Tortoise sweeps provide insights into Aldabra’s populations |24 February 2020
Although a big sweep on Aldabra may sound like a major annual cleaning session, it’s actually the time of year when we get down and personal with the tortoises. Aldabra hosts the largest population of giant tortoises in the world, and while there has been a lot of research on these tortoises, we still lack some basic information on them in the wild.
When you think about a human population, some of the most important statistics are sex ratios (how many males compared to females), the age structure of the population (toddlers, teenagers, adults, retirees), and other particulars such as average height. For tortoises, this is also important to know. It has been shown statistically that tortoises on different parts of the atoll have physical differences. In this case, the tortoises on Picard (north-west of Aldabra) are much larger than tortoises in the south-east part of the atoll, despite being the same species!
It’s all about resources; the tortoises on Picard have more food and water year-round allowing them to grow larger, while the tortoises in the south-east have more limited resources, thereby staying smaller. What we don’t know is the differences in growth rates, whether the sex ratios differ, and whether the population is made up of younger, older, or similar aged tortoises. Although the tortoise population trend is stable, there is still much more to learn to understand and conserve these gentle giants.
The SIF team on Aldabra has started to collect information on tortoise population demographics since 2017 via ‘sweep’ surveys. Our research staff go into the field armed with measuring tapes, to pre-designated areas in which all tortoises within that area are measured and assessed. In November, the team measured 225 tortoises on Cinq Cases in two sweep zones and 190 tortoises on Picard in four areas. The tortoises on Picard are mostly marked from previous studies, so we can collect information on individuals, which is incredibly useful data. Of those 190 tortoises, 127 had an ID, which will allow us to look at individual growth rates, survival and movements.
At the other end of the atoll at Cinq Cases, exciting developments are being made to the programme to allow us to also follow tortoises there at an individual-level. Overall, all the effort that has gone and continues to be put into the programme, will help us to understand the population in the two very different locations of the atoll. We will keep you updated as the programme continues to develop, and when the next sweeps occur in June!
Contributed by SIF (Seychelles Islands Foundation)