Reproductive instinct in animals

Love, drama… or pure biology?

Humans find it difficult not to attribute familiar emotions to the behaviour of our pets. When we see a cat rubbing against furniture, meowing intensely or adopting mounting postures, it is easy to think that she is ‘looking for affection’ or ‘missing something’. The same is true of female dogs that become more restless, affectionate or vocal during heat.

However, these behaviours are not expressions of affection, sadness or romantic desire, but programmed biological responses. They are regulated by reproductive hormones such as oestrogen and progesterone, which activate instinctive circuits without any emotional or social intention, unlike in humans.

Understanding this difference not only allows us to take better care of our pets, but also to avoid decisions based on misguided projections, such as allowing a litter “so she doesn’t feel empty” or delaying sterilisation out of guilt. True empathy comes from knowledge: knowing how their biology works allows us to act responsibly, without imposing needs on them that they do not have.

What seems like “love”… and what is actually biology 🧠🐾

When we live with animals, our brain does something almost inevitable: anthropomorphism, that is, interpreting their behaviour as if it were human emotional decisions (“she’s sad”, “she wants to be a mother”, “she feels lonely”, “she’s falling in love”). It’s understandable… but when it comes to reproduction, that interpretation often fails.

In most cases, what we see during oestrus is not “romance” or a conscious desire for motherhood/fatherhood. It is the visible expression of an exquisitely regulated biological system: the hypothalamic-pituitary-gonadal axis (a neuroendocrine circuit connecting the brain and ovaries/testicles). In other words: the brain releases signals, the gonads produce reproductive hormones, and those hormones change the body… and also behaviour.

The “star” hormones in females are:

• Oestrogens tend to ‘trigger’ oestrus behaviours: vocalisation, restlessness, searching, marking, receptivity.
• Progesterone dominates after ovulation and prepares the uterus for possible gestation, whether or not pregnancy occurs.

None of this requires mental planning. It is physiology.

Dogs and cats do not function the same way: cycles, ovulation and ‘season mode’

The female dog’s cycle: an internal clock with numbers (and lots of progesterone)

In female dogs, ovulation is spontaneous: the cycle occurs even if there is no male present. And, although each animal varies, there are fairly consistent averages described in veterinary literature:

• Proestrus: around 9 days on average (with ranges from a few days to several weeks). Bleeding, enlarged vulva and interest in males without allowing mounting are usually seen.
• Oestrus: also around 9 days on average (variable). Here she is receptive; ovulation occurs in this phase (usually shortly after the LH peak).
• Diestrus: lasts approximately 2 months even if there is no pregnancy; progesterone remains high and then gradually decreases.
• Anestrus: resting phase of variable duration.

Translation into “human” terms: even if there are no suitors, the bitch’s body follows its biological calendar.

The cat: ‘spring mode’, repeated heat and induced ovulation

Cats tend to behave as a seasonal polyestrous species (more reproductive activity with more hours of daylight). In addition, many cats experience induced ovulation: ovulation is triggered mainly after copulation, by a neuroendocrine reflex that causes a peak in LH (luteinising hormone).

This explains why, if there is no ovulation/gestation, a cat can repeat oestrus with a frequency that seems endless to humans: cycles can typically repeat every 14–21 days in season.

Did you know?

If your cat is “in endless heat,” sometimes it’s not “capricious”: light rules. With more hours of daylight (or a lot of artificial lighting at home), some cats go into “season mode” and repeat cycles more easily. 🌞 

So no, it’s not that “she can’t get over it because she’s desperate”. It’s that her physiology, under certain conditions, resets the programme.

Behaviour: what you see, what her body is doing

‘She’s doing strange things’: what you see is not an emotional message, it’s an ethological pattern

Many behaviours associated with heat are intense. In female cats: loud meowing, rubbing, rolling on the floor, raised pelvis posture. In female dogs: restlessness, increased demand for contact, changes in appetite, escapism, selective attention to smells.

Your vet advises you 🩺

When a female dog is in heat, the risk of escape increases: double leash + well-fitted harness on walks, and extra vigilance at doors/garages. It is not disobedience: it is reproductive motivation. 

In ethological terms, it fits with patterns of reproductive behaviour modulated by hormones and chemical signals (including pheromones). It may sound like “drama” to us, but for the animal it is usually a “physiological state” with its biological script.

The great myth: “if she doesn’t have puppies, she will suffer”

There is no solid basis for claiming that a dog or cat ‘needs to be a mother’ to be psychologically well. What is well documented is that repeated exposure to hormonal cycles throughout life is associated with specific health risks. And here the numbers are important.

The numbers that matter: real risks and why the decision is no longer emotional

Pyometra: when the uterus goes from being an organ to an emergency

Pyometra is a potentially serious uterine infection associated with the oestrus phase (high progesterone) and changes in the endometrium.

In a large population analysis conducted in a country with a high proportion of intact females, it was estimated that approximately 23–24% of female dogs could develop pyometra before the age of 10. Furthermore, the risk varied greatly by breed, with ranges described from 10% to 54%.

Put simply: leaving a female dog intact is not just about ‘letting her have her heat’. It is accepting a real, and not insignificant, probability of serious uterine pathology throughout her life.

Pseudopregnancy: ‘phantom motherhood’ exists… but it’s not sadness

Pseudopregnancy (or false pregnancy) is very confusing because it seems emotional: nesting behaviour, adoption of objects as “pups”, and even lactation. However, it is an endocrine phenomenon (related to progesterone and prolactin) and can present mainly with behavioural signs.

In a large survey of clinicians, the questionnaire was sent to 2,000 veterinarians and 397 responded (a response rate of 19.8%): 96% reported having seen dogs with pseudopregnancy and behavioural changes even without physical signs in the last year, and in some cases behavioural problems such as aggression were described; yet only about half routinely asked about behavioural changes during consultations.

In a broad survey of clinicians, 96% reported having observed dogs with pseudopregnancy in the last year. And in a study focusing on reproductive cases seen in consultation, a frequency of 30.81% was described within the set of reproductive pathologies evaluated (i.e., not “of all bitches,” but within the reproductive problems that came to consultation).

Moral of the story: it is not “sadness at not having been a mother”. It is biology acting as if it had been.

Mammary tumours: the data that most influences decisions

Here, the classic evidence is widely cited because it provides very specific figures on the impact of the timing of sterilisation.

In a historical epidemiological study, it was reported that, compared to intact females, the relative risk of mammary tumours was:

• 0.5% if sterilised before the first heat
• 8% if sterilised after the first heat
• 26% if sterilised after the second heat

Furthermore, in female dogs, mammary tumours are not always malignant: a ratio of approximately 50% benign and 50% malignant is often reported, which is reason enough to take this very seriously.

In cats, the scenario is usually more aggressive: it is estimated that 80–90% of feline mammary tumours are malignant. And prevention through early sterilisation is particularly effective: sterilisation before 6 months of age is associated with a reduction in risk of around 91%, and sterilisation before 12 months of age with a reduction of approximately 86%. In contrast, the preventive benefit decreases significantly when sterilisation is performed at an older age.

In a questionnaire-based case-control study, 308 cases and 400 controls were analysed, with a response rate of 58%; intact female cats were overrepresented (odds ratio 2.7) and it was estimated that sterilisation before 6 months reduced the risk by 91%, and before 12 months by 86%, showing a very marked preventive effect of early sterilisation.

So… what about the “guilt” of sterilising?

Guilt often stems from a misconception: that sterilisation ‘takes something emotional away’ from the animal. In reality, what is reduced is repeated exposure to hormonal cycles and well-known medical consequences.

This does not mean that there is a one-size-fits-all rule: age, breed, size, lifestyle and medical history all play a role. But it does mean that the narrative of ‘she needs to be a mother’ does not hold up in the face of the data.

Final thought (to remember without drama)

If your pet seems to be starring in a soap opera during heat… she’s probably just following a hormonal script written by evolution 😅.

Fun fact

In dogs and cats, much of the “drama” of heat is a perfect example of how hormones can modulate behaviour: appetite, sleep, social tolerance, marking… without any conscious plan. It’s biology with a loudspeaker. 

And when you look at the numbers — pyometra occurring in around 20–25% of dogs under 10 years of age in some populations, mammary tumours with risks that vary greatly depending on the timing of sterilisation, and very high malignancy in cats — the decision ceases to be sentimental and becomes informed.

Complete references and recommended reading

Schneider, R., Dorn, C. R., Taylor, D. O. N. (1969) – Factors influencing the occurrence of mammary tumours in bitches – Journal of the National Cancer Institute – Classic epidemiological study demonstrating the decisive influence of the timing of sterilisation on the risk of canine mammary tumours – https://academic.oup.com/jnci/article-abstract/43/6/1249/910225

Egenvall, A., Hagman, R., Bonnett, B. N., Hedhammar, Å., Olson, P., Lagerstedt, A. S. (2001) – Incidence of pyometra and risk factors in bitches – Journal of Veterinary Internal Medicine – Large-scale population analysis estimating the probability of developing pyometra before 10 years of age in unspayed bitches – https://pubmed.ncbi.nlm.nih.gov/11817057/

Hagman, R. (2018) – Pyometra in the bitch: pathogenesis, diagnosis and treatment – Veterinary Clinics of North America: Small Animal Practice – Comprehensive review of the hormonal mechanisms involved in pyometra and their clinical relevance – https://pubmed.ncbi.nlm.nih.gov/23279529/

Root Kustritz, M. V. (2007) – Reproductive control in bitches – Merck Veterinary Manual – Practical review of the canine oestrus cycle and its clinical and preventive implications – https://www.merckvetmanual.com/dog-owners/reproductive-disorders-of-dogs/management-of-reproduction-in-dogs

Salmeri, K. R., Bloomberg, M. S., Scruggs, S. L., Shille, V. M. (1991) – Long-term effects of gonadectomy in dogs – Journal of the American Veterinary Medical Association – Analysis of the physiological and behavioural consequences of sterilisation in dogs – https://pmc.ncbi.nlm.nih.gov/articles/PMC7158189/

American College of Veterinary Surgeons (n.d.) – Mammary tumours in dogs – ACVS – Small Animal Surgery – Clinical summary on the frequency, biological behaviour and surgical treatment of canine mammary tumours – https://www.acvs.org/small-animal/mammary-tumors/

Sorenmo, K. U., Kristiansen, V. M., Cofone, M. A., Shofer, F. S., Breen, A. M., Langeland, M., Mongil, C. M., Grondahl, A. M., Teige, J. (2009) – Biological characteristics of mammary tumours in female dogs – Veterinary and Comparative Oncology – Study describing the proportion of benign and malignant mammary tumours and their clinical behaviour – https://www.msdvetmanual.com/reproductive-system/mammary-tumors-in-dogs/mammary-tumors-in-dogs

Misdorp, W. (2002) – Mammary tumours in cats – Veterinary Pathology – Key reference documenting the high malignancy of feline mammary tumours – https://www.msdvetmanual.com/reproductive-system/mammary-tumors-in-cats/mammary-tumors-in-cats

Goericke-Pesch, S., Hoffmann, B. (2017) – Pseudopregnancy in bitches: endocrine basis and behavioural manifestations – Veterinary Evidence – Evidence-based review of the frequency and clinical expression of canine pseudopregnancy – https://veterinaryevidence.org/index.php/ve/article/download/711/1118/6080

Hagman, R., Lagerstedt, A. S., Hedhammar, Å., Egenvall, A. (2014) – Influence of breed and age on the risk of pyometra – Acta Veterinaria Scandinavica – Study demonstrating the wide variability in the risk of pyometra among dog breeds – https://eprints.gla.ac.uk/135010/

Concannon, P. W., Lein, D. H., Hodgson, B., Sarma, K. (1980) – Induction of ovulation in cats through copulation – Reproduction in Domestic Animals – Experimental study demonstrating how the number of mounts influences ovulation and conception rates in cats – https://onlinelibrary.wiley.com/doi/10.1111/rda.14638