Western fence lizards – a small lizard common across western USA – defend their territories with great vigour and determination. During the breeding season, these charismatic little lizards appear to dance to deter rivals and win mating opportunities. By bobbing their heads up and down, doing push-ups, and waving their tails, they show off the iridescent blue scales they sport on their necks and bellies.

These displays provide an honest signal to other lizards about their health and strength. Weaker rivals retreat when faced with the displays of stronger males. But when rivals are evenly matched, the displays may escalate into brawls. Western fence lizards are known for aggressively defending their patch.

But an unwanted passenger could be devastating some lizards’ attempts to reproduce. Western fence lizards are the main host of juvenile Western blacklegged ticks. Up to 130 of these voracious blood-suckers have been found on some unlucky individuals.

The level of harm exerted by tick infestation is the topic of recent research conducted by Dr Gita Kolluru, Dylan Lanser, and Dr Larisa Vredevoe at California Polytechnic State University in San Luis Obispo. Their experimental trials offer novel information about the behaviour of this lizard species in response to tick parasitism.

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The researchers first selected pairs of lizards with similar competing abilities – which the team predicted by measuring their size, tail and colouration. By ensuring the lizards were evenly matched, the researchers ensured that any differences observed would most likely be due to the impacts of tick infestation. One male from each pair was infested with lab-reared ticks for a week before the trials began. On the day of the contest trials, males were introduced to each other in an outdoor arena.

Infested lizards struggled to keep up with their tick-free counterparts. A key determinant of competition outcomes for Western fence lizard males is their aggressiveness. The trials revealed that infested males would often retreat after the pair’s display behaviours, opting not to escalate the rivalry to a physical battle. Once they had retreated, infested males were less likely to instigate another competition.

This observation may be partly explained by blood loss. Although the ticks used in the trials were tiny juveniles, they drank enough blood from the infested lizards to reduce the lizards’ levels of ‘haemoglobin’ – the oxygen-carrying compound of blood. This means that infested lizards can struggle to get enough oxygen to their muscles during intensive displays and battles. Previous research suggests that every 25% haemoglobin lost equates to a 20% reduction in stamina.

Additionally, lizards – like other reptiles – rely on external heat sources to control their body temperature. Cold lizards are sluggish and slow. Lizards typically bask on top of rocks to warm themselves in the sun. Competing lizards often reach body temperatures of 40 degrees Celsius. Infested lizards, however, showed a preference for cooler body temperatures. Their lower body temperatures may also contribute to their hindered competing abilities.

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In addition to the impacts of blood loss and behavioural changes, tick bites themselves could be physically damaging. Ticks often preferentially attach inside a small skin fold at the base of the lizard’s neck – called the ‘nuchal pocket.’ Skin inside this fold is often softer than on other areas of the lizards’ body and easier to bite. Ticks are also protected from the elements inside the pocket.

The researchers found that ticks that didn’t attach inside the nuchal pocket would often attach on the eyelid or thin ear membrane. Damage and scarring from tick attachment to delicate sensory organs could impair competing ability. As such, some hypothesize that the nuchal pocket may have evolved to attract ticks away from sensory organs.

In contrast to this idea, Dr Kolluru, Dr Vredevoe, and Lanser didn’t find any relationship between attachment outside of the nuchal pocket and competing ability during their trials.

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The team’s trials demonstrate how tick infestation could be impairing the ability of Western fence lizards to defend territories. As aggressive defence is the main strategy for gaining access to mating opportunities, tick-infested lizards may be siring fewer offspring than their tick-free rivals. Defending larger territories may give male lizards a greater chance of securing mates; however, by covering more ground they could be increasing their risk of encountering ticks.

The timing of infestation may also be important. Infestation in early spring could have dire consequences for males battling to secure territories ahead of the mating season. Males infested later in the season have a greater chance of securing some mating opportunities in already-established territories before becoming infested.

The ticks themselves rely on the lizards for dispersal. Hindering the lizards’ ability to defend larger territories may be reducing the ticks’ opportunities to disperse. In contrast, the preference for cooler temperatures exhibited by infested lizards could be benefitting the ticks by keeping them safe from drying out in hot, dry environments. Dr Kolluru, Dr Vredevoe, and Lanser identify a potential future research avenue investigating this relationship from the ticks’ perspective.

Additionally, the researchers plan to explore the lizard-tick relationship further by experimentally investigating the impact of tick infestation on the courtship displays males use when trying to win the affections of a female lizard. Lizards can see ticks on each other at close range. Both female lizards and rival males may be adjusting their responses to other lizards based on their tick status. The researchers plan to untangle these effects by using artificial ticks to create sham infestations.