"Their dogs eat fish."
Translation:Sus perros comen pescado.
It is as you say, "sus" = "their", and it only has to match the name it goes with: su perro (singular, "their dog"), sus perros (plural, their dogs).
Now, about the "come" and "comen" (present, indicative):
Usted (in Spain: formal, singular "you"), él/ella/ello (he/she/it) come.
Ustedes (in Spain: formal, plural "you"), ellos/ellas (they) comen.
Tú (in Spain: informal, singular "you") comes.
Vosotros (in Spain: informal, plural "you"): coméis.
Hope that cleared it out a bit.
Yes, it changes depending on the sex of the animal and it is for all animals, not just pets. But keep in mind that even if most animals change that way (male/female: perro/perra (dog), gato/gata (cat), oso/osa (bear), pavo/pava (turkey), pato/pata (duck)), some change differently just as in English (caballo/yegua (horse/mare), gallo/gallina (rooster/hen)) and some do not change but just get a "macho" (male) or "hembra" (female) after the name. Some animals that only have a noun to both males and females are: araña (spider), hiena (hyena), foca (seal), rata (rat).
So it depends of each animal... I do not know of a general rule, but it seems to me that mammals are the ones getting the different male/female names, even if there are exceptions.
Out of curiosity, how would you refer to an animal that is neutral in grammatical gender when you need to identify it as either female/male? For example, when referring to a spider's gender, I'd just call it "female [or male] spider." Is it the same in Spanish? Do you just use macho/hembra, or are those reserved for specific animals?
I wrote "su perros comen pescado". The answer is sus. My first thought was "ah, because we are talking about more than one dog, it is sus. and if it were a single dog, it would be su perro." But the tip Duolingo is giving me is "pay attention to gender".
So which is it? Is it "sus" because it's plural? Or is it a gender thing?
I had the same problem, but I see no response here yet. I thought I've seen in other practice here that 'los pescados' can mean fish (as food) generally. (Yo como los pescados = I eat fish. If that is correct (of course maybe it's not), why is "Sus perros comen los pescados." incorrect?