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Diet

Thorectes lusitanicus is remarkably different from the other dung beetles in the Palearctic region because it is polyphagous, meaning it feeds on many different types of food. While some tropical dung species consume carrion and fruit, Palaearctic dung beetles primarily consume the dung of herbivores and omnivores. Like other Palaearctic dung beetles, T. lusitanicus has exhibited specific dung preferences. In the case of this species, T. lusitanicus prefers rabbit dung over cow dung, which researchers hypothesize is related to the relatively low water content of rabbit dung^[5].

While T. lusitanicus larvae rely on dung consumption, adult T. lusitanicus have been observed consuming acorns, fungi, fruits, carrion, and the dung of various mammals^[1][6]. Researchers used to believe that dung beetles did not consume seeds, but more recent laboratory studies have since shown that adult T. lusitanicus prefer acorn consumption over the consumption of large herbivore dung^[1]. Furthermore, this beetle species exhibit preferences regarding the oak species of the acorns it consumes, which aligns with the patterns observed in acorn-feeding vertebrates^[5]. In order to consume an acorn, a T. lusitanicus beetle will slightly bury it in the soil and then gnaw at one of the ends. After the few weeks it takes to fully consume the acorn, the beetle may stay inside the empty pericarp and hibernate^[1]. However, since dung is required for nesting and larvae feeding and acorns are only abundant seasonally, researchers hypothesize that dung remains the main food source of wild T. lusitanicus throughout the year^[1].

Benefits of Acorn Consumption

Acorns have two primary types of polyunsaturated fatty acids that adequately satisfy the fatty acid nutritional requirement of most insects. They also contain larger amounts of proteins and lipids than cow dung, which is often used as a comparative diet in laboratory studies that investigate the effects of an acorn diet on Thorectes lusitanicus^[5]. Recent studies have discovered that not only does acorn consumption satisfy a dietary need for the species, but it also leads to many physiological and developmental advantages in T. lusitanicus^[5][6].

As one may expect, consumption of these more favorable nutrients found in acorns has been experimentally shown to significantly increase fat body development in T. lusitanicus. Hemolymph composition, which is related to thermal tolerance in beetles, is dependent on the fatty acid and protein synthesis occurring in the fat body. Therefore, acorn consumption also affects hemolymph composition and has been shown to increase resistance to cold temperatures in T. lusitanicus. Laboratory experiments even show that the T. lusitanicus beetles feeding on acorns were more active than those feeding on cow dung at both colder and more moderate temperatures. These acorn-fed beetles also showed greater ovary development as well as larger and more numerous oocytes, or developing eggs, compared to the beetles fed with cow dung^[2]. In laboratory studies, T. lusitanicus fed with acorns and presented with a generalist pathogen had significantly higher levels of phenoloxidase, an enzyme that is commonly utilized to measure the immunity of insects, compared to dung-fed T. lusitanicus. This indicates acorn consumption provides greater resistance to generalist pathogens than cow dung consumption^[7].

Researchers suggest the seasonality of acorn abundance may align with the periods where T. lusitanicus requires the greatest amount of energy since the acorns are abundant in autumn-winter, which is usually when this species reproduces and hibernates. Increased thermal tolerance and immunity increase the survival of T. lusitanicus throughout the winter, especially because starvation is known to decrease insect immunity. Furthermore, these changes may also allow T. lusitanicus to prolong its period of activity before going into hibernation^[7].

Seed Dispersal

Like some tropical dung beetles, the Palaearctic Thorectes lusitanicus beetles can act as seed dispersers by burying dung that contains seeds within the soil, which increases the likelihood that those seeds become established seedlings^[1]. T. lusitanicus is unique in that it also acts as a seed disperser by burying acorns directly because they do not always eat the entire acorn. They have been observed collecting acorns and only partially consuming them before burying them in the soil, meaning they are secondary dispersers of oak acorns^[6]. By eating only a small portion of the acorn (and sometimes none at all), these beetles help the seed germinate and become a seedling, which confers ecophysiological and reproductive advantages for the oak tree^[1][7]. Burying the acorns protects them from more efficient seed predators, leads to the developed seedlings having deeper roots, and increases their survivorship^[1][5]. When compared to rodents, well-known oak seed dispersers, T. lusitanicus was qualitatively more effective. While rodents dispersed greater numbers of acorns, they also consumed more of them and buried them in less suitable micro-habitats^[13]. This has important implications for the conservation of oak forests in the Mediterranean, and researchers suggest that preservation of the interactions between T. lusitanicus and oak species is essential for Mediterranean forest conservation^[1].

Flight

While most dung beetles have long flight wings and can fly several miles at a time, the entire Thorectes genus is apterous, meaning these species have lost their capacity to fly^[2][9]. Some researchers hypothesize that dry and arid conditions facilitated the adaptation of fused elytra, as this fusion reduces water loss and allows water to be utilized for continuous respiration, which is utilized as a mechanism of thermoregulation^[11]. Other researchers are interested in the evolutionary relationship between apterism and acorn consumption in Thorectes lusitanicus. They hypothesize that the apterism may have led to the selection of alternative mechanisms to elevate body metabolism, such as the inclusion of acorns in their diet. Recent phylogenetic evidence indicates that two other dung beetles in the Geotrupidae family also consume acorns: Thorectes baraudi, a close relative of T. lusitanicus, and Mycotrupes lethroides, a very distant relative of T. lusitanicus. Since the three species are not all closely related yet all are apterous, this may provide evidence that apterism can lead to diet shifts^[2].

Olfaction

Insects heavily rely on their olfactory sense to detect and locate food. Laboratory experiments with Thorectes lusitanicus indicate that this species utilizes antennae chemoreceptors when searching for acorns. These experiments also revealed that this acorn selection is mediated by the volatile compound emitted by the acorns since T. lusitanicus was still capable of selecting acorns when the odor is mixed with dung odors^[5].