By Dr. Becker
The sex ratio of a species defines the proportion of males to females. In humans, it hovers around 105:100, male to female.1 Among other species, the ratios vary. Birds tend to have male-skewed sex ratios while many mammals are female-skewed.
There are even species, such as some marsupials, that have populations of only pregnant females, because the males die after mating.2
It’s known that adult sex ratios may change animal behavior and mating preferences. For instance, in bird species the more prevalent sex tends to be the one raising the offspring.3
What’s not known, however, is exactly why adult sex ratios may be unbalanced. Researchers from the University of Bath in England recently attempted to find out.
Sex Chromosomes May Help Explain Unbalanced Sex Ratios
In mammals, including humans, females have two X sex chromosomes while males have one X and one Y. The sex that has two different chromosomes is known as the heterogametic sex; in humans, that would be males, but not all species work this way.
In birds, for instance, females have ZW chromosomes while males have ZZ. In this case, the females are the heterogametic sex.
Interestingly, a recent study published in Nature found that tetrapod species with female heterogamety (females with two different chromosomes) tend to have a male-biased adult sex ratio (more than 55 percent male).4 (Tetrapods include four-limbed mammals, birds, reptiles and amphibians.)
On the other hand, species with male heterogamety tend to have a more female-biased adult sex ratio (more than 56 percent female). According to the study, “The genetic sex-determination system explains 24% of interspecific variation in ASRs in amphibians and 36% in reptiles.”5
It’s unknown why sex chromosomes may be involved in skewing sex ratios, but it’s possible the heterogametric sex may have a lower chance of surviving to birth or to adulthood.
Unnatural Causes May Also Skew Sex Ratios, Putting Species at Risk
Heterogamety may play a natural role in sex selection, but unnatural causes can also skew a species’ sex ratio. For instance, in crocodiles the sex of hatchlings is determined by temperature during incubation of the eggs.
Warming temperatures could tip the sex ratio of crocodiles and threaten their survival.
Similarly, temperature also plays a role in the sexes of baby sea turtles. When the nest temperature is about 84.2 degrees F, the hatchlings will be about half male and half female.
As temperatures rise, more females are born, and once the temperature gets to 87.8 degrees F virtually all females will be born.6,7 Study author Graeme Hays told The Guardian:8
“Over the next 20 to 30 years, it’s not going to create problems,’ Hays said. ‘In fact, there’s going to be a benefit to the turtles, because there’s going to be more females produced, which means more females laying eggs.
More females will lead to a population expansion. But ultimately, if you extrapolate long enough into the future … once you get 100 years or more into the future, then things start to look serious.
You have so few males left that it’s likely to be a problem. There will be heaps of female but not enough males to fertilize all those eggs.’”
Among Sumatran elephants, meanwhile, tusk size likely plays a role. Sumatran elephants are not known for their tusks, which are rarely very long on males and may be so short they appear hidden on females.
However, even a short tusk draws the attention of poachers looking to sell the ivory illegally. As a result, male Sumatran elephants are being increasingly targeted, with each death skewing the sex ratio further among the remaining animals and interfering with breeding rates.
Environmental Chemicals May Also Be Skewing Sex Ratios
Pollution is another factor. Endocrine-disrupting chemicals and hormones (both synthetic and natural) from livestock operations are increasingly contaminating waterways.
Purdue University researchers found that when minnow embryos were raised in water taken from streams contaminated by concentrated animal feeding operations (CAFOS), 60 percent developed into males and 40 percent females, rather than the typical 50/50 split.9
Also concerning, the maximum hormone loads in the streams occur in the spring, which is also when fish spawn, hatch and develop. Luke Iwanowicz, a research biologist with the U.S. Geological Survey told Environmental Health News:10
“This type of study was long overdue. Anytime you see skewed sex ratios and population effects, it’s concerning.”
Sex Ratios Should Be Factored Into Conservation Efforts
Researchers from the University of California, Davis and the University of Colorado in Boulder suggested extinction prediction models should be re-evaluated to include additional factors relevant to extinction.
Specifically weather, the proportion of males compared to females, differences in reproductive success, birth rates and other random factors were advised to be included.11
When these factors are accounted for, certain species may be at risk of extinction much sooner than is currently recognized. Veterinary Practice News reported:12
“They urgently recommended a re-evaluation of the risks to wildlife based upon the proportion of males compared with females in a dwindling population, and the differences in reproductive success between individuals and birth rates in the group.
When … [researchers] factored these additional aspects into risk assessments for particular species, they found that the danger of extinction substantially increased.
They demonstrated that missing factors such as the number of males to females and variations in the number of offspring are capable of causing unexpected large swings in the size of a population.”