Some males really go to town when putting on a show for the ladies. Male peacocks flounce around with elaborate feather trains while male dear strut their stuff with impressive antler racks. However, other males prefer to serenade the ladies. ‘Most frog songs fall into this rubric with males exhibiting more showy and elaborate forms than females,’ says Elizabeth Leininger from St. Mary's College of Maryland, USA. However, she adds that showy male traits can be lost as well as gained, ‘either by females assuming the male character or via the loss of masculine traits,’ she says. While most Xenopus males regale the ladies with rapid and elaborate calls, the males of two distantly related species – X. boumbaensis and X. borealis – have relinquished the more complex details of their mating calls. Intrigued by mechanisms that led to the loss of male characteristics and knowing that the larynx probably plays a key role in forming those differences, Leininger and Darcy Kelley from Columbia University, USA, decided to compare the male and female larynges of X. boumbaensis and X. borealis that had lost the male characteristics in their calls.
Measuring the relative sizes of the frogs’ larynges, the duo could see that the male X. boumbaensis’ larynges were ten times the size of the females’ because they had more muscle fibres, the muscle cells were colossal and they had a larger cartilage box. Meanwhile, male X. borealis’ larynges were only five times larger than their females’. Next, Kelley and Leininger began comparing the muscle fibre types in the frogs' larynges – there are two types of muscle fibre, ones that contract and relax rapidly (fast-twitch) and slower contracting (slow-twitch) fibres – to look for differences between the males and females. Teaming up with Ken Kitayama, Kelley and Leininger could see that muscle fibre distribution in the larynges of male X. borealis was similar to that of the female muscle fibres; both had a mixture of fast- and slow-twitch fibres, which would naturally restrict the laryngeal muscle contraction and prevent X. borealis males from producing the rapid and complex courtship calls produced by other Xenopus males. However, when they investigated the larynges of both sexes of X. boumbaensis, they found that the males’ larynges were nothing like the females’; the male laryngeal muscle fibres are exclusively fast twitch and looked more like the larynges of other Xenopus males.
Next, the duo focused on nerve stimulation of the laryngeal muscles to find out how the muscle physiology varied between the males and females of each species. The intensity of most Xenopus males’ calls increases toward the end of a trill and Kelley and Leininger explain that this occurs when closely timed electrical stimuli reinforce the earlier stimuli (known as potentiation) to trigger progressively stronger contractions and louder calls. When the team measured the characteristics of the laryngeal muscle contraction in response to nerve impulses, they found potentiation in male X. boumbaensis, but less in the females. However, the male X. borealis’ response to electrical stimulation was more like that of the females, lacking the potentiation that increases the volume of the mating call in other Xenopus males.
So, the loss of male features in the X. borealis mating call is associated with the feminisation of their larynges. And, although the X. boumbaensis larynx retains many of the physical characteristics associated with other Xenopus males, Leininger explains that they do not have the appropriate patterns of input from the brain to produce the elaborate calls used by the majority of Xenopus to woo the ladies.