Meandering worms: mechanics of undulatory burrowing in muds

Recent work has shown that muddy sediments are elastic solids through which animals extend burrows by fracture, whereas non-cohesive granular sands fluidize around some burrowers. These different mechanical responses are reflected in the morphologies and behaviours of their respective inhabitants. However, Armandia brevis, a mud-burrowing opheliid polychaete, lacks an expansible anterior consistent with fracturing mud, and instead uses undulatory movements similar to those of sandfish lizards that fluidize desert sands. Here, we show that A. brevis neither fractures nor fluidizes sediments, but instead uses a third mechanism, plastically rearranging sediment grains to create a burrow. The curvature of the undulating body fits meander geometry used to describe rivers, and changes in curvature driven by muscle contraction are similar for swimming and burrowing worms, indicating that the same gait is used in both sediments and water. Large calculated friction forces for undulatory burrowers suggest that sediment mechanics affect undulatory and peristaltic burrowers differently; undulatory burrowing may be more effective for small worms that live in sediments not compacted or cohesive enough to extend burrows by fracture.     

Burrow morphology and utilization of the goby (Parapocryptes serperaster) in the Mekong Delta,Vietnam

Some fish species living in mudflats construct burrows for dwelling and hiding. The goby Parapocryptes serperaster is a burrowing fish in mudflats of many estuaries in South East Asia. This study was carried out in the Mekong Delta, Vietnam, to examine burrow morphology and usage by this species. Morphology of the burrows constructed by P. serperaster was investigated by resin castings in situ to obtain the physical structure and configuration of each burrow. Fish from the burrows were caught and measured before burrow casts were made. Fish burrows comprised several openings, a few branching tunnels and multi-bulbous chambers. The surface openings were circular, and the shapes of branching tunnels were nearly round. The burrows had interconnected tunnels and various short cul-de-sac side branches. The burrow structure differed between fish sizes, but burrow dimensions were positively correlated with fish size, indicating that larger fish can make larger and more sophisticated burrow. The burrow structure and dimensions were not different between the dry and wet seasons. Laboratory observations showed that P. serperaster used body movements to dig burrows in the sediment. Burrows could provide a low-tide retreat and protection from predators, but were not used for spawning and feeding for this goby species. This study indicates that the burrowing activity of gobies is an important adaptation for living in shallow and muddy habitats.     

A comparison of crayfish burrow morphologies: Triassic and Holocene fossil,paleo‐ and neo‐ichnological evidence,and the identification of their burrowing signatures

The architectural and surficial morphologies of crayfish burrows from the Upper Triassic Chinle Formation and Holocene sediments were compared in order to determine: 1) if Triassic burrows could truly be attributed to crayfish activity; 2) how comparable the burrowing mechanisms are; and 3) whether or not a common set of burrowing signatures could be identified for both ancient and modern freshwater crayfish. Materials used in this study include burrows from the members of the Upper Triassic Chinle Formation, casts of modern burrows constructed by Procambarus clarkii Hobbs and Procambarus acutus acutus (Girard) in the laboratory, and casts of naturally constructed modern burrows of Cambarus diogenes di‐ogenes (Girard).

Triassic and Holocene crayfish burrow morphologies exhibit simple to complex architectures, varying degrees of branching, chamber, and chimney development. They also exhibit relatively textured surficial morphologies (bioglyphs) such as scrape and scratch marks, mud‐ and lag‐liners, knobby and hummocky surfaces, pleopod striae, and body impressions. Holocene crayfish construct distinctive burrows due to their conservative limb arrangement, functional morphology, and behavior with respect to environmental stimuli. Similarities between Holocene and Triassic crayfish burrows suggest that extant and Triassic crayfish employed identical burrowing mechanisms. Features of the surficial and architectural morphologies impart a distinctive signature to burrows of both ancient and modern freshwater burrowing crayfish.

Burrowing signatures of crayfish can be used to identify new and previously misinterpreted continental trace fossils. These are useful in studies of the paleohydrogeology, paleoclimatology and paleoecol‐ogy of burrow‐bearing deposits.     

Application of morphologic burrow interpretations to discern continental burrow architects: Lungfish or crayfish?

A methodology for trace fossil identification using burrowing signatures is tested by evaluating ancient and modern lungfish and crayfish burrows and comparing them to previously undescribed burrows in a stratigraphic interval thought to contain both lungfish and crayfish burrows. Permian burrows that bear skeletal remains of the lungfish Gnathorhiza, from museum collections, were evaluated to identify unique burrow morphologies that could be used to distinguish lungfish from crayfish burrows when fossil remains are absent. The lungfish burrows were evaluated for details of the burrowing mechanism preserved in the burrow morphologies together forming burrowing signatures and were compared to new burrows in the Chinle Formation of western Colorado to test the methodology of using burrow signatures to identify unknown burrows.

Permian lungfish aestivation burrows show simple, nearly vertical, unbranched architectures and relatively smooth surficial morphologies with characteristic quasi‐horizontal striae on the burrow walls and vertical striae on the bulbous terminus. Burrow lengths do not exceed 0.5 m. In contrast, modern and ancient crayfish burrows exhibit simple to highly complex architectures with highly textured surficial morphologies. Burrow lengths may reach 4 to 5 m.

Burrow morphologies unlike those identified in Gnathorhiza aestivation burrows were found in four burrow groups from museum collections. Two of these groups exhibit simple architectures and horizontal striae that were greater in sinuosity and magnitude, respectively. One of these burrows contains the remains of Lysoro‐phus, but the burrow surface reveals no reliable surficial characteristics. It is not clear whether Lysorophus truly burrowed or merely occupied a pre‐existing structure. The other two groups exhibit surficial morphologies similar to those found on modern and ancient crayfish burrows and may provide evidence of freshwater crayfish in the Permian.

Burrows from the Upper Triassic Chinle Formation in western Colorado exhibit simple to moderately complex architectural morphologies, ranging from predominantly vertical, unbranched, with little or no chamber development to predominantly vertical, few branches, and with minor chamber development. Surficial burrow morphologies are moderate to highly textured. The burrows have scrape marks, scratch marks, mud and lag‐liners, knobby surfaces, pleopod striae, and body impressions.

Although no fossil remains of the burrowing organism were found within or associated with the Chinle burrows from western Colorado, the similarity of architectural and surficial burrow morphologies to those in the Chinle of Canyonlands, Utah and to modern crayfish burrows, clearly indicates that the Colorado burrows are the product of burrowing crayfish rather than lungfish. Evaluation of burrowing signatures preserved in the architectural and surficial burrow morphologies is a very useful tool to compare and contrast Chinle burrows from different regions on the Colorado Plateau. Documentation of crayfish burrows in the Chinle of Utah and Colorado strongly suggests that other large‐diameter Chinle burrows elsewhere on the Colorado Plateau and in stratigraphically equivalent units may also be the product of crayfish activity.     

Short-term influence of recolonisation by the polycheate worm [2pt] Nereis succinea on oxygen and nitrogen fluxes and [2pt] denitrification: a microcosm simulation

The short-term effects of sediment recolonisation by Nereis succinea on sediment-water column fluxes of oxygen and dissolved inorganic nitrogen, and rates of denitrification, were studied in microcosms of homogenised, sieved sediments. The added worms enhanced oxygen uptake by the sediments, due to the increased surface area provided by the burrow walls and the degree of stimulation was stable with time. Similarly, ammonium fluxes to the water column were stimulated by N. succinea, but declined over the 3 day incubation in all microcosms including the controls. Nitrate fluxes were generally greater in the faunated microcosms, but highly variable with time. Denitrification rates were positively stimulated by N. succinea populations, denitrification of water column nitrate was stimulated 10-fold in comparison to denitrification coupled to nitrification in the sediments. Rates of denitrification of water column nitrate were not significantly different from rates in undisturbed sediment cores with similar densities of N. succinea, whereas rates of coupled nitrification–denitrification were 3-fold lower in the experimental set-up. These results may reflect the relative growth rates of nitrifying and denitrifying bacteria, which allow more rapid colonisation of new burrow surfaces by denitrifier compared to nitrifier populations. The data indicate that recolonisation by burrowing macrofauna of the highly reduced sediments of the Sacca di Goro, Lagoon, Italy, following the annual dystrophic crisis, may play a significant role in the reoxidation and detoxification of the sediments. The increased rates of denitrification associated with the worm burrows, may promote nitrogen losses, but due to the low capacity of nitrifying bacteria to colonise the new burrow structures, these losses would be highly dependent upon water column nitrate concentrations.     

Continuously measured changes in redox potential influenced by oxygen penetrating from burrows of Callianassa subterranea

A continuously recording multiple redox sensor was used to measure oxygenation effects in otherwise anoxic sediments. The sensor reacted within three minutes (increase > 50 mV) to oxygen penetrating into the sediment. Different patterns of oxidation/reduction were observed along burrows of sediment-dwelling infauna. Examples of an increase of more than 600 mV lasting 50 hours and short, 1 to 4 hours long, oxidation pulses of 280 mV increase are given. Redox patterns similar to the ones observed along natural burrows were generated using artificial burrows. Oxygen penetrated up to 1 mm by diffusion from burrow walls into the adjacent sediment, whereas elevated Eh values were measured within 3 mm of the burrow. The oxygenation effect of a burrowing shrimp, Callianassa subterranea, was studied in a 3-month experiment with ten sensors and five animals. For one individual we recorded 108 events with Eh increasing by more than 300 mV. It was estimated that below every square meter of sediment surface 0.7 m². burrow surface are subjected to oxygenation at least once per day by the population of Callianassa in the southern North Sea.     

The Contribution of Crab Burrow Excavation to Carbon Availability in Surficial Salt-marsh Sediments

Geomorphology, vegetation and tidal fluxes are usually identified as the factors introducing variation in the flushing of particulate organic matter (POM) from tidal marshes to adjacent waters. Such variables may, however, be insufficient to explain export characteristics in marshes inhabited by ecosystem engineers that can alter the quantity and quality of POM on the marsh surface that is subject to tidal flushing. In this study we evaluated the balance between transfer of buried sedimentary organic carbon (C) to the marsh surface due to crab excavation (measured from the mounds of sediment excavated from burrows) and outputs of C from the surface due to sediment deposition within crab burrows (estimated from sediment deposited within PVC burrow mimics), in a Southwestern Atlantic salt marsh supporting dense (approximately 70 ind m⁻²) populations of the crab Chasmagnathus granulatus. C excavation by crabs was much greater than deposition of C within crab burrow mimics. Per area unit estimates of the balance between these two processes indicated that crabs excavated 5.98 g m⁻² d⁻¹ and 4.80 mg m⁻² d⁻¹ of total and readily (10 d) labile C, respectively. However, sediments excavated by crabs showed a significantly lower content of both total and readily-labile C than sediment collected in burrow mimics. This indicates that ecosystem engineering by burrowing crabs causes a net decrease in the concentration of C in the superficial sediment layers and, thus, an overall decrease in the amount of C that can be washed out of the marsh by tidal action. Incorporating the in situ activities of ecosystem engineers in models of marsh export should enhance understanding of the function of marshes in estuarine ecosystems.     

Influence of age and environmental factors on burrow-making behavior of the short-tailed cricket,Anurogryllus muticus (De Geer) (Orthoptera: Gryllidae)

For the short-tailed cricket, Anurogryllus muticus, burrow-making behavior is essential. All nymphal instars construct burrows, but in the adult stage the rate of burrowing behavior is age dependent. Increases in photophase and light intensity stimulate burrowing, and the explicit negative phototaxis is correlated with the cricket's inability to exist under dry conditions. Ingestion of substrate during burrow construction may serve to acquire additional moisture. There is no evidence of burrow recognition, and crickets can construct a burrow when needed. The natural distribution of burrows at the plot investigated on Moorea supports the notion thatA. muticus builds burrows where the preferred food plantAlysicarpus vaginalis is most abundant. By minimizing the traveling distance to food sources when foraging they can retreat to their burrow again.     

Burrow structure and foraging costs in the fossorial rodent,Thomomys bottae

Summary A model for calculating the energy cost of burrowing by fossorial rodents is presented and used to examine the energetics of foraging by burrowing. The pocket gopher Thomomys bottae (Rodentia: Geomyidae) digs burrows for access to food. Feeding tunnels of Thomomys are broken into segments by laterals to the surface that are used to dispose of excavated soil. Energy cost of burrowing depends on both soil type and on burrow structure, defined by the length of burrow segments, angle of ascent of laterals, depth of feeding tunnels, and burrow diameter. In a desert scrub habitat, Thomomys adjust burrow segment length to minimize cost of burrowing. Observed segment lengths (mean=1.33 m) closely approximate the minimum-cost segment length of 1.22 m. Minimizing energy expended per meter of tunnel constructed maximizes efficiency of foraging by burrowing in the desert scrub. Burrow diameter and cost of burrowing increase with body size, while benefits do not, so foraging by burrowing becomes less enconomical as body size increases. Maximum possible body size of fossorial mammals depends on habitat productivity and energy cost of burrowing in local soils.     

The effects of temperature and salinity on ventilation behavior of two species of ghost shrimp (Thalassinidea) from the northern Gulf of Mexico: a laboratory study

Thalassinidean shrimp are among the most important bioturbators in coastal ecosystems. The species Lepidophthalmus louisianensis and Callichirus islagrande are found in dense aggregations (up to 400 burrows m⁻²) along sandy and muddy shores of the northern Gulf of Mexico. These shrimp actively ventilate their burrows to provide oxygen and eliminate wastes. In doing so, they expel nutrient-rich burrow water to the overlying water column, potentially altering nutrient cycling and benthic primary productivity. To develop a mechanistic understanding of the role of burrowing shrimp in nutrient processes, we must first examine how changes in environmental conditions alter the frequency, strength, and duration of ventilation. Field measurements of burrow temperature and salinity suggest that the burrow serves as a buffer from the highly variable conditions found in these estuarine, intertidal habitats. Temperatures at sediment depths >30 cm were generally warmer in winter and cooler in summer than at the sediment surface. Burrow salinities, measured at low tide, were consistently higher than adjacent open water. We used these measurements to parameterize laboratory studies of burrow ventilation in artificial burrows made of plastic tubing and in more natural sediment mesocosms, and studies of oxygen consumption in small glass containers. Rates of oxygen consumption and burrow ventilation by L. louisianensis were lower than those of C. islagrande, perhaps reflecting a lower overall activity rate in the former species which resides in less permeable sediments. Generally, increased temperature had a significant positive effect on oxygen consumption for both species. Salinity had no effect on oxygen consumption by L. louisianensis, reflecting the ability of this species to exist in a wide range of salinities. In contrast, oxygen consumption rates of C. islagrande, which is less tolerant of low salinity, were significantly higher at 35‰ than at 20‰. Ventilation rates were highly variable, and shrimp in artificial burrows tended to have consistently higher ventilation rates than those in sediment mesocosms. There is a trend toward more frequent ventilation at 30 °C for both species. Salinity had no effect on ventilation for either species. Our results suggest that thalassinideans exhibit highly variable and species-specific ventilation patterns that are more likely to be affected by temperature than salinity. Increased ventilation at higher temperatures seems to coincide with increased oxygen consumption at these temperatures, although a similar finding was not made for salinity treatments.     

Direct evidence of a mesopelagic fish,Melanostigma atlanticum (Zoarcidae) spawning within bottom sediments

Synopsis Adult Melanostigma atlanticum were observed living 15 to 32 cm deep within sediment box cores from 350 m depth in the Laurentian Trough of the Maritime Estuary of the St. Lawrence. The fish were found in fluid pockets or burrows of brown-coloured (oxygenated) silty clay within the anoxic zone of the sediment. In August 1983, four individuals (3 male and 1 female) were recovered from pockets located 15 and 32 cm below the sediment surface. All specimens were greater than 100 mm in length, had empty stomachs, and the female carried no eggs. In July 1985, 6 individuals (1 male and 5 females) were found in a head to tail arrangement within a single burrow at 18–20 cm depth. Three of the females carried eggs, while two appeared to have already released their eggs. A cluster of about 50 large eggs, some of which appeared to have been fertilized, was also found in the burrow. Most of the stomachs were empty, but one contained two copepods, another a proceroid cestode, and others contained between 4–22 parasitic hemiurid trematodes. These observations indicate that burrowing into bottom sediments is important in the reproductive behaviour of M. atlanticum.     

COMPACT DISC REVIEW

ABSTRACT

In previous studies, calling sites of two species of burrowing frogs Eupsophus in southern Chile have been shown to amplify conspecific vocalizations generated externally, thus providing a means to enhance the reception of neighbour's vocalizations in breeding aggregations. In the current study the amplification of vocalizations of Eusophus roseus was investigated to explore the extent of sound enhancement reported previously for two congeneric species. Advertisement calls broadcast through a loudspeaker placed in the vicinity of a burrow, monitored with small microphones, are amplified by up to 14 dB inside cavities relative to outside. The fundamental resonant frequency of burrows, measured with broadcast noise and pure tones, ranges from 345–1335 Hz; however it is not correlated with burrow length. The spectra of incoming calls are altered inside burrows by predominantly increasing the amplitude of lower relative to higher harmonics. The call amplification effect inside burrows of E. roseus parallels the effect reported previously for two congeneric species and reinforces the suggestion that sound enhancement inside calling sites has a widespread effect on signal reception by burrowing animals.     

Behavior of the burrowing shrimp Alpheus macellarius in varying gravel substrate conditions

The behavior of the male snapping shrimp, Alpheus macellarius (Decapoda: Alpheidae), was studied in tank experiments with four levels of gravel content for five consecutive days. Markov-chain type analyses identified significant behavioral states that were grouped into six behavioral classes (Wandering, Surveying, Grooming and Foraging, Resting, Hidden). Significant trends in the durations and frequencies of the classes were found across days and periods, with shrimps mainly wandering and burrowing during the first day. Grooming, foraging and resting became more frequent during succeeding days. Shrimps built burrows in the morning and then groomed and foraged mostly in the afternoon. This periodicity implies a greater priority for the shrimp to construct and maintain a burrow rather than forage. Gravel content did not significantly influence burrowing behavior, but marked variations were noted in burrowing success, burrow structure and stability. Animals in 15% and 25% gravel substrates produced more burrows of greater complexity, which lasted longer than those of the shrimps in 0% and 5% gravel setups. Feeding was mainly sediment scavenging suggesting the predominance of a deposit-feeding trophic mode. Modifications in burrow construction and adaptations in burrowing and feeding indicate the ability of A. macellarius to respond to different environmental conditions.     

Large tetrapod burrows from the Middle Triassic of Argentina: a behavioural adaptation to seasonal semi‐arid climate?

Krapovickas, V., Mancuso, A.C., Marsicano, C.A., Domnanovich, N.S. & Schultz, C.L. 2013: Large tetrapod burrows from the Middle Triassic of Argentina: a behavioural adaptation to seasonal semi‐arid climate? Lethaia, Vol. 46, pp. 154–169. We report the discovery of large burrow casts in the early Middle Triassic Tarjados Formation, at Talampaya National Park, north‐western Argentina. Facies analysis indicates the burrows are preserved in sandbars deposited by an ephemeral river under semi‐arid and seasonal climatic conditions. The structures are mostly preserved in longitudinal cross‐section and consist of an opening, an inclined tunnel (ramp), and a terminal chamber. The ramp is 8–14 cm in height, up to 130 cm in length and penetrates 49–63 cm bellow the palaeosurface with an inclination of 22°–30°. We studied burrow cast dimensions, overall architectural morphology, surficial marks, and compared them with other large burrows of both invertebrate and vertebrate origin. A tetrapod origin of the burrow casts was established based on: distinctive architecture, and size, which is more than twice the most common size range for large terrestrial invertebrate burrows. Comparison with other Upper Permian and Triassic tetrapod burrows allows us to identify three general morphological groups: (1) simple inclined burrows; (2) helical burrows; and (3) burrow network complexes, representing different behaviours. A study of tetrapod body fossils preserved within other Upper Permian and Triassic burrows shows that the Tarjados structures were most likely produced by non‐mammalian cynodonts. The environmental and climatic context suggests that aridity and seasonality played a fundamental role selecting burrowing behaviour in therapsids and that by the Early–Middle Triassic their burrowing behaviour attained a complexity comparable to modern mammals. □Argentina, behaviour, palaeoclimate, Permo‐Triassic, Tarjados Formation, Tetrapod burrows.     

Microhabitat and rhythmic behavior of tiger beetle Callytron yuasai okinawense larvae in a mangrove forest in Japan

Mangrove forests are regularly flooded by tides at intervals of approximately 12.4 h (tidal rhythm). Larvae of the tiger beetle Callytron yuasai okinawense in a mangrove forest made shallow burrows in mounds up to 1 m in height constructed by the mud lobster Thalassina anomala. No larval burrows were observed on the forest floor, which was very muddy even during low tide. Some larvae plugged the burrow openings before they were submerged at high tide. The mean interval between consecutive burrow plugging events was 12.37 h, which is similar to the period of tidal cycles. Nine out of 30 larvae plugged the burrow openings even when the burrows did not become submerged. Plugging behavior may be governed by an endogenous biological clock, or may be a response to exogenous information about tidal level (e.g. moisture seeping through the ground).     

An assessment of the breeding range,colony sizes and population of the Westland petrel (Procellaria westlandica)

Abstract

The Westland petrel (Procellaria westlandica) is an endemic New Zealand species and one of the very few burrowing seabird species still breeding on mainland New Zealand. It nests only on a series of coastal ridgelines near to Punakaiki on the West Coast of the South Island. Between 2002 and 2005, surveys were undertaken at 28 of the 29 known colonies. The area occupied by the colonies was 73 ha; most colonies had fewer than 50 burrows, but six colonies had 201–500 burrows and four colonies had more than 1000 burrows. We find that the current breeding range of Westland petrel and the location of individual colonies are similar to those reported in both the 1950s and 1970s. Based on total burrow counts at 28 colonies and burrow occupancy rates determined by annual monitoring, the annual breeding population is estimated to be between 2954 and 5137 breeding pairs.     

Conichnus Männil 1966, not conichnus Myannil 1966

Vertically curved, unbranched, strongly lined burrows are prominent features of muddy outer‐shelf/offshore deposits of Cretaceous to Pleistocene age. Such burrows are usually called Terebellina, but use of this ichnogenus should be avoided for two reasons: 1) the type species of Terebellina is a large agglutinated foraminiferid referable to Bathysiphon, not a trace fossil; and 2) most trace fossils identified with Terebellina are either examples of Palaeophycus (in cases where burrows are essentially horizontal and occur individually) or Schaubcylindrichnus (where burrows are oblique to stratification and occur in aggregations or loose clusters). The replacement name S. freyi is proposed for this latter category of burrow.

These light‐colored, grain‐lined burrows are conspicuous structures in the dark‐colored, outer‐shelf mudrocks of the lower Pleistocene Rio Dell Formation. The burrows are part of a typical “distal Cruziana ichnofacies”; association of structures, including Teichichnus, Planolites, Palaeophycus, and Chondrites, occurring in thoroughly bioturbated background sediments. Schaubcylindrichnus freyi in the Rio Dell is interpreted as the dwelling structure of a gregarious, endobenthic organism, comparable to the lined burrows produced by the deposit‐feeding polychaete Clymenella.     

When seasnake meets seabird: Ecosystem engineering,facilitation and competition

Ecosystem engineers such as burrowing seabirds can increase habitat availability for sympatric taxa – but only if the burrow's owner allows other species to use the newly created shelter site. Our studies on a small Pacific island suggest that an avian burrower (the wedge‐tailed shearwater Puffinus pacificus) is both a facilitator and a competitor for amphibious seasnakes. Video camera inspection of 102 burrows revealed frequent usage of these burrows as retreat sites by the snakes, with Laticauda laticaudata restricted to burrows <4 m from the water's edge, whereas Laticauda saintgironsi often used burrows further inland. Snakes never occupied burrows that contained adult shearwaters, suggesting active burrow defence by the birds. Model snakes that we inserted into burrows were attacked, especially on the head and upper body, and we found one snake pecked to death outside a burrow. Wedge‐tailed shearwaters act as facilitators, creating a thermally favourable microhabitat and substantially enhancing habitat suitability for snakes; but they are also competitors, aggressively competing with snakes for occupancy of the resource that has been created.     

Burrow plasticity in the deep-sea isopod Bathynomus doederleini (Crustacea: Isopoda: Cirolanidae)

We investigated whether the deep-sea isopod Bathynomus doederleini has the capacity to change burrow length in response to changes in environmental conditions. We observed burrowing behavior in individuals that were placed on substrates with either simple (ST) or complex (CT) surface topographies. Individuals in the ST group (N = 10) constructed seven burrows. The mean ratio of the burrow length to body length was 1.8. The individuals in the CT group (N = 10) constructed eight burrows with a mean ratio of burrow length to body length of 2.5. Thus the burrows were significantly longer in the CT group. In addition, the isopods in the CT group often incorporated a chamber in the mid-section of the burrow. Our results may be used to infer the determinants of burrow morphology and speculate about the lifestyle of this species in the deep sea.     

Development of an index of abundance for pygmy rabbit populations

The pygmy rabbit (Brachylagus idahoensis) is a cryptic, burrowing lagomorph of conservation concern for which an efficient method to monitor populations is needed for conservation planning. We developed an index of abundance based on density of active burrow systems at 7 sites (57.2–118.5 ha) in east central Idaho. We conducted censuses of burrow systems and used mark-resight surveys of 80 radio-collared individuals to estimate density of rabbits. At 5 sites, we also used a second method to estimate rabbit numbers based on presence of tracks in snow around burrow systems. We evaluated patterns of burrow use by individuals and examined the relationship between vegetation structure and density of rabbits. Density of active burrow systems varied from 0.19 to 3.46 per ha, and density of rabbits ranged from 0.02 to 0.46 per ha. Number of burrow systems used by individuals increased with density of available burrows, which supported a nonlinear relationship between abundance of burrows and rabbits. Population density increased curvilinearly with density of active burrows accounting for >75% of the variation (r² = 0.79) in population estimates across sites. We documented a positive relationship between visual obstruction of vegetation and density of rabbits across 6 of the study sites. Our results suggest that density of burrows can serve as an index for monitoring changes in abundance of pygmy rabbits in east central Idaho and that this index also might be useful for monitoring changes in relative abundance over time at other locations. To assess abundance at larger spatial scales or across different regions, the index should be calibrated under regional conditions and site-level covariates should be evaluated. © 2011 The Wildlife Society.