the birds' burrows for shelter when available, or dig their own. The seabirds' guano helps to maintain invertebrate populations on which tuatara predominantly prey; including beetles, crickets, spiders, wētās, earthworms, and snails. Their diets also consist of frogs, lizards, and bird's eggs and chicks. Young tuatara are also occasionally cannibalized. The diet of the tuatara varies seasonally and they mainly only consume fairy prions and their eggs in the summer. In total darkness no feeding attempt whatsoever was observed and the lowest light intensity at which an attempt to snatch a beetle was observed occurred under 0.0125 lux. The eggs and young of seabirds that are seasonally available as food for tuatara may provide beneficial fatty acids. Tuatara of both sexes defend territories, and will threaten and eventually bite intruders. The bite can cause serious injury. Tuatara will bite when approached, and will not let go easily. Reproduction Tuatara reproduce very slowly, taking 10 to 20 years to reach sexual maturity. Though their reproduction rate is slow, tuatara have the fastest swimming sperm by two to four times compared to all reptiles studied earlier. Mating occurs in midsummer; females mate and lay eggs once every four years. During courtship, a male makes his skin darker, raises his crests, and parades toward the female. He slowly walks in circles around the female with stiffened legs. The female will either submit, and allow the male to mount her, or retreat to her burrow. Males do not have a penis; they have rudimentary hemipenes; meaning that intromittent organs are used to deliver sperm to the female during copulation. They reproduce by the male lifting the tail of the female and placing his vent over hers. This process is sometimes referred to as a "cloacal kiss". The sperm is then transferred into the female, much like the mating process in birds. Along with birds, the tuatara is one of the few members of amniota to have lost the ancestral penis. Tuatara eggs have a soft, parchment-like 0.2 mm thick shell that consists of calcite crystals embedded in a matrix of fibrous layers. It takes the females between one and three years to provide eggs with yolk, and up to seven months to form the shell. It then takes between 12 and 15 months from copulation to hatching. This means reproduction occurs at two- to five-year intervals, the slowest in any reptile. Wild tuatara are