and May (spring-summer) for North Atlantic sharks and June and July (winter) for Southern Hemisphere sharks. In the western North Atlantic, birth occurs well offshore in the Sargasso Sea at depths around . Both sexes grow at similar rates until the onset of maturation, with females maturing later and at a larger size than males. In the first four years of life, the annual growth rate is and similar in both hemispheres; thereafter, sharks from the western South Pacific begin to grow slower than those from the North Atlantic. In the North Atlantic, males mature at a fork length of and an age of 6–11 years, and females at a fork length of and an age of 12–18 years. In the Southwest Pacific, males mature at a fork length of and an age of 8–11 years, and females at a fork length of and an age of 15–18 years. The oldest porbeagle on record was 26 years of age and measured long. The maximum lifespan of this species appears to be 30–40 years in the Atlantic, but could be as much as 65 years in the South Pacific. Thermoregulation Like other members of its family, the porbeagle is endothermic; metabolic heat generated by its red muscles is conserved within the body by specialized systems of blood vessels called retia mirabilia (Latin for "wonderful nets"; singular rete mirabile), that act as highly efficient countercurrent heat exchangers. The porbeagle has several rete mirabile systems: the orbital retia accessing its brain and eyes, the lateral cutaneous retia accessing its swimming muscles, the suprahepatic rete accessing its viscera, and the kidney rete. Among sharks, the porbeagle's capacity for elevating body temperature is second only to the salmon shark's. Its red muscles are located deep within the body, adjacent to the spine, and its lateral rete is composed of over 4,000 small arteries arranged in bands. It has one of the highest core temperatures within its family, 8–10 °C (14–18 °F) warmer than that of the surrounding water. Being warm-bodied may allow this shark to maintain higher cruising speeds, hunt in deep water for extended periods of time, and/or enter higher latitudes during winter to exploit food resources not available to other sharks. The orbital retia of the porbeagle can raise the temperature of its brain and eyes by 3–6 °C (5–11 °F), and likely serve to buffer those sensitive organs against the large temperature