Instincts: Not Passed On Genetically?

Human babies are endowed with rooting or suckling instincts and grasping instincts, from the moment they are born. Yet it seems inconceivable that such minute behaviors should express themselves in every single member of an entire species! Considering infant instincts, there is no chance to learn the instinct from the environment or other organisms. As a result, the instinct must somehow be passed on physically. How would such tiny, specific behaviors be passed on physically, through the broad brush of the genome?

Instincts should be investigated for their implications on Lamarckian natural selection. Lamarck held that evolution selects for specific behaviors. Since instincts have a murky physical basis, and are often essential to survival, somehow minute behaviors play an important role in evolution. However, natural selection only selects for phenotypes. How are behaviors then selected for?

Since genes are the only physical data passed on in sexual reproduction, one would conclude that instincts are somehow encoded in our genetic makeup. A fragment of gene code, or genotype, is usually expressed as a physical trait, or phenotype. These phenotypes commonly affect behavior in particular through chemistry or neural structure. However, for these phenotypes to regulate such fine movement and behavior seems rather unlikely, and is not well understood by the scientific community.

Some readers might point to transgenerational epigenetics as a way to explain behavior transmission. By methylating certain genes, they are not expressed and effectively inactive. There is some evidence that epigenetics influence the inheritance of behaviors in mice and C. elegans [1], but the evidence is not substantial.

Let us consider some reflexes, instead of instincts. Reflexes are involuntary reactions to external stimuli. For example, we reflexively flinch our hand away upon touching a hot stove. Many motor reflexes are mediated by the spinal cord. That is to say, there are direct neural connections made between the spinal neurons and motor neurons. Other reflexes involve the brain. For example, the flinch reflex when something flashes into your visual field is wired into the superior colliculus, but it must interact with the motor cortex [2]. So these reflexes are polysensory and polygenetic. In other cases, the genotype is known exactly. For example, the location of the photic sneeze reflex gene is known precisely [2]. The common thread in all the reflexive mechanisms is that they are well defined phenotypes because the behaviors have unique physical basis, and have unique connections between neurons and brain areas. As a result, a genotype is a well defined way to pass on the behavior.

Surprisingly, instincts may have origins that are not related to the genotype at all. In a serendipitous experiment, Kuo was inserting catheters into the hearts of chicken embryos. But the chicks did not peck out of their shell or peck for food after being let out [3]. It turns out when he moved the head of the embryo to reach the heart, the chicks could not hear their heartbeat anymore! The chicks learned the pecking instinct by nodding their head to their own heartbeat. So it is self-interaction of an organism with itself that gives rise to such finely tuned instincts!

Self-interaction generates instincts in other organisms. Rats with funnels around their head, when later raising children, did not groom or lick their babies [3]. This occurs because rats learn to groom by licking their own paws! The funnel disrupted the self-interaction, and the instinct was not formed.

So instincts can be passed on without any explicit genomic instructions or physical traits. This seems uncannily like Rupert Sheldrake’s controversial idea of a morphogenetic field, in which living organism have an nonphysical field around their genome that carries behavior. While I don’t advocate this theory, we should remember that not all behavior can be boiled down to chemistry, and stay open to new viewpoints.

1. Justin Ma’s Quora response to “What role does epigenetics play in animal instincts?”
2. Simon Cooke’s Quora response to “What is the biological basis for instinct?”
3. Joyce Schenkein’s Quora response to “How are instincts genetically passed down as information?”

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