During three field seasons, totalling 20 months between 1984-87, the ecological separation of four species of small Cathartid vultures was investigated at Hato Masaguaral in the flat savannas or Llanos of central Venezuela, South America. The focus of the study was an analysis of the role of migrant Turkey Vultures Cathartes aura meridionalis, in the scagenging guild of resident vultures, particularly in relation to a resident subspecies of Turkey Vulture C. a. ruficollis. Ecological separation between different taxa was achieved by differential habitat use by foraging vultures, feeding on different types of carcases and at different times. Where mixed-species flocks fed at carrion, social dominance mediated competition; heavier taxa were not necessarily dominant. During the dry season, migrant Turkey, Lesser Yellow-headed Cathartes burrovianus and Black Vultures Coragyps atratus foraged at highest densities in open savanna habitats. By contrast, resident Turkey and King Vultures Sarcor amphus papa foraged almost entirely in closed gallery forest. In the wet season, the numbers of Cathartes vultures declined by 6 fold in the absence of migrant Turkey Vultures which returned to North America. There was a distinct habitat shift in resident Turkey Vultures at the end of the dry season from gallery forest to open savanna habitats; conversely at the end of the wet season birds moved back into the gallery forest. Some residents may themselves have been migratory, moving into the ranch area at the end of the dry season, but this did not explain the decrease of Cathartes vulture density between seasons. The changes coincided with the departure of the majority of migrants in the spring and with the influx of migrants in the autumn, respectively. The core areas used by radiotragged resident Turkey Vultures shifted seasonally from gallery forest to open savanna. Most marked migrant Turkey Vultures were not resighted, suggesting that they passed through the study area on their way further south or that they had large home ranges. Compared to migrant Turkey Vultures a larger proportion of resident Turkey and Black Vultures were resighted. Comparative observations at bait sites in gallery forest and open savanna showed that carcases were detected more quickly, group sizes of migrant Turkey Vultures were greater, and agonistic encounter rates were higher in the open. Both migrant and resident Turkey Vultures occurred at similar densities over the forest but most carcases were first located by residents. Migrants won almost all agonistic encounters initiated against resident Turkey Vultures. The feeding rate of resident Turkey Vultures was significantly and negatively affected by the numbers of migrant Turkey and King Vultures, but not by other residents present at carcases. While the body condition of migrant Turkey Vultures was low after autumn migration from North America, the condition of birds trapped in subsequent months improved. However, the body condition of residents was below average throughout the dry season when migrants were present. By contrast, in the wet season when migrants were absent, the body condition of residents was above average. The wing-loading of different taxa was related to their foraging strategies; low wing-loading enabled some taxa (e.g. resident Turkey Vultures) to fly at low altitude over vegetation and forage using olfaction. Others, with higher wing-loading depended on sight for foraging (directly by observing carcases in the open or by watching the activities of other vultures) and on thermals to remain aloft (e.g. Black Vultures). Results suggested that, although there may be seasonal changes in food supply which may explain wet season changes in foraging behaviour, resident Turkey Vultures were forced to forage in gallery forest during the dry season to reduce interference competition from migrants.

Nephrops norvegicus, like other lobsters and crayfish, react to threatening stimuli by producing of the tail-flip escape response. This response, which is important in both predator avoidance and capture by trawling, takes the form of repeated flexions and extensions of the abdomen, produced by the deep abdominal flexor and extensor muscles. Much research has been concentrated on both the metabolic and neuronal factors controlling and limiting the tail-flip swimming of decapod Crustacea, but little attention has been focused upon the interactions between these two areas. This study has examined the tail-flip swimming of N.norvegicus in terms of both metabolic and neuronal limitations. Results have indicated that prolonged swimming and its recovery are limited neither by the availability of energy (from direct, stored sources - endogenous muscle ATP; or from indirect sources - by glycolytic production of ATP from D-glucose),nor by alteration of inter- or intra-cellular conditions by the build up of the glycolytic end product L-lactate. However, the part played by phospho-L-arginine (another short-term energy reserve in muscle) in the limitation of tail-flipping has not been unequivocally defined, and the possibility remains that there may be some metabolic influence in determining the endurance and recovery of swimming. Despite the lack of definitive evidence against any metabolic limitation of escape swimming, the results of further experiments suggest that neuronal factors play a major role in the limitation of tail-flipping. These experiments were designed to assess the importance and site of action of neuronal factors, in particular habituation, in swimming. Furthermore, the waning of tail-flipping in response to repeated stimulation appears to represent true habituation. By the use of both histochemical and morphological techniques, the roles of the muscles of the thoraco-abdominal joint in both tail-flipping and postural control have been partially elucidated. These results suggest both bracing and steering functions for the larger muscles of the two groups examined (thoraco-abdominal extensors and abdominal abductors) and postural roles for the smaller muscles. These investigations also identify suitable areas for further research, not least an examination of the innervation and recruitment of these muscle groups during tail-flipping in order to determine unequivocally their bracing and steering functions.

This study examines the dynamics of the Atlantic salmon (Salmo salar L.) population of the Foyle catchment in Ireland, through the analysis of long-term datasets and extensive field data. In chapter 1 the current conservation status of the salmon is discussed with particular reference to the Foyle. Chapter 2 considers the interaction between commercial netting and recreational sport angling and the effect of total combined exploitation on an independent measure of population size (redd counts) using a 49 year dataset. The evidence suggests that year class strength is the principal modulator of variation in commercial catches, sport angling catches and spawning escapement. Chapter 3 examines the role of density-dependence in the Foyle salmon population. A Ricker density-dependent model showed that spawning adult population size significantly predicted variation in the resultant filial generation; however, a significant amount of variation (ca. 68%) remained unexplained. Environmental factors were significant in explaining some of the remaining variance and these influences were linked to specific life stages. Chapter 4 investigates the effect of marine climatic conditions in the North Atlantic on the abundance of returning migrant Atlantic salmon, using a 126 year dataset of commercial catches and an index of climate in the northern hemisphere, the North Atlantic Oscillation (NAOI). The NAOI when below 0.151 explained a significant proportion of variation in five year running mean catches of migrant Atlantic salmon returning to the River Foyle. This indicates that a significant proportion of the variance in population size in the past was the result of variability in conditions in the marine environment. However, when the NAOI was above 0.151 this relationship uncoupled. Chapter 5 tests the capacity of local instream and broadscale catchment characteristics to predict 0+ salmon abundance within the Foyle area. This chapter also highlighted the potential detrimental impact of increasing urbanisation on the salmon stocks of the Foyle area.

Individuals both within and between populations can vary markedly in a number of traits, including behaviour, life-history patterns and morphology. These differences do not simply reflect noise around a mean, but reflect real and important variability that can have a number of important implications. Such individual variability is especially prevalent among larval fish, which undergo significant changes in size, anatomy, physiology and morphology as they develop into adults. The potential for fish to develop differing behaviours and morphologies has important implications in the aquaculture environment. For example, some fish may be aggressive and/or cannibalistic while others are not, or some fish may have a propensity to take risks while others shy away from risk. Elucidation of the mechanisms underlying these differences could enable the farmer to mitigate the development of behaviours and morphologies that are not conducive with welfare and production. Such information would be especially useful in the rearing of species such as Atlantic cod, which are highly cannibalistic in the larval and early juvenile period of development. The purpose of the work outlined in this thesis was to elucidate certain aspects of individual variability in predominantly larval cod that relate to the culture of this species. An introduction to the subject area is provided in Chapter 1. Fish husbandry techniques followed the standard procedure employed at the two study sites and are described in detail in chapter 2. This chapter also describes the morphometric technique used to analyse morphology, principal component analysis of linear measurements, and discusses the reasons for adopting this technique. Chapters 3 and 4 examine morphological development in larval cod reared under standard culture conditions and using common commercial feeds, for the purpose of elucidating developments in trophic morphology that could potentially relate to the development of cannibalism in this species. Chapter 3 specifically examines patterns of change in head shape in larval Atlantic cod and the extent to which head development varies within a cohort, while chapter 4 examines the effect of diet on the development of head morphology in larval cod. The former of these studies identified clear and consistent patterns of growth in various measures of head structure (and especially eye diameter). This was with the exception of jaw width, which developed in opposition to these measures. Periods of rapid change in head morphology coincided with points at which the larval diet changed and may have been caused by this change. Growth of the head and the post cranium was highly variable, especially in the latter stages of larval development and investment in head growth relative to post-cranial growth increased over the first two thirds of larval development, remaining constant thereafter. The second of these studies found that fish fed different prey types developed different head morphology. Specifically, fish fed small prey developed more fragile heads and larger eyes relative to jaw width than fish fed larger prey. Analysis of the head morphology of dead fish indicated that at least some of these differences resulted, not from the death of certain morphotypes, but from a phenotypically plastic response to the different diets. The morphology of a small number of cannibalistic larvae analysed during the study indicated that fish fed the larger prey developed morphology comparable with that of cannibalistic morphs. In the study detailed in chapter 5, aggressive interactions in larval cod were quantified in order to determine whether these interactions represented an early form of cannibalism or a battle for resources. Attacks where characterised by brief, one-way, nips by an attacker to a victim. Fish also commonly exhibited a pattern of burst swimming (darts) that appeared to reflect a generalised escape response. This darting behaviour was not affected by the presence of food, but was more common in fish fed the higher prey densities. Conversely, overall levels of prey did not affect the incidence of aggressive attacks, although analysis was confounded by a decline in levels of aggression with increasing fish density. The frequency of nips was highest when food was absent and nips were preferentially directed at the tail of victims, to victims of a smaller or similar size than the attacker and to victims that showed abnormal body posture. These findings indicated that at least some attacks by larval cod represented an early attempt at cannibalism. Chapter 6 details a study in which differences in the risk taking behaviour of one-year old cod of different stock and/or family origin were examined. Fish of North-eastern Arctic stock origin were found to be more prone to take risks than fish of Norwegian coastal stock origin. Furthermore, although there were no significant differences in risk-taking between families of North-eastern Arctic stock origin, a weakly significant difference existed between families of fish originating from coastal stock. The weight and condition of fish was significantly smaller in fish that emerged to escape than in fish that avoided risk and these factors may have contributed to the observed behavioural differences between stocks and families. Cortisol levels did not vary between risk avoiders or risk takers, but were significantly higher in control fish of North-eastern Arctic stock origin compared to control fish of coastal origin. These results provided evidence for a heritable component to risk-taking in cod. The results of the aforementioned studies have important implications, particularly for the culture of cod and these implications are discussed in Chapter 7, together with a summary of the objectives and findings of each study. The future studies that are prompted by these findings are also considered.

This thesis is an examination of developmental trade-offs occurring in the pupal stage of caddis flies, using both an experimental and correlational approach. Resources available at metamorphosis were manipulated by forcing caddis fly larvae, Odontocerum albicorne, to divert more silk, and hence protein, into their larval cases. The resultant adult forms had shorter wings and lighter thoraces (somatic investment) than control individuals, but abdomen mass (reproductive investment) was not affected. The results of this experiment could be due to mechanistic constraints of the developmental pathways during pupation, because resources from the breakdown of certain structures may be 'earmarked' for incorporation into specific adult structures. Equally, the observed response could represent a strategic preservation of reproductive reserves. O. albicorne is a short lived species which mates soon after adult emergence and so sacrificing somatic investment may have less drastic affects on fitness. To examine whether resource allocation pathways during development of the adult form are flexible in caddis flies, the same resource depletion experiment was performed on the caddis fly species, Glyphotaelius pellucidus. Adult life span is much longer in this species, which is an adaptation to loss of larval habitat for many months of the year. Here, somatic investment would be predicted to be of much greater importance in the life history, and so if the response to the nutrient deficiencies was strategic, somatic reserves would be expected to be preserved at the expense of reproductive potential. The pattern of resource allocation in the resultant adults was in line with predicted life history requirements, as abdomen mass was sacrificed so as to maintain thoracic mass.

This study constructed phylogenies for the Nectariniidae from up to 20 parameters derived from quantitative analyses of their vocalisations and used these to test hypotheses on whether or not the origin of sunbirds is African or Asian. In addition to standard sonographic measurements, this study is the first to use entropy values derived directly from sonograms as an objective way of separating calls from songs and simple songs from complex songs. The affinities of the genera of flowerpeckers were firmly established on vocal grounds and concurred with their traditional taxonomy. The vocal phylogeny failed to separate sunbirds from spiderhunters but showed that the Asian group of sunbirds emerged as the basal clade and therefore the most primitive clade of the sunbird lineage. This is in agreement with the phylogenetic outcome of recent DNA studies. It is therefore concluded that sunbirds may have originated on the Indian subcontinent, reached Indian Ocean islands including Madagascar and then penetrated into Africa, with the possibility that some oceanic island species were later derived from secondary invasions from Africa. The results further suggest that the clade of the Gulf of Guinea group of species may have arisen from a common ancestor from the African continent. The Indian Ocean island species also separated as a distinct group. Mean bill lengths and body sizes of different taxa were analysed in relation to the vocalisations. The results indicated significant relationships between bill length and body size and song complexity, with the latter higher in the more advanced genera.

Birds can influence the embryonic development through incubation behaviour because avian embryos develop using parent’s body heat. Although previous studies assumed that incubation behaviour influences hatching patterns, few studies have studied the effect of incubation behaviour during egg-laying and early incubation on hatching patterns due to difficulties to determine onset of incubation during egg-laying. I investigated whether incubation behaviour during egg-laying and early incubation affects hatching patterns in gulls using measurements of mean nest attendance and daily change of nest attendance. Hatching patterns were influenced by incubation behaviour during egg-laying and early incubation behaviour. As parents spent more time in their nests, a brood hatched more synchronously and hatching success of the first-laid eggs increased when gulls laid relatively smaller first-laid eggs than other pairs. Within-clutch variation in eggshell colour related to daily change of incubation behaviour. This might relate to hormonal change during egg-laying. Increase of prolactin initiates incubation and accompanies decline of steroid hormones which relate to accumulation of eggshell pigments. Hatching patterns may also be influenced by accelerated development of last-laid eggs. When eggs were swapped to increase interval between eggs, last-laid eggs of herring gulls accelerated their development to catch up. Accelerated development may increase the survival of chicks from last-laid eggs by reducing the disadvantage of small size within a clutch. However, the costs of accelerated development seem to appear during the embryonic period. Hatching success was low in eggs with accelerated development, although there were no differences in growth rate and early nestling survival between accelerated and control last-laid eggs. Eggshell characteristics might be a factor affecting hatching patterns because they are related to embryonic metabolism. Hatching duration was not related to eggshell thickness and total functional area, but chicks which hatched from eggs with higher proportion of mammilllary cone contact area took longer to hatch. Chicks hatched from thicker eggshells showed longer “head plus bill” at hatching and grew faster in skeleton size after hatching. Diet during egg-laying and early incubation affected nest attendance. Females which consumed more marine food during egg formation had lower nest attendance during egg-laying and early incubation. This may relate to longer foraging time required to obtain marine food. In conclusion, this thesis suggests that parents can influence hatching patterns by altering incubation behaviour during egg-laying and early incubation and hatching patterns also may be affected by accelerated development of last-laid eggs, diet during egg-laying and early incubation and eggshell characteristics (proportion of mammillary cone contact area).

Telomeres, the repetitive DNA sequences that cap eukaryotic chromosomes, are thought to play an important role in linking life conditions and senescence. In vertebrate somatic cells, telomeres shorten at each cell division, and the rate at which they do so has been linked to cellular and organismal senescence. Although telomeres generally shorten with age in vertebrates, in most species studied there is considerable variation between same age individuals. In this thesis, I examined the telomere dynamics of various avian species, investigating both the causes of variation in telomere length among individuals and what effect this variation has on attributes such as survival rates. Previous studies have shown that most telomere loss occurs in young individuals and it thus makes sense that early life conditions are responsible for much of the inter-individual variation in telomere length. I investigated this idea by studying chick telomere dynamics in a wild population of lesser black-backed gulls Larus fuscus. There was considerable variation in hatching telomere length among individuals and much of this variation was related to circumstances during embryonic growth. Larger hatchlings had shorter telomere lengths, suggesting that embryonic growth rate could have affected telomere attrition. Independent of this trend, males had longer telomeres at hatching than females. Although telomere length did decrease with age post-hatching, these initial variations remained consistent during the initial post-hatching period. The relationship between early life conditions and telomere length was investigated further with a longitudinal study of telomere length in chicks of the European shag Phalacrocorax aritotelis. A previous study on this population of birds had shown that telomere length declines with age within individuals over a period of several years. However no change in telomere length was detected over a period of 11-13 days during the chick period. Body size had no effect on telomere length, but males did have longer telomere than females. These initial chapters investigate telomere length in chicks; however there are very few studies that investigate telomere length over the entire lifespan of long-lived species. I thus next examined the telomere dynamics of two species of long-lived seabird, the northern and southern giant petrels (Macronectes spp.). In both giant petrel species, telomeres were shorter in adults than chicks, but there was no trend for adult telomere length to decrease with age. In southern giant petrels, there was a significant relationship (independent of age and sex) between an individuals telomere length and whether it was still alive 8 years after it was initially sampled. This relationship was not present in northern giant petrels, possibly due to a smaller sample size. The results thus support both the idea that most telomere loss occurs in young individuals and that telomere length may be an indicator of life expectancy. Various methods exist to measure telomeres. As the number of taxa whose telomere dynamics are being studied increases, it becomes increasingly important to know which methods are the best to use and to what extent these methods are applicable across species. These questions were investigated in relation to work conducted on the telomere dynamics of the blue-footed booby Sula nebouxxi. Both the TRF and qPCR techniques were used to measure booby telomeres, but problems arose with both methods. It is possible that these problems occurred because blue-footed boobies have a particularly large amount of interstitial telomeric DNA, although a more detailed analysis of booby telomeres would be necessary to determine this. These findings suggest that standardised methods to measure telomeres cannot necessarily be applied to every new species whose telomere dynamics are studied. The evidence presented here suggests that the study of telomere dynamics can be a very powerful tool for behavioural ecologists. It now seems possible that telomeres might provide both a way of measuring the long-term costs of early life-conditions and a way to measure the quality of an individual. However, further research is still needed to fill in the considerable gaps in our knowledge and fully exploit the potential telomeres have for behavioural ecology.

Supertrees are a useful method of constructing large-scale phylogenies by assembling numerous smaller phylogenies that have some, but not necessarily all, taxa in common. Birds are an obvious candidate for supertree construction as they are the most abundant land vertebrates on the planet and no comprehensive phylogeny of both extinct and extant species currently exists. In order to construct supertrees, primary analysis of characters is required. One such study, presented here, describes two new partial specimens belonging to the Primobucconidae from the Green River Formation of Wyoming (USA), which were assigned to the species Primobucco mcgrewi. Although incomplete, these specimens had preserved anatomical features not seen in other material. An attempt to further constrain their phylogenetic position was inconclusive, showing only that the Primobucconidae belong in a clade containing the extant Coraciiformes and related taxa. Over 700 such studies were used to construct a species-level supertree of Aves containing over 5000 taxa. The resulting tree shows the relationships between the main avian groups, with only a few novel clades, some of which can be explained by a lack of information regarding those taxa. The tree was constructed using a strict protocol which ensures robust, accurate and efficient data collection and processing; extending previous work by other authors. Before creating the species-level supertree the protocol was tested on the order Galliformes in order to determine the most efficient method of removing non-independent data. It was found that combining non-independent source trees via a “mini-supertree” analysis produced results more consistent with the input source data and, in addition, significantly reduced computational load. Another method for constructing large-scale trees is via a supermatrix, which is constructed from primary data collated into a single, large matrix. A molecular-only tree was constructed using both supertree and supermatrix methods, from the same data, again of the order Galliformes. Both methods performed equally as well in producing trees that fit the source data. The two methods could be considered complementary rather than conflicting as the supertree took a long time to construct but was very quick to calculate, but the supermatrix took longer to calculate, but was quicker to construct. Dependent upon the data at hand and the other factors involved, the choice of which method to use appears, from this small study, to be of little consequence. Finally an updated species-level supertree of the Dinosauria was also constructed and used to look at diversification rates in order to elucidate the “Cretaceous explosion of terrestrial life”. Results from this study show that this apparent burst in diversity at the end of the Cretaceous is a sampling artefact and in fact, dinosaurs show most of their major diversification shifts in the first third of their history.

Anthelmintic resistance in parasitic nematodes of small ruminants is widespread and, in some parts of the world, threatens the sustainability of sheep production. The mechanisms whereby parasitic nematodes become resistant to anthelmintics, particularly ivermectin, remain to be determined. The majority of studies to date have investigated target site mutations; relatively little attention has been paid to the role of gene expression changes. The present study focused on Teladorsagia circumcincta; the predominant parasitic gastrointestinal nematode species in the UK and the predominant resistant species. The role of changes in gene expression were investigated in an ivermectin-susceptible isolate (CVL) and a multidrug resistant isolate (MOTRI), utilising a range of molecular biological techniques. In the first experiment, a panel of novel putative ivermectin resistance genes were identified from T. circumcincta, comprising 11 partial P-glycoprotein (Pgp) and 3 partial Cytochrome P450 (CYP) sequences. Both Pgps and CYPs have been implicated in the handling and metabolism of xenobiotics in other biological systems, but have not been investigated in T. circumcincta to date. Initial results, using semi-quantitative PCR identified changes in expression of this panel of genes between the CVL and MOTRI isolates. Constitutive differences in expression of the Pgps and CYPs between CVL and MOTRI were determined using the ΔΔCt TaqMan® real-time PCR method. A statistically significant increase in expression was observed for TeciPgp-9 NBD2 across all life-cycle stages but most notably in eggs (55-fold increase). A statistically significant reduction in expression of TeciPgp-2 NBD2 was observed in all but the adult stages of MOTRI compared to CVL. Analysis of a 208 base pair sequence of TeciPgp-9 NBD2 identified high levels of polymorphism, with at least four non-coding SNPs evident in the MOTRI isolate. These results merit further investigation. Inducible changes in the expression of the Pgps and CYPs were investigated in MOTRI before and after ivermectin treatment, using real-time PCR. Statistically significant fold changes in expression in most of the genes occurred in at least one life-cycle stage. Inducible expression of TeciPgp-2 NBD2 and TeciPgp-9 NBD2 was investigated further by comparing adult MOTRI parasites with those recovered three days after in vivo ivermectin exposure, and by exposing pools of MOTRI xL3 to ivermectin in the larval migration inhibition test. The survivors of ivermectin exposure exhibited a statistically significant reduced 13.68-fold expression of TeciPgp-2 NBD2 compared to MOTRI. Similarly, the MOTRI xL3 able to migrate in the presence of ivermectin in the LMIT had a 1.88-fold reduction in TeciPgp-2 NBD2 expression compared to MOTRI xL3 unexposed to ivermectin. These results indicate that inducible changes in TeciPgp-2 NBD2 and TeciPgp-9 NBD2 expression can occur, but the experimental design is critical to being able to identify the changes. In a more global approach, the transcriptomic response of MOTRI adults to in vitro ivermectin exposure was investigated using Roche 454 sequencing, generating 98,685 novel EST sequences, providing an important resource for a genome resource-poor organism. Objective bioinformatic analysis of the two datasets revealed statistically significant differences in the mean expression levels of the KEGG orthologous groups for ‘translation’, ‘amino acid metabolism’ ‘carbohydrate metabolism’ and ‘xenobiotic degradation and metabolism’. On combining the two datasets, and through application of a novel statistical method, 16 clusters of ESTs were identified as containing statistically significant differences in the mean proportion of exposed reads compared to unexposed reads under the conservative model, whilst a further 355 clusters were found to have statistically significant differences under the liberal model. One-way suppression subtractive hybridisation (SSH) was used to identify genes exhibiting increased expression in MOTRI adults compared to CVL adults. 28 contiguous sequences were identified from the SSH experiment; 6 contiguous sequences were selected for validation; 5 of these results were confirmed using semi-quantitative PCR. Each contig was BLAST searched against the Roche 454 dataset; contig SSH14 aligned most closely to one of the statistically significant clusters in the conservative model, SSHs 5, 6, 10 and 23 aligned most closely to statistically significant clusters in the liberal model. This suggests that changes in expression in these sequences occur both constitutively, between CVL and MOTRI isolates, and inducibly, following ivermectin exposure. This work has shown that changes in gene expression, particularly the constitutively reduced expression in TeciPgp-2 NBD2 and the constitutively increased expression in TeciPgp-9 NBD2 (coupled with the presence of SNPs) could play a role in allowing multidrug resistant T. circumcincta to survive ivermectin exposure. Roche 454 sequencing and SSH approaches identified gene expression changes associated with in vitro ivermectin exposure and ivermectin resistance. These could form the basis of a novel panel of candidate resistance genes whose altered expression profiles may allow multidrug resistant T. circumcincta to survive ivermectin exposure by some, as yet identified, mechanism. Finally, we have also shown that a multidrug T. circumcincta isolate is affected by ivermectin exposure and that changes in gene expression could have a role to play in the ivermectin resistance phenotype in T. circumcincta. The genetic changes underpinning these changes in gene expression remain to be elucidated, and need to be investigated in other isolates. These changes could form the basis of an ivermectin resistance molecular marker, to monitor the spread of resistance, and to evaluate management practices aimed at delaying its spread.