Pyramidal growth syndrome is an observable malformation of the carapace, and has long been established as an important indicator of the quality of husbandry (Wiesner et al., 2003). In reality, mild pyramiding is not be harmful to the tortoise (Innis, 1994), however, it is important to notice pyramiding as soon as possible so poor husbandry can be corrected before more serious issues develop.
It is interesting to hear what new tortoise keepers have learned about pyramiding. The current most popular response is that pyramiding is in fact, caused by lack of humidity. Actually, this is only the most current belief among the public. Among the scientific community, however, you will be able to uncover loads of hypotheses, theories, and ideas about how pyramiding arises. There is a rich history of studies focusing on pyramiding that all come to different conclusions. When we read one paper, per-say the one stating that the cause is related to humidity, it may seem we have found all our answers. However, when we step back and consider all scientific findings as a whole, we can see that we have a lot of ideas but no real concrete answers.
Lets take a look at the history of pyramidal research – what are some of the past and present findings? During the 1980’s, the strongest belief was that pyramiding was due to a high protein diet (Steams, 1989; Highfield, 1990). Steams (1989) proposed that excess dietary protein can cause pyramiding. Highfield (1990) subsequently reported that hatchling sulcata tortoises raised on an alfalfa pellet substrate developed more pyramiding than those that were raised on newspaper, even though both groups were fed a low protein vegetable based diet. He speculated that ingestion of the substrate (which was the high protein alfalfa) may have contributed to the shell deformities. [ Really?? ]
Long-time keepers questioned this hypothesis (as they should) because they were not observing a direct relationship between high protein diets and pyramiding. There seemed to be more factors involved. For instance, a personal communication in 1993 with keepers at the Wildlife Survival Center, stated that their young radiated tortoises did not follow this relationship. Their radiated tortoises had always been kept indoors for their first 2 years of life, before going outdoors full-time. Their diet had always been 18% protein. The keepers noticed that the pyramiding was mild to severe during the first two years, then would greatly improve, if not disappear, after going outdoors full-time, despite being fed the same high protein diet. They had suspected that the most important factors were exposure to natural sunlight and more exercise (Innis, 1994; Personal communication with J. Iaderosa, 1993).
Wiesner and Iben (2003) later conducted a study to examine the effect of dietary protein level and environmental humidity on the degree of pyramidal growth. Out of three groups, they found that dry environmental conditions (24.3–57.8% and 30.6–74.8% relative humidity) produced taller pyramids than humid conditions (45‐99% relative humidity). Variable dietary protein, however, resulted in only a minor, positive impact on pyramidal growth. (Wiesner and Iben, 2003). These findings have flourished into a wide-spread belief that pyramiding is, in fact, caused by lack of humidity. But I’m sure if you asked the folks back in 1990, they would tell you that pyramiding is, in fact, caused by a high protein diet.
Despite the explosion of newbie keepers believing that lack of humidity is the true cause, scientists still continue to research alternative causes. Why would they continue to research alternatives when this is apparently a closed case? Other causes that have been studied, include lack of calcium (Liesegang et al., 2007) or the use of supplemental heat at night (Heinrich et al., 2016). These studies found pyramiding was correlated with something other than high protein diets or the lack of humidity.
Image from Heinrich et al. (2016), which found that supplemental heat at night caused pyramiding in leopard and sulcata tortoises even when humidity was kept consistent between the two groups (control group / no supplemental heat, left; treatment group / supplemental heat, right).
The conclusion is that, so far, no one really knows what “causes” pyramiding. The studies outlined so far are faulty for one reason – most were looking for a single environmental cause to pin-point. Pyramiding is a symptom of poor care, and poor care can be caused by any husbandry-related factor.
The most well-rounded and most believable hypothesis as to where pyramiding arises stems from the research done by Alibardi (2005), who studied the proliferation of the epidermis (beta-keratin cells) of chelonians, not necessarily in regards to pyramiding. However, what he found was that the proliferation of the epidermis in scutes of the carapace and plastron in the tortoise (Testudo hermanni) corresponded with seasonal changes. During fall/winter (resting season), only few beta-keratin cells were produced. When growth is resumed in spring (growing season), however, cell proliferation is intense and centralized around hinge regions and tips of scutes. Numerous beta-keratin cells are produced around the hinge regions, forming a new thick corneous layer around the hinge regions, which constitute the growing rings of scutes (see diagrams below). The genes involved in beta-keratin production are largely regulated by hormonal and seasonal changes. In other words, the beta-keratin will be deposited periodically, whether or not a tortoise continues to grow. If they are healthy and quickly growing tortoises, as they should be in the wild, their body volume will increase, allowing the new beta-keratin cells to be deposited in new growth areas. If we consider captive-raised tortoises, which are almost always going to experience less-than-ideal conditions compared to their wild counterparts, there may be a lag in growth. The less increase in body volume, the greater the chances that beta-keratin will be deposited under old-growth, causing some buildup of beta-keratin. Less-than-ideal conditions that may hinder the growth of a tortoise’s body mass may include: high protein, lack of humidity, deficiencies or excess vitamins, minerals, UV light, exercise, water, overall food intake, inappropriate artificial heat (all factors that have been previously suggested to “cause” pyramiding).
Altered image extracted from Alibardi (2005), which shows a schematic representation of how pyramiding may build each growing season if the tortoise mass does not increase.
Altered image extracted from Alibardi (2005), which shows a schematic representation of the hinge region (spreading, actively growing region between scutes). This region will proliferate beta-keratin cells and push them outwards. If old growth remains in its place, it will be uplifted, thereby forming pyramids. This process is regulated by hormonal changes and seasonal patterns.
References:
Alibardi, L. (2005) Proliferation in the epidermis of chelonians and growth of the horny scutes. Journal of morphology. 265:52-69.
Bauer, T. (2019) Nutrition and husbandry conditions of Palearctic tortoises (Testudo spp.) in captivity. Journal of Applied Animal Welfare Science, Vol. 22, No. 2.
Heinrich, M. L., & Heinrich, K. K. (2016). Effect of supplemental heat in captive African leopard tortoises (Stigmochelys pardalis) and spurred tortoises (CENTROCHELYS sulcata) on growth rate and carapacial scute pyramiding. Journal of Exotic Pet Medicine, 25(1), 18–25. doi:10.1053/j.jepm.2015.12.005
Liesegang, A., Hatt, J. M., & Wanner, M. (2007). Influence of different dietary calcium levels on the digestibility of Ca, Mg and P in Hermann’s tortoises (Testudo hermanni). Journal of Animal Physiology and Animal Nutrition, 91(11–12), 459–464. doi:10.1111/jpn.2007.91.issue-11-12
Wiesner, C., & Iben, C. (2003). Influence of environmental humidity and dietary protein on pyramidal growth of carapaces in African spurred tortoises (Geochelone sulcata). Journal of Animal Physiology and Animal Nutrition, 87(1–2), 66–74. doi:10.1046/j.1439-0396.2003.00411.x
Steams, B. 1989. The captive status of the African spurred tortoise (Geochelone sulcata)’, recent developments. International Zoo Yearbook, 25:87-98.
Highfield, A. 1990. Keeping and Breeding Tortoises in Captivity. England, R and A Publishing Limited.
Personal communication. J Iaderosa. 1993. Curator, Wildlife Survival Center, St. Catherine’s Island, GA.