Rhodes HJ et al. (2014), Male-male clasping may be part of an alternativ...

XB-ART-49059

PLoS One 2014 Jan 01;95:e97761. doi: 10.1371/journal.pone.0097761.

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Male-male clasping may be part of an alternative reproductive tactic in Xenopus laevis.

Rhodes HJ , Stevenson RJ , Ego CL .

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Male Xenopus laevis frogs have been observed to clasp other males in a sustained, amplectant position, the purpose of which is unknown. We examined three possible hypotheses for this counter-intuitive behavior: 1) clasping males fail to discriminate the sex of the frogs they clasp; 2) male-male clasping is an aggressive or dominant behavior; or 3) that males clasp other males to gain proximity to breeding events and possibly engage in sperm competition. Our data, gathered through a series of behavioral experiments in the laboratory, refute the first two hypotheses. We found that males did not clasp indiscriminately, but showed a sex preference, with most males preferentially clasping a female, but a proportion preferentially clasping another male. Males that clasped another male when there was no female present were less likely to "win" reproductive access in a male-male-female triad, indicating that they did not establish dominance through clasping. However, those males did gain proximity to oviposition by continued male-male clasping in the presence of the female. Thus, our findings are consistent with, but cannot confirm, the third hypothesis of male-male clasping as an alternative reproductive tactic.

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Species referenced: Xenopus laevis

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	Figure 1. Clasping behavior within male-male pairs is repeated reliably from night to night. A. Clasping behavior was observed for one randomly selected male in each pair for two subsequent nights (N1 and N2). Behavior on the two nights was strongly correlated (Spearman correlation coefficient = 0.8783, p,0.001, n = 25). Darker gray symbols indicate multiple, overlapping data points (e.g., there are three data points at 100,100). Inset shows an expansion of the 0–10% range. B. Four pairs of frogs were observed on more than two nights, either 3–4 subsequent nights (M97, M100 and M91) or three nights spaced over nearly 3 weeks (males were returned to their separate home tanks between night 4 and night 19). Again, we saw consistent behavior. doi:10.1371/journal.pone.0097761.g001
	Figure 2. Most male X. laevis showed a preference for clasping female conspecifics, but some showed an apparent preference for males and some made little effort to clasp. Examination of the total time spent clasping the male and female conspecifics (measured as % of observations) showed that a subset of males spent considerable time clasping the conspecific male, even in the presence of a female. Red symbols indicate animals that showed a preference for clasping the conspecific male (chi-square tests, p,0.0001); black symbols indicate a preference for clasping the female (chi-square tests, p,0.0001); blue symbols indicate animals that showed no significant preference (p.a). doi:10.1371/journal.pone.0097761.g002
	Figure 3. Male-male clasping cannot be explained by size discrimination. The smaller frog in a male-male pair was no more likely to clasp his partner than the bigger frog in the pair (Mann- Whitney U, p = 0.326, n = 14 per category). Box plot shows median, 25th and 75th percentiles; whiskers are 10th and 90th percentiles. doi:10.1371/journal.pone.0097761.g003
	Figure 4. Clasping behavior of ‘‘winner’’ and ‘‘loser’’ males. A) Night 2 clasping behavior was used to determine whether males won or lost the reproductive encounter with the female. The male that predominantly clasped the female in each pair was declared the winner (W); see results for specific criteria. B) Winner males were significantly less likely to have clasped the other male on Night 1 than loser males (Mann-Whitney U test, p, 0.001, n = 14 per category) indicating that initiating and sustaining male-male clasps is associated with losing rather than winning primary reproductive access during male-male competition. Box plot shows median, 25th and 75th percentiles; whiskers are 10th and 90th percentiles. doi:10.1371/journal.pone.0097761.g004
	Figure 5. Peripheral males appear to employ different tactics to gain reproductive opportunities. A) The losing male may clasp the winning male or B) he may assume a non-optimal position clasping the female. For both A and B, inset shows outlines of individuals with the female in blue, the winning male in yellow, and the losing male in green. A and B are different triads; eggs from ongoing oviposition can be seen in the upper right corner of A. C) Three distinct patterns of behavior are apparent for losing males on Night 2, Male-directed clasping (M; as seen in A), Female-directed clasping (F; as seen in B) or no/little clasping (N). D) Male-male clasping on Night 1 (with no female present) varied significantly with Night 2 outcome (loser, winner) and with the primary tactic of the loser (Kruskall-Wallis p,0.001, significant differences for post hoc pairwise comparisons indicated with different letters; F: n = 4, M: n = 5, N: n = 5) Box plot shows median, 25th and 75th percentiles; whiskers are 10th and 90th percentiles. doi:10.1371/journal.pone.0097761.g005

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