I was in a mood recently. A mood to rummage through hard disks to look at old photos.
I do this every once in a while, stumbling across images I like and wondering, "Hmmm, why didn't I ever process that?"
This usually leads to a bit of futzing, after which I either get distracted or grow weary of fiddling, leaving the photos to be re-discovered another day.
This time I pushed through.
One of the photos I dug up was this image of a pair of sperm whales (Physeter macrocephalus), with the nearer of the two just commencing her dive to forage. I know the whale is a female because this angle provides a clear view of the distinguishing bits, specifically the whale's mammary slits, which is probably why I took the photo in the first place.

Look at the lower ventral area (belly), just forward of where the whale's powerful caudal area (rear end) starts.
See the three grooves?
The two smaller ones on either side of the longer middle one are the mammary slits—charging stations for baby sperm whales.
There are other considerations that provide clues to a given whale's gender, but the only sure-fire way to know is to see this area.
Speaking of which, here is another image I came across—a young humpback whale (Megaptera novaeangliae) playing while her mother chills in the background.

I once heard an "expert" (picture me using fingers to make exaggerated air quotes) assert to a group of people that it is not possible to distinguish the sex of a young whale until it's grown to a certain (unspecified) size (that makes it an adult).
Uh huh.
The mammary slits are present from birth. Provided you can see the correct area, ascertaining female or not is easy.
This little one was having a blast. Seen here, she had just used her fluke (tail) to spin in a circle, pivoting around her upper body region. Mommy was relaxed, conserving her energy for the months of child-rearing that still lay ahead.
Last was this photo of a Bryde's whale (Balaenoptera edeni).

I took this photo waaaaaay back in 2009. We had spotted a pair of these rorquals traveling at high speed in open ocean.
"How fast?" you might wonder.
I'm not exactly sure, but we kicked up quite a wake keeping up. Online references say this species have been clocked up to 13 knots (24kph, 15mph). I'd guess our pair were going a constant five to 10 knots for the more than an hour that we followed. The whales demonstrated curiosity. They pulled up beside our boat two or three times (at full speed of course), giving us a clear look.
My friend Julia and I attempted a single entry. That is all we could do because the whales had taken us far out to sea. We had to go home.
As soon as we hit the water, I shot down to get below the froth from the boat. One whale spun around like Kramer (from the sitcom Seinfeld). I took this photo. The whale shot off in much less than the blink of an eye.
This image has been on my mind ever since.
There was an issue at the time that made it difficult to process the file in a way that would look decent.
We were in open ocean. I was probably 10-15m down. Though it was a clear day, the sun was low. The sharp angle meant diminished light underwater.
The way most digital camera sensors work, you have half the pixels sensitive to green light, a quarter to red, a quarter to blue.
The situation that day meant basically no red light, minimal if any green, and substantially reduced blue due to the low angle of the sun. So out of the 21 Megapixels available to record light (Canon 5DMkii), only a small fraction received stimulation, and those only at suboptimal levels.
That makes for noise (file yuckiness).
Software has improved a lot in 15 years. When I gave the file another look in the latest version of Lightroom, voilà! I was able to reduce the noise enough to make for a decent image. Finally.
More interesting however are the pock marks all over the whale. See them?
Those are the result of cookie cutter sharks (Isistius brasiliensis).
These small sharks live several kilometers (a long way, for you mile-people) down in the ocean. They wait for nightfall to migrate up in search of fine dining. Once cookie cutters find their targets, these sharks (that you may have never heard of before) latch on with their jaws and liberate a chunk of flesh by twisting their bodies. Chomp-twist-extract-swallow. In that order.
Like mosquitoes. Except in the ocean. With teeth. And carnivorous.
I'd like to tell you that you don't have anything to worry about.
OK. I'll do that for the sake of your peace of mind, but whatever you do, don't search for "cookie cutter shark attack human" and definitely don't look at any of the pictures.