by Catriona Nguyen Robertson
RSV Science Communication Officer

Sex is difficult – even at the best of times. Unrequited love is hard. And human activity can create barriers to make it even harder for other animals.

Animals are choosy when it comes to selecting a sexual partner, and usually it is the females who tend to choose. Females are selective about their choice of sexual partner. They may seek superior territory, food security, nurturing fathers, or good genes.

Males across the animal kingdom have evolved traits that help them be chosen: striking colours, elaborate tails, beautiful song, and others. The advantages provided by these traits increase reproductive success and are therefore selectively passed down generations in a process called sexual selection.

Sexual selection can occur both intra- and inter-sexually. During intrasexual selection, members of the same sex attempt to outcompete rivals for mates. This is typically responsible the evolution of armaments to increase their chances of success such as larger beetle horns and deer antlers, or larger body size. By contrast, intersexual selection results from mate choice, where certain behaviours or characteristics (e.g. mating calls and bright colours) are considered ideal.

But human waste can interfere with this process.

Invasive species brought by people into new areas are detrimental to native flora and fauna, urbanisation drowns out mating calls, and pharmacological waste changes reproductive behaviours of aquatic life. Plus, climate change makes things difficult for all species.

Human waste impacting the reproductive success of other animals is not a recent phenomenon. Last decade, ecologists on a field trip in WA found golden brown beer bottles strewn across the ground – with which male Australian jewel beetles were attempting to mate. The similarity of these “empties”, in both colour and texture, to the female Australia jewel beetle is striking. Because the bottles were larger, they were ‘super sexy stimuli’ for the males. Not only did this mean that males were confused and not successfully mating, but they were also more vulnerable to predators in the time that they desperately clung on to bottles.

While beer bottles have since been changed, more recently, Bob has been interested in emerging contaminants that pollute our waterways.

Prescription drugs enter our water supply as people release trace amounts in their urine or flat-out flush unused medication down the sink or toilet. Around 50-60% of the active ingredients of some pharmaceuticals are flushed out away, such as oestrogen in the birth control pill. We are literally medicating our waterways.

A major problem with pharmaceuticals is that the receptors on our cells that drugs are designed to target tend to be evolutionarily conserved among different animal groups. Medicines that are developed for humans can therefore also have the same or a similar effect on other species – like fish.

Professor Bob Wong (Monash University) and other researchers around the world have seen abnormalities in the genitalia of both terrestrial and aquatic life due to exposure to drugs that interfere with the reproductive hormone system (endocrine-disrupters) like oestrogen and the plasticizer, BPA. These endocrine-disrupters are feminising male fish, alligators, turtles and frogs, such that it becomes difficult for them to reproduce. Entire species are at risk.

But it is not all bad news. As an example of where our waste is not entirely negative, in Mexico, some birds are incorporating cigarette butt fibres into their nests to aid fledgling success as the chemicals inhibit the growth of nest parasites. In addition, some birds incorporate colour from straws, glass, and plastic in their nests.

It is important to remember that no species exists in a vacuum. Reproduction is essential for the survival of animals and conservation of biodiversity, yet it is easily disturbed by our actions. We should not be making sex any more challenging.

Ockham’s Razor at the Royal Society of Victoria.

The simplest explanation is often the best. ABC Radio National’s Ockham’s Razor has returned to the Royal Society of Victoria. Seven incredible tales of science and endeavour were presented at the podcast and recorded for the podcast live. Hosted by Tegan Taylor and produced by James Bullen, Ockham’s Razor is a soap box for all things scientific.

Professor Bob Wong’s talk can be heard online now at https://www.abc.net.au/radionational/programs/ockhamsrazor/sex-in-a-changing-world/13869892.