This is the first in a three-part guest series by Dr Carin Bondar, author of The Nature of Sex. It's available now in the UK, Australia and New Zealand.
The process of sex is all about maximizing one’s biological fitness. Now "fitness" in biology doesn't mean how long you live - all evolution cares about is how much you've reproduced. Here’s where the process takes an interesting turn: the value of the sexual cells (called gametes) between males and females is dramatically different. Males have sperm, which is abundant and cheap. Females have eggs, which are low in number and very expensive to produce.
This generally means that males are very promiscuous, and females are very choosy. If a female has already mated with a desired male, or if she has the ability to store sperm and has already received enough, it makes sense for her to spend her energy on other biologically relevant tasks like finding food or avoiding predators. This is, of course, bad news for any male who has yet to make a deposit in her sperm bank. The drastically different needs of males versus females sets the scene for some pretty astonishing and drastic strategies for sexual success.
Before any one individual can even hope to be sexually successful, they’ve got to identify and secure at least one willing partner. In this first article I’m going to outline just a few of the many ways that make it difficult for many of our animal cousins to even find a mate, much less copulate with them!
The meet
As Humans we take it for granted that we have a pretty easy time finding potential partners. Whether it’s a high school sweetheart, someone you met in a social club or sports gathering, someone you met through friends or someone you met on a dating site, there is a myriad of ways that members of our species can find a sexual partner. For other animals it’s just not that straightforward, especially for younger individuals without much experience. Sometimes the best idea is to follow the lead of someone else. I’ll borrow this quote from Milan Kundera (1978):
“Women don’t look for handsome men, they look for men with beautiful women”
Female Caribbean guppies (Poecilia species) engage in mate-choice copying both in the wild and in the lab, showing an overwhelming preference for males associated with other females – regardless of the quality of those males! It’s interesting to note how easily females of many species can be ‘fooled’ by biologists into choosing a partner who is clearly of a lower social or biological rank. Clearly the opinions of the so-called ‘beautiful women’ are important to ladies from a wide range of species.
Another potential problem when it comes to finding a mate is noise. Many ecosystems are veritable symphonies of animal noises; however, animals have evolved in the context of this natural noise, and so they have developed a wide range of strategies in order to deal with it. This is not the case with all of the noise that is produced by our species in our ever-greater pursuit of urbanizing the planet. Several bird and insect species have to completely change the components of their mating calls in order to be heard over the deafening noises created by industry and traffic. Changes to frequency, volume and note composition are common in urban areas where males of many species are simply trying to find a member of the opposite sex. In fact for many bird species in large urban centers, a greater number of males remain unpaired in the breeding season.
It's not easy with this racket going on: urban environments can be noisy. millions27/123rf.com
Human courtships are generally free from predation pressure – we don’t need to worry about being killed and eaten in the middle of trying to woo a member of the opposite sex. The unfortunate reality for the rest of the animal kingdom is that it’s a dangerous world out there, and animals are often forced into making a choice between being sexually successful and remaining hidden from predators. In many cases across several diverse animal groups, the very structures that make males attractive to females make them more susceptible to predation as well (think of the peacock). This is the age old conflict between the forces of natural selection versus those of sexual selection. Sure, the biggest, boldest, brightest males are the ones favored by females, but a beautiful boy has got to be very careful about flashing his sexual signals in the presence of a predator.
Wolf spider males (members of the Lycosidae family) can be either ornamented with large black brushes on their forelegs that are made even more enticing by a vibratory display, or without ornamentation. When faced with a predation threat, both showy and non-showy males cease their locomotory and courtship activities; however, brush-legged (ornamented) males take far longer to initiate courtship subsequent to a predation threat compared to their plainer counterparts. Perhaps not surprisingly, adorned males show a greater response to a predation threat because they have more to lose. If you’ve got expensive assets to protect, it makes sense to have a larger repertoire of responses to protect them.
More to lose? A male ornamental wolf spider. Cathy Keifer/123rf.com
As if dealing with noise and predation were not difficult enough, many species engage in direct sexual deception. For example, red-sided garter snakes (Thamnophis sirtalis parietalis) hibernate in large groups (tens of thousands) over cold Canadian winters. Upon waking, the snakes rapidly move into reproduction mode, and their massive squiggly orgies are quite a tourist draw for many small towns in the province of Manitoba. When the snakes first emerge from their chilly slumber, they are stiff and cold, and therefore more susceptible to avian predators like crows. Males will emit female pheromones to draw in some snuggle-time from other (already warmed) males and speed their warming process. This kind of homosexual foolery has direct benefits to the males that receive the warming; they quickly stop emitting the pheromones once their body temperatures are appropriate for courting the ladies. Imagine going to all the trouble of courting a ‘female’ only to discover that ‘she’ is a fella! What a dirty trick!
When you’re under the weather with the flu or some other kind of ailment, it’s very unlikely that you’ll take this opportunity to try and find yourself a sexual partner. Who wants to engage in courtship with a snotty nose or a need to vomit? Further, what kind of partner will be remotely interested in your current disgusting condition? Although we humans don’t give it a second thought, animals have to make tough decisions about courtship and mating based on their current level of health. In many cases it’s impossible for individuals to allocate resources to both healing and sexual reproduction, which is horrible news if they are either single-time or seasonal breeders.
Tree lizards (Urosaurus ornatus) are an aggressive species that frequently engage in intra-species brawls. Indeed, individuals in the wild are often found with wounds on their external surfaces. When subjected to experimental wounds (‘subcutaneous’ biopsies), researchers found that female tree lizards invest more in immune response than in reproduction. This was measured by counting the number and size of egg follicles within their bodies, and the level of reproductive hormones in the blood. When females were provided with unlimited food, they were able to allocate energy to both food and reproduction. However, when females were provided with limited food they were unable to allocate energy to reproduction. That is hardly surprising: when one has a wound to heal or a broken bone to repair, energy should be allocated to getting better rather than getting lucky.
Up all night to get healthy. Carin Bondar
So there you have it. A very scant set of examples that I hope has opened your mind to the enormous level of potential pitfalls that animals face even before the act of sex is on the table. Next week I’ll move into the fascinating phase two of the sexual process: the act of sex itself!
The Nature of Sex is now available in the UK, Australia and New Zealand. Stay tuned for release dates in the USA and Canada in spring 2016.
