Beyond the myth

Why does breeding not improve health?

It is common for owners to observe certain behaviours during heat—such as intense vocalisations, mounting postures, or increased restlessness—and interpret that their dog or cat ‘needs’ to mate or ‘wants’ to have puppies or kittens. This interpretation, although understandable from a human perspective, does not reflect the biological reality of the animal.

Sexual behaviour in domestic females does not arise from a conscious desire or emotional deprivation, but is driven by natural, cyclical hormonal changes. During oestrus, hormones such as oestrogen and progesterone trigger a series of behaviours that have a single purpose: to facilitate reproduction and ensure the survival of the species.

This is not a case of “frustrated desire” or unfulfilled emotional needs. In fact, many females exhibit this behaviour even when isolated or without contact with males, demonstrating that these are automatic bodily responses, not emotional choices.

Understanding this difference is essential for making responsible, guilt-free decisions. Not breeding them does not harm or frustrate them. On the contrary, spaying them in time saves them stress, avoids risks and significantly improves their quality of life.

Where the myth comes from: ‘humanised’ motherhood is not science

The belief that ‘parenting is good’ does not come from veterinary data, but from an emotional interpretation. We tend to think that, like humans, animals need to experience motherhood in order to develop psychologically. However, this idea clashes head-on with biology.

In dogs and cats, maternal behaviour does not arise from a conscious desire or a long-term emotional construct. It is regulated by very specific hormonal peaks — especially prolactin and oxytocin — that appear during pregnancy and lactation and gradually decrease afterwards. Endocrinological studies show that once these hormone levels normalise, maternal behaviours and emotional states associated with the absence of offspring do not persist.

For this reason, a female who has not given birth does not experience frustration, emotional emptiness or affective deprivation. In fact, comparative behavioural assessments between sterilised females without previous offspring and females who have given birth find no significant differences in indicators of well-being, emotional stability or sociability. What is sometimes interpreted as a ‘positive change’ after a litter often simply coincides with the natural maturation associated with age, not with the fact of having given birth.

An evolutionary perspective: what Darwinism helps us understand:

If we broaden our view and step outside the domestic context, the argument becomes even more solid from an evolutionary point of view. Dogs are descended from wolves, a social species that lives in hierarchical packs. In these natural structures, only one female—the alpha wolf—reproduces regularly, while most of the females in the group do not have access to reproduction throughout their lives.

Ethological studies of wild wolf populations show that, under natural conditions, between 70% and 85% of adult females in a pack never reproduce. From a Darwinian perspective, this data is crucial.

If not reproducing caused significant physical or psychological deterioration, this characteristic would have been a clear evolutionary disadvantage, and natural selection would have tended to progressively eliminate it.

However, the opposite is true: non-reproducing female wolves have a life expectancy comparable to that of reproducing females, actively participate in hunting, defending their territory and indirectly caring for their young, and are in fully functional physical condition. Evolution has therefore selected females that are biologically viable and healthy even in the total absence of reproduction.

From this point of view, it is inconsistent to think that a domestic dog ‘needs’ a litter for her health, when in her species of origin most females never breed and do not become ill as a result. Evolutionarily, not reproducing is not a disadvantage; in many contexts, it is a stable strategy.

What the clinical evidence says: breeding does not provide benefits and does accumulate risks

From a medical point of view, allowing a litter before sterilisation does not provide any measurable physical or psychological benefits. On the contrary, each uninterrupted hormonal cycle has a cumulative effect on health.

Your vet advises: prevention over ‘natural’ behaviour

Many vets point out that avoiding repeated hormonal cycles is similar to reducing exposure to ‘high peaks’ of hormones, which over time can contribute to problems such as mammary tumours or pyometra. Therefore, sterilisation is not a punishment or an ‘irrelevant procedure’, but a preventive measure that makes medical sense. 

The epidemiological data is particularly clear on some key points:

Mammary tumours

• Classic studies and modern reviews show that the risk of mammary tumours in female dogs is:
  • less than 0.5% if sterilised before the first heat,
  • around 8% after the first heat,
  • and exceeds 25–26% after the second heat.
  • Each ovarian cycle exponentially increases the cumulative risk.
  • Furthermore, in a clinical study with population follow-up of 293 female dogs throughout their lives, spayed females had an approximately 4-fold reduction in the risk of dying from mammary cancer, expressed as OR = 0.23 (with a 95% confidence interval of 0.11 to 0.47), compared to intact females.

Pyometra

• It is estimated that between 20% and 25% of intact female dogs will develop pyometra before the age of 10, with a mortality rate that can reach 5–10% even with appropriate surgical treatment. Spaying virtually eliminates this risk.
• In European research aimed at studying populations of intact female dogs, pyometra is described as one of the most common diseases in intact female dogs, with a very consistent figure: approximately 1 in 4 intact females (≈25%) will suffer from it before the age of 10.
• Pseudopregnancy:
• Up to 50–75% of intact female dogs show signs of clinical pseudopregnancy throughout their lives, with physical and behavioural manifestations that may recur after each heat.

For this reason, early sterilisation, ideally before the first or second heat, is now considered one of the preventive measures with the greatest positive impact on life expectancy and quality of life in small animal veterinary medicine. It is not a “radical” decision, but an intervention based on decades of clinical and population data.

In fact, a clinical analysis of more than two million dogs and cats showed that sterilised animals tend to live longer: in a very large set of records, it reported that sterilised female dogs lived around 23% longer than unsterilised ones, and sterilised female cats around 39% longer, on average.

The reality behind the litter: physical and emotional impact

Real physical changes: pregnancy is not neutral

Motherhood is not a harmless process for a dog or cat’s body. Each pregnancy places a significant metabolic, immunological and structural burden on the animal, even when everything appears to be progressing normally.

Clinical studies describe that, after giving birth:

• energy requirements can increase by up to 300% during lactation,
• the risk of mastitis can affect between 10% and 20% of lactating females,
• and disorders such as puerperal hypocalcaemia occur more frequently in small breeds and large litters.

In addition, mammary tissue undergoes permanent structural changes after each lactation, which partly explains the relationship between number of litters, advanced age and tumour risk. The impact is cumulative: the more pregnancies, the greater the physiological cost in the medium and long term.

Emotional aspect: they do not “need to be mothers” to be balanced

One of the most frequently repeated arguments is that “after giving birth, they become calmer”. However, behavioural studies show no stable or lasting improvements in behaviour associated with breeding.

‘Curious fact’ – Non-permanent behavioural changes

Some people believe that a female who has “not given birth” becomes frustrated or anxious. The reality is that behaviours associated with the oestrous cycle tend to disappear after sterilisation, not because they “fulfil a need”, but because they are no longer dominated by fluctuating hormones.

On the contrary, the following are frequently described in clinical practice:

• episodes of anxiety during lactation,
• partial or total rejection of offspring,
• compulsive or aggressive behaviours,
• and symptoms consistent with postpartum depression, especially in stressful contexts.

When greater calm is observed after a litter, this usually coincides with natural maturation associated with age, not with the reproductive experience itself. Emotional stability depends much more on socialisation, environment, mental stimulation, bonding with the guardian, and overall health.

Key idea to remember

Allowing your pet to have a litter does not improve their health or behaviour. On the contrary, it delays a preventive decision that could save their life. Choosing to spay or neuter your pet at the right time is not going against the animal’s nature. It is, in fact, in line with what evolutionary biology, veterinary medicine and scientific evidence have been demonstrating for decades.

Sources and recommended reading

Tumours of the mammary gland in dogs – Journal of Veterinary Internal Medicine. Epidemiological review of the risk of mammary tumours and their relationship to the timing of sterilisation. https://onlinelibrary.wiley.com/doi/10.1111/jvim.12562

Canine pyometra: pathogenesis, diagnosis and treatment – Theriogenology. Clinical and statistical analysis of pyometra in intact bitches. https://www.sciencedirect.com/science/article/pii/S0093691X12001240

Reproductive suppression in wolves – Behavioural Ecology and Sociobiology. Studies on hierarchy, reproduction and health in wolf packs. https://link.springer.com/article/10.1007/s00265-009-0857-6

Effects of ovariectomy on canine health and longevity – Frontiers in Veterinary Science. Modern review on sterilisation and long-term health. https://www.frontiersin.org/articles/10.3389/fvets.2020.00342/full

Canine pseudopregnancy: clinical aspects and prevalence – Journal of Small Animal Practice. Clinical data on pseudopregnancy and its impact. https://onlinelibrary.wiley.com/doi/10.1111/jsap.13724