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Drought is a prominent feature of Hawaiʻi’s climate. However, it has been over 30 years since the last comprehensive meteorological drought analysis, and recent drying trends have emphasized the need to better understand drought dynamics and multi-sector effects in Hawaiʻi. Here, we provide a comprehensive synthesis of past drought effects in Hawaiʻi that we integrate with geospatial analysis of drought characteristics using a newly developed 100-year (1920–2019) gridded Standardized Precipitation Index (SPI) dataset. The synthesis examines past droughts classified into five categories: Meteorological, agricultural, hydrological, ecological, and socioeconomic drought. Results show that drought duration and magnitude have increased significantly, consistent with trends found in other Pacific Islands. We found that most droughts were associated with El Niño events, and the two worst droughts of the past century were multi-year events occurring in 1998–2002 and 2007–2014. The former event was most severe on the islands of O’ahu and Kaua’i while the latter event was most severe on Hawaiʻi Island. Within islands, we found different spatial patterns depending on leeward versus windward contrasts. Droughts have resulted in over $80 million in agricultural relief since 1996 and have increased wildfire risk, especially during El Niño years. In addition to providing the historical context needed to better understand future drought projections and to develop effective policies and management strategies to protect natural, cultural, hydrological, and agricultural resources, this work provides a framework for conducting drought analyses in other tropical island systems, especially those with a complex topography and strong climatic gradients.

Drought is a prominent feature of Hawaiʻi’s climate. However, it has been over 30 years since the last comprehensive meteorological drought analysis, and recent drying trends have emphasized the need to better understand drought dynamics and multi-sector effects in Hawaiʻi. Here, we provide a comprehensive synthesis of past drought effects in Hawaiʻi that we integrate with geospatial analysis of drought characteristics using a newly developed 100-year (1920–2019) gridded Standardized Precipitation Index (SPI) dataset. The synthesis examines past droughts classified into five categories: Meteorological, agricultural, hydrological, ecological, and socioeconomic drought. Results show that drought duration and magnitude have increased significantly, consistent with trends found in other Pacific Islands. We found that most droughts were associated with El Niño events, and the two worst droughts of the past century were multi-year events occurring in 1998–2002 and 2007–2014. The former event was most severe on the islands of O’ahu and Kaua’i while the latter event was most severe on Hawaiʻi Island. Within islands, we found different spatial patterns depending on leeward versus windward contrasts. Droughts have resulted in over $80 million in agricultural relief since 1996 and have increased wildfire risk, especially during El Niño years. In addition to providing the historical context needed to better understand future drought projections and to develop effective policies and management strategies to protect natural, cultural, hydrological, and agricultural resources, this work provides a framework for conducting drought analyses in other tropical island systems, especially those with a complex topography and strong climatic gradients.

Drought is a prominent feature of Hawaii's climate, however, the biological, ecological, cultural, and socioeconomic impacts of drought in Hawaii are not well understood. This paper provides a comp...

We examined the behaviour of humpback whales (Megaptera novaeangliae) in Hawaiian waters in relation to time of day using visual observations from shore platforms and acoustic observations using hydrophones. As humpbacks are not observed to feed in Hawaiian waters, we assumed that the data were free from the influence of diel cycles of prey availability. The majority of mothers with calves were likely to be escorted by one or more male consorts only after 0700 h. The percentage of pods with three or more adults increased across the day and the rates of male-male agonistic behaviour typical of multiple-adult pods also increased. Noon observations were characterized by high rates of behaviour such as breaching that may serve as visual and/or acoustic signals. No diel variation in the number of singers was observed, suggesting that the number of singers was independent of the late afternoon peak in male-male competition within surface-active groups. The results suggest that the whales were resting in the dawn hours, behaviour related to mating begins shortly after sunrise, and that male-male competition peaked in the afternoon.

Bottlenose dolphins (Tursiops truncatus) use short, wideband pulses for echolocation. Individual waveforms have high-range resolution capability but are relatively insensitive to range rate. Signal-to-noise ratio (SNR) is not greatly improved by pulse compression because each waveform has small time-bandwidth product. The dolphin, however, often uses many pulses to interrogate a target, and could use multipulse processing to combine the resulting echoes. Multipulse processing could mitigate the small SNR improvement from pulse compression, and could greatly improve range-rate estimation, moving target indication, range tracking, and acoustic imaging. All these hypothetical capabilities depend upon the animal's ability to combine multiple echoes for detection and/or estimation. An experiment to test multiecho processing in a dolphin measured detection of a stationary target when the number N of available target echoes was increased, using synthetic echoes. The SNR required for detection decreased as the number of available echoes increased, as expected for multiecho processing. A receiver that sums binary-quantized data samples from multiple echoes closely models the N dependence of the SNR required by the dolphin. Such a receiver has distribution-tolerant (nonparametric) properties that make it robust in environments with nonstationary and/or non-Gaussian noise, such as the pulses created by snapping shrimp.

Object classifiers that attempt to mimic dolphin echolocation require an auditory weighting function representative of dolphin peripheral auditory processing. An evolutionary program (EvPg) was used to fit the frequency-dependent output of a bank of bandpass filters to the auditory sensitivity of the bottlenose dolphin, Tursiops truncatus. Pseudo-Gaussian (PG) and rounded exponential (ROEX) functions were used to describe individual filter shapes. Variables determining the number of filters per model, overall filter shape and amplitude scaling were submitted to the EvPg for optimization. Maximum deviation (P e ) between model output and the sensitivity of the dolphin was used as a measure of similarity between the two, i.e., lower P e indicated a greater similarity. The number of filters converged upon 37 for all ROEX models and ≤ 45 for all PG models. The P e of the best-performing PG model was 0.08, and for all ROEX models was 0.13. Greatest deviations typically occurred below 5 kHz and above 1...

Abstract Every winter, (male) humpback whales (Megaptera novaeangliae) produce long complex songs. Song content is dynamic and singers incorporate changes as they occur, thus song is shared through cultural transmission. We compared songs recorded in winter migratory termini in Tonga, New Caledonia, Eastern Australia, and on migration paths off Eastern Australia and New Zealand, in the winter of 1994. Seven themes were shared by all regions, with an additional two themes shared by all but Tonga. Differences in regional variants were most pronounced between Tongan and Eastern Australian song. New Caledonian and Kaikouran song were more similar to songs from Eastern Australia rather than Tonga. These regional differences were stable across the season. The results suggest some migratory exchange among widely separate wintering regions of Area V, consistent with tag recovery data, but the time and location at which song sharing occurs remains speculative.

The relationships between acoustic features of target echoes and the cognitive representations of the target formed by an echolocating dolphin will influence the ease with which the dolphin can recognize a target. A blindfolded Atlantic bottlenose dolphin (Tursiops truncatus) learned to match aspect-dependent three-dimensional targets (such as a cube) at haphazard orientations, although with some difficulty. This task may have been difficult because aspect-dependent targets produce different echoes at different orientations, which required the dolphin to have some capability for object constancy across changes in echo characteristics. Significant target-related differences in echo amplitude, rms bandwidth, and distributions of interhighlight intervals were observed among echoes collected when the dolphin was performing the task. Targets could be classified using a combination of energy flux density and rms bandwidth by a linear discriminant analysis and a nearest centroid classifier. Neither statistical model could classify targets without amplitude information, but the highest accuracy required spectral information as well. This suggests that the dolphin recognized the targets using a multidimensional representation containing amplitude and spectral information and that dolphins can form stable representations of targets regardless of orientation based on varying sensory properties.

The synthesis and biological properties of a novel series of selective calcium-independent phosphodiesterase inhibitors are described. These compounds also inhibit the specific binding of [3H]rolipram to rat brain membranes and exhibit efficacy in preclinical models of antidepressant activity in mice, such as reducing immobility in the forced-swim test and reversing reserpine-induced hypothermia. Imidazolidinones 4 and 16 were found to be the most potent compounds studied.

The development of a unique dolphin biomimetic sonar produced data that were used to study signal processing methods for object identification. Echoes from four metallic objects proud on the bottom, and a substrate-only condition, were generated by bottlenose dolphins trained to ensonify the targets in very shallow water. Using the two-element ('binaural') receive array, object echo spectra were collected and submitted for identification to four neural network architectures. Identification accuracy was evaluated over two receive array configurations, and five signal processing schemes. The four neural networks included backpropagation, learning vector quantization, genetic learning and probabilistic network architectures. The processing schemes included four methods that capitalized on the binaural data, plus a monaural benchmark process. All the schemes resulted in above-chance identification accuracy when applied to learning vector quantization and backpropagation. Beam-forming or concatenation of spectra from both receive elements outperformed the monaural benchmark, with higher sensitivity and lower bias. Ultimately, best object identification performance was achieved by the learning vector quantization network supplied with beam-formed data. The advantages of multi-element signal processing for object identification are clearly demonstrated in this development of a first-ever dolphin biomimetic sonar.

Some humpback whales migrate annually from Antarctic feeding grounds to the seas around the Tongan Islands to give birth and mate. The Tongan humpbacks are considered part of Southern Hemisphere Group V that splits during migration, some swimming to Eastern Australia and others to various Polynesian Islands. During this time long complex songs are produced. The song is thought to be a male breeding display and may serve either as an intra-sexual or an inter-sexual signal or both. It is in a constant state of change that occurs every season. Since these changes are directional they cannot be described by drift, and singers incorporate changes as they occur, thus song must be shared through cultural transmission. This investigation describes the cultural changes that occurred in 158 songs recorded from Tongan humpbacks through the 1990s. The rate of change differed within years, some themes were retained for as much as five years and others were lost after only two years. The farther apart the years the less similar are the songs, as in the humpback songs of the Northern Hemisphere. The largest number of changes seems to have occurred in the early 1990s where all themes seemed to have been lost and new ones originated. What initiates these changes remains speculative, but we assess some hypotheses in relation to humpback whale behaviour and cultural transmission in avian song.

Bottlenose dolphins (Tursiops truncatus) detect and discriminate underwater objects by interrogating the environment with their native echolocation capabilities. Study of dolphins' ability to detect complex (multihighlight) signals in noise suggest echolocation object detection using an approximate 265-micros energy integration time window sensitive to the echo region of highest energy or containing the highlight with highest energy. Backscatter from many real objects contains multiple highlights, distributed over multiple integration windows and with varying amplitude relationships. This study used synthetic echoes with complex highlight structures to test whether high-amplitude initial highlights would interfere with discrimination of low-amplitude trailing highlights. A dolphin was trained to discriminate two-highlight synthetic echoes using differences in the center frequencies of the second highlights. The energy ratio (delta dB) and the timing relationship (delta T) between the first and second highlights were manipulated. An iso-sensitivity function was derived using a factorial design testing delta dB at -10, -15, -20, and -25 dB and delta T at 10, 20, 40, and 80 micros. The results suggest that the animal processed multiple echo highlights as separable analyzable features in the discrimination task, perhaps perceived through differences in spectral rippling across the duration of the echoes.

A biomimetic neural network was used to model the ability of a bottle nosed dolphin to recognize aspect-dependent geometric objects. Each echo train was recorded and an Integrator Gateway Network (IGN) was trained to discriminate among the objects using spectra of the object echoes. The IGN classifies objects using an average-like sum of the spectra from successive echoes. However, combining echoes may reduce classification accuracy if the spectra vary from echo to echo. The dolphin and the IGN learned to recognize the geometric objects, even though orientation was free to vary. The process of recognition using cumulated echoes was robust with respect to nonstationary raw input. The results were interpreted as evidence for the formation of aspect-independent representations of the objects. >

Steller sea lion (Eumetopias jubatus) mothers and pups establish and maintain contact with individually distinctive vocalizations. Our objective was to develop a robust neural network to classify females based on their mother-pup contact calls. We catalogued 573 contact calls from 25 females in 1998 and 1323 calls from 46 females in 1999. From this database, a subset of 26 females with sufficient samples of calls was selected for further study. Each female was identified visually by marking patterns, which provided the verification for acoustic identification. Average logarithmic spectra were extracted for each call, and standardized training and generalization datasets created for the neural network classifier. A family of backpropagation networks was generated to assess relative contribution of spectral input bandwidth, frequency resolution, and network architectural variables to classification accuracy. The network with best overall generalization accuracy (71%) used an input representation of 0-3 kHz of bandwidth at 10.77 Hz/bin frequency resolution, and a 2:1 hidden:output layer neural ratio. The network was analyzed to reveal which portions of the call spectra were most influential for identification of each female. Acoustical identification of distinctive female acoustic signatures has several potentially important conservation applications for this endangered species, such as rapid survey of females present on a rookery.

The Department of Defense lacks refined capability to assess and predict potential impact of tactical and experimental sound sources in the presence of marine mammals. Although there is sustained concern over the effects on marine mammals of man made sound in the oceans, there is very little direct information about what sound frequency-intensity combinations damage marine mammal hearing. Our broad objective is to transition information about effects of DoD sound types on marine mammal auditory anatomy and acoustic ecology to predictive models and mitigation tools. This effort responds directly to the DoD capability to comply with the National Environmental Policy Act requirements and will contribute directly to answering the National Council's Research Needs related to the effect of low-frequency sound on marine mammals (1994, 2000). This final report summarizes the accomplishments of SERDP Project CS-1082 spanning the lifetime of the project from FY98-FY-00.

Dolphins demonstrate an adaptive control over echolocation click production, but little is known of the manner or degree with which control is exercised. Echolocation clicks (N approximately 30,000) were collected from an Atlantic bottlenose dolphin (Tursiops truncatus) performing object discrimination tasks in order to investigate differential click production. Seven categories of clicks were identified using the spectral conformation and relative position of -3 and -10 dB peaks. A counterpropagation network utilizing 16 inputs, 50 hidden units, and 8 output units was trained to classify clicks using the same spectral variables. The network classified novel clicks with 92% success. Additional echolocation clicks (N24,000) from two other dolphins were submitted to the network for classification. Classified echolocation clicks were analyzed for animal specific differences, changes in predominant click type within click trains, and task-related specificity. Differences in animal and task performance may influence click type and click train length.

CS-1082 was a FY98 New Start. Our broad objective is to transition information about effects of DoD sound types on marine mammal auditory anatomy and acoustic ecology to predictive models and mitigation tools. Currently, the DoD lacks scientifically defensible information concerning the safe operation of many of their training and testing systems in the presence of marine mammals. There is only very little direct information about what sound frequency-intensity combinations may damage the hearing of marine mammals. This effort responds directly to the DoD capability to comply with the National Environmental Policy Act requirements and will contribute directly to answering the National Research Council's Research Needs related to the effect of low-frequency sound on marine mammals (1994). This project consists of three inter-related tasks. Task 1 consists of otopathological analyses of marine mammal ears. Task 2 consists of otopathological analyses of baleen whale ears, the results of which will motivate development of a biomimetic model of baleen whale hearing and responsiveness to DoD sound types. Task 3 utilizes predictions about sensitivity generated in Task 2, plus statistical sampling models and acoustical classification algorithms, to develop a capability to automate the use of the U.S. Navy's Integrated Undersea Surveillance System (IUSS) for mapping the distribution of whales.

This study demonstrated the ability of a false killer whale (Pseudorca crassidens) to discriminate between two targets and investigated the parameters of the whale's emitted signals for changes related to test conditions. Target detection performance comparable to the bottlenose dolphin's (Tursiops truncatus) has previously been reported for echolocating false killer whales. No other echolocation capabilities have been reported. A false killer whale, naive to conditioned echolocation tasks, was initially trained to detect a cylinder in a "go/no-go" procedure over ranges of 3 to 8 m. The transition from a detection task to a discrimination task was readily achieved by introducing a spherical comparison target. Finally, the cylinder was successfully compared to spheres of two different sizes and target strengths. Multivariate analyses were used to evaluate the parameters of emitted signals. Duncan's multiple range tests showed significant decreases (df = 185, p less than 0.05) in both source level and bandwidth in the transition from detection to discrimination. Analysis of variance revealed a significant decrease in the number of clicks over test conditions [F(5.26) = 5.23, p less than 0.0001]. These data suggest that the whale relied on cues relevant to target shape as well as target strength, that changes in source level and bandwidth were task-related, that the decrease in clicks was associated with learning experience, and that Pseudorca's ability to discriminate shapes using echolocation may be comparable to that of Tursiops truncatus.

More than 20 years have elapsed since the first detailed descriptions of humpback whale song (Payne and McVay, 1971; Winn, Perkins and Poulter, 1971) and despite considerable attention, the function of song remains elusive. In this paper, we review the literature on humpback whale song and describe general methods used to collect data on free-ranging whales. Then, we present the results of three current studies that have used different methods to shed light on the function of whale song.

Observations of captive and wild cetaceans provide abundant evidence that vision is an important sensory modality that functions in proximal social and predator/prey interactions, despite the limitations imposed on optic signals in water (Mitchell, 1970, 1972; Otte, 1974; Herman and Tavolga, 1980; Madsen and Herman, 1980). New views of the importance of vision in the life of the dolphin and of the ability to use visual information for carrying out complex cognitive tasks have been advanced by Herman and his colleagues (Madsen & Herman, 1980; Mobley & Herman, 1985; Herman, 1986, 1987; Herman et al., 1989a, 1989b; also see Herman, this volume for an extensive review and analysis of the cognitive data). These new views form the basis for much of the discussion in this chapter.

Steller sea lions (Eumetopias jubatus) breed and rear their young in coastal rookeries dispersed along the northern tier of the North Pacific. Population densities are high and no allomaternal behavior occurs, providing strong selection pressure for mother–pup recognition processes. Mothers and pups establish and maintain contact with individually distinctive vocalizations. Our objective is to understand the acoustic features that serve to identify individual females, and develop a ruggedized computer system to perform acoustic recognition of females in the field. We have cataloged almost 2000 contact calls from 46 females in 1998 and 25 in 1999. Each female is visually identified by marking patterns, which provides the ground truth for acoustic identification. Acoustic properties of the calls were measured and presented to several statistical classifiers. Representations of the calls had to be robust with respect to acoustical variability introduced by motivational changes as the mother and pup regained proximity. The calls, classifiers, and results of generalization tests will be described, and a concept for the field system will be discussed. [Research supported by ONR Research Opportunities for Program Officers award N00014‐00‐1‐0114 to R. H. DeFran, Cetacean Behavior Laboratory, SDSU.]

Many species of baleen whales reside or migrate annually through California waters. During these times some species of vocalizing whales can be detected, identified, and tracked with SOSUS arrays. Historical research by W. Cummings documented strong seasonal changes in narrow‐band ambient sound associated with then unidentified sounds. Contemporary research shows these sounds to be blue and fin whale vocalizations. The current study compares historical and contemporary narrow‐band ambient measurements with respect to mysticete residency and movement through southern California waters. Seasonal peaks in ambient noise agree with earlier reports, but increased anthropogenic sound reduced the magnitude of the effect. Potential sources of changes in ambient noise and their effects on the whales are discussed. [Research supported by the Strategic Environmental Research and Development Program, Project CS‐1082.]

The hypothesis that echolocating dolphins best receive acoustic signals over the pan bones of the lower jaw is widely accepted. Studies in echolocation and hearing have assumed that those areas serve as the dolphin’s peripheral hearing system. The research that established that model, however, does not exclude other potential sound reception sites and suggests that additional areas of the head may be acoustically sensitive and perhaps frequency dependent. Using jawphones, relative hearing thresholds for representative frequencies (10, 30, 60, and 90 kHz) were behaviorally measured at over 40 sites on a dolphin’s head. Iso‐sensitivity curves were constructed and projected onto the image of a dolphin’s head based on these measurements. The results suggest sensitivity to high frequency along the lower jaw with greater sensitivity forward of the pan bone area, sensitivity to low frequency around the external auditory meatus, and an acoustic asymmetry with greater sensitivity favoring the right side of the head. These results may correlate to underlying anatomical features and suggest a more complex peripheral hearing system than has been previously assumed.

Modeling cetacean hearing is an important step in advancing models of echolocation and in predicting the potential impact of oceanic noise upon cetaceans. Modeling the response of cetacean ears to acoustic input can be a difficult task and the performance of such models is often sub‐optimal. Evolutionary programming was employed as a systematic method of optimizing the response of a bank of pseudo‐Gaussian filter shapes to known audiometric functions in order to model the resonant systems of the basilar membrane. The response of the filter banks was tested against normalized audiograms of the bottlenose dolphin (Tursiops truncatus) and the predicted frequency dependent sensitivity of the Humpback whale (Megaptera novaeangliae). Filter banks were created that demonstrated comparable frequency responses to tonal stimuli across the (predicted) range of hearing in both species. The potential for incorporating these ear models into models of echolocation and their use in predicting the sensitivity of baleen wh...

Baleen whales live over extensive home range and time scales. Study of how these animals use their vocalizations for communication requires massive data sampling over long periods. This paper describes a system for automating the sampling and analysis of baleen whale calls. Many species produce stable, homogeneous call structures which lend themselves to automated species identification. We have benchmarked a series of bioacoustical call identification algorithms against a set of blue and fin whale calls while systematically manipulating the signal‐to‐noise ratio. Blue (Balaenoptera musculus) and fin (B. physalus) whale calls are very stereotypical. Blue whale ‘‘A’’ and ‘‘B’’ calls have fundamental frequencies of approximately 17 Hz, narrow bandwidth, well‐defined harmonic structure, and typical duration of 15–25 s. Fin whale ‘‘pulses’’ have fundamental frequencies of approximately 17 Hz, but are broadband in nature and short (approximately 1‐s) duration. The results demonstrated a typical tradeoff of speed versus accuracy. The best algorithm was inserted into an underwater sound recording system and its signal‐detection theoretic performance was quantified. Results will be discussed with respect to technological, ecological, and conservation aspects of baleen whale bioacoustics. [Project CS‐1082 of the Strategic Environmental Research and Development Program.]

The biological sonar system of bottlenose dolphins is adapted for cluttered, high noise, and extremely reverberant shallow‐water environments such as bays, estuaries, and near‐shore waterways. Echolocation behavior and signals are plastic, modified in‐stride during encounters with novel targets and in novel surroundings. Moreover, biosonar target discrimination performance provides an existence proof for biological sonar‐based recognition of various kinds of targets. For the dolphin this is accomplished in part by auditory neural computations that are not yet fully understood. A model of adaptive target classification processes was developed using heuristics based on dolphin psychophysics and echolocation behavior. Echoes were recorded while a dolphin ensonified several aspect‐dependent targets during a target discrimination task. A computational model of dolphin ear filters was adapted to include filter gain recursively scaled to acoustical characteristics of local ambient noise and reverberation. Stocha...

Contemporary anti‐invasion mines have aspect‐dependent shapes, making discrimination of mines from nonthreat objects, and classification of mines, a difficult task for human mine countermeasures personnel. Shallow‐water (SW) noise and bottom reverberation substantially degrade the acoustic structure of mine echoes. Traditional MCM target strength and FFT classifiers are not effective under these conditions. The testing of a novel neural network classifier to solve the task of classifying mines in the SW acoustic environment was begun. Three mine types were ensonified at 1 deg rotations using synthetic dolphin clicks. A learning vector quantization network was trained to classify the mines using the Choi–Williams joint time‐frequency distributions of echoes. A training set of 36 echoes per mine (5, 15, 25,..., 355 deg) was created, with the remaining echoes (0, 10, 20,..., 350 deg) reserved for generalization testing. The network correctly classified the mines at novel orientations with 85% accuracy. Perfo...

Humpback whales (Megaptera novaeangliae) migrate annually between high‐latitude summer feeding waters and low‐latitude winter mating waters. During migration and in winter waters, adult [male] humpbacks produce long complex songs. Songs gradually change during each singing season, and singers incorporate changes as they occur. Classification of song units using neural networks and cepstral cross correlation reveals that variability in song characteristics within region is lower than variability between regions. These regional differences in song characteristics can be considered to be dialects that are maintained through cultural transmission of regional variants. Thus differences in songs from different regions may be an index of reproductive isolation. Cladistic patterns of geographic and temporal variation in songs support a model of limited migratory exchange among widely separate wintering regions. [Work supported by UARC, SPREP, WDCS, PDCT.]

Rats were trained in a three-alternative spatial delayed matching-to-sample task in which the samples were rewarded forced choices of one arm of a three-arm starburst maze and retention was indicated by returning to that arm following a delay or retention interval. If the rat made an error on its first free choice of a trial, the chosen arm was blocked off and the rat was allowed a second choice between the remaining two arms. In two experiments, we investigated rats’ performance following presession treatment with scopolamine hydrobromide or following one of several control treatments presented in a repeated-measures design. In the first experiment, rats were given 0.1-mg/kg scopolamine hydrobromide, 0.1-mg/kg scopolamine methylbromide, or an equal volume of saline vehicle. In the second experiment, we tested new groups of rats under the same drug conditions as in Experiment 1 and in an additional condition without drugs, but with a longer retention interval. Experimental sessions were separated from one another by at least 1 rest day and 1 drug-free short-delay reminder session. Choice accuracy was lower following injections of scopolamine hydrobromide relative to the other conditions. Proactive interference, in the form of intrusions by previous samples or intrusions of the previous choices, however, was unaffected by drug treatment. These results suggest that scopolamine interferes with choice accuracy by decreasing the retention or encoding of information about the location of the sample, rather than by decreasing the animal’s ability to differentiate the responses made on one trial from those made on the previous trial.