Female whale sharks grow slower yet larger than males

A new study that tracked the development of whale sharks over ten years has produced the first evidence that males and females of this species develop differently. The investigation revealed that even though the male sharks grow at a faster rate, the females ultimately outgrow them. 

The research, published by Frontiers, shows that male whale sharks grow at a faster rate, but their body development plateaus around the time they reach 8 or 9 meters. The female sharks develop more slowly, gradually reaching an average length of about 14 meters to overtake the males.

Study lead author Dr. Mark Meekan is a fish biologist at the Australian Institute of Marine Science (AIMS). He noted that whale sharks have been reported up to 18 meters long.

“That’s absolutely huge – about the size of a bendy bus on a city street. But even though they’re big, they’re growing very, very slowly. It’s only about 20cm or 30cm a year.”

The research was conducted along Western Australia’s Ningaloo Reef during 11 seasons from 2009 to 2019. The experts tracked the growth of 54 whale sharks based on the unique “fingerprint” of spots that can be used to identify individuals.

According to AIMS marine scientist Dr.  Brett Taylor, the team recorded more than 1, 000 whale shark measurements using stereo-video cameras.

“It’s basically two cameras set up on a frame that you push along when you’re underwater,” explained Dr. Taylor. “It works the same way our eyes do – so you can calibrate the two video recordings and get a very accurate measurement of the shark.”

For the females, there are huge advantages to being big, said Dr. Meekan. “Only one pregnant whale shark had ever been found, and she had 300 young inside her. That’s a remarkable number, most sharks would only have somewhere between two and a dozen.”

Dr. Meekan suggested the massive females are probably getting big because of the need to carry a whole lot of pups.

Ningaloo Reef in Western Australia is one of the only places in the world where whale sharks appear regularly near the shore. Swimming with the sharks at Ningaloo Reef boosts the local economy by $24 million a year. However, the iconic fish were listed as endangered in 2016.

Dr Meekan said the new findings have huge implications for conservation, with whale sharks threatened by targeted fishing and ships strikes.

“If you’re a very slow-growing animal and it takes you 30 years or more to get to maturity, the chances of disaster striking before you get a chance to breed is probably quite high, and that’s a real worry for whale sharks.”

Dr Meekan said the findings also explains why groups of whale sharks that gather in tropical regions are made up almost entirely of young males. “They gather to exploit an abundance of food so they can maintain their fast growth rates.”

According to Dr. Taylor, the discovery that whale sharks plateau in their growth goes against everything scientists previously thought.”This paper has really re-written what we know about whale shark growth.”

The study is published in the journal Frontiers in Marine Science.

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By Chrissy Sexton, Earth.com Staff Writer