Fog-basking behaviour and water collection efficiency in Namib Desert Darkling beetles

Background: In the Namib Desert fog represents an alternative water source. This is utilised by Darkling beetles (Tenebrionidae) that employ different strategies for obtaining the fog water. Some dig trenches in the sand, while others use their own bodies as fog collectors assuming a characteristic fog-basking stance. Two beetle species from the genus Onymacris have been observed to fog-bask on the ridges of the sand dunes. These beetles all have smooth elytra surfaces, while another species with elytra covered in bumps is reported to have specialised adaptations facilitating water capture by fog-basking. To resolve if these other beetles also fog-bask, and if an elytra covered in bumps is a more efficient fog water collector than a smooth one, we examined four Namib Desert beetles; the smooth Onymacris unguicularis and O. laeviceps and the bumpy Stenocara gracilipes and Physasterna cribripes. Here we describe the beetles' fog-basking behaviour, the details of their elytra structures, and determine how efficient their dorsal surface areas are at harvesting water from fog. Results: The beetles differ greatly in size. The largest P. cribripes has a dorsal surface area that is 1.39, 1.56, and 2.52 times larger than O. unguicularis, O. laeviceps, and S. gracilipes, respectively. In accordance with earlier reports, we found that the second largest O. unguicularis is the only one of the four beetles that assumes the head standing fog-basking behaviour, and that fog is necessary to trigger this behaviour. No differences were seen in the absolute amounts of fog water collected on the dorsal surface areas of the different beetles. However, data corrected according to the sizes of the beetles revealed differences. The better fog water harvesters were S. gracilipes and O. unguicularis while the large P. cribripes was the poorest. Examination of the elytra microstructures showed clear structural differences, but the elytra of all beetles were found to be completely hydrophobic. Conclusions: The differences in fog water harvesting efficiency by the dorsal surface areas of beetles with very different elytra surface structures were minor. We therefore conclude that the fog-basking behaviour itself is a more important factor than structural adaptations when O. unguicularis collect water from fog

Honeybee navigation: distance estimation in the third dimension

Honeybees determine distance flown by gauging the extent to which the image of the environment moves in the eye as they fly towards their goal. Here we investigate how this visual odometer operates when a bee flies along paths that include a vertical component. By training bees to fly to a feeder along tunnels of various three-dimensional configurations, we find that the odometric signal depends only upon the total distance travelled along the path and is independent of its three-dimensional configuration. Hence, unlike walking desert ants, which measure the distance travelled in the horizontal plane whilst traversing undulating terrain, flying bees simply integrate the image motion that is experienced on the way to the goal, irrespective of the direction in which the image moves across the eyes. These findings raise important questions about how honeybee recruits navigate reliably to find the food sources that are advertised by scouts

Polarized light detection in spiders

Copyright © 2001 The Company of BiologistsWe describe here the detection of polarized light by the simple eyes of spiders. Using behavioural, morphological, electrophysiological and optical studies, we show that spiders have evolved two different mechanisms to resolve the e-vector of light. Wolf spiders (Lycosidae), are able to turn in response to rotation of a polarized pattern at the zenith of their visual fields, and we also describe a strip in the ventral retina of the principal (anterio-median) eyes that views this location and has receptors tiered into two layers. This provides each pair of receptors with a similar optical solution to that provided by the ‘dorsal rim area’ of the insect compound eye. In contrast, gnaphosid spiders have evolved a pair of lensless secondary eyes for the detection of polarized light. These two eyes, each sensitive to orthogonal directions of polarization, are perfectly designed to integrate signals from the larger part of the sky and cooperate to analyse the polarization of light. Built-in polarizers help to improve signal purity. Similar organisation in the eyes of several other spider families suggests that these two mechanisms are not restricted to only a few families.Marie Dacke, Thuy A. Doan and David C. O’Carrol

A model of cue integration as vector summation in the insect brain

Ball-rolling dung beetles are known to integrate multiple cues in order to facilitate their straight-line orientation behaviour. Recent work has suggested that orientation cues are integrated according to a vector sum, that is, compass cues are represented by vectors and summed to give a combined orientation estimate. Further, cue weight (vector magnitude) appears to be set according to cue reliability. This is consistent with the popular Bayesian view of cue integration: cues are integrated to reduce or minimize an agent's uncertainty about the external world. Integration of orientation cues is believed to occur at the input to the insect central complex. Here, we demonstrate that a model of the head direction circuit of the central complex, including plasticity in input synapses, can act as a substrate for cue integration as vector summation. Further, we show that cue influence is not necessarily driven by cue reliability. Finally, we present a dung beetle behavioural experiment which, in combination with simulation, strongly suggests that these beetles do not weight cues according to reliability. We suggest an alternative strategy whereby cues are weighted according to relative contrast, which can also explain previous results

The Dung Beetle Dance: An Orientation Behaviour?

An interesting feature of dung beetle behaviour is that once they have formed a piece of dung into a ball, they roll it along a straight path away from the dung pile. This straight-line orientation ensures that the beetles depart along the most direct route, guaranteeing that they will not return to the intense competition (from other beetles) that occurs near the dung pile. Before rolling a new ball away from the dung pile, dung beetles perform a characteristic “dance,” in which they climb on top of the ball and rotate about their vertical axis. This dance behaviour can also be observed during the beetles' straight-line departure from the dung pile. The aim of the present study is to investigate the purpose of the dung beetle dance. To do this, we explored the circumstances that elicit dance behaviour in the diurnal ball-rolling dung beetle, Scarabaeus (Kheper) nigroaeneus. Our results reveal that dances are elicited when the beetles lose control of their ball or lose contact with it altogether. We also find that dances can be elicited by both active and passive deviations of course and by changes in visual cues alone. In light of these results, we hypothesise that the dung beetle dance is a visually mediated mechanism that facilitates straight-line orientation in ball-rolling dung beetles by allowing them to 1) establish a roll bearing and 2) return to this chosen bearing after experiencing a disturbance to the roll path

Markfuktighetsförhållanden och jordartsfördelningens påverkan på kväve- och fosforförluster från skog och sankmark

Övergödningen utgör ett allvarligt hot mot Östersjön och för att kunna åtgärda problemen krävs effektiva metoder för uppskattning av näringsbelastning och källfördelning. Ökad förståelse för bakomliggande orsaksfaktorer till diffusa näringsförluster från skog och sankmark kan förbättra skattningen av belastning och bidra till riktade åtgärder. I studien undersöktes markfuktighetsförhållanden och jordartsfördelning som potentiella förklaringsfaktorer till förluster av totalkväve (TN) och totalfosfor (TP) från skog och sankmark i sydöstra Sverige. De data som analyserades insamlades senhösten 2016 då provtagningar genomfördes i slumpmässigt utvalda källvattendrag vid utloppet från 100 små delavrinningsområden dominerade av skog och sankmark. Avrinningsområden avgränsades utifrån den nya nationella höjdmodellen (NNH) med två meter upplösning, men gränsdragningen visade sig komplicerad och det krävdes omfattande manuell bearbetning. Endast i ca 50 % av de studerade avrinningsområdena bedömdes avgränsningen vara representativ efter justeringar, men förbättringspotential identifierades i övergripande justeringar för hydrologiska hinder i NNH. Vidare testades om statistiska samband kunde identifieras mellan Topographic wetness index (TWI), Depth-to-water index (DTW) och jordartsfördelning i områdena med de uppmätta halterna av TN och TP. I fördelningen av TN och TP identifierades extremt avvikande värden (outliers) och resultaten från variansanalyserna kunde inte påvisa några samband. Utan outliers korrelerade DTW och TP med högre halter TP i avrinningen från fuktiga områden än från torra områden, vilket däremot inte kunde påvisas med TWI. Det påvisades även högre halter TN i avrinningen från områden utan sandiga jordarter eller förekomst av leriga jordarter, än från områden med sandiga jordarter. Områdena var dock mycket homogena och för mer robusta resultat kan det vara relevant att studera områden som uppvisar större variationer i de studerade egenskaperna. Studien jämförde även uppmätta halter TN och TP i sydöstra Sverige mot beräknade PLC6-typhalter och fann att typhalterna överskattade TN och gav ett rimligt men något underskattat värde för TP. Vidare studier krävs för att kunna bedöma om markfuktighetsförhållanden och jordartsfördelning kan förbättra modelleringen av TN och TP från skog och sankmark.Sources of nutrient loads to the Baltic sea must be identified to implement appropriate measures preventing further eutrophication. Diffuse loads from forests and wetlands constitute a major source of natural nutrient pollution and identification of its controlling factors can improve modelling of nutrient losses across Sweden. This study used the Swedish national digital elevation model to delineate small forested watersheds in the south east of Sweden. In these watersheds the impact of wetness indices and soil distribution were tested on the concentrations of total nitrogen (TN) and total phosphorus (TP) measured downstream. The watersheds delineations were evaluated for quality, and it was suggested that improvements are required to the digital elevation model for hydrological consistency, as ~50 % were deemed inaccurate. The ANOVA results did not detect statistically significant correlations between the topographic wetness index (TWI), depth-to-water index (DTW), or soil distribution and TN or TP respectively. However, when ANOVAs were repeated excluding extreme outliers to ensure homogeneity of variances, the results showed a significant correlation between DTW and TP, and between soil distribution and TN. These findings indicate that catchments with low soil wetness index and high share of sand soils may show lower nutrient losses from forests and wetlands. However, further studies are required to evaluate sources and effects of extreme values of TN and TP, as well as whether soil wetness and distribution can improve modelling of nitrogen and phosphorus to better predict losses to the Baltic sea

A new galloping gait in an insect

An estimated three million insect species all walk using variations of the alternating tripod gait. At any one time, these animals hold one stable triangle of legs steady while swinging the opposite triangle forward. Here, we report the discovery that three different flightless desert dung beetles use an additional gallop-like gait, which has never been described in any insect before. Like a bounding hare, the beetles propel their body forward by synchronously stepping with both middle legs and then both front legs. Surprisingly, this peculiar galloping gait delivers lower speeds than the alternating tripod gait. Why these beetles have shifted so radically away from the most widely used walking style on our planet is as yet unknown

Dung beetles use their dung ball as a mobile thermal refuge

At midday, surface temperatures in the desert often exceed 60°C. To be active at this time, animals need extraordinary behavioural or physiological adaptations. Desert ants, for instance, spend up to 75% of their foraging time cooling down on elevated thermal refuges such as grass stalks [1]. Ball-rolling dung beetles work under similar thermal conditions in South African savannahs. After landing at a fresh dung pile, a beetle quickly forms a dung ball and rolls it away in a straight line, head down, walking backwards [2]. Earlier studies have shown that some dung beetles maintain an elevated body temperature to gain a competitive advantage [3], [4] and [5], and that heat shunting may prevent overheating during flight [6] and [7]. However, we know little about the behavioural strategies beetles might employ to mitigate heat stress while rolling their dung balls. Using infrared thermography and behavioural experiments, we show here that dung beetles use their dung ball as a mobile thermal refuge onto which they climb to cool down while rolling across hot soil. We further demonstrate that the moist ball functions not only as a portable platform, but also as a heat sink, which effectively cools the beetle as it rolls or climbs onto it

Weighted cue integration for straight-line orientation

Animals commonly integrate multiple sources of information to guide their behavior. Among insects, previous studies have suggested that the relative reliability of cues affects their weighting in behavior, but have not systematically explored how well alternative integration strategies can account for the observed directional choices. Here, we characterize the directional reliability of an ersatz sun at different elevations and wind at different speeds as guiding cues for a species of ball-rolling dung beetle. The relative reliability is then shown to determine which cue dominates when the cues are put in conflict. We further show through modeling that the results are best explained by continuous integration of the cues as a vector-sum (rather than switching between them) but with non-optimal weighting and small individual biases. The neural circuitry in the insect central complex appears to provide an ideal substrate for this type of vector-sum-based integration mechanism