Caenorhabditis elegans shows that sex differences in a nervous system are not just a matter of having extra neurons. The worm has a large neural core shared by males and self-fertilizing hermaphrodites, plus sex-specific cells, connections, gene-expression programs and physiological states. That combination offers a compact way to study how biological sex can reshape behavior while leaving most of a neural network common to both sexes.
Do male and hermaphrodite worms have different brains?
They have partly different nervous systems, but “female” is not the accurate term for the second sex in this species. C. elegans has males and self-fertilizing hermaphrodites. Both sexes retain a substantial shared network. A widely cited account describes a core nervous system of exactly 294 neurons, supplemented by eight hermaphrodite-specific neurons and 89 male-specific neurons. A later 2018 synthesis lists the same eight hermaphrodite-specific neurons but 91 male-specific neurons. The difference reflects updated anatomical accounting, not a contradiction about the basic biology.
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| Feature | Hermaphrodite | Male |
|---|---|---|
| Shared neural core | 294 neurons in the account summarized by Portman; the shared network is supplemented by sex-specific cells | |
| Sex-specific neurons | 8 hermaphrodite-specific neurons | 89 in the 2007 account; 91 in the 2018 synthesis |
| Representative innate behavior | Egg laying | Copulation and other male behaviors |
The important point is not which historical count is selected. It is that a mostly shared network can be modified with a relatively small number of dedicated cells and still generate markedly different innate behaviors.
Three layers of sex difference in the nervous system
1. Cell inventory
Some neurons exist only in one sex. Hermaphrodite-specific HSN and VC neurons participate in egg-laying circuitry. Male-specific sensory, interneuron and motor neurons support courtship, mating and copulation. These cells provide dedicated routes for behaviors that the other sex does not perform in the same way.
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2. Circuit wiring
Sex differences also occur among neurons present in both sexes. Reviews describe sex-specific structure and physiology in shared neurons, along with different synaptic connections. A shared cell can therefore sit in a different circuit depending on sex: it may receive an input that is absent in the other sex, form a different output connection or be recruited by a different behavioral pathway.
3. Cellular state
The same neuron can operate differently because its molecular and physiological state differs. A 2025 single-cell atlas found differentially expressed genes in all 62 analyzed clusters of sex-shared neurons. The study also linked sex-differential expression of neuromodulators to neuronal output. This distributes sexual dimorphism across the nervous system rather than concentrating it in a small, isolated “sex circuit.”
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How the circuits produce different behavior
Hermaphrodite egg laying
HSN and VC neurons are part of the hermaphrodite egg-laying system. Their presence and connections help organize the timing and control of releasing eggs, illustrating how a small set of sex-specific cells can be integrated with a broadly shared motor and sensory network.
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Male mating behavior
Male-specific sensory neurons detect relevant cues, interneurons process them and motor neurons drive copulation-related actions. The male system is therefore not merely a hermaphrodite circuit with one behavior switched on; it includes dedicated sensory, decision and motor components that interface with shared circuitry.
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How sex is established during development
The X-to-autosome signal
The worm’s sex-determination pathway begins with the ratio of X chromosomes to autosomes. That signal controls a genetic cascade whose central transcriptional regulator is tra-1. Reviews describe TRA-1 as the master regulator of essentially all somatic sexual characteristics.
Cell-autonomous action in neurons
TRA-1 acts cell-autonomously in nearly all sexually dimorphic somatic cells, including neurons. In practical terms, a neuron can receive the sex-determination program directly rather than being shaped only by hormones or instructions from a distant organ. This provides a mechanism for sex-specific changes in neuronal identity, connectivity and function.
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Programmed cell death sets the neuron complement
Sex-specific programmed cell death helps establish which neurons survive. The developmental examples include survival of hermaphrodite HSN neurons and male CEM neurons. Eliminating or retaining particular cells is one route by which two sexes can begin with different circuit inventories.
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NeuroPAL and coordinated maturation
NeuroPAL imaging made male-specific neuronal identities visible in living animals. It showed that male-specific neurons are generated throughout larval development but complete terminal differentiation in a coordinated wave during the fourth larval stage. That wave coincides with the appearance of sexual organs, suggesting that neural maturation and sexual maturation are developmentally synchronized rather than independent schedules.
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What the whole-animal connectome adds
A 2019 Nature study provided quantitative wiring maps for both adult sexes. Its central statement was: “Here we describe connectomes for both adult sexes of the nematode Caenorhabditis elegans, an important model organism for neuroscience research.” The maps connect sensory inputs to end-organ outputs and reveal sex differences at several levels, including dimorphic shared neurons and sex-specific connections.
One especially useful result is that sex-specific inputs enter central shared circuitry. Sexual and non-sexual pathways can converge on common neurons and network hubs, so a circuit used for general-purpose processing may also carry information relevant to mating. This is a wiring explanation for how sex-specific behavior can influence, and be influenced by, broader sensory and motor functions.
What this teaches about sex differences more generally
- Sex differences can be layered. A complete explanation may require cell inventory, synaptic wiring and cellular state, not just a count of neurons.
- Shared circuitry does not imply identical function. The same named neuron can have different partners, gene expression and physiological output in the two sexes.
- Small dedicated populations can have large behavioral effects. A handful of sex-specific neurons can connect to a much larger common network.
- Development matters. Sex-specific fate decisions, programmed cell death and coordinated maturation build the adult circuit over time.
- Integration is as important as separation. Connectomes show sex-specific and general-purpose pathways meeting inside shared processing networks.
These are mechanisms and testable principles, not evidence that humans possess the same circuits. Human nervous systems are vastly more complex, and no single C. elegans study establishes a direct one-to-one translation to human sex differences, medical outcomes or behavior. The worm is most valuable as a tractable system for asking how genetic sex-determination programs alter cells and circuits.
A practical framework for comparing the two worm sexes
| Comparison axis | Hermaphrodite | Male |
|---|---|---|
| Neuron inventory | Shared core plus eight hermaphrodite-specific neurons, including HSN and VC examples | Shared core plus 89 male-specific neurons in the 2007 account or 91 in the 2018 synthesis |
| Developmental timing | Sex-specific developmental programs establish hermaphrodite identities | Male-specific neurons are produced during larval development and terminally differentiate in a coordinated fourth-larval-stage wave |
| Synaptic connectivity | Shared and hermaphrodite-specific pathways support egg-laying control | Sex-specific sensory, interneuron and motor pathways support copulation and connect into shared central circuits |
| Gene expression and neuromodulators | Sex-shared neurons can carry hermaphrodite-biased molecular states | Sex-shared neurons can carry male-biased molecular states; the 2025 atlas found differential expression in all 62 analyzed shared clusters |
| Behavioral output | Egg laying and other hermaphrodite behaviors | Copulation and other male behaviors |
Why the worm remains a powerful model
C. elegans combines identifiable neurons, a mapped connectome, accessible genetics and observable innate behaviors. Researchers can therefore connect a sex-determination gene such as tra-1 to a specific cell, synapse or behavioral output with unusual precision. The resulting lesson is broader than nematode biology: nervous-system sex differences can be built by changing who is present, who is connected and how each cell is tuned.
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