The research builds on an intriguing earlier finding. In a previous experiment on the same species, researchers discovered that males seemed to exercise a form of cryptic choice in favour of females whose false heads had been experimentally ablated. Virgin females with a damaged false head received larger ejaculates during mating than females with intact false heads. Because butterfly ejaculates can contain nutritious, hormone-like and protective components that benefit the female, larger ejaculates represent a tangible mating advantage. The researchers proposed that a damaged false head might signal to a male that the female had survived a predator attack, demonstrating an inherited ability to deflect predators that would be valuable for his offspring.

That discovery raised an obvious question: if males favour females with damaged false heads, do females reciprocate by favouring males with damaged false heads? To find out, the team designed an experiment in which virgin females were presented with either a male whose false head had been surgically removed or a male whose false head was left intact. The butterflies used were the offspring of females collected in the Botanical Garden of the National Autonomous University of Mexico (UNAM), a species whose larvae feed mainly on Echeveria gibbiflora, a succulent plant in the family Crassulaceae.

The experimental design was deliberately paired. Each replicate consisted of two mating cages, one containing a virgin female with a male whose false head had been ablated and the other containing the same setup with a control male whose false head was untouched. To ablate the false head, males were briefly immobilised in a freezer at minus four degrees Celsius for about five minutes, after which the false head region was removed with microscissors. Control males underwent the same handling procedure but without the ablation. The researchers chose this paired design rather than exposing a female to two males simultaneously because male C. xami defend mating territories, meaning that in nature females never encounter more than one male at a time when seeking a mate.

The mating cages were cylindrical structures made of mesh cloth and metal wire, 25 centimetres in diameter and 60 centimetres tall. They were hung on a metal frame in a garden of the Instituto de Ecología at UNAM in Mexico City, situated next to the Pedregal de San Ángel, the natural habitat where C. xami lives. Replicates were set up between 11:00 and 14:00 hours, and if a pair mated before the end of this period the replicate was concluded early. Males and females within each replicate were matched as closely as possible for age and wing length, the latter being a reliable predictor of body weight in this species. No individual was exposed to a potential mate more than once, regardless of whether mating occurred.

When a couple mated, the researchers recorded the duration of copulation and transferred the female to a one-litre white plastic container with a piece of the larval food plant for egg laying. The female was placed under a 75-watt light bulb for two hours over two consecutive days, conditions under which females lay most of their eggs in the first two days after mating. The researchers then compared the proportion of females mating in each treatment, copula duration and the number of eggs laid, using appropriate statistical tests to verify that the paired matching of age and wing length had been successful.

The results were clear. Overall, only 12 out of 38 pairs mated, a rate of 31.6 percent. Five females, or 26.3 percent, mated with males whose false heads had been ablated, while seven females, or 36.8 percent, mated with males whose false heads were intact. This difference was not statistically significant. Copula duration was also remarkably similar between the two groups, with a median of 30 minutes for males with ablated false heads compared to 34 minutes for control males. The number of eggs laid by females mated to each type of male was likewise very similar, with medians of 23 and 18 eggs respectively. In short, the absence of a false head on a male had no measurable effect on his mating success or on his mate’s subsequent reproductive output.

To strengthen these findings, the researchers also reported the results of a previous pilot experiment using the same paired design, in which only mating success was recorded. In that experiment, 12 out of 40 pairs mated, a rate of 30 percent that was almost identical to the main experiment. The proportion of couples mating was exactly the same in both treatments, with 6 out of 20 pairs mating in each group. However, the pilot experiment had an important limitation: due to logistical constraints, 80 percent of its replicates used sibling pairs, which is why the researchers classified it as preliminary rather than definitive evidence.

The findings are somewhat unexpected given the earlier discovery of male choice in the same species. The researchers acknowledge that their sample size was limited, but note that it was very similar to that of the previous male mate choice experiment. One notable difference was the lower proportion of matings observed in the current study compared to the earlier work, where 24 out of 40 pairs, or 60 percent, mated. The researchers cannot fully explain this discrepancy, as butterfly rearing, experimental design and facilities were the same in both studies. They speculate that males might be more sensitive to manipulation than females, although they consider it extremely improbable that any specimen was harmed, or alternatively that females pay more attention to other traits such as territory size and quality, rendering the state of the male’s false head less important in their mating decisions.

Perhaps the most compelling explanation offered by the researchers relates to the courtship behaviour of this species. Male C. xami defend mating territories, and when a female enters a territory the male flies behind her, staying less than 10 centimetres behind. When the female perches, the male walks beside her until reaching a parallel, head-to-head and tail-to-tail position, at which point copulation takes place. This means that in nature, males can see the rear end of a female, where the false head is located, while females most of the time presumably see only the front part of a male. As a result, females may simply be unable to evaluate the condition of a male’s false head during courtship, explaining why males use this trait in their mate choice while females apparently do not. The study highlights how the same ornamental structure can be shaped by natural selection in one sex and by sexual selection in the other, and how the choreography of courtship itself can determine which signals are visible to which partner.

Subject of Research: The role of the false head trait in female mate choice of the butterfly Callophrys xami

Article Title: Lack of evidence that the false head of male Callophrys xami (Reakirt, 1867) ( Lepidoptera , Lycaenidae ) is a sexually selected trait

Article References: Cruz-Avendaño, C., Medina, C., Cordero, C., & de Gasperin, O. (2025). Lack of evidence that the false head of male Callophrys xami (Reakirt, 1867) (Lepidoptera, Lycaenidae) is a sexually selected trait. Nota Lepidopterologica, 48, 269-275. https://doi.org/10.3897/nl.48.171025

Image Credits: AI Generated

DOI: 10.3897/nl.48.171025

Keywords: Callophrys xami, false head, Lycaenidae, sexual selection, mate choice, butterflies, cryptic choice, predator deflection, copula duration, ejaculate, courtship behaviour, Mexico