Biomedical subjects

C Michael Bull

Publications and source records attributed to C Michael Bull.

7 recordsLinked to original sources

PubMed2007

Susceptibility to infection by a haemogregarine parasite and the impact of infection in the Australian sleepy lizard Tiliqua rugosa.

The Hamilton and Zuk hypothesis on haemoparasite-mediated sexual selection and certain studies of fitness are based on the assumption that blood parasite infections are detrimental to their hosts. However, there are few reports that have demonstrated harmful effects of endemic blood parasites on fitness in wild populations, and it has even been suggested that they may be non-pathogenic. In this paper, we show that individuals of the Australian sleepy lizard (Tiliqua rugosa) have smaller home ranges when they are infected with the haemogregarine blood parasite Hemolivia mariae than when no infection can be detected. An apparently contradictory result was that lizards with larger home ranges were more susceptible to infection under experimental exposure to Hemolivia. We propose that lizards sacrifice defence against pathogens by increased activity, perhaps associated with maintaining home ranges and mating opportunities. As a consequence, they gain higher parasite loads, which in turn inhibit their activity. In this case, the parasite-host interaction may act as a buffer of lizard activity.

Animals↗

PubMed2006

Associations between blood parasite infection and a microsatellite DNA allele in an Australian scincid lizard (Egernia stokesii).

We used blood samples from 175 individuals of the Australian lizard Egernia stokesii to determine infection status of three apicomplexan blood parasites from the genera Hemolivia, Schellackia, and Plasmodium and to determine genotypes at 12 microsatellite DNA loci. We found one significant association between genotype and infection status. For locus Est4, individuals carrying allele 159 had lower prevalence of infection with Hemolivia (14.3% of 28 lizards) than individuals that did not carry the allele (58.4% of 89 lizards). We interpret this as a linkage to a functional gene associated with parasite resistance. We found no evidence among seven lizard populations that the frequency of allele 159 was related to the population prevalence of Hemolivia infection and discuss several explanations of that pattern.

Animals↗

PubMed2006

Interactions between climate, host refuge use, and tick population dynamics.

The relationship between Australian sleepy lizard (Tiliqua rugosa) microhabitat use and tick (Amblyomma limbatum) population dynamics was investigated. Over 3 years (2002-2004) between 23 and 50 lizards were radio-tracked up to four times a week to record microhabitat use and each fortnight to determine tick loads. Daily maximum temperature was highly predictive of lizard microhabitat use. In hotter fortnights lizards used larger bushes and burrows for refuge. Peak background tick infestation levels and pulses of attachment coincided with higher ambient temperature. Male ticks attached throughout the year independent of season. Engorged females detached late in spring, summer and autumn, when climate regularly restricted lizards to a few thermally conservative refuges. Peak nymph and larval attachment occurred over summer and into autumn. Climate-dependent timing and type of host refuge use may influence tick population density. In more temperate summers lizards may avoid refuges with potentially high parasite loads.

Animals↗

PubMed2006

Transmission mode and distribution of parasites among groups of the social lizard Egernia stokesii.

We explored patterns of infection of three apicomplexan blood parasites with different transmission mechanisms in 46 social groups across seven populations of the Australian lizard, Egernia stokesii. There was higher aggregation of infections within social groups for Hemolivia, transmitted by ticks, and Schellackia, either tick-transmitted or directly transmitted from mother to offspring, than for Plasmodium, with more mobile dipteran vectors. Prevalence was not related to group size, proximity to other groups or spatial overlap with adjacent groups for any of the parasites. However, for Hemolivia, groups with higher levels of relatedness among adults had higher parasite prevalence. Living in social groups leads to higher risk of infection for parasites with low transmission mobility. An unanswered question is why so few lizard species tolerate these risks to form stable social aggregations.

Animals↗

PubMed2006

Influence of drying time on nematode eggs in scats of scincid lizard Egernia stokesii.

Two pharyngodonid nematode species, Pharyngodon tiliquae and Thelandros trachysauri, infect the Australian lizard Egernia stokesii (gidgee skink) in populations from South Australia. Eggs are detected in lizard scats that are deposited in piles outside the rock crevice refuges that the lizards occupy. Eggs were isolated by salt flotation from fresh scats and from scats that had been dried in simulated field conditions for 7, 14, 21, and 28 days. Egg counts decreased with drying time for both nematode species, but T. trachysauri eggs were still detected after 28 days of drying, whereas P. tiliquae eggs were rarely detected after 14 days. These results suggest that egg counts can be used to infer host infection status only from relatively fresh scats and that eggs of the 2 species persist in a state where they can be detected by standard flotation techniques, for different times.

Analysis of Variance↗

PubMed2005

The influence of parasites on the retention of long-term partnerships in the Australian sleepy lizard, Tiliqua rugosa.

Many studies have shown that potential partners are less likely to be chosen for mating if they are infected with parasites, although most of those studies have considered short-term choices. This paper shows that the Australian sleepy lizard, Tiliqua rugosa (Scincoidea), retains long-term pair fidelity for up to 21 years. However, in some cases pairs separate, and abandoned males have significantly higher tick loads in their last year with their previous female partner than did males that retained their partners from 1 year to the next. These data are consistent with the hypothesis that pair bonds are retained because individuals, both with low infection levels, are less likely to become infected if they remain with the same sexual partner.

Animals↗

PubMed2002

Stable social aggregations in an Australian lizard, Egernia stokesii.

Social groups in many animal species contain family members, and are maintained by parental care, and then tolerance of related individuals in the group. Lizards rarely show prolonged parental care, and rarely form stable social aggregations, although cohesive groups have been reported in the Australian skink genus, Egernia. A population of Egernia stokesii was surveyed for six seasons on Camel Hill, South Australia. At this site individuals occupied rock crevices for refuges on an isolated rocky outcrop of about 1.5 ha. The population was divided into 17 stable social groups, each containing 2-17 individual lizards. Surveyed juveniles took more than 5 years to reach mature size, and most juveniles and subadults remained in the social group of their parents for that period and longer. There were 2-8 permanent adult members of each group. Group members shared common crevice refuges, basked close together (and sometimes on top of each other) and defecated in common scat piles. There was low mortality after the first 2 years of life, and low dispersal either into or out of the population. Some individuals were "floaters" that did not belong to a social group. Over the study a number of these became established in groups. The social structure of these lizards resembles the family groups reported in many species of birds and mammals where group members help to raise the offspring of relatives. The low level of parental care in lizards suggests that the evolution of this form of social organisation in lizards has resulted from different processes than in other vertebrate taxa.

Aging↗