The curious case of inter-species sex



14, Jan 2017

The sexual interactions initiated by a male Japanese macaque towards female sika deer have made news and also surprised researchers. Inter-species sexual behavior has been observed in the past but usually among members of closely-related species, some of which has led to a new species in the evolutionary tree. More than one reason has been suggested to explain such behavior which challenges the mainstream “copulation for procreation’s sake” theory.

What makes the recently observed inter-species behavior odd?

According to the researchers who observed and analyzed this behavior:

  • While sexual activity has been observed between members of different species in the past, it occurs between species that are closely related to one another and has been documented among fishes and insects.
  • The fewer cases of heterospecific mating where individuals belong to distant species were observed in animals born and brought up in captivity.
  • But the recently observed sexual advances of a male Japanese macaque towards several female sika deer on Yakushima Island, Japan, is the second such observation of sexual activity between distant species in the wild.
  • It is also the first such observation which did not lead to death or severe injury to either of the species. While the sika deer varied in the favorability of their responses to the macaque, the macaque in this case, definitely did not employ aggressive or harmful tactics; it did not force itself on the deer.
  • It is also the first such sexual interaction observed between these two species.

Why might inter-species sex in animals be occasionally beneficial?

While the latest report on sexual behavior between individuals of two different and distant species suggests we have a lot more to learn about non-reproductive sexual behavior in animals, the question on inter-species sex is not a new one.Though various reasons are put forth, a definite answer or set of answers still elude us.

Homoploid hybrid speciation

  • Research has increasingly found hybridization or inter-species breeding to result in a newer species which can evolve into a separate species. While this hybrid speciation was known to happen in case of plants, more such species have been recognized in animals too where it is usually Homoploid – the offspring retain the same number of chromosomes as present in the parents.
  • As evolution in animals was thought to happen through “splitting” of a single species into two over a long period, interspecies sex between members of different evolutionarily close species was considered a sexual blunder till research later proved it could be – on rare occasions – instrumental to their evolution.
  • While detection of hybrid-formed species was made difficult by their great degree of physical resemblance to their parent species, advances in genetic technologies have made it easier to spot the hybrid.
  • There are great many chances for hybrid species to be introduced, especially in a population of diverse and fast evolving groups of animals, according to biologist James Mallet.“Sex with another species may be very occasionally quite a good idea”, he told the National Geographic.

In a 2007 Nature article, Mallet said, “On average, 10 percent of animal species and 25 percent of plant species are now known to hybridize.”

Among arguments against hybrid speciation is the observation that there is nearly negligible chance of the newly formed species to not mate with members of the parent species and thereby maintain their identity as a distinct species. So it remains an exception rather than the rule.

When has mate deprivation hypothesis proven true?

Note: A hypothesis is in no way a concrete explanation. It is made based on limited evidence and serves as a starting point.

  • At first glance, one learns that mate-deprivation hypothesis was first put forth by Randy Thornhill, a professor of biology who sought to explain rape by men through an evolutionary perspective. Introduced in the 80s, the hypothesis said that male rapists were more likely to be men who could not enter into sexual relationships with the women they chose.A subsequent test proved this hypothesis false and it crumbled against the counter-argument that in several instances, male rapists are rich and successful with ample opportunities to have sexual relationships with the women of their choice.
  • This mate-deprivation hypothesis seems to be the best explanation so far for the macaque’s behavior, according to the researchers. They write, “In our case study, the most realistic hypothesis would be the ‘mate deprivation hypothesis’, which states that males with limited access to females are more likely to display this behaviour.
  • According to them, the macaque seems to be a peripheral male or was one in a group of peripheral males. Peripheral males occupy a low status in macaque society and are usually said to be denied breeding rights. This could have led to the interaction in order to abate sexual frustration, says Cedric Sueur, an animal behaviorist and one of the researchers.
  • Japanese macaques live in multi-male, multi-female groups, where there is an increased chance of sexual competition and this increases support to the mate-deprivation hypothesis.
  • But with only a few such interactions observed and all of them being carried out by a single male, Sueur said further studies are needed to better understand the cause for this behavior.

Where are the other explanations focused at?

The researchers considered a few other possibilities which have been suggested before in an attempt to understand inter-species sexual behavior.

  • The macaque thought the deer was one of its own species. This incomplete species recognition is commonly observed among species closely related to one another such as a coyote and a dog. One reason put forth to explain what apparently seems a “blunder” is how choosiness differently affects the male and female of the species. For one sex (usually males) a greater benefit ensues from mating with multiple partners and being less choosy. Therefore, even a female member of a different species which bears a resemblance to the male’s species may be considered a mating partner. But the evolutionary and morphological difference between the macaque and sika deer makes this explanation unlikely.
  • The macaque was using the interaction as a practice session. Given the harmony observed between these two species – deer eat the food dropped by macaques from tree tops and macaques have been observed grooming deer – and that this technique of learning has been observed in other animals before, this possibility was considered. Again the counter-argument that macaques as highly social beings have several opportunities to learn copulation from members of their own is hard to counter.
  • Hormonal surge and bond between these two species might have acted together to trigger the non-violent sexual advances. These factors are said to have facilitated the copulation between Atlantic spotted dolphins (Stenella frontalis) and bottlenose dolphins (Tursiops truncatus).

Other reasons (put forth by programmer Tim B on StackExchange) suggested for inter-species sexual behavior are as a way to pass the time, social bonding, claiming dominance over another species or any other individual or mutual benefits such as increased fertility.

Who else are said to have been observed exhibiting this behavior?

  • Sexual activity has occurred between a female eland (large antelope) and an ostrich, a male dog and a chicken, a male monkey and a snake, and a female chimpanzee and a cat, according to reports cited by biologist and sexologist Alfred Kinsey.
  • Male grasshoppers of the Tetrix ceperoi species are frequently observed attempting to mount male as well as female members of other species and also flies but the larger females usually discourage them.
  • Male spider mites of the Panonychus citri species copulate with female spider mites of the Panonychus mori species nearly as frequently as they do with their own species, though an offspring never results.
  • The only other case of sexual encounter – at least unexpected for the human observers – highlighted between distant species in the wild are instances of Antarctic fur seals coercing king penguins which resulted in the latter’s deaths.
  • The penguins are prey for the seals and the observation that the seal "was trying to court the penguin as if it were a female seal" mystified researchers.
  • Sea lions and sea otters have also been witnessed sexually harassing other types of seals on occasion and causing the death of their targets.

Evolutionary biologist Axel Hochkirch said, “Insects, spiders, worms, frogs, birds and fish also engage in inter-specific sexual activities” and while hybrids such as the mule have resulted when the mates belong to closely-related species, mixed mating that is sure not to yield a progeny remains an unexplainable anomaly.

How has inter-species sex usually been studied?

Where animals are concerned, the discussion usually centers on the impact on fitness of the species and hybridization.

  • In the macaque-sika deer interaction too, further research has been suggested to understand the long-term and short-term effects of inter-species’ sexual interactions on the macaque males. The tentative suggestion is that while long-term fitness level may not be hampered, in the short-term, energy loss, gametes and time may place such a male at a disadvantage.
  • Reproductive interference – an umbrella term used to describe reproductive activities between individuals of different species that result in the loss of fitness of one or both species – is an actively researched area. Apart from a detrimental impact on fitness, its influence on species displacement and thereby sexual exclusion, and patterns of co-existence is also being studied.

Zoophilia and Bestiality

Zoophilia is sexual attraction aroused in humans towards animals and bestiality is any sexual act between humans and animals.

  • According to Wikipedia, several branches of science – psychology, sexology, ethology and anthropology – have touched on zoophilia. In the fifth edition of Diagnostic and Statistical Manual of Mental Disorders, zoophilia is listed as one of the “other specific paraphilic disorder”.
  • Ethics, philosophy, law, animal rights and animal welfare are other branches that can produce different perspectives on zoophilia.
  • Recent research has begun to explore the conjecture that in the absence of sadism there are at least some animals which find pleasure in a zoophilic relationship.

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Tags | inter-species sex macaque sika deer