Tortoises are personal pets that have gained immense popularity around the world in recent years. And as evidenced by the proliferation of books and websites catering to the subject of captive tortoise maintenance, keepers of tortoises are constantly seeking information about providing their pets with optimal conditions. Many species adapt well to captivity, but pet tortoises are very much wild animals. Knowledge about their wild counterparts, therefore, may help tortoise keepers go a long way in satisfying the needs of their pet tortoises. The purpose of this article is to discuss aspects of the biology of wild tortoises – including evolutionary history, life history traits, physiological ecology, and social behaviors and communication – that can help keepers of tortoises provide a better existence for both captive and wild tortoises. In my second post, I discuss life history traits of tortoises.
Tortoises exhibit a unique and interesting suite of life history traits that make them one of the most remarkable groups of organisms on the planet, but also one of the most vulnerable ones. Life history traits are those aspects of an organism’s biology that define how it lives. These traits include longevity, growth, age at maturity, reproductive strategy, and survival. The diversity of life on planet Earth is defined, in part, by the multitude of expressions of life history traits. In this section, I provide a review of the life history of tortoises, with recommendations for pet tortoise owners that consider life history trait expression in tortoises.
We know that tortoises are exceptionally long-lived organisms, with some species attaining lifespans of more than 250 years. A number of captive tortoises have lived remarkably long lives, including Adwaita, an Aldabra giant tortoise that lived to be at least 255 years old; Tu’I Malila, a radiated tortoise that lived at least 188 years; Harriet, a Galápagos tortoise that lived to be at least 175; and Timothy, a Mediterranean spur-thighed tortoise that lived to be at least 165 years old. Two living captive Aldabra giant tortoises, including Jonathan, estimated to be 182 years old, and Esmeralda, estimated to be 170 years old, further attest to the longevity of tortoises.
Tortoises exhibit two additional life history traits that are inextricably linked with their long lifespans, including slow growth and delayed sexual maturity. We know from numerous field studies that tortoises grow very slowly and take a long time to mature – up to 20 years in desert tortoises and up to 40 years for Galápagos giant tortoises (Turner et al. 1987; MacFarland et al. 1974). Tortoises are indeed built for long, slow lives. Though Aesop focused on the tortoise’s relatively slow locomotion in his Tortoise And The Hare fable, a much more remarkable fact of the biology of tortoises is their slow-motion lives, particularly with respect to their rates of growth and maturity.
Another life history trait exhibited by all organisms is reproduction, and tortoises are no less remarkable when it comes to the expression of this trait. Tortoise species exhibit a wide range of reproductive strategies in terms of numbers of eggs and clutches produced per year, but by-and-large exhibit low fecundity or low reproductive rates compared to most other organisms. Under natural conditions in the habitats where they are adapted to live, most tortoise species lay one clutch of eggs per year, whereas others may lay up to two or three (Wilbur and Morin 1988). Clutch sizes vary considerably, from the single-egg clutch of the speckled padloper (Loehr et al. 2004) to 40 or more eggs per clutch in the African spurred tortoise (Holt 2009).
The last important life history trait to consider in this discussion is survivorship. In general, organisms fall into one of three survivorship categories: Type I, characterized by high survivorship during the early and middle portions of the organisms life and low survivorship in late life; Type II, characterized by generally equal survivorship rates through the life of an organism, and Type III, characterized by low survivorship during the early life stages of an organism, and high rates of survival later in life. Long-lived mammals such as humans, elephants, and whales, as well as many other species exhibiting intensive, long-term parental care behaviors that enhance the survival of their offspring, are considered Type I survivorship organisms. Type II organisms exhibit essentially equal rates of survival during all life stages; examples include small vertebrates such as lizards, songbirds, and rodents. Nearly all organisms that display Type III survivorship exhibit relatively short lives and huge bursts of reproductive output, with most of their offspring succumbing to mortality during the early life stages. Organisms such as dandelions, house flies, and corals fall into this category. Oddly enough, so do tortoises. The youngest life stages of tortoises, particularly eggs and hatchlings, exhibit very low rates of survival, whereas larger, older individuals can expect much higher survival rates. Some field studies suggest that as much as 80-90% of tortoise eggs that are laid are eaten by predators (Landers et al. 1980; Vilardell et al. 2008). Hatchling and juvenile tortoises also suffer high rates of predation (Epperson and Heise 2003; Swingland and Coe 1979; Pike and Seigel 2006; Perez-Heydrich et al. 2012), so that perhaps as few as one egg out of 100 may develop into a fully mature adult tortoise. So how is it that tortoises, which exhibit incredibly long lives, fall into the same survivorship category with short-lived flies and annual plants? The answer is in their reproductive output. With their low rates of annual fecundity, tortoises can match the massive reproductive output of other organisms displaying Type III survivorship, but only with repeated reproduction over many, many years. Thus, in comparison to insects and annual plants, tortoises live in slow motion. Tortoise populations depend on long lives of adults with many years of reproduction in order to remain viable (cf. Brooks et al. 1991; Congdon et al. 1993). This is why poaching and collection of tortoises for food and the pet trade can be devastating to wild populations. If you remove an adult, 20-year old female from a population, you are denying perhaps 80 years of reproductive output that she would be giving to that population.
As keepers of pet tortoises, we should be prepared to provide care for our pet tortoises for a very long time. Accordingly, it would be prudent for most pet tortoise keepers to identify and train younger family members to care for our pets when we are gone, as our tortoises – given proper care – will surely outlive us. We should expect that our tortoises will grow and mature slowly, and that slow growth is not an effect of improper care or diet, but rather a natural aspect of tortoise biology. Fast rates of growth in captive tortoises may, in fact, be detrimental to their health (Ritz et al. 2012). Finally, because the life histories of tortoises require that individuals within wild populations reproduce annually over long periods of time to ensure the long-term viability of populations, we must be absolutely sure that our activities as pet tortoise keepers do not impact wild populations. Purchase of wild-caught animals only serves to decimate wild populations, and we should repudiate this and other activities associated with the exploitation of wild tortoises.
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