Countermanding in rats as a practical model for investigation of adaptive control of behaviour, lifespan changes in behavioural control and neurotransmitter function

The countermanding paradigm was designed to investigate the ability to cancel a prepotent response when a stop signal is presented and allows estimation of the stop signal response time (SSRT), an otherwise unobservable behaviour. Humans exhibit adaptive control of behaviour in the countermanding task, proactively lengthening response time (RT) in expectation of stopping and reactively lengthening RT following stop trials or errors. Human performance changes throughout the lifespan, with longer RT, SSRT and greater emphasis on post-error slowing reported for older compared to younger adults. Inhibition in the task has generally been improved by drugs that increase extracellular norepinephrine. The current thesis examined a novel choice response countermanding task in rats to explore whether rodent countermanding performance is a suitable model for the study of adaptive control of behaviour, lifespan changes in behavioural control and the role of neurotransmitters in these behaviours. Rats reactively adjusted RT in the countermanding task, shortening RT after consecutive correct go trials and lengthening RT following non-cancelled, but not cancelled stop trials, in sessions with a 10 s, but not a 1 s post-error timeout interval. Rats proactively lengthened RT in countermanding task sessions compared to go trial-only sessions. Together, these findings suggest that rats strategically lengthened RT in the countermanding task to improve accuracy and avoid longer, unrewarded timeout intervals. Next, rats exhibited longer RT and relatively conserved post-error slowing, but no significant change in SSRT when tested at 12, compared to 7 months of age, suggesting that rats exhibit changes in countermanding task performance with aging similar to those observed in humans. Finally, acute administration of yohimbine (1.25, 2.5 mg/kg) and d-amphetamine (0.25, 0.5 mg/kg), which putatively increase extracellular norepinephrine and dopamine respectively, resulted in RT shortening, baseline-dependent effects on SSRT, and attenuated adaptive RT adjustments in rats in the case of d-amphetamine. These findings suggest that dopamine and norepinephrine encouraged motivated, reward-seeking behaviour and supported inhibitory control in an inverted-U-like fashion. Taken together, these observations validate the rat countermanding task for further study of the neural correlates and neurotransmitters mediating adaptive control of behaviour and lifespan changes in behavioural control.

Intraspecific Behavioural Diversity in a Freshwater Zooplankton: A study of the fitness consequences of vertical migration behaviour for distinct morphs of Daphnia pulicaria

The literature on niche separation and coexistence between species is large, but there is widespread variation in behavioural strategy between individuals of the same species that has received much less attention. Understanding what maintains this diversity is important because intraspecific behavioural diversity can affect population dynamics and community interactions. Multiple behavioural strategies can arise either as phenotype-dependent ‘conditional strategies’, where phenotypic variation causes individuals to adopt different strategies for optimizing fitness, or as internally-independent ‘alternative strategies’, where multiple fitness peaks exist for individuals and strategic ‘choice’ remains plastic. Though intraspecific variation in stable phenotypes is known to maintain intraspecific behavioural diversity through conditional strategies, when internal conditions are highly plastic or reversible, it is not clear whether individual behaviours are maintained as conditional strategies, or as alternative strategies of equal fitness. In this study, I combine an observational and experimental approach to identify the likely mechanisms maintaining behavioural diversity between hemoglobin-rich and hemoglobin-poor morphs in a natural population of Daphnia pulicaria. In Round Lake, individuals with low hemoglobin migrate daily from the hypolimnion to the epilimnion, whereas individuals with high hemoglobin remain in the hypolimnion. Using high-resolution depth and time sampling, I discovered behavioural diversity both within and among hemoglobin phenotypes. I tested the role of hemoglobin phenotype in maintaining behavioural diversity using automated migration robots that move individuals across the natural environmental gradients in the lake. By measuring the fitness of each morph undergoing either a natural migration behaviour, or the migration of the opposite morph, I found that the fitness of hemoglobin rich and poor morphs in their natural behaviour does not differ, but that Hb-rich individuals can obtain equal fitness from either behaviour, while Hb-poor morphs suffer substantial drops in survivorship in the alternate migration behaviour. Thus, migration behaviour in this system exists as a conditional strategy for some individuals, and as alternative strategies of equal fitness for others. The results of this study suggest that individual limits in the expression of highly flexible internal conditions can reinforce intraspecific behavioural diversity. Few studies have measured the fitness consequences of switching migration strategies and this study provides a rare example in the field.

Transferability of Regional Permafrost Disturbance Susceptibility Modelling Using Generalized Linear and Generalized Additive Models

To effectively assess and mitigate risk of permafrost disturbance, disturbance-p rone areas can be predicted through the application of susceptibility models. In this study we developed regional susceptibility models for permafrost disturbances using a field disturbance inventory to test the transferability of the model to a broader region in the Canadian High Arctic. Resulting maps of susceptibility were then used to explore the effect of terrain variables on the occurrence of disturbances within this region. To account for a large range of landscape charac- teristics, the model was calibrated using two locations: Sabine Peninsula, Melville Island, NU, and Fosheim Pen- insula, Ellesmere Island, NU. Spatial patterns of disturbance were predicted with a generalized linear model (GLM) and generalized additive model (GAM), each calibrated using disturbed and randomized undisturbed lo- cations from both locations and GIS-derived terrain predictor variables including slope, potential incoming solar radiation, wetness index, topographic position index, elevation, and distance to water. Each model was validated for the Sabine and Fosheim Peninsulas using independent data sets while the transferability of the model to an independent site was assessed at Cape Bounty, Melville Island, NU. The regional GLM and GAM validated well for both calibration sites (Sabine and Fosheim) with the area under the receiver operating curves (AUROC) N 0.79. Both models were applied directly to Cape Bounty without calibration and validated equally with AUROC's of 0.76; however, each model predicted disturbed and undisturbed samples differently. Addition- ally, the sensitivity of the transferred model was assessed using data sets with different sample sizes. Results in- dicated that models based on larger sample sizes transferred more consistently and captured the variability within the terrain attributes in the respective study areas. Terrain attributes associated with the initiation of dis- turbances were similar regardless of the location. Disturbances commonly occurred on slopes between 4 and 15°, below Holocene marine limit, and in areas with low potential incoming solar radiation

Redistribution of Soil Organic Matter by Permafrost Disturbance in the Canadian High Arctic

With increased warming in the Arctic, permafrost thaw may induce localized physical disturbance of slopes. These disturbances, referred to as active layer detachments (ALDs), redistribute soil across the landscape, potentially releasing previously unavailable carbon (C). In 2007–2008, widespread ALD activity was reported at the Cape Bounty Arctic Watershed Observatory in Nunavut, Canada. Our study investigated organic matter (OM) composition in soil profiles from ALD-impacted and undisturbed areas. Solid-state 13C nuclear magnetic resonance (NMR) and solvent-extractable biomarkers were used to characterize soil OM. Throughout the disturbed upslope profile, where surface soils and vegetation had been removed, NMR revealed low O-alkyl C content and biomarker analysis revealed low concentrations of solvent-extractable compounds suggesting enhanced erosion of labile-rich OM by the ALD. In the disturbed downslope region, vegetation remained intact but displaced material from upslope produced lateral compression ridges at the surface. High O-alkyl content in the surface horizon was consistent with enrichment of carbohydrates and peptides, but low concentrations of labile biomarkers (i.e., sugars) suggested the presence of relatively unaltered labile-rich OM. Decreased O-alkyl content and biomarker concentrations below the surface contrasted with the undisturbed profile and may indicate the loss of well-established pre-ALD surface drainage with compression ridge formation. However, pre-ALD profile composition remains unknown and the observed decreases may result from nominal pre-ALD OM inputs. These results are the first to establish OM composition in ALD-impacted soil profiles, suggesting reallocation of permafrost-derived soil C to areas where degradation or erosion may contribute to increased C losses from disturbed Arctic soils.