Monk Seal Monday #62: Male Mobbing

Earlier this year, we reported on male competition taking places along the shoreline on Kauai’s south side. It can get pretty aggressive–and another good reason to steer clear of monk seals in the water.

Now, we want to share about another kind of aggression among monk seals. This interaction generally but not always takes place between a group of males and a single female and is called male mobbing. Like “cruising males,” the perpetrators tend to be a group of competing sub-adult males.

In 2016, RK28 was observed with large wounds and abscesses on her back. It was determined these wounds were caused by male monk seals who had attempted to mount her and while doing so, biting her repeatedly on the back. The wounds can be pretty severe and certainly disconcerting-looking to our eyes. For good reason, it turns out.

According to the NOAA Pacific Islands Fisheries Science Center’s website, “Events involving multiple and single adult male Hawaiian monk seals exhibiting aggression towards adult females and immature seals has led to a significant number of severe injuries and deaths. Unfortunately, the loss of even a single female, and the loss of her lifetime reproductive potential, represents a significant setback to population recovery of this endangered species.”

Luckily, RK28 went on to give birth in 2018, and she’s thought to be pregnant again this year. If so, she may pup any time now.

Last week, one of our diligent volunteers ran across a video on Instagram, presumably taken in the waters off the nearby islands of Niihau and Lehua. In the caption, tour boat captain @crazy_capt_r writes, “This was an incredible experience! This is a very dramatic scene of the Hawaiian monk seal’s [sic] mating. In the video there are 9 males vying to mate with 1 female. She slips away from one male only to be grabbed by one of the other males waiting nearby. This is only a partial video of the whole ordeal. This went on for at least 30 minutes before they dispersed into deeper water.”

Here’s a link to the video. Warning: it might be disturbing to some people.

Tracy Mercer with the Hawaiian Monk Seal Science Team reviewed the video and confirmed the behavior is more consistent with male mobbing than mating. She also reported that she’s seen many females on Niihau with the scars from mobbing events. So, it’s not all that unusual, especially in pockets of the population when males greatly outnumber females. In fact, there have been instances over the years in the Northwestern Hawaiian Islands, where approximately 85 percent of the species’ population resides, that have led to the demise of enough females to cause NOAA to intervene.

Also from the NOAA PIFSC website:

Over an 11 year period from 1984-1994, 37 male seals were selectively removed from Laysan Island to restore a balanced sex ratio. These seals were translocated to Johnston Atoll (n=9) or the main Hawaiian Islands (n=21), placed into captivity (n=5), or died (n=2). Following removal, instances of injury or death from multiple male aggression events drastically declined. The removal of these males from the Laysan Island population has contributed to the restoration of a balanced sex ratio and has proven a valuable mitigation strategy.

Single male aggression events have most notably occurred at French Frigate Shoals and more recently at Kure Atoll. Intervention efforts include hazing of identified aggressors, translocating pups from areas where aggressive males frequent, treating injured seals when appropriate and removal of the adult male. The 3 adult males at French Frigate Shoals observed to repeatedly target pups, were translocated to Johnston Atoll (n=2 in 1998) or euthanized (n=1 in 1991). One adult male was brought into permanent captivity in 2013 after he had been observed injuring pups at Kure Atoll. This mitigation strategy effectively reduced pup deaths as a result of adult male aggression at this site.

But here’s the interesting thing. Tracy also reported the actual mating behavior and practices of Hawaiian monk seals is a bit of a mystery. Presumably it takes place in the water, not on land. In the numerous decades of Hawaiian monk seal research, no one on staff with the Hawaiian Monk Seal Science Team has witnessed the actual mating of monk seals. A few non-staff people have reported mating taking place nearshore. But that hasn’t been scientifically confirmed.

To read more about adult male aggression, click here. And if you see a female with fresh wounds on her back, please report it to the Kauai Hawaiian Monk Seal hotline at 808-651-7668. This will allow the animal’s health to be assessed. If the injuries are severe, she may be treated with antibiotics to prevent the wounds from becoming infected. Then again, Hawaiian monk seals have an amazing natural ability to heal. We’ve seen it time and again.