Clarke H. Scholtz

Dung Beetles Use the Milky Way for Orientation

When the moon is absent from the night sky, stars remain as celestial visual cues. Nonetheless, only birds, seals, and humans are known to use stars for orientation. African ball-rolling dung beetles exploit the sun, the moon, and the celestial polarization pattern to move along straight paths, away from the intense competition at the dung pile. Even on clear moonless nights, many beetles still manage to orientate along straight paths. This led us to hypothesize that dung beetles exploit the starry sky for orientation, a feat that has, to our knowledge, never been demonstrated in an insect. Here, we show that dung beetles transport their dung balls along straight paths under a starlit sky but lose this ability under overcast conditions. In a planetarium, the beetles orientate equally well when rolling under a full starlit sky as when only the Milky Way is present. The use of this bidirectional celestial cue for orientation has been proposed for vertebrates, spiders, and insects, but never proven. This finding represents the first convincing demonstration for the use of the starry sky for orientation in insects and provides the first documented use of the Milky Way for orientation in the animal kingdom.

A phylogenetic analysis of dung beetles (Scarabaeinae : Scarabaeidae): unrolling an evolutionary history

The phylogeny of the Scarabaeinae, the largest and most important group of dung feeding beetles, is hypothesised based on 200 morphological characters of 50 taxa, representing nearly one quarter of the known genera. We present a drastically different picture of evolution of this highly successful group of beetles than those previously proposed. It is apparent that gross morphology is correlated with either rolling or tunnelling but does not accurately reflect evolutionary history. Results indicate that there are not two separate clades of dung beetles, the rollers and tunnellers, but that rolling behaviour has evolved several times from ancestral tunnellers. The Dichotomiini, Canthonini, and Coprini are poly- or paraphyletic, whereas each of the remaining nine tribes appear as well supported monophyletic clades (the monophyly of the Gymnopleurini was not tested). The genera traditionally included in the Dichotomiini are the oldest and most basal lineages and all other clades, including those of the Canthonini, evolved from ancestral dichotomiine lineages either directly or indirectly. New interpretations of the evolution of rolling, its possible loss, nesting and feeding behaviours, and future changes in classification are discussed. Evidence supports the origin of the Scarabaeinae before the Tertiary and subsequent vicariance of many clades via the breakup of Gondwanaland.

Lunar orientation in a beetle

Many animals use the suns polarization pattern to orientate, but the dung beetle Scarabaeus zambesianus is the only animal so far known to orientate using the million times dimmer polarization pattern of the moonlit sky. We demonstrate the relative roles of the moon and the nocturnal polarized–light pattern for orientation. We find that artificially changing the position of the moon, or hiding the moons disc from the beetles field of view, generally did not influence its orientation performance. We thus conclude that the moon does not serve as the primary cue for orientation. The effective cue is the polarization pattern formed around the moon, which is more reliable for orientation. Polarization sensitivity ratios in two photoreceptors in the dorsal eye were found to be 7.7 and 12.9, similar to values recorded in diurnal navigators. These results agree with earlier results suggesting that the detection and analysis of polarized skylight is similar in diurnal and nocturnal insects.

Twilight orientation to polarised light in the crepuscular dung beetle Scarabaeus zambesianus

SUMMARY The polarisation pattern of skylight offers many arthropods a reference for visual compass orientation. The dung beetle Scarabaeus zambesianus starts foraging at around sunset. After locating a source of fresh droppings, it forms a ball of dung and rolls it off at high speed to escape competition at and around the dung pile. Using behavioural experiments in the field and in the laboratory, we show that the beetle is able to roll along a straight path by using the polarised light pattern of evening skylight. The receptors used to detect this skylight cue can be found in the ommatidia of the dorsal rim area of the eye, whose structures differ from the regular ommatidia in the rest of the eye. The dorsal rim ommatidia are characterised by rhabdoms with microvilli oriented at only two orthogonal orientations. Together with the finding that the receptors do not twist along the length of the rhabdom, this indicates that the photoreceptors of the dorsal rim area are polarisation sensitive. Large rhabdoms, a reflecting tracheal sheath and a lack of screening pigments make this area of the eye well adapted for polarised light detection at low light levels. The fan-shaped arrangement of receptors over the dorsal rim area was previously believed to be an adaptation to polarised light analysis, but here we argue that it is simply a consequence of the way that the eye is built.

Discontinuous Gas‐Exchange Cycles in Scarabaeus Dung Beetles (Coleoptera: Scarabaeidae): Mass‐Scaling and Temperature Dependence

Although discontinuous gas exchange cycles (DGC) are known from many insects, the effects of body size and temperature on DGC have not been widely examined. Here, these effects are investigated in five Scarabaeus dung beetle species from mesic and xeric habitats. The investigation tests two hypotheses: that previous estimates of the scaling exponents for the DGC and its characteristics are more broadly applicable to insects, and that, in response to temperature, both DGC frequency and the quantity of CO2 emitted during the open (O) phase (O‐phase emission volume) are modulated. Like previous workers, we find that V̇co2 scaled as mass0.968 and that O‐phase emission volume scaled as mass0.833. However, temperature‐associated increases in V̇co2 (Q10s of 2.19–2.65) were modulated mostly by increases in DGC frequency since O‐phase volumes remained constant across temperature. Flutter (F)‐phase and O‐phase durations were closely coupled to DGC duration, although the relationship between closed (C)‐phase duration and DGC duration was less pronounced. We show that ventilation phase coefficients, previously considered a measure of the proportional duration of each phase of the DGC, calculated from the slopes of these relationships are a measure of change in phase duration with change in DGC duration and not a measure of the way in which total DGC duration is apportioned among phases. We suggest that proportions be used to estimate the contribution of each of the phases to the total duration of the DGC.

Phylogeny and systematics of the Trogidae (Coleoptera: Scarabaeoidea)

Abstract. A cladistic analysis of the Trogidae using eighteen adult characters resulted in a monophyletic group from which the genera Glaresis Erichson, Afroglaresis Petrovitz and Cryptogenius Westwood are excluded. The Trogidae as here defined comprises three genera, Trox Fabricius (with two subgenera, Trox and Phoberus Macleay), Omorgus Erichson (with three subgenera, Omorgus, Afromorgussubg.n. and Haroldomorgussubg.n.) and Polynoncus Burmeister.

The basal phylogeny of Scarabaeoidea (Insecta : Coleoptera) inferred from larval morphology

Larvae of 60 genera representing the following families and subfamilies of Scarabaeoidea were studied and analysed phylogenetically: Lucanidae (Aesalinae, Nicaginae, Syndesinae, Lampriminae, Lucaninae), Passalidae (Passalinae, Aulacocyclinae), Trogidae, Pleocomidae, Geotrupidae (Taurocerastinae, Lethrinae, Geotrupinae), Bolboceratidae, Ceratocanthidae, Hybosoridae, Glaphyridae, Scarabaeidae (Aphodiinae, Scarabaeinae, Melolonthinae, Dynastinae, Cetoniinae). Seventy-eight larval morphological characters were employed in the analysis. Our data confirm that Dascillidae are not closely related to Scarabaeoidea. The monophyly of the superfamily is supported by 20 apomorphic character states, 18 of them unique. Monophyly of the following scarabaeoid clades is supported (with the number of larval synapomorphies followed by the bootstrap value in parentheses): Scarabaeoidea without Passalidae (6/67), Passalidae (9/100), Pleocomidae (11/93), Trogidae (8/93), Glaphyridae (10/96), Lucanidae (9/95), Ceratocanthidae + Hybosoridae (5/74), Scarabaeinae (9/98). The family Ceratocanthidae was found to be paraphyletic with respect to Hybosoridae. Monophyly of the family Scarabaeidae is not supported. The resolution of the basal parts of the strict consensus tree is higher when using Dascillidae + Eulichadidae v. Agyrtidae + Helophoridae as an outgroup, but the differences in topology become insignificant after bootstrapping. It is suggested that larval morphology alone is not an adequate tool to address basal relationships of Scarabaeoidea and a total evidence analysis should be performed.

Dung feeding in adult scarabaeines (tunnellers and endocoprids): even large dung beetles eat small particles

Abstract 1. The maximum size of ingested particles was determined in 15 species of adult dung beetle (Scarabaeidae: Scarabaeinae) by mixing small latex or glass balls of known diameter into the dung used as food. Twelve species (tribes Coprini, Onitini, Oniticellini, and Onthophagini) were tunnellers (making dung stores for feeding and breeding in the soil below the pat) and three species (tribe Oniticellini) were endocoprids (feeding and breeding in the dung pat itself).

Lethal and sublethal effects of ivermectin on the dung-breeding bettles Euoniticellus intermedius (Reiche) and Onitis alexis Klug (Coleoptera, Scarabaeidae)

Abstract The lethal and sublethal effects of ivermectin residues in cattle dung on the two dung-breeding beetles Euoniticellus intermedius (Reiche) and Onitis alexis Klug were bioassayed in the laboratory. A single standard injection of cattle with ivermectin (200 μg kg−1) prevented adult emergence of E. intermedius from dung collected 2–7 days after treatment. Fewer beetles emerged from the dung of treated animals compared with the dung of control animals, collected 1 and 14 days after ivermectin injection. Adult emergence of O. alexis was reduced between 2 and 7 days after treatment. Dung from treated cattle prolonged the development of E. intermedius for 28 weeks and that of O. alexis for up to 21 weeks after injection. Specimens of E. intermedius that developed in dung collected 1 and 14 days after treatment were less productive than their controls in their first breeding week but not thereafter. It is suggested that cattle should be treated with ivermectin before or after the peak activity period of dung beetles to reduce effects on dung beetle populations.

How dim is dim? Precision of the celestial compass in moonlight and sunlight

Prominent in the sky, but not visible to humans, is a pattern of polarized skylight formed around both the Sun and the Moon. Dung beetles are, at present, the only animal group known to use the much dimmer polarization pattern formed around the Moon as a compass cue for maintaining travel direction. However, the Moon is not visible every night and the intensity of the celestial polarization pattern gradually declines as the Moon wanes. Therefore, for nocturnal orientation on all moonlit nights, the absolute sensitivity of the dung beetles polarization detector may limit the precision of this behaviour. To test this, we studied the straight-line foraging behaviour of the nocturnal ball-rolling dung beetle Scarabaeus satyrus to establish when the Moon is too dim—and the polarization pattern too weak—to provide a reliable cue for orientation. Our results show that celestial orientation is as accurate during crescent Moon as it is during full Moon. Moreover, this orientation accuracy is equal to that measured for diurnal species that orient under the 100 million times brighter polarization pattern formed around the Sun. This indicates that, in nocturnal species, the sensitivity of the optical polarization compass can be greatly increased without any loss of precision.