For captive animals, engaging in natural behaviour is a pillar of the animal welfare framework. But when it comes to sex, one important behaviour has been largely ignored, and sometimes even punished: masturbation.

Solo sex is surprisingly common across the animal kingdom. It is well documented in primates. Tortoises are surprisingly vocal during their solo lovemaking endeavours, if not very graceful. Camels masturbate by rubbing their penises in the sand and porcupines make inventive use of all sorts of objects.

Our new study could change how other scientists view masturbation in birds and improve their welfare.

Masturbation also seems to be common in birds. A quick internet search brings up an abundance of video clips on social media and dedicated posts on bird-keeping forums, largely from worried or bemused hobbyist bird keepers.

It has often been treated as an abnormal problem behaviour in captive birds (particularly parrots). Folklore husbandry has assumed it is the undesirable outcome of stress, bad health or poor environment. Bird keepers often therefore discourage masturbation via punishment or veterinary interventions such as diet or care changes and, sometimes, even drugs and surgery. Despite the welfare implications, masturbation in birds had been largely unexplored by the scientific community.

We set out to change that, by investigating the distribution and evolutionary history of masturbation in birds for the first time. We studied 120 species of bird across 22 major groups, gathering data from the scattered scientific literature, online reports and community forums, and surveys of bird experts.

Our study found that masturbation is widespread across birds with a strong evolutionary history, meaning that it’s an ancient trait probably similar in closely related species. Although we found more records of masturbation in male birds, it occurs in both sexes and across all age groups.

Solo sex also seems to be linked to species that mate with multiple partners, supporting the idea that it might help to increase reproductive success when there is a high degree of competition over fertilisation. For instance, in males it may flush out old sperm to leave newer (better condition) sperm for mating. In females it may increase sexual arousal to help with sneak mating with males other than their partner.

Wild behaviour

Crucially, we discovered that masturbation is actually less common in captivity than the wild, and more common in birds reared by their own parents than by humans. What this tells us is that masturbation in birds is neither an unnatural behaviour, nor a consequence of captivity. Given this finding, it is important that birds are not prevented from masturbation. Of course, as with any behaviour, there may be extreme cases where chronic masturbation could indicate underlying health or husbandry issues.

Avian self-pleasure is usually a rather inelegant affair, in which a bird rubs their cloaca (a shared orifice for both excretion and reproduction) against an object, like a branch, twig or toy. This is often accompanied by a lot of flapping and self-satisfied vocalisation.

One potential reason for the lack of scientific studies exploring avian masturbation may be because the cloaca is thought to have fewer nerve clusters, and therefore lower sensitivity, than our own genitals.

Clearly however, birds are getting some satisfaction from masturbation, so perhaps there is more to a bird’s sensations during sex than has previously been recognised. Further exploration of this could have important implications for both welfare and captive breeding programmes. While sexual pleasure may not be exactly the same experience as for mammals, it is wildly premature to dismiss the idea that birds also feel pleasure.

The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.

In the early 1990s, crime-loving television audiences could choose mainly between cozy, fictional detective series such as Columbo and Murder, She Wrote. The US docuseries Unsolved Mysteries brought a few real cold-case investigations to light, but coverage of forensic science on screen was still relatively simple.

Then, in May 1991, The Silence of the Lambs was released. Based on Thomas Harris’s 1988 novel, this big-budget thriller was darker, more disturbing and psychologically complex than most crime films of the time.

The protagonist, FBI trainee Clarice Starling (played by Jodie Foster), is a young woman working in a predominantly male environment – who is often underestimated by her colleagues. When she discovers key evidence through a suspenseful process of extraction from a young victim’s mouth, viewers are introduced to a field of criminal investigation they may never have considered before: forensic entomology.

Some kind of seed pod?

No, sir … that’s a bug cocoon.

Entomology – the scientific study of insects – is one of the oldest branches of the natural sciences. And the application of insects to criminal cases dates back almost as far. In the forensic text The Washing Away of Wrongs (1247), written by Chinese investigator Sung T’zu, flies attracted to traces of blood on a sickle helped identify a murderer.

However, it was not until the late 19th century that forensic entomology was formalised as a scientific discipline – thanks largely to the studies of Jean Pierre Mégnin. Influenced by his experiences on the battlefield, the French vet began investigating which insects were attracted to animal and human remains at different stages of decomposition.

These days, carrion insects are used to tell criminal investigators about the time since a victim’s death, whether their body has been moved, and if any drugs or toxins have contributed to their death.

Human remains are commonly colonised by blowflies and their maggots. But in The Silence of the Lambs, Starling was faced with something more unusual: the cocoon of a death’s-head hawkmoth (Acherontia atropos).

The cocoon, which the serial killer inserts into his victims’ throats, is identified by two entomologists who are clearly not forensically trained. Otherwise, they would have thought twice before cutting open the only piece of insect evidence without seeking permission for such a destructive analysis.

The film introduces murderous concepts such as “staging” – the intentional alteration of a crime scene – and a perpretrator’s modus operandi and criminal signature, relating to any distinctive methods they use.

Today, many of us working in forensic entomology and taphonomy (the study of what happens to organisms between death and discovery) are still told our work is “just like The Silence of the Lambs”. But 35 years after the film’s release, forensic entomology is no longer limited to microscopes, forceps and entomologists working alone.

Today’s criminal investigations often feature complex interactions between environmental conditions, decomposition processes and human activity. This makes collaborations between multiple scientific (and non-scientific) disciplines essential.

How the science has progressed

In the two decades preceding the film’s release, the biomedical and life sciences journal PubMed listed 37 publications on the subject of forensic entomology. Since then, there have been more than 1,800.

Methods used for insect identification and age estimation have changed dramatically. Today, molecular and chemical techniques can identify insect species and determine their stage in the lifecycle and geographic origin. These techniques are especially useful in cold cases or poorly preserved crime scenes, where samples may have been damaged or improperly stored.

Insects are also playing an increasingly accurate role in determining the time of death. As well as feeding on decomposing remains, they help spread the bacteria and other microorganisms involved in decomposition. These microbial communities change in predictable ways over time – even in extreme environmental scenarios – offering investigators a further indicator of the postmortem interval.

Chemical profiling of insect cuticular hydrocarbons (insect skin) provide definitive species and age signatures. These can reduce the risk of error associated with identification by people, and the time and costs of DNA sequencing.

Forensic entomology has also expanded into areas such as entomotoxicology, where insects feeding on decomposing remains are analysed for the presence of drugs, toxins or other chemical compounds. It is even possible to recover the DNA of the individual on whom an insect has been feeding directly from that insect’s gut contents.

In The Silence of the Lambs, investigators assume that “water leaves no trace evidence of any kind”. Yet today, aquatic forensic researchers examine not only insects but crustaceans, microorganisms and bone proteins associated with decomposing remains in water.

Revisiting the moth cocoon scene

The film’s infamous moth cocoon scene – which saw the extracted evidence collected with forceps, then taken for visual inspection at a museum – would be approached rather differently today.

Firstly, spoons are now recommended over forceps to avoid damaging the sample. Modern forensic practice aims to preserve specimens by taking photographs before any manipulation. Where possible, insects are reared to the adult stage which is often easier to identify with certainty.

Rather than opening the cocoon, it could be compared as is with museum reference collections or analysed using technology such as hyperspectral imaging. This would confirm the species and estimate its developmental stage without altering the evidence.

Many high-profile cases, including some wrongful convictions, have demonstrated how forensic entomology can be a key tool in the investigation of current and historical crimes – as well as of natural disasters and war crimes.

However, technological advances are not enough. The reliability of forensic entomology depends on appropriate crime scene protocols, evidence collection, ongoing research and, perhaps most importantly, specialist training and attention to detail. These qualities are certainly embodied by agent Starling.

But there is another major difference since the film came out in 1991. Unlike Starling’s experience, women now represent a major part of the forensic science workforce. They contribute to a discipline that has become far more diverse, collaborative and scientifically advanced than the one portrayed in The Silence of the Lambs.

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Noemi Procopio receives funding from UK Research and Innovation (UKRI) via a Future Leaders Fellowship, from National Institute of Justice (NIJ), and from Science and Technology in Policing.

Paola A. Magni does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.