connectivity of individuals in networks constructed from patterns of common crevice use and observed patterns of parasitism by three blood parasites (Hemolivia, Schellackia and Plasmodium) and an ectoparasitic tick (Amblyomma vikirri). Data came from a 1-year mark-recapture study of four populations. Transmission networks were constructed to represent possible transmission pathways among lizards. Two lizards that used the same refuge within an estimated transmission period were considered connected in the transmission network. An edge was placed between them, directed towards the individual that occupied the crevice last. Social networks, a sub-set of same-day only associations, were small and highly fragmented compared with transmission networks, suggesting that non-synchronous crevice use leads to more transmission opportunities than direct social association. In transmission networks, lizards infested by ticks were connected to more other tick-infested lizards than uninfected lizards. Lizards infected by ticks and carrying multiple blood parasite infections were in more connected positions in the network than lizards without ticks or with one or no blood parasites. Our findings suggest higher levels of network connectivity may increase the risk of becoming infected or that parasites influence lizard behaviour and consequently their position in the network.

Monogamy in lizards

Monogamy is relatively rarely reported in taxa other than birds. The reproductive system of many lizard species appears to involve multiple mating partners for both the male and the female. However, short-term monogamous relationships have been reported in some lizard species, either where the male defends a territory that is only occupied by a single adult female, or where males stay with females for a period of time after mating, apparently to guard against rival males. There are a few reported cases of more prolonged monogamous relationships in lizards, with the Australian sleepy lizard, Tiliqua rugosa, the best studied example. Adult males and females of this species form monogamous pairs for an extended period before mating each spring, and they select the same partner in successive years. The paper reviews possible functions of monogamy in this and other lizard species, and suggests that the additional perspective from studying lizards may enrich our overall understanding of monogamous behaviour.

Social monogamy and extra-pair fertilization in an Australian lizard, Tiliqua rugosa

Abstract This study investigates social monogamy in the Australian sleepy lizard, Tiliqua rugosa. At a 70-ha site near Mount Mary, South Australia, we radio tracked 55 adult female and 39 adult male lizards during their spring activity periods. Each lizard was observed in 1–5 years. Females were observed with a single male partner on an average of 10.8 days per year, although in 17.3% of cases, females were observed on 2 or fewer days with a male. The most intense pairing period each year was 15 September–15 November when females were with male partners on an average of 36% of observation days. Partnerships lasted an average of 43.3 days each year. After mating in early November, the pairs separated. Observations of females pairing with other males were rare. Most males (82%) were also consistently monogamous, although 7 were observed pairing with 2 females within one season. To investigate paternity, we allowed 21 gravid females to give birth to 42 offspring in the laboratory. We determined genotypes at five polymorphic microsatellite DNA loci for the females, their male partners and their offspring. Four litters (19%) and 6 of the offspring from those litters (14.3%) showed evidence of extra-pair fertilization (EPF). Although the sample sizes are small, females of polygynous males were more likely to experience EPF.

Adding burrows to enhance a population of the endangered pygmy blue tongue lizard, Tiliqua adelaidensis

The endangered pygmy blue tongue lizard, Tiliqua adelaidensis, occupies spider burrows in a population near Burra, South Australia. In each of twelve 20 × 20 m plots at that population, we added 36 artificial burrows, providing about a ten fold increase in suitable burrows for lizards. Over three surveys during the spring and summer of 2001/2002 there were significant increases in lizard numbers in the experimental plots relative to the controls, both for adult lizards and for new recruits after clutches were produced. This local increase in population density may be due to lizards locating suitable burrows more easily where burrow numbers were supplemented. The increased availability of high quality burrows may also reduce mortality among lizards searching for suitable burrows. Additional burrows also led to an increase in local density of a burrow-dwelling centipede that is a potential predator of lizards, but there was no evidence of predation, and any negative predation impact was outweighed by the beneficial effect of providing more burrows. Overall the results suggest that adding artificial burrows could enhance local population density and recruitment success, and that this could be a valuable tool in the conservation management of this endangered species.

Home range overlap of mothers and their offspring in the sleepy lizard, Tiliqua rugosa

Abstract This paper reports a field investigation of interactions between juveniles and their mothers in the Australian sleepy lizard, Tiliqua rugosa. In their first spring season, juvenile lizards maintain home ranges largely within the home range of their mother. Juvenile home ranges are significantly smaller than those of adult males and females, and juveniles move significantly less often and significantly shorter distances than adults. While siblings were never found together in the spring, they showed a significant tendency to be closer to each other than if they were randomly located in their home ranges. Juveniles and mothers were never found together, nor was there any evidence for any positive (or negative) spatial association. Nevertheless, the extended tolerance of home range overlap represents a greater degree of mother-offspring association than has been previously reported for other lizards. Despite this, the level of parental care can only be described as minimal.

Discrimination between related and unrelated individuals in the Australian lizard Egernia striolata

Abstract. We examined evidence for kin-related signal discrimination in the Australian scincid lizard Egernia striolata, using 41 juvenile lizards from 14 litters from a population in the Mount Lofty Ranges, South Australia. We characterised each juvenile at four polymorphic microsatellite DNA loci, determined genetic relatedness among pairs of juveniles, and assigned for each litter one other litter that was genetically closest and one other litter that was genetically most distant. Lizards responded significantly more strongly, with tongue flicks and time in contact, to skin secretion stimuli from more distant than genetically close, unfamiliar conspecific juveniles. When presented with an intruder, juvenile lizards basked for shorter periods when the intruder was more distantly related, and aggressive lizards directed more aggressive acts towards more distantly related than closely related conspecific intruders. We deduce from these results that there is kin recognition in juvenile E. striolata, and that they are able to discriminate related from unrelated individuals among unfamiliar conspecifics.

Burrow choice by individuals of different sizes in the endangered pygmy blue tongue lizard Tiliqua adelaidensis

The pygmy blue tongue lizard Tiliqua adelaidensis had been considered extinct until its recent rediscovery near Burra in the mid-north of South Australia. The lizards apparently rely on spider burrows as refuge sites. In this paper we describe the dimensions of burrows occupied by lizards. Larger lizards occupy deeper burrows with wider entrance diameters than do smaller juvenile lizards. When offered a choice of artificial burrows in the field, all lizards preferred vertical over angled burrows, while adult lizards preferred deeper burrows than did juveniles. In laboratory trials with artificial burrows of equal depth, lizards tended to prefer the narrowest burrow they could fit into. We suggest that lizards are constrained by a preference for a deep burrow to reduce the risk of being dug out, and to avoid climatic extremes, and a preference for a narrow burrow in which they can block predator access. The result is that smaller lizards have different optimal burrow dimensions than larger lizards. To maintain a viable population of lizards a variety of burrow sizes is needed. Conservation strategies for this species may include maintaining a diverse community of hole digging spiders, or providing a variety of sizes of artificial burrows.

Using social networks to deduce whether residents or dispersers spread parasites in a lizard population.

1. Heterogeneity of host behaviour can play an important role in the spread of parasites and pathogens around wildlife populations. Social networks have previously been suggested to represent transmission pathways within a population, but where the dynamics of host-parasite interactions are difficult to observe, networks may also be used to provide insights into transmission processes. 2. Pygmy bluetongue lizards, Tiliqua adelaidensis, occupy individual territories, live exclusively in burrows constructed by spiders in Australian native grasslands and are hosts to a tick, Bothriocroton hydrosauri, and a nematode, Pharyngodon wandillahensis. 3. On five monthly occasions, the locations of all individual lizards in three study plots were used to construct weighted, undirected networks based on proximity of adjacent burrows. 4. The networks were used to explore alternative hypotheses about the spread of each parasite through the population: that stable population members that remained in the same burrow over the study period played a major role in influencing the pattern of infection or that dispersing individuals played a more significant role. 5. For ticks, host individuals that were infected were more connected in the network than uninfected hosts and this relationship remained significant for connections to residents in the population, but not for connections to dispersers. 6. For nematodes, infected and uninfected hosts did not differ in their overall strength of connection in the network, but infected hosts were more connected to dispersers than were uninfected hosts, suggesting that lizards moving across the population are the major agents for the transmission of nematodes. 7. This study shows how network analyses can provide new insights into alternative pathways of parasite spread in wildlife populations, where it is difficult to make direct observations of transmission-related behaviours.

Use of burrows by the endangered pygmy blue-tongue lizard, Tiliqua adelaidensis (Scincidae)

The pygmy blue-tongue lizard, Tiliqua adelaidensis, had been considered extinct until its recent rediscovery near Burra in the mid-north of South Australia. The lizards apparently rely on spider burrows with a single entrance as refuge sites. In this paper we describe observations from all-day video recordings of the entrances of burrows occupied by lizards on 31 days across the spring and early summer of 1996. The lizards spent most of each day either retreated down the burrow or basking at the burrow entrance. Early in the season, when temperatures were cooler, lizards spent more of each day basking, and more of their basking time fully emerged from the burrow, than later in the season. Results are consistent with the hypothesis that the lizards thermoregulate by moving in and out of the burrow. Lizards also fed by making short excursions from the burrow, often to catch passing invertebrate prey, although they also fed on flowers early in spring. Later in the season lizards were more likely to vacate their burrows for longer times, or to move away from their burrows, and mating activity was observed when males approached burrows occupied by females. The observations suggest that pygmy blue-tongue lizards rely heavily on burrows for many activities, and that any conservation management scheme for this endangered species will rely on maintaining an adequate supply of burrows.

Social network structure and parasite infection patterns in a territorial reptile, the tuatara (Sphenodon punctatus)

We investigated whether the parasite load of an individual could be predicted by its position in a social network. Specifically, we derived social networks in a solitary, territorial reptile (the tuatara, Sphenodon punctatus), with links based on the sharing of space, not necessarily synchronously, in overlapping territories. Tuatara are infected by ectoparasitic ticks (Amblyomma sphenodonti), mites (Neotrombicula spp.) and a blood parasite (Hepatozoon tuatarae) which is transmitted by the tick. We recorded the location of individual tuatara in two study plots twice daily during the mating season (March) in 2years (2006 and 2007) on Stephens Island, New Zealand. We constructed weighted, directed networks to represent pathways for parasite transmission, where nodes represented individual tuatara and edges connecting the nodes represented the extent of territory overlap among each pair of individuals. We considered a network-based hypothesis which predicted that the in-strength of individuals (the sum of edge weights directed towards a node) in the derived network would be positively related to their parasite load. Alternatively, if the derived social network did not reflect actual parasite transmission, we predicted other factors such as host sex, size or territory size may better explain variation in parasite infection patterns. We found clear positive relationships between the in-strength of tuatara and their tick loads, and infection patterns with tick-borne blood parasites. In particular, the extent that individuals were connected to males in the network consistently predicted tick loads of tuatara. However, mite loads of tuatara were significantly related to host sex, body size and territory size, and showed little association with network measures. The results suggest that the pathway of transmission of parasites through a population will depend on the transmission mechanism of the parasite, but that social networks provide a powerful predictive tool for some parasites.