Tierra de Oro Laboratory

Physiology and Ecology in the Pinyon-Juniper Savannah

Beetles on the Edge?

Physiology and Temperature Sensitivity of a Big Beetle and its Mimic

As temperatures get warmer everywhere, including here in the Santa Fe area, how well will beetles be able to adapt? Ground-dwelling arthropods such as Tenebrionid beetles play a huge role in nutrient cycling in arid environments, so the loss of common species will have effects that cascade through the ecosystem.

Large black beetle held in fingers.
Eleodes obscura. This subspecies, E. o. dispersa is distinguished by the relatively low ridges on the elytra with many, irregular punctures. Note the fingers stained brown by defensive secretions from the beetle.

Eleodes obscura is a large (> 3 cm), common, widespread species of beetle. They can be found from northern Mexico to Southern Washington, and east to Montana (see filled symbols in the map below). There are currently four recognized subspecies based on the patterning of the elytra. The subspecies at the lab, E. o. dispersa, is characterized by comparatively weak ridges on the elytra. Like all species of Eleodes, E. obscura produces defensive compounds from glands in the rear of the abdomen.

Map of Western North America, showing distributions of subspecies of two species of darkling beetle. Red diamonds and green circles cluster, indicating some subspecies overlap.
Map of Southwestern United states, showing the distributions of subspecies of Eleodes obscura and Philolithus elatus. There are two pairs of subspecies that cluster relatively well: P. e. elatus (open red diamonds) overlaps with E.o. glabriuscula (solid red diamonds) and P. e. infernus (open green circles) corresponds with E. o. dispersa (solid green circles). The overlap of the other subspecies is less clear. Two subspecies, E. o. obscura and P. e. difformis have relatively narrow distributions, but only overlap with two other subspecies, P. e. compar and E. o. sulcipennis. which have wide distributions. Adapted from Brown (1971) and Trplehorn et al. (2015).
Philolithus elatus, a mimic of E. obscura. The elytral pattern on this subspecies, P. e. infernus, resembles that of E. o. dispersa, although the prothorax looks very different. 7/29/24.

The species Philolithus elatus is supposed to be a Batesian mimic of Eleodes obscura. Philolithus belongs to a subfamily of Tenebrionids that do not produce defensive compounds. Instead, P. elatus physically resembles E. obscura, and it is thought that this resemblance deters predators from eating them. When an undefended species (mimic) looks like one that is defended (model), it is referred to as Batesian mimicry, and it is thought that the mimic will have an advantage because it does not need to expend energy in producing an actual defense.

Like E. obscura, P. elatus has multiple recognized subspecies (open symbols on map). Brown (1971) reported a correspondence between the distribution of the subspecies of E. obscura and P. elatus, such that those with the most resemblance were found together. For example, the elytra of E. o. dispersa (filled green circles on map) have less distinct grooves and more little punctures than subspecies found elsewhere. Philolitus elatus infernus (open green circles on map), the subspecies whose range overlaps that of E. o. dispersa, has a similar pattern on its elytra. It has also been reported that P. elatus mimics the defensive behavior of E. obscura, which performs “headstands” to raise its abdomen and deploy its defensive glands.

The model-mimic relationship between the two species allows us to test the hypothesis that chemical defense has a metabolic cost. It seems reasonable that maintaining and filling the glands will cost energy. Can this cost be measured?

The goal of this study is to better understand the relationship between the two species, and experiments are ongoing to answer the folowing questions:

  1. How closely does the headstanding behavior of P. elatus match that of E. obscura? We have been performing field observations to compare the responses of the two species to an approaching threat.
  2. Is there a measurable difference in metabolic rate, measured as oxygen consumption, between the two species? Resting oxygen consumption is being measured using coulometric respirometry.
  3. Is there a difference between the heat tolerance of the two species? Oxygen consumption is being measured at multiple temperatures to determine if they have different susceptibility to thermal stress.
  4. Are the lifespans of the two species different? Adult Eleodes obscura are known to live for several years, while relatives of P. elatus live only a single season. Is P. elatus also short-lived?