Calcar, the Tarsal Spur in Chameleons

Abstract

Chameleons of the genus Chamaeleo exhibit unique secondary sexual characteristics, including the male tarsal spur, calcar, a caudal protrusion located on the hind feet. This article investigates the morphology, ontogeny, and potential functions of the tarsal spur, focusing on species differentiation and its implications for chameleon research and breeding. Despite its prevalence in males, the spur's role remains largely speculative, with alternative traits likely influencing sexual selection.

Keywords: Chameleons, tarsal spur, sexual dimorphism, Chamaeleo, morphology, ontogenesis, sexual selection

Introduction

The tarsal spur in chameleons stands out as a fascinating feature of sexual dimorphism within the reptile clade. Specifically, male chameleons of certain species possess this unique appendage, which remains enigmatic in terms of its functional significance and evolutionary implications. The tarsal spur is a skin duplicature - caudal protrusion of the tarsus, located at the plantar level of the hind feet, and is covered with scales analogous to those of surrounding structures. In contrast to Tilbury (2010) and Tolley & Herell (2014), stating the tarsal spur is a caudal protrusion of the tarsal bone, there is none.

Calcar Meaning

The Latin word calcar primarily means "spur," referring to a pointed, often metal device worn on the heel of a boot, used by horse riders to urge their horses forward. The term is derived from the Proto-Indo-European root kʲel-, which denotes action or movement. There is a possible association with the Greek word kálakes (κάλακες, pronounced "ká-lakes"), which also means "spur," though this connection is less clearly defined. This ties back to the function of spurs in encouraging movement and speed.

In anatomy, the term calcar refers to a small anatomical prominence or projection. It can also describe a spur or spurlike projection found in various biological contexts, such as on the base of a petal or on the wing or leg of a bird. In anatomy, it may refer to specific structures, such as a spur of cartilage in bats.

Chameleons with Calcar in Scientific Name

There are four occurrences of the word "calcar" in the taxonomy of chameleons:

• Chamaeleo calcaricarens BÖHME, 1985 is a species ocuring in Ethiopia, Eritrea, Djibouti and Somalia, a sister species of Chamaeleo africanus LAURENTI 1768, differing from the latter mainly by absence of the tarsal spur in male.
• Chamaeleo calcarifer PETERS 1871 is a sister species of Chamaeleo calyptratus DUMÉRIL & BIBRON 1851, ocuring in the Kingdom of Saudi Arabia, previously considered its subspecies.
• Chamaeleo calcaratus MERREM 1820 is a synonym of Chamaeleo africanus LAURENTI 1768
• Chamaeleo calcaratus PETERS 1870 is a synonym of Chamaeleo calyptratus DUMÉRIL & BIBRON 1851.

Morphology and Distribution

The tarsal spur is present in select species of the genus Chamaeleo, notably: Chamaeleo africanus, C. arabicus, C. calcarifer, C. calyptratus, C. dilepis, C. gracilis, C. martensi, C. monachus, C. necasi, C. roperi, C. ruspolii, and C. zeylanicus. Importantly, this characteristic is primarily documented in males, although variations exist among species. 

During embryonic development, the tarsal spur is evident from the early stages and grows in size during proportionally with the individual, maintaining its relative dimension compared to other body parts throughout ontogenesis (Harris & Pough, 2007). As a distinct feature, the spur serves as a diagnostic trait crucial for species identification—facilitating differentiation between closely related taxa such as C. calcarifer and C. africanus (Lutz & Tinsley 2006) and C. dilepis and C. incognitus (Necas 2025a).

Sexual Dimorphism and Variation

In many Chamaeleo species, the spur's presence is consistently recorded in males, while females typically lack this structure. However, notable exceptions exist, such as in Chamaeleo zeylanicus and certain populations of C. dilepis, where females may exhibit varying spur presence and size (Norris & Hall 2020; Tilbury 2010). In this context, sexual dimorphism is not absolute, suggesting complex evolutionary dynamics influencing spur expression.

It has been reported (Necas & Dvorak 2020) that, due to inbreeding and incorrect incubation temperatures, the presence or absence of the tarsal spur in the Yemen chameleon, Chamaeleo calyptratus, can be altered. The following aberrations are documented: 

• presence of an underdeveloped tarsal spur in both males and females, 

• absence of the tarsal spur in males, 

• presence of a fully developed tarsal spur in females,

• unilateral expression of fully developed tarsal spur on one hind foot, while the spur is absent on the other one (Necas 2025b).

Due to this variations, the sex determination of chameleons of the genus Chamaeleo having spurs is problematic and not 100% reliable, despite the hit rate is high. The more inbred the genome becomes, the more aberrant the expression of the spurs are and the more spurless males and more spur-wearing females are present (P. Necas, own observations). All studies therefore, using the spur-based sex determination method as a solely one used (like Andrews 2005) is to be considered with caution, as the results might not be correct. 

Functional Significance

Despite its morphological distinction, the functional role of the tarsal spur remains largely speculative. Observational studies have yet to demonstrate any active use of the spur in behavioral interactions, mating displays, or territorial disputes. Furthermore, neither male selection based on spur presence nor female selection based on spur absence has been corroborated by evidence. This is particularly perplexing given that male chameleons often approach females from various angles, obscuring the spur's visibility (Köhler & Weichert, 2022). Instead, more prominent traits—such as casques, crests, body shapes, and vibrant color patterns—likely serve as the primary determinants for mating selection and behavioral displays, overshadowing the potential influence of the tarsal spur.

Implications for Breeding and Genetic Studies

The tarsal spur plays a significant role in sex differentiation within chameleon breeding programs, especially in captivity. It provides a reliable phenotypic marker for identifying male specimens, thus facilitating controlled breeding strategies (Henderson & al. 2019). However, genetic studies indicate that factors such as inbreeding and improper incubation temperatures can result in aberrations concerning the tarsal spur in specific species, notably the Yemen chameleon (Chamaeleo calyptratus)(Necas & Dvorak 2020). Documented anomalies include underdeveloped spurs in both sexes, absences of spurs in males, the emergence of fully developed spurs in females, and unilateral spur expression on hind feet.

Hind Leg Spurs in Birds and Mammals

The tarsal spur of chameleons is indeed a unique morphological feature, as there is no homologous anatomical structure like it among reptiles. While there are spurs in some amphibians, birds and mammals, these structures are not homologous to the chameleon's spur.

In birds, spurs are generally situated on the hind limbs and can be found in species like the domestic chicken (Gallus gallus domesticus) and certain species of pheasants. These spurs are typically bony projections located on the tarsometatarsus and serve multiple functionalities. They are primarily used in combat during mating displays and territorial disputes, helping males establish dominance over rivals (Harrison 2000).

In mammals, similar spur-like structures are present in monotremes, such as the platypus (Ornithorhynchus anatinus) and echidna (Tachyglossus aculeatus). The platypus spur is a bony structure associated with a venomous gland, capable of producing toxins that can be delivered via the spur during aggressive encounters or threats (Jones et al. 2013). The kick delivered by the spur can cause mechanical injury combined with envenomation, which can be extremely painful. Echidna spurs, however, lack this venomous capability and are primarily used in male-male interactions (Griffiths 2006).

In contrast, the spurs of chameleons consist solely of dermal tissue, lacking a bony base, and have no functional association with any venomous gland. They are not utilized as weapons for defense or aggression, which distinguishes them from the spur-like structures found in birds and mammals.

Human Spurs

Interestingly, even humans possess a spur, albeit in a different context. The spur-like protrusion of the calcaneus, known in Latin as calcar calcanei, serves important mechanical functions in the body (Perry, 2000). Located at the heel, this structure acts as a base of support for the heel during walking, enduring significant mechanical stress from the weight of the body. The calcaneus also serves as the attachment point for the Achilles tendon (tendo Achillis), which is known as the strongest tendon in the human body. This tendon plays a critical role in locomotion, enabling powerful movements such as jumping and running (Khan et al. 2009).

Though the calcaneal spur is often externally invisible in certain ethnic groups (such as Romani people or some Indian tribes), it can be more conspicuous as a heel protrusion in others, like populations from sub-Saharan Africa , indigenous populations of the Americas, south Asian populations. Among these groups, structural adaptations may arise from selective pressures favoring specific forms of walking or running.

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