New World dung beetle (Coleoptera: Scarabaeidae: Scarabaeinae) colonization of a recent Miocene insular territory: The case of Costa Rica

Costa Rica emerged from the seas as a new geological territory during the Miocene as an insular archipelago. It later became part of a continental area once it became a segment of Central America. Two dung beetle genera that colonized this new territory from South and North America, Canthidium and Onthophagus (Coleoptera: Scarabaeidae: Scarabaeinae), are here studied, in the first analysis of a volcanic paleo-archipelago, colonized from its emergence, and then later becoming terra firma. To assess their biodiversity distribution patterns, we analyzed the effect of biogeography, ecosystem origins, and body size on their altitudinal distribution patterns in three geographic basins of Costa Rica. Based on 32 years of collecting representing more than 158,000 specimens from 1017 localities, we undertook Generalized Linear Models of the two dung beetle genera to assess the effects of biodiversity and biogeographical distribution patterns. Canthidium and Onthophagus species ranged from 0 to 3000 m a.s.l., with an abrupt diversity decline at altitudes above 1500 m. Endemic species tended to show a higher altitudinal mean with a narrow altitudinal band distribution than non-endemic dung beetle species. Although there was a trend of decreasing species body size with the increase in altitude, such a trend depended on the distribution pattern of the species group. This possible insular-mediated endemicity mechanism has generated baffling biodiversity levels, considered the highest worldwide per unit area. Costa Rica is an expanse represented by a geographic overlap of two or more temporally disjunct areas and is not part of a natural transition zone. The effect of the insular Miocene origin of Costa Rica still pervades today, reflected by different insular syndromes shown by the dung beetle fauna. The importance of geological origins in generating biodiversity seems to have been an underrated criterion for conservation biology practices and should be considered ex officio.

The evolution of Bolbites onitoides (Coleoptera: Scarabaeidae: Phanaeini): its phylogenetic significance, geographical polychromatism and the subspecies problem

The distribution of the iridescent dung beetle Bolbites onitoides can be divided, based on dorsal colouration, into a blue western-half and a red eastern-half. This has raised the question, in 1959, of whether the two colour variants could represent distinct subspecies. Having examined ~1200 specimens and evaluated the proposal under both an ontological and an operational subspecies concept, we conclude that B. onitoides should continue to be treated as a single monotypic species because: (1) two kinds of colour intermediates were found living among populations of the two main variants; (2) the distribution of the variants overlaps; (3) no other characters were seen to vary consistently in accordance with the colour variants; (4) the overall geographical pattern can be explained by phenomena other than (incipient) speciation, such as phenotypic plasticity and distinct selective regimes; and (5) colour has been extensively shown not to be a reliable indicator of speciation processes among dung beetles. By comparing our findings with other cases of polychromatism among scarabaeines, we discuss publications proposing subspecies taxa relying uniquely upon colour variants. We contend that, due to the often continuous, populational, polytopic and, several times, clinal nature of the intraspecific geographical variation, subspecies classification schemes should not be established simply to classify variation across a species range without a commitment to the reality as evolutionary entities of the taxa being proposed. The alternative stance championed by many contemporary authors to give trinomina to conspecific (meta)population lineages, in turn, may eventually prove to be adequate, but we express some of our concerns as to the feasibility of this practice. Whether these intraspecific taxa should be ranked in a Linnaean hierarchy – e.g. as subspecies – is equally an open question. We also elaborate on an evolutionary scenario where the role of the iridescence in sexual selection, as hypothesized in a previous work, may be an exaptation, not an adaptation, among the horned Phanaeina.

The relationship between altitudinal gradients, diversity, and body size in a dung beetle (Coleoptera: Scarabaeinae: Onthophagus) model system

Mountainous regions represent an excellent model to test ecological hypotheses encompassing assemblage diversity and body traits of species. Among insects, there is no uniform body size pattern across temperature gradients, suggesting that processes controlling body size may differ among species. The aim of this study was to explore diversity and body size patterns of dung beetle species of the genus Onthophagus Latreille, 1802 across altitudinal gradients at two mountains in Mexico. Tropical mountain species were sampled from 2200 to 3400 m a.s.l. In both mountains, there was a decrease of richness and abundance of Onthophagus with increasing altitude. There were contrasting relationships between beetle body size and altitude, which varied depending on the data set analyzed. Furthermore, interspecific variations of body size were not related to the number of altitudinal bands recorded by each species. Species from high altitudes were also recorded at lower mountain altitudes, suggesting that diversity at the highest altitudes represents a subgroup of lowland diversity. This may indicate that the fauna inhabiting higher elevations could be the most eurytopic one. Here we present an example of an assemblage that partially contradicts Bergmann’s rule, with contrasting effects of altitude on dung beetle body size.