The impact of glide phases on the trackability of hydrodynamic trails in harbour seals (Phoca vitulina).

The mystacial vibrissae of harbour seals (Phoca vitulina) constitute a highly sensitive hydrodynamic receptor system enabling the seals to detect and follow hydrodynamic trails. In the wild, hydrodynamic trails, as generated by swimming fish, consist of cyclic burst-and-glide phases, associated with various differences in the physical parameters of the trail. Here, we investigated the impact of glide phases on the trackability of differently aged hydrodynamic trails in a harbour seal. As fish are not easily trained to swim certain paths with predetermined burst-and-glide phases, the respective hydrodynamic trails were generated using a remote-controlled miniature submarine. Gliding phases in hydrodynamic trails had a negative impact on the trackability when trails were 15 s old. The seal lost the generated trails more often within the transition zones, when the submarine switched from a burst to a glide moving pattern. Hydrodynamic parameter analysis (particle image velocimetry) revealed that the smaller dimensions and faster decay of hydrodynamic trails generated by the gliding submarine are responsible for the impaired success of the seal tracking the gliding phase. Furthermore, the change of gross water flow generated by the submarine from a rearwards-directed stream in the burst phase to a water flow passively dragged behind the submarine during gliding might influence the ability of the seal to follow the trail as this might cause a weaker deflection of the vibrissae. The possible ecological implications of intermittent swimming behaviour in fish for piscivorous predators are discussed.

Hydrodynamic determination of the moving direction of an artificial fin by a harbour seal (Phoca vitulina).

Harbour seals can use their vibrissal system to detect and follow hydrodynamic trails left by moving objects. In this study we determined the maximum time after which a harbour seal could indicate the moving direction of an artificial fish tail and analysed the hydrodynamic parameters allowing the discrimination. Hydrodynamic trails were generated using a fin-like paddle moving from left to right or from right to left in the calm water of an experimental box. The blindfolded seal was able to recognise the direction of the paddle movement when the hydrodynamic trail was up to 35 s old. Particle Image Velocimetry (PIV) revealed that the seal might have perceived and used two different hydrodynamic parameters to determine the moving direction of the fin-like paddle. The structure and spatial arrangement of the vortices in the hydrodynamic trail and high water velocities between two counter-rotating vortices are characteristic of the movement direction and are within the sensory range of the seal.

Tracking of biogenic hydrodynamic trails in harbour seals (Phoca vitulina).

For seals hunting in dark and murky waters one source of sensory information for locating prey consists of fish-generated water movements, which they can detect using their highly sensitive mystacial vibrissae. As water movements in the wake of fishes can persist for several minutes, hydrodynamic trails of considerable length are generated. It has been demonstrated that seals can use their vibrissae to detect and track hydrodynamic trails generated artificially by miniature submarines. In the present study, we trained a harbour seal to swim predefined courses, thus generating biogenic hydrodynamic trails. The structure of these trails was measured using Particle Image Velocimetry. A second seal was trained to search for and track the trail after the trail-generating seal had left the water. Our trail-following seal was able to detect and accurately track the hydrodynamic trail, showing search patterns either mostly congruent with the trail or crossing the trail repeatedly in an undulatory way. The undulatory trail-following search pattern might allow a seal to relocate a lost trail or successfully track a fleeing, zigzagging prey fish.

Mental rotation of perspective stimuli in a California sea lion (Zalophus californianus).

The time it takes humans to discriminate rotated objects from their mirror images increases linearly with the rotation angle. This phenomenon is probably due to an analogue mode of visual information processing during which an object's mental representation is rotated in a time-consuming process called mental rotation. As the speed of mental rotation in humans depends on rotation axis, we tested the ability of a California sea lion to mentally rotate perspective line drawings of three-dimensional objects about four axes. In a matching-to-sample experiment the animal was presented with the image and a mirror image of a block sample that had previously been shown upright. Both image and mirror image were rotated by a multiple of 60 degrees about the object's x-, y-, z-axis, or a skew axis (an axis oblique to these standard orthogonal axes). The animal's choice and reaction times were recorded using a computer-controlled touch-screen device. Mean reaction times and errors generally increased with angular disparity supporting the model of mental rotation for three-dimensional objects. Linear regression analysis of mean reaction times yielded high correlation coefficients only for three axes. The slope of reaction time functions indicated the highest mental rotation speed for the skew axis. This contrasts with the priority of mental rotation axes in humans suggesting that due to special ecological demands a different mode of orientation invariance evolved in marine mammals.

The envelope glycoprotein of human endogenous retrovirus HERV-W induces cellular resistance to spleen necrosis virus.

Human endogenous retrovirus type W (HERV-W) envelope glycoprotein (Env) has recently been reported to induce fusion in cells expressing the RD-114 and type D retrovirus receptor (RDR) and to serve as a functional retroviral envelope protein. In this report, another biological function for HERV-W was demonstrated by testing its ability to protect cells against retroviral infection. Spleen necrosis virus (SNV), a gammaretrovirus was chosen for testing resistance because it uses RDR to enter cells. An HERV-W Env expression plasmid was transfected into canine osteosarcoma cells (D-17), which are permissive for SNV infection. Cell fusion assays were performed to demonstrate biological function of HERV-W Env in D-17 cells. The presence of HERV-W env sequences was confirmed in stably transfected cell clones by using polymerase chain reaction. Viral infectivity assays were performed with SNV and amphotropic Murine leukemia virus (MLV-A) pseudotyped vector viruses to measure titers in D-17 cells expressing HERV-W Env and in negative control cells. The HERV-W Env caused fusion of D-17 cells in culture and greatly reduced infection by SNV vector virus. A 1000- to 10,000-fold decrease in SNV infectivity was observed for D-17 cells expressing HERV-W Env as compared to D-17 cells that were not expressing HERV-W Env. In contrast, infection by MLV-A pseudotyped vector virus was not significantly reduced. Thus, HERV-W Env confers host cell resistance to infection by SNV. This is the first report of a human endogenous retrovirus gene product blocking infection by any exogenous retrovirus.

Hydrodynamic trail-following in harbor seals (Phoca vitulina).

Marine mammals often forage in dark or turbid waters. Whereas dolphins use echolocation under such conditions, pinnipeds apparently lack this sensory ability. For seals hunting in the dark, one source of sensory information may consist of fish-generated water movements, which seals can detect with their highly sensitive whiskers. Water movements in the wake of fishes persist for several minutes. Here we show that blindfolded seals can use their whiskers to detect and accurately follow hydrodynamic trails generated by a miniature submarine. This shows that hydrodynamic information can be used for long-distance prey location.

Selective heating of vibrissal follicles in seals (Phoca vitulina) and dolphins (Sotalia fluviatilis guianensis).

The thermal characteristics of the mystacial vibrissae of harbour seals (Phoca vitulina) and of the follicle crypts on the rostrum of the dolphin Sotalia fluviatilis guianensis were measured using an infrared imaging system. Thermograms demonstrate that, in both species, single vibrissal follicles are clearly defined units of high thermal radiation, indicating a separate blood supply to these cutaneous structures. It is suggested that the high surface temperatures measured in the area of the mouth of the follicles is a function of the sinus system. In seals and dolphins, surface temperature gradually decreased with increasing distance from the centre of a follicle, indicating heat conduction from the sinus system via the follicle capsule to adjacent tissues. It is suggested that the follicular sinus system is a thermoregulatory structure responsible for the maintenance of high tactile sensitivity at the extremely low ambient temperatures demonstrated for the vibrissal system of seals. The vibrissal follicles of odontocetes have been described as vestigial structures, but the thermograms obtained in the present study provide the first evidence that, in Sotalia fluviatilis, the follicles possess a well-developed sinus system, suggesting that they are part of a functional mechanosensory system.

Ambient temperature does not affect the tactile sensitivity of mystacial vibrissae in harbour seals.

Vibrissae provide pinnipeds with tactile information primarily in the aquatic environment, which is characterized by its high thermal conductivity and large potential cooling power. Since studies of thermal effects on human tactile sensitivity have revealed that cooling below normal skin temperature impairs sensitivity, the present study investigates the tactile sensitivity of the vibrissal system of harbour seals at varying ambient temperatures. Using plates bearing gratings of alternating grooves and ridges, the texture difference thresholds of two adult seals were determined under water. We took advantage of the natural difference in ambient temperature between summer and winter. Mean water temperature was 1. 2 degreesC during the winter and 22 degreesC during the summer. During the cold season, the thermal status of both seals was examined using an infrared-sensitive camera system. The texture difference threshold of both seals remained the same (0.18 mm groove width difference) under both test conditions. The thermographic examination revealed that the skin areas of the head where the mystacial and supraorbital vibrissae are located show a substantially higher degree of thermal emission than do adjacent skin areas. This suggests that, in the vibrissal follicles of harbour seals, no vasoconstriction occurs during cold acclimation, so that the appropriate operating temperature for the mechanoreceptors is maintained.

Mental rotation in a California sea lion (Zalophus californianus).

Mental rotation is a widely accepted concept that suggests an analogue mode of visual information-processing in certain visuospatial tasks. Typically, these tasks demand the discrimination between the image and mirror-image of rotated figures, for which human subjects need an increasing reaction time depending on the angular disparity between the rotated figures. In pigeons, tests of this kind yielded a time-independent rotational invariance, suggested as being the result of a non-analogue information-processing that has evolved in response to the horizontal plane that birds perceive from above while flying. Given that marine mammals use the water surface as the horizontal plane for orientation while diving, the ability of a California sea lion to mentally rotate two-dimensional shapes was tested. Using a successive two-alternative matching-to-sample procedure, the animal had to decide between the image and mirror-image of a previously shown sample. Both stimuli were rotated by a multiple of 30 degrees with respect to the sample. The animal's reaction time was measured by a computer-controlled touch-screen device, rewarding the animal for pressing its snout against the stimulus matching the sample. A linear regression analysis of the animal's mean reaction time against the angular rotation of the stimulus yielded a significant correlation coefficient. Thus, the present data can be explained by the mental rotation model, predicting an image-like representation of visual stimuli in this species. The present results therefore correspond well with those found for human subjects, but are inconsistent with the data reported for pigeons.