Sexual selection, as I described in my previous blog, includes male-male competition and female mate choice. Both males and females want to maximize their reproductive success, but current research suggests that the strategies of males and females can create more of a conflict than a mutually beneficial system. Dr. Bob Wong from Monash University gave a recent seminar on his research regarding this sexual conflict.
Are Dominant Males the Best Lovers?
It is easy to assume that if reproductive success supports males to be dominant, then females must prefer the dominant males. There are many examples of the male-male competition “winners” to be the female preferred choice. In junglefowl the females mate with the winner of a cock fight. Similarly, the most dominant elephant seal gets to mate with the majority of females. A study with Alpine ibex demonstrated that these sheep must be of very high quality to win head-butting fights which may explain the female preference for the dominant male (click here for video on ibex). However, there are other examples that suggest dominance does not always correlate with female choice.
Wong investigated sexual selection in Pacific blue-eyes to answer questions about female choice. After determining which of two males was dominant, Wong presented these males to a female (only the female could see both). The females did have a preference, but there was no significant difference between the dominant or subordinate males. So what do the females prefer? Further experimentation revealed that the females preferred the males that courted more vigorously, not necessarily a dominant male which typically has larger and longer fins. The consequence of this female choosiness is that the females will spawn sooner with their preferred male and have higher hatching success.
Another example of this scenario is shown in the red-collared widowbird where the bird with the biggest red badge is more dominant, but research shows females actually select for the longest tails. In some cases, male-male competition could actually hamper female mate choice as shown in quacking frogs where too many males can cause them to try and force copulation on females, occasionally killing them in the process. Clearly, dominant males do not necessarily make the best lovers.
Are Males Sexually Permissive?
Sexual signals can be very costly to produce and maintain such as the intricate vocalization of the quacking frogs. Wouldn’t it be beneficial for males to be choosy as well? Wong explored this question with Desert gobies (click here to see feature video on Wong’s work with desert gobies). These unique fish live in extremely salty water that can greatly vary in temperature, and the males actually look after the eggs. Experimental research determined that the males directed more courtship behaviors towards larger females, which produce the most eggs. However, with such small habitats, maybe the gobies would just have to go with the first female they see. Wong looked at this by presenting a large and small female in different sequences. It turns out the order of the presentation is important and if a smaller female was presented after a larger female, courtship behaviors are greatly reduced. To further investigate female encounter rates Wong also found that the interval time between encounters is also influential. Short intervals created a significant difference in courtship behaviors for small or large females, but long intervals (24 hours) produced no significant difference. Wong’s research with desert gobies suggests that males can be sexually permissive and that the female encounter rate definitely makes a difference.
A species of squid (southern bottletail) has also shown to prefer proportionally larger females, but for a very different reason. Male sperm is thought to be very cheap to produce, but the male squid preference for larger female is actually related to sperm investment. The female squid consume male spermatophores after mating which is used for growth and future reproduction. Males prefer larger females because they eat fewer spermatophores. It would seem that males can be choosy, but their reasons may vary.
As Wong’s research suggests, there is certainly a sexual conflict with many animal species. Dominant males are not always the female’s preferred choice and may even negatively affect it. Additionally males may be choosy to maximize their own reproductive success. It is likely that there are benefits for male-male competition and female choice, but perhaps this greatly varies between species. In humans it is easy to see that female and male preference can be widely different which is typically attributed to personality. Perhaps there are personality differences as well for other species in regards to sexual selection? Evidently, there is still a lot left to be understood about sexual selection.
- Secondary sexual characters signal fighting ability and determine social rank in Alpine ibex (Capra ibex); Bergeron et al., 2010
- Sexual selection of multiple handicaps in the red-collared widowbird: Female choice of tail length by not carotenoid display; Pryke et al., 2001
- Sequential male mate choice in a fish, the Pacific blue-eye Pseudomugil signifer; Wong et al., 2004
- Superior fighters make mediocre fathers in the Pacific blue-eye fish; Wong, 2004
- Female sand gobies prefer good fathers over dominant males; Forsgren, 1997
- Strategic male signalling effort in a desert-dwelling fish; Wong, 2009
- The interval between sexual encounters affects male courtship tactics in a desert-dwelling fish; Svensson et al., 2010
- Strategic male mate choice minimizes ejaculate consumption; Wegener et al., 2012