Exaggerated secondary sexual characteristics are apparently costly and seem to defy natural selection. This conundrum prompted Charles Darwin to propose the theory of sexual selection. Accordingly, exaggerated secondary sexual characteristics might be ornaments on which female choice is based and/or armaments used during male-male competition. Males of many cichlid fish species, including the adaptive radiation of Nicaraguan Midas cichlids, develop a highly exaggerated nuchal hump, which is thought to be a sexually selected trait. To test this hypothesis, we conducted a series of behavioral assays in F2 hybrids obtained from crossing a species with a relatively small hump and one with an exaggerated hump. Mate-choice experiments showed a clear female preference for males with large humps. In an open-choice experiment with limited territories, couples including large humped males were more successful in acquiring these territories. Therefore, nuchal humps appear to serve dual functions as an ornament for attracting mates and as an armament for direct contest with rivals. Although being beneficial in terms of sexual selection, this trait also imposes fitness costs on males possessing disproportionally large nuchal humps since they exhibit decreased endurance and increased energetic costs when swimming. We conclude that these costs illustrate trade-offs associated with large hump size between sexual and natural selection, which causes the latter to limit further exaggeration of this spectacular male trait.

Characterization and quantification of the nuchal hump

Lateral photographs of the left side of 70 males and 54 females were taken using a Panasonic LUMIX DMC-FZ62 camera. Specimens were illuminated using two fluorescent lamps positioned 30 cm above the specimen. As a size standard, a ruler was positioned close to the fish in all photographs. Using ImageJ, body size was determined by measuring standard body length in reference to the size standard and hump size was quantified by measuring hump angle. However, as hump size could be affected by changes in body size, we also determined two other measurements for hump size corrected for the size of the fish: corrected maximum hump height (maximum hump height - standard body length), as well as corrected hump area (hump area / head area).

Intersexual selection – humps as ornaments

Mate-choice experiments were conducted in large 2,000-liter tanks (170 x 160 x 80 cm, tanks were not filled to maximum capacity), water temperature was maintained at 28°C (±1°C) and the experimental tanks were divided into three compartments: one large compartment (170 x 80 x 80 cm) and two small ones (85 x 80 x 80 cm). The large compartment was connected to each of the small compartments by a convex gate, the size of which could be adjusted to allow the selective passage of the smaller females but restricting that of large males. Because in our experimental set-up males could not see each other, female choice was most likely based only on morphological differences and potentially behavioral differences and not on the outcome of intrasexual aggression. Mate choice was tested in 20 different females exposed to 20 unique male pairs (i.e., no female or male was tested repeatedly). A female was introduced into the experimental tank and allowed to explore the entire set-up for 12 hours. After this acclimation period, two males were added to the tank, each confined to one of the small compartments. Males for each trial were closely size-matched, the smaller male was on average only 4.04% ± 3.13 (mean difference ± standard deviation of difference, n = 20 pairs) smaller than the larger male. In contrast, differences in hump size among males within one trial were pronounced. The males with the smaller hump sizes had on average 21.98% ± 8.77 (mean difference ± standard deviation of difference, n = 20 pairs) smaller humps than the males with the larger hump sizes. Males were randomly assigned to either of the two compartments. The female was allowed to choose between the two males by entering and leaving their respective compartments. The position of the female was monitored three times daily (approximately every four hours at 9am, 1pm and 5pm) and females were assumed to have chosen a mate if they stayed for two consecutive days with the same male (note that the female had the option of not choosing either male by remaining in the large compartment, however, all tested females chose one of the two males). On the second evening, the female was removed from the selected male and moved back into the large compartment to verify her choice once more. In all trials, the female went back to the same male’s compartment from which she had been removed, affirming her choice. The next morning, the final position of the female was recorded and subsequently used for the analysis of female choice. In most cases, eggs were laid within this period (total duration of the experiment was 3 days) on flowerpots in the male’s compartment. The experiment was terminated after 20 trials. Lateral photographs of the left side of all studied individuals were taken before the experiment, and hump size and body size were measured. 

Intrasexual selection – humps as armaments

In this experiment, we explored the ability of males to acquire and maintain a breeding territory in a crowded group tank with only two of such territories (constituted by a flower pot for spawning). We asked if the hump size of males that succeeded in acquiring a territory differed from that of non-successful males. Midas cichlids are highly territorial fish. Only the most dominant males are able to acquire and maintain a territory. Even if a female chooses a specific male, couples will be able to spawn only if the male can defend the territory from intruders. Therefore, although we are aware of the contribution of female choice in this experiment, successful spawning events can also be attributed to the males’ ability of defending a territory. Prior to the experiment, hump size and body size of all fish were determined. Males and females were assigned to each trial as to maximize variation in hump size between individuals in the particular trial. Within the experiment, male hump sizes ranged from 9.6° to 26.4°. Differences in female hump sizes ranged from 10.5° to 18.6°. To minimize the effect of body size on mate-choice, particularly large or small individuals were excluded from the experiment, resulting in a range of body size from 25.01 cm to 28.54 cm in males, and from 17.26 cm to 21.94 cm in females. We conducted 21 trials, each with eight sexually mature males and four mature females present. All fish were introduced into a 2,000-liter experimental set-up (170 x 160 x 80 cm) in which water temperature was maintained at 28°C (±1°C). The experimental tank was divided into three compartments: one large compartment (170 x 80 x 80 cm) and two small compartments (85 x 80 x 80 cm), each with a flower pot and separable from the large compartments by controllable gates. Successful males were defined as those that acquired a territory, initiated courtship behavior, attracted a female to their territory and proceeded to egg laying.

Swimming performance – energetic costs and endurance

Swimming performance was measured in a 185-liter swim tunnel (Loligo Systems, Denmark). Flow speeds in the tunnel (10 – 225 cm s^-1) were generated by a motor (Loligo Systems, Denmark). Water temperature was 25.5 ± 0.5°C. A mirror adjusted at a 60° angle above the swim tunnel allowed dorsal filming of the fish. All trials were recorded using a video camera (Panasonic Full HD; HC-V110). The swimming performance of 51 males was measured in three consecutive phases. Fin beats could not be accurately measured for one male that was subsequently excluded. Phase I consisted of a 15-minute acclimation period with no flow in the tunnel. In phase II, we investigated if males with large nuchal humps incurred higher energetic costs during swimming due to potential effects of the hump on the hydrodynamics of the fish. As proxy for energetic costs, we used the number of pectoral fin beats per period of time . Fish were allowed to swim at an initial flow speed of 15 cm s^-1 for 10 minutes. Flow speed was then increased in 15 cm s^-1 increments every 10 minutes to a final velocity of 60 cm s^-1. Total number of fin beats (left + right pectoral fin) were manually counted over a period of 93 seconds at 60 cm s^-1.

During phase III, we determined critical sustained swimming speed (U_crit, Brett 1964). Flow speed started at 62.5 cm s^-1, and velocity was increased in 2.5 cm s^-1 increments every 5 minutes until the fish was exhausted. The experiment was terminated when the fish drifted back onto the grid of the swim tunnel and remained there for 1 minute. Our experiments might not provide a comparable measure of U_crit to studies using riverine species such as trout (Beamish et al. 1989), due to the use of shorter incline steps compared to those studies and due to differences in velocity inclines between phase II and phase III. However, as we were interested only in relative performance of fish with varying hump sizes, our results provide a useful proxy for individual endurance. After exhaustion, fish were removed from the swim tunnel, photographs of the fish were taken, and hump size and body size were determined. Critical swimming speed was estimated as U_critaccording to an equation from Brett (1964):

U_crit = u_i  + ((t_i / t_ii) * u_ii)

with u_i being the highest velocity increment that could be maintained for the entire 5 min time interval, t_i the time interval spent at the maximum exhaustion velocity, t_ii the time interval of each increment (5 min) and u_iithe velocity increments (2.5 cm s^-1). 

Characterization and quantification of the nuchal hump 

Data quantifying morphological measurements in 70 male and 54 female Midas cichlids. ID = individual identifier 1-124, sex = sex of specimen (F = female, M= male), HA = hump angle in °, SL = standard body length in cm, cor. hump area = corrected hump area ( hump area / head area), cor. maximum hump height = corrected maximum hump height (maximum hump height/ standard body length).

Intersexual interaction – humps as ornaments

Data acquired during the female choice experiment for 20 trials. The female was allowed to choose between two males : one with a large hump and one with a small hump. Trial = identifier trial 1-20, Outcome = male with the larger hump was selected (1) or male with the larger hump was rejected (0) by the female, Size = male with the larger hump was larger (1) or male with the larger hump was smaller (0) than the male with the smaller hump, Side = male with the larger hump was in the left (0) or in the right (1) compartment, deltaSize = difference in body size: male with the larger hump - male with the smaller hump.

Intrasexual selection – humps as armaments

Data acquired during the group mate-choice experiment for 21 trials. We determined if the hump size of males that succeeded in acquiring a territory differed from that of non-successful males. Couple = identifier trial 1-21, devmale_HA = deviation in hump angle of the male that formed the couple from the male group mean during the respective trial, devmale_SL = deviation in standard body length of the male that formed the couple from the male group mean during the respective trial, devfemale_HA = deviation in hump angle of the female that formed the couple from the female mean during the respective trial, devfemale_SL = deviation in standard body length of the female that formed the couple from the female group mean during the respective trial, newcomer_male = variable scoring if the male that formed the couple was newly introduced to the set-up (1) or already present during the previous trial (0).

Swimming performance – energetic costs and endurance

Swimming performance measured for 51 males. As proxy for swimming performance we determined the critical swimming speed (U_crit) and the number of fin beats for a time interval of 93s. Fin beats could not be accurately measured for one male that was subsequently excluded. ID = individual identifier 1-51, HA = hump angle of the tested individual, SL = standard body length of the tested individual, Ucrit = critical swimming speed of the tested individual, Finbeats = number of finbeats of the tested individual over a period of 93s.