Turtle and tortoise mating systems revolve around the number or quality of a female’s mates, the timing of her remating decisions, her capacity for sperm storage, and the fitness ramifications of long reproductive life.
Every tortoise is a turtle but all turtles are not tortoises…
Although turtles and tortoises typically do not display pair bonds or family group affiliations, social organizations exist in some species. For example, dominance hierarchies have been described in gopher tortoises, and individuals of this species as well as snapping turtles (Chelydra serpentina) also defend home ranges that may be important in mate access or control.
Male dominance hierarchies also exist in wood turtles (Clemmys insculpta), and male rank has been shown to affect reproductive success.
Males who consistently win fights against other, usually smaller, males enjoy a higher dominance rank and greater access to extended copulation with females.
Based on DNA paternity data, high-ranking males were found to father a significantly greater number of offspring than those of lower rank, as recorded by Galbraith in 1991.
An important aspect of turtle reproductive biology is the ability of females to store viable sperm in their oviducts for long periods of time. Females of many other taxa have evolved this ability as well.
In many social insects, for example, a queen mates prior to entering the nest and then uses only this initial sperm to fertilize eggs throughout her reproductive lifetime, which may last for years and involve the production of thousands or millions of progeny.
Among the vertebrates, turtles and snakes can store sperm for by far the longest periods.
In species representing these two groups, reports exist of a female’s continued offspring production for up to 4 and even 7 years have been recorded, respectively, following isolation from males.
To date, only a handful of surveys have examined genetic paternity in turtle broods from nature, but virtually every study has documented multiple paternity.
Tortoises are polygamous, meaning they mate with many partners. Size not age determines when it is time to reproduce. Wild tortoise is often over 20 years old and in some species even 40 years old. While in contrast, captive-raised animals reach sexual maturity between 10 and 20 years old. With some breeders pushing their animals at even five years old. We do not recommend this, however.
Dominant males mate more often than less aggressive males. The female tortoise is able to store sperm in her cloaca, fertilizing her eggs for up to four years after copulation.
Click to watch Leo displaying dominancy and he is half as small as the male he is fighting. Plus, they are not even the same species. Yet, testosterone is being created.
The stimulatory effect of testosterone on male sexual activity is one of the clearest examples linking hormones and behaviors. However, this relationship is complex in Chelonians.
The male tortoise is most aggressive during mating season. When another male is encountered, he bobs his head in warning, stands tall and attacks. He uses his head and neck in an effort to flip his rival.
A persistent male can even do serious damage to a female’s shell. This is also the time that males can become really aggressive toward each other. Males attempt to tip opponents on their backs by getting under an opponent’s shell or by using their gular (throat) spurs. Gulars can do serious damage to a rival’s shell and skin, and a tortoise unable to right itself in a sunny area can overheat and die.
Courtship occurs during spring and summer. In fact, both male and female tortoises can be aggressive, but the female usually gives in eventually. The male circles the female, often nodding his head and biting at her legs and the edges of her carapace.
He rams into her, attempting to trap her, so she can be mounted. Their mating is a noisy affair, filled with the sounds of hissing and grunting, while the male vigorously stamps his hind feet.
Having more than one male in a group can stimulate breeding vigor due to the competition among males. However, in large tortoise groups, the strongest male can also dominate the other males and be the only one mating with the females.
During a one-year period, blood samples were collected from male and female G. nigra living under seminatural conditions on Santa Cruz Island, Galápagos. Plasma steroid hormones were measured by radioimmunoassays (RIAs).
In males, plasma testosterone and corticosterone increased a few months before the onset of the mating season. Peak levels were observed while most copulations occurred and environmental temperatures were highest.
Both testosterone and corticosterone showed low levels during the cold and dry nesting season and high levels during the hot and rainy mating season. In females, testosterone and corticosterone also rose during the hot and rainy mating season.
Both hormones peaked during the second half of the mating season and decreased during the cooler dry season. Female estradiol levels increased at the onset of the mating season, reaching the highest level at the peak of the mating season, which coincided with the highest annual temperatures measured.
Estradiol slowly decreased within the next months and rapidly dropped at the onset of the nesting season when temperatures decreased. Progesterone levels were high close to the time of ovulation and showed clearly elevated levels at the beginning of the nesting season after some females had laid their first clutch.
Progesterone decreased during the nesting season when ambient temperatures began to decrease and reached minimal levels in the post-breeding period shortly before the onset of the next mating season.
There were significant annual variations in plasma testosterone in both males and females.
Plasma corticosterone was generally higher in males than in females and varied throughout the year in both sexes.