How do I tell if my hamster is a boy or girl, especially with a tail lump?

You can make this determination by looking at the distance between the urinary and anal openings. In male gerbils, this space is larger than it is in females. This method works best when you have other gernils for comparison. Males will also have prominent and readily identifiable testicles as well as a prominaent scent gland that is larger than a female's scent gland.

With these pictures, I would guess she is pregnant. Gestation for a hamster is 15-18 days. On the third photo, it appears as if her mammary glands are developed. Usually hamsters start building a nest site in a corner. I would definitely separate her from the male as they can be aggressive during birthing. It is important to allow the female to have a quiet, minimally disturbed environment, especially during their first litter, to decrease the chance of rejection of babies by the mother.

It can be quite difficult to an untrained eye with a rabbit that young, but is possible. It's best to have one person hold the bunny upside down and hold the back legs, so it can't kick. Once the bunny is calm, look for the penis (testicles will not be present) or vagina. Do a google image search of "how to sex a rabbit" to find good examples of the difference in anatomy. Below is a good link that explains how to look for the anatomy: http://www.wikihow.com/Determine-the-Sex-of-a-Rabbit

It is difficult to tell exactly from the picture you provided but I think the area you are concerned with maybe your hamsters testicles. It is normal for testicles to descend and the hair of the scrotum be thinner than the hair on the rest of the body. Depending on the age of your hamster this could be what is going on. If you would like to post another question or do a consult we may be able to help you further with what is going on. Alternatively, you could take Ant in to be examined by your veterinarian to make sure there is nothing to worry about. I hope this helps. Thanks for using Petco Pet Education Center, formerly Petcoach.

It's impossible to tell from the pictures, i would need to check her myself in order to tell you if it's a male or female. here is a website that can help you sexing Dust: thegerbils.com/breeding2sex.htm

Based on the distance between Comet's anus and genitals, I agree that he appears to be male. The picture is quite blurry when I try to zoom in, but it appears that from the top down, you can see his anus, scrotum, then penile opening (the penis itself is contained inside of this prepuce). I would expect more of a vertical opening for the vulva without anything above it in a female. I've included an article that has a rudimentary drawing that's pretty helpful. This is unrelated, but I also see rice-like particles on Comet's hind end. These appear to be tapeworm segments, which can be acquired by eating either tapeworm infected fleas or rodents. I'd recommend asking Comet's vet about deworming him for tapeworms based on this picture. https://www.petcoach.co/article/sexing-kittens/

I can't see based on the photos. I need to see lower down below the opening to the urethra. Sometimes though this is still difficult as the male guinea pigs can pull the testicles into the abdomen. Try holding Buddy up and taking another photo. You can likely also bring him into a pet store or a vet's office and they will likely be able to tell. I hope this helps and best of luck!