122 Sharks: The Animal Answer Guide
producing more than live-bearing species. In many species, litter size in-
creases as a female grows because larger females have a larger uterus that
will accommodate more pups.
Within these numbers lie some interesting examples. Sand Tigers pro-
duce two young per reproductive cycle. That number isn’t arbitrary. A fe-
male shark has two uteruses. Several fertilized eggs are released into each
uterus, but the first one to hatch internally is destined to be the only one to
survive. The firstborn Sand Tiger devours all its uterine siblings, engaging
in what is called intrauterine cannibalism. After finishing off its brothers
and sisters, it then feeds on the eggs that the mother continues to produce.
At the end, only two well-fed, meter-long babies emerge, one from each
uterus. Sharks are the only animals known to engage in intrauterine can-
nibalism.
Most live-bearing elasmobranchs have low average birth rates, produc-
ing fewer than a dozen young each breeding cycle, which is seldom less
than a year and often longer. Intermediate numbers characterize a smaller
set of species, such as Nurse Sharks and Tope. A few species are more pro-
lific breeders; these include the Prickly Sharks, hexanchid cow sharks, Ti-
ger and Blue sharks, and Whale and Pacific Sleeper sharks.
Number of young produced by some live-bearing sharks
Species No. of pups per litter
Threshers, Longfin Mako, pseudotriakid 2
gollumsharks, several centrophorid
gulper sharks
Carcharhinids, except for Tiger and Blue sharks 3–10
Spiny Dogfish 6–7 every 2 years
Basking Shark 6
Frilled Shark 6–12 every 1–2 years
Many triakid houndsharks 5–15
Pristiophorid saw sharks 6–19
Sharpnose Sevengill Sharks (Heptranchius perlo) 6–20 annually
Sawfishes 8–20 pups every other year
Scalloped Hammerhead 1–41 pups every other year (av. = 16)
Squalid sharks Up to 32
Nurse Shark 25–50
Tope 6–50 (increasing with size of mother)
Hexanchid cow sharks 20–100
Prickly Shark 114
Tiger and Blue sharks 70–135
Whale and Pacific Sleeper sharks 300
Guitarfishes and stingrays 6 per year
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