The Japanese rhinoceros beetle (Trypoxylus dichotomusTrypoxylus dichotomus (Linnaeus, 1771)), also known as the Japanese rhino beetle, the Japanese horned beetle, or by its Japanese name (, or ), is a species of rhinoceros beetle. They are commonly found in continental Asia in countries such as China, the Korean peninsula, Japan, Vietnam, Burma, Thailand, and Taiwan. In these areas, this species of beetle is often found in broad-leaved forests with tropical or sub-tropical climates. This beetle is well known for the prominent cephalic horn found on males. Male Japanese rhinoceros beetles will use this horn to fight other males for territory and access to female mating partners. Upon contact, males will attempt to flip each other onto their backs or off of their feeding tree. In response to selective pressures, smaller male T. dichotomus have adapted a "sneak-like behavior". These smaller beetles will attempt to avoid physical confrontation with larger males and try to mate with females.
List of subspecies Trypoxylus dichotomus corniculatus Trypoxylus dichotomus dichotomus: Mainland China, the Korean Peninsula Trypoxylus dichotomus septentrionalis: Honshu, Shikoku, Kyushu, Tsushima Island Trypoxylus dichotomus shennongji Trypoxylus dichotomus shizuae: Yakushima Island, Tanegashima Island Trypoxylus dichotomus takarai Okinawa Trypoxylus dichotomus tsuchiyai: Kuchinoerabu Island Trypoxylus dichotomus tsunobosonis: Taiwan Trypoxylus dichotomus xizangensis
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Hwang, Seul-Ma-Ro, The Dynastini of the World Coleoptera: Scarabaeidae: Dynastinae. Published by Nature & Ecology, Korea, 2011
Description These beetles have a dark brown and red appearance. However, their bodies can appear to be black without direct light. On average, males tend to measure between 40 and 80 mm, while females are typically smaller, growing between 35 and 60 mm long. Male A. dichotoma have a distinct sexually dimorphic horn protruding from the base of its head which can reach a length of up to one-third of its body length. The length of the male T. dichotomus elytra has been recorded to be between 19 and 33 mm and the male horn can range between 7 and 32 mm. As the horn is a sexually dimorphic trait, only male Japanese rhinoceros beetles will grow one. This cephalic horn is typically somewhat thin and "pitchfork shaped". This appendage acts as a lever arm and is commonly used as a tool to fight other males for access to territory and females. Despite the large size of the cephalic horn, male Japanese rhinoceros beetles are still capable of flight; male and females have been reported to fly at similar average speeds. Males with proportionally large horns compared to their body size possess larger wings to compensate.
Geographic range T. dichotomus can be found widely distributed throughout Asia, including China, Japan, Taiwan, Vietnam, Myanmar, Laos, India, Thailand, and the Korean Peninsula.
Habitat This beetle species prefers to live in broad-leaved forests with tropical or sub-tropical climates. They can also be found often in mountainous environments. Across populations and regions, male beetles can vary greatly in size and horn performance, and it is suggested that differences are due to relative intensities of selection.
Bark-carving behavior Adult Japanese rhinoceros beetles un-burrow from the earth during the summer months between June and August. They prefer to congregate on wounded tree trunks. Quercus acustissima, Quercus serrata, and Quercus mongolica grosseserrata are the most common trees they choose. A tree wound is caused by boring insects which break through the exterior of the tree and feed on the nutrient-rich sap on the interior. Adult T. dichotomus takes advantage of the easily accessible food and consume the exposed tree sap. A subspecies of T. dichotomus known as Trypoxylus dichotomus septentrionalis exhibits bark-carving behavior. This variety of Japanese rhinoceros beetle does not require other insects to breach the tough arboreal exterior to access sap. Notably, these beetles conduct this behavior on Fraxinus griffithii trees, which have a thinner bark than the aforementioned species; this thinner exterior is considerably easier to cut through. These beetles cut into the tree by using their clypeus as a chisel. They hold on tightly to the tree and move their head back and forth to cut into the bark. For a short time, sap flows out of the newly made wound, and the Japanese rhinoceros beetle can feed. After a few minutes, the sap stops flowing, so the beetle begins to carve again.
Life cycle Female T. dichotomus beetles oviposit by scattering their eggs in the humus portion of soil during July and September. The larvae feed on the humus, develop into the third instar phase and pupate during June-July of the following year. Adult beetles emerge from the soil in the few months after pupating. It takes T. dichotomus beetles 1 year to develop into adults after being laid as eggs.
Larval behavior
Chemical cues Larval aggregation in T. dichotomus is driven by chemical cues. The larvae in this species burrow into the dirt, so chemical and acoustic cues are more relevant than visual cues. Studies have shown that chemical cues are necessary for larvae gathering. Larvae with nonfunctioning chemosensory organs cannot aggregate, so chemical cues are likely an important signal guiding larval aggregation. Notably, first instar larvae do not aggregate with other larvae because they demonstrate cannibalistic tendencies at this stage. Second and third instar larvae are not cannibalistic, so they aggregate normally. These larvae also do not discriminate based on kinship when they group. They will group at conspecific larvae but do not demonstrate preferences based on shared genetic similarities.
Mature larvae have been shown to construct pupal cells close to groups of larvae that are already living in the soil. These larvae recognize chemicals produced by other larvae and use these signals to determine where they will make their pupation site. One main benefit of grouping is an increase in diet quality for the larvae. Increase larval activities, like ingesting humus or burrowing, increase symbiotic microorganism activity in the nearby soil. This increase in symbiotic microorganism activity creates a more nutritious diet for the larvae to consume. Two potential costs of group living in these larvae include increased risk of fungal epidemic and increased risk of predation. Metarhizium fungal infection is lethal for T. dichotomus larvae, and grouping together leaves the larvae more susceptible to infection. Mogera imaizumii are able to quickly detect and consume these larvae, so living together leave them more vulnerable to being consumed.
Burrowing T. dichotomus larvae remain buried in the soil until they emerge to breed in the summer. Despite this behavior, the shape of this beetle is not well suited for burrowing. The transverse sectional diameter of the A. dichomota last instar larva is around 20 mm which creates resistance as the larvae move through the soil. To make up for this inefficiency, A. dichotoma developed a rotational burrowing technique. These larvae possess a C-shaped body, so they burrow into the ground by rotating and using their tails to kick soil upward. Kicking the soil up helps the larvae sink into the ground by fluidizing the soil, reducing resistance, and allowing them to burrow more effectively.
Genetics
The horn found in male T. dichotomus is a well-studied example of an exaggerated trait that evolved through intrasexual selection. This horn is a sexually dimorphic trait, which means there must be a sex-determination gene involved in its development and evolution. The doublesex/Mab-3 related (DMRT) transcription factor family is a family of genes that are heavily involved in sexually dimorphic traits. These genes are evolutionarily conserved across many taxa, including worms, mammals, and beetles. The specific variant of doublesex implicated in the Japanese rhinoceros beetle is known as Td-dsx which stands for T. dichotomus dsx homologue. During development, alternative splicing of Td-dsx results in the formation of male and female isoforms of the gene. Td-dsx expression at the horn-forming area of the head increases during the prepupal stage, resulting in horn development in male T. dichotomus. Expression of male and female isoforms of Td-dsx increases during this time, but only the male isoform leads to horn growth. Knockdown of the male isoform of Td-dsx has shown to result in no horn growth in male Japanese rhinoceros beetles.
Physiology
Japanese rhinoceros beetles have rounded pupal horns that have been shown to transform into an angular adult horn following adhesion and shrinkage-based stimuli. This occurs as a part of the beetles' natural metamorphosis as they come into a new exoskeletal morphogenesis. The mechanical mechanisms underlying such a physiological transformation are still relatively unstudied and unknown, but researchers have confirmed that physical stimulation causes pupal remodeling in Japanese rhinoceros beetles. It is entirely possible that a similar mechanism exists in closely related species, which would indicate a genetic underpinning to the morphological mechanism. It is thought that cell division and internal pressure are contributing factors to the alterations of epithelial layers after ecdysis.
Mating
Male-male interactions
T. dichotomaus are well known for their male aggressive behavior. Males will often use their large horns to fight other males over territory and the access to female Japanese rhinoceros beetles. These beetles will often fight on the trunks of host trees to determine who will keep or gain the territory. The goal of these fights is to uproot the opposition and either throw the other male onto their back or off the tree outright. This aggressive behavior has been broken down into four stages.
Stage 1 This stage is known as the encounter, and consists of two males seeing each other but not yet making physical contact.
Stage 2 This stage is known as shoving, and occurs when the males make physical contact and begin to shove each other with their horns. This stage is the most important because the beetles will analyze each other. At this time, each beetle will figure out the size of the opponent and decide if they want to fight or flee.
Stage 3 This stage is known as prying. At this point, the males will use their horns to try and flip the other onto their backs. The battling beetles will proceed to step 4a or 4b depending on the size differences.
Stage 4 Stage 4 consists of 4a and 4b. During stage 4a, known as chasing, if the horn length and/or body size is considerably large, the larger male will chase after the smaller one. The smaller male will retreat. During stage 4b, known as flipping, if the horn length and/or body size is small or negligible, the beetles will fight until one is flipped. This process takes considerably more time and energy than stage 4a.
Sneak-like behavior Smaller T. dichotomus can make use of alternative reproductive behaviors to circumvent horn-to-horn combat. One of these alternative behaviors has been described as "sneak-like behavior" of which there are three variations. The first sneak-like behavior occurs when a male approaches another male from behind while the latter male has already assumed a mounted position on a female. The former beetle will try to use its horn to separate the latter beetle from the female. The second situation occurs when a male approaches another male while the latter male is positioned face-to-face with a female. In this case, the former male will attempt to mount the female. The third type of sneak-like behavior occurs when two males are fighting over a female and a third male attempts to mount the female.
Female-female interactions