There are two major families of views on our relationship to the biological world. On animalism, we are animals of the species homo sapiens. Animalism comes in two varieties: physicalist animalism says that we are purely physical animals and dualist animalism says that some or all animals, including all of us, have non-physical features such as non-physical mental states or a soul (of a Cartesian or an Aristotelian sort). On colocationism, wherever one of us is present, there is an animal of the species homo sapiens present as well, but we are not identical to such an animal. There are multiple varieties of colocationism. On the constitution view, we are wholly constituted by our associated animals. Typically, such constitution theorists are physicalists—the animals are purely physical and hence so are we. The other main variety of colocationism is further-aspect dualist colocationism on which our associated animals are purely physical, but we are not. This includes a view on which we are souls (which count as located wherever the ensouled bodies are), a view on which we are a composite of an animal and a soul and a view on which we are partly constituted by an animal and partly constituted by a non-physical aspect. The debate on animalism versus colocationism is thus to a significant degree orthogonal to the debate between physicalists and dualists.
If animalism is true, then a normal adult, say Sally, used to be a fetus, and to have killed that fetus would have been to kill Sally, and it would have deprived Sally of even more than killing Sally now would. Thus, animalism strongly suggests that abortion is wrong, though violinist-type arguments could be used to try to resist that conclusion. On the other hand, colocationist views are much more congenial to pro-choice philosophers, and hence appear to be somewhat dominant in the pro-choice moral philosophy scene. For if colocationism is true, then it could be that the human animal existed significantly before Sally came to be colocated with it, and if so, then killing that human animal in abortion would not have been a killing of Sally. Though, a colocationist could also think that colocation started at fertilization and hence a killing of the fetus would also be a killing of the colocated Sally.
So whether animalism or colocationism is the right metaphysics of us is very relevant to the moral status of abortion.
Now I will cautiously wade into waters that are rather unfamiliar to me, and I apologize if I use terminology in non-standard ways. The question of animalism versus colocationism appears to be very relevant to the question of transgender realism. Let Type I Transgender Realism (1TR) be the view that some people literally are men in female bodies or women in male bodies. Let Type II Transgender Realism (2TR) be the claim that some people who had female bodies and felt that they were or should be men are now, after gender reassignment surgery and hormonal treatment, literally men, and some people who had male bodies and felt that they were or should be women are now, after gender reassignment surgery and hormonal treatment, literally women. If 1TR is true, so is 2TR: surely a man in a female body does not cease to be a man after the body is surgically modified to be more male-like. But at the same time, the law in a number of jurisdictions tracks 2TR but not 1TR, requiring surgery for legal classification as male or female.
Now, it seems very plausible that whether a human animal is male or female (or hermaphrodite) depends on biological criteria very much like those by which we ask whether an elephant or a gecko or maybe even a plant is male or female (or hermaphrodite). These criteria do not depend on psychological states but on whether the organism is such that it should produce its own sperm or such that should produce its own eggs (or both). It is also very plausible that men are male (though they may be more or less feminine) and women are female (though they may be more or less masculine). So if we are human animals, then whether we are male or female, and hence whether we are men or women, depends solely on biological criteria, and 1TR is false.
Moreover, if we are human animals, then 2TR is also false, at least given the current surgical methods. If we remove a mouse's female reproductive system and reshape what remains to look like male genitalia, and treat with hormones, what we have is a female mouse that has lost its reproductive system and behaves like a male. It might be more complicated if a functioning male reproductive system is transplanted. But I think it would still be true that the resulting mouse isn't such that it should produce sperm. Moreover, the mouse doesn't produce its own sperm—it produces the donor's sperm. Here's another route to the conclusion that even a functioning male reproductive transplant doesn't turn the female mouse male. After mere removal of a female (respectively, male) mouse's reproductive system, what we have is a female (respectively, male) mouse that is missing a reproductive system. But now imagine two identical twin female mice, A and B. Both have their female reproductive systems removed. But B then has a male reproductive system added, and then removed. If B became male upon addition of the male reproductive system, then B should still be male after removal thereof—a male does not cease to be a male after losing the reproductive system, but becomes a mutilated male. But A and B may be exactly alike at the end of suffering all this cruelty. It would then be odd to say that of two exactly similar mice, one is male and one is female. So we should say that they are both female, and hence B was female all along, even while having the male reproductive system.
Maybe an animalist could get out of this argument by distinguishing between sex and gender, and denying the idea that a man is an adult male human and a woman is an adult female human. Instead, perhaps, a man is an adult masculine human and a woman is an adult feminine human. The appeal to non-human animals in my argument then becomes irrelevant because only human animals can be men and women. On this story, there will be a disnalogy between the triple of terms "human", "woman" and "man" and triples like "chicken", "hen" and "rooster". A hen is a female chicken, but a woman need not be a female human. While this animalist-compatible view would let one preserve 1TR and 2TR, it would not be compatible with the aspiration that "a woman in a man's body" may have to be really female. It is my impression it is more the genderqueer than the transgendered who use phrases like "male woman" or "female man". Besides the idea of literally male women and female men seems problematic.
On the other hand, if colocationism is true, it is much easier to hold to 1TR and 2TR. Sure, Sally's associated animal (the animal that she is partly or wholly constituted by) may be male, but perhaps maleness and femaleness in a human person is not simply determined by whether the human animal is male or female. Colocationism could allow one to hold to 1TR without revisionary biology and without the oddness of saying that Sally is a male woman. Moreover, colocationism makes it plausible that sexual reassignment surgery could be a valuable thing: it is fitting that a man be associated with a male animal and a woman with a female animal, and while my arguments above suggest that surgery will not change the sex of the associated animal, it could somewhat improve the fit between the person and the associated animal.
Of course, colocationism by itself does not imply 1TR or 2TR: one could still think that a person is a man if and only if the person is associated with an adult male human animal and that a person is a woman if and only if the person is associated with an adult female human animal. But colocationism opens options beyond that.
So the debate between animalism and colocationism is not only highly relevant to the abortion debate but also to the question of transgender realism. Settling the question between the animalists and colocationists would not completely settle the latter two questions, but it would lead to significant progress.
Let me end by saying, without argument, that we are primates and all primates are animals. Hence animalism is true.