Archives of The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 31, Number 6;   June, 2004

Desert Tortoise Social Behavior

by Mark Massar

The study plot is located on the eastern border of the National Training Center (north of Baker, CA) in an area proposed for military training. This is an extreme desert region with little rainfall, sparse vegetation, and extreme summer temperatures - not unexpected given its proximity to Death Valley. Tortoises are on the edge of their distribution here. A few miles to the east is the Baker Sink - an area too hot and dry for tortoises. Because of the extreme conditions, tortoise densities are correspondingly low. On the plot, a series of small washes oriented north-south separate ridges of desert pavement. Here tortoises use as coversites small caves eroded into the hardened, calcified walls of the washes.

Twenty tortoises (10 males and 10 females) have been fitted with radio transmitters. Their home ranges have been delineated. Over 280 coversites, mostly caliche caves, have been identified and uniquely labeled with metal tags. Tortoises use multiple coversites within their home range, sometimes sharing them with other tortoises. Within one wash wall may be a complex of multiple caves, with several tortoises living side by side.

We concentrated our observing during the months of September and October to correspond with the peak of the mating season. Although tortoises engage in mating from the time they emerge from hibernation (March) until they return to hibernation (November), mating activity peaks in September and October. This is when male testosterone levels are at their highest (male desert tortoises have the highest levels of testosterone known for any vertebrate animal). As it turns out, male and female reproductive cycles are asynchronous. In males, gamete maturation occurs in early summer; in females ovarian growth and egg laying occur in the spring. Male testes are fully regressed in spring and are thus incapable of producing sperm. Female tortoises overcome this obstacle by storing sperm from fall mating in tubules located within the oviduct. Females may use the stored sperm to fertilize their eggs the following spring. It is not known for how long female desert tortoises can store sperm, whether they can actively chose sperm from certain males to fertilize their eggs, or whether the eggs from a single clutch have multiple paternities. These phenomena have been observed in other animals. Although spring mating appears to have no obvious benefit for the males, it may be important for proper growth and development of female reproductive organs and gametes.

I tracked 6 female tortoises, all located in the southwestern portion of the study plot. Because of the ruggedness of the terrain, the heat, and the time it would take to visit all the transmittered female tortoises on the plot, I instead focused on these 6, all of which were located fairly close together. I would walk in a large loop visiting each female in succession, scanning the surrounding area visually with binoculars and electronically with the antennae searching for nearby male tortoises. If no male's were present, I would move on to the next female. When a male was found near a female, I would start focal observations on the animals, precisely recording the behavior of the individuals. I was looking for behavioral patterns by the female which might indicate female choice for particular males. Behavioral patterns such as continuously turning her shell away from the advancing male, dropping her shell tightly to the ground, or quickly seeking refuge under a bush or coversite, would likely indicate rejection of the male by the female. Often times, a female simply remained sequestered in her coversite while the male courted her at the cave's entrance. Because males are generally much larger than females, females would often be in caves too small for the males to enter. The best the male could hope for in this situation is for the female to accept his advances and come out of her cave. This would be a clear sign of female choice, and I've observed this on occasion.

On one occasion a male tortoise (Male 61) performed head bobs for two hours at the entrance of Female 41's cave before she would exit. Mating took place in the lower branches of a creosote bush for precisely 19 minutes, after which time the female seemed to have lost interest and quickly retreated back to her cave, followed closely by the male. The male continued headbobbing at the cave's entrance, despite the apparent lack of interest on the part of the female. He remained at the cave the rest of the day, blocking its entrance, possibly doing this to prevent other males from gaining access to the female. We've noticed this burrow blocking behavior on numerous occasions with many of the male tortoises.

When two male tortoises encounter each other, a dramatic battle often ensues. This is especially the case when the two are alpha males and when a female is nearby. Fights involve rapid head bobbing, biting, and ramming in which males charge each other with their gular horns (often resulting in one or both males being flipped onto their carapace). Desert tortoises maintain dominance hierarchies. By recording the outcomes of fights, we are beginning to map out the dominance hierarchy of the males on the study plot. One thing we've noticed is that it appears that the alpha males are spending a disproportionate amount of their time engaged in battles and patrolling their territories instead of mating, while the smaller males are mating with the females.

Because tortoises are such long-lived animals (80-100 years), a clear picture of their social structure could take years. This project was started just three years ago. I'll be out there again next year, with my five-foot diameter shade umbrella and bottles of sunscreen, living among the tortoises.