Mating System and Reproduction
Females incubate the eggs on their bodies for 7 months after which they give birth to live babies. The average clutch size is 29 babies but it is tightly linked to size of the female. Smaller animals have small clutches (8 to 15) of small babies (mean 150 g) while larger females may have larger clutches (85) of much larger (300 g) babies. During the time she is pregnant, the female does not feed. This is a considerable investment of energy since she is undergoing quite a strong metabolic activity developing the babies. It is possible that the reason that females do not feed is to prevent the risk of being injured by her prey (Rivas 2015, 2020).
Clearly if the female were to be wounded then she and all her clutch could be in jeopardy. Animals that us this strategy are known as "capital breeders" since the animals relay on the stored fat and reserves in order to breed and not on the income that it may gather during the pregnancy. Female anacondas do not breed every year because the investment done in every mating event is so large (about 35% of her body weight) that the female cannot recover her condition in the short time between parturition (September to December) and the new mating season (February to May). In average a little more than one third of the females breed in any given year. Smaller females breed every other year while larger animals need even longer to recover from their reproductive events.
Likely because they do not feed during pregnancy, a breeding anaconda might eat some of her suitors (photo Luciano Candisani). This creates an interesting evolutionary pressure on males. They want to court the female and mate as much as possible with her but they need to be careful to split and leave the breeding ball at the first hint that the female is no longer mating and has decided to take a last snack before her long fast of pregnancy.
This behavior has been found to occur not only in green anacondas but also in Yellow and Beni Anacondas as well.
Anacondas make breeding aggregations of one female and several males. Despite the uneven sex ratio, no conflicts, fights, or agonistic interactions occur between the males other than perhaps pushing each other away from the mating position.
I also found that anacondas have an amazing Sexual Size Dimorphism (SSD) where the female is much larger than the males. Indeed anacondas show the largest SSD found in any tetrapod. This SSD is surprising due to the high likelihood that males compete physically for the females (by pushing each other). Physical competition would produce selection pressure for large size in males, however this is obviously not seen in anacondas.
One of the aims of my research is to explain why we find this SSD despite the physical competition among males. So far I have found that males seem to rely on tactile cues in order to identify who the female is in the breeding ball. I have found evidence that larger males are mistaken for females and courted by smaller males. Courted males, as well as courting males, will be in disadvantage. Thus, there seems to be an optimal male size where it can out compete other males but it is not too large to be mistaken (click here to read the whole article).
At first I was surprised that anacondas seemed to possess the only polyandrous mating system among snakes, where many males mated with one female. At the time I started the study, it was believed that polyandry was constrained in snakes because of phylogenetic reasons (it was not considered to be present in the lineage to begin with).
Yet, when I found out that anacondas were polyandrous, I started looking with more attention at the mating system of other snakes. To my surprised, I discovered that anacondas were not alone in being polyandrous. Furthermore, there were many more snakes in this same mating system. In fact, after looking at the data with my eyes wide open, I concluded that polyandry is, in fact the dominant mating system in snakes (click here for the whole article)