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Notes on snake control in Ibiza and Formentera (2/3)

18 July 2023


To access the first part of this entry:

Notes on snake control in Ibiza and Formentera (1/3).

To access the third part of this entry:

Notes on snake control in Ibiza and Formentera (3/3)


Biometrics, sexual maturity and body condition

Studying the biometry of the captures (taking standardised measurements and biological processes of the individual), was a very interesting and useful work, since only with the body size of the snake (LHC or length from the snout to the cloaca, not counting the tail, which can be longer in males than in females) and thanks to previous studies (see references at the end of the text), it is possible to deduce if a specimen is an adult (sexually mature), not counting the tail, which can be longer in males than in females) and thanks to previous studies (see references at the end of the text) it is possible to deduce whether a specimen is an adult (sexually mature) or a juvenile (sexually immature), and therefore whether it is capable of reproducing or not. According to recent studies (see references at the end of the third part of the entry), in Eivissa snakes reach sexual maturity at 62.8 cm in the case of females, and at 50 cm in the case of males. The average LHC of the specimens we analysed was 69.0 cm, an adult size for both sexes.

Regarding sex, the percentage of the sample was fairly even. Of the total sample, 47 individuals could be sexed, 26 were males and 21 were females, with the majority of the total number of captures (62.5 %) being sexually mature individuals. However, while 92.3 % of the males were found to be mature, only 52.4 % of the females were mature (at least on the basis of the biometric data available to date). In the same way that the capture of mature adults of both sexes indicates in itself the possibility that reproduction is taking place on the island (apart from the fact that it has been confirmed for years), the capture of small individuals that have been born recently allows us to confirm the time of year in which they were born. Thus obtaining information on the biological cycle of the species, which can be useful for documenting the abnormality or otherwise of its biological cycle in an environment for which it is catalogued as an invasive species and in which variations could arise with respect to its area of origin as it expands and settles in the territory.

The smallest individual (29.8 cm in length and 7.9 g in weight) had a size within the neonate range (23.7 - 30.8 cm; 6.1 - 9.0 g) and was captured on 23 November in Zone C (in this particular case, in a private home located in Sant Jordi de Ses Salines). The measurements of this specimen indicate that it must have been born between October and November, well after the usual date, both in Ibiza (September) and on the mainland (late August-early September). In fact, on the Iberian Peninsula, laying seems to take place at the end of June, while in Ibiza evidence has been found of later egg-laying, around the month of July. In addition, oviductal eggs have been found developed from May to July, and even evidence of a possible second spermatogenesis in males in early autumn. All this seems to support the expansion of the reproductive period of the species in Eivissa, and, although we still do not have enough data to be able to affirm this, we have confirmed the presence of hatchlings on dates that coincide with these studies that affirm a lengthening of the reproductive period, as well as the presence of highly developed follicles from May to July.

snake killing company IbizaComparison between an adult and a juvenile horseshoe snake, both are placed on the same plane.

But how to differentiate females from males? In general, males may be larger, but size is not a valid criterion as the difference is not so clear and there are many intermediate specimens that could correspond to both females and males. The external appearance is also not useful to differentiate them, as it seems that there is no sexual dimorphism, apart from a possible longer tail in males, and the narrowing at the beginning of the tail in the case of females; but both characteristics may not be clearly seen and in juveniles may even be imperceptible. Therefore, the only way to be 100% reliable is to look at the genitalia. Females have an oviduct leading to the cloaca, so externally only one orifice is visible. Males also have no external gonads, but they do have a hemipenis (in a cavity next to the cloaca), which can be brought out when mating. There is a technique commonly used to determine the sex of snakes, which consists of pressing at a certain angle to pull out the hemipenis, and thus confirm whether it is a male or not. Another way is to insert a probe through the cloaca towards the tail, to see if there is a hole inside where the hemipenis hides, and if it does not enter, it means that there is no cavity and therefore it is a female. Finally, the most reliable method is to locate the testicles in males, and the follicles or oviductal eggs in females (obviously, this implies sacrificing the animal first). 

Hemipenis of a male horseshoe snake.Hemipenis of a male horseshoe snake.

snake eggs IbizaEggs already developing inside a female horseshoe snake.

snake eggs and folliclesIn this picture, we can see both developing eggs and unfertilised follicles.

female snake tailNarrowing at the beginning of the tail, typical of a female, however, this feature does not always have to be so noticeable, so it is not a particularly reliable method of sexing individuals.

Using a combination of all these techniques, we could see that the sex ratio is apparently unequal in the early summer, while in the later months it seems to even out (figure 3). But in any case, comparisons with other years are lacking to rule out something purely accidental, as well as having a larger sampling unit to be able to draw conclusions that are backed by more solid data.

snake control ibizaNumber of monthly catches from April to December, separated by sex (total green, maroon males, orange females).

Sexually active individuals were captured in all sampling zones. We identified highly developed follicles (diameter between 10.87 mm and 19.87 mm) in 4 females captured in sub-zones 1 and 2 of Zone A during the months of May, June and July, which indicates that they are reproducing in a consolidated manner next to the Natural Park (see part 1 of this entry for more information on the 3 sampling zones and sub-zones). The different physical barriers existing between sub-zones 1 and 2 of Zone A and the natural park are not a sufficient means of containment, so the strategies for snake control in the protected area of Ses Salines should include active control of the species in all the areas adjacent to the park. Ignoring this need will be a big mistake, as it is most likely that all areas will act as a reservoir for each other, if sufficient control is not applied simultaneously in all adjacent areas.


Body fat

Another aspect that we found particularly striking is the surprising amount of fat that some individuals can have. We measured the amount of body fat in all the specimens studied, as has been done in other studies, using a scale of 4 categories, with 0 being the absence of fat bodies and 4 the maximum, when fat bodies continuously and abundantly cover the internal viscera. In general, fat content was quite high, with a mean of 3.08 out of a maximum of 4. There seems to be no relationship between body size (LHC) and the amount of fat bodies, but it has been suggested in other studies that in females they may decrease during the reproductive period due to the cost of reproduction. In our case, the fat loss that can be observed in June and July is quite equal between males and females (figure 4). The minimum in August cannot be taken into account as indicative, since, as in the case of December, we only have one specimen analysed and this may not be representative at all.

cuerpos grasos culebra herraduraEvolution of the fat bodies by month and sex, without a doubt, the limited sample size does not allow any solid conclusions to be drawn categorically. Evolution from April to November, orange females, maroon males and total green colour.

General appearance and fat distribution in an adult horseshoe whip snake.General appearance and fat distribution in an adult horseshoe whip snake.

what a snake's blubber looks likeDetail of the appearance of the fat of a horseshoe whip snake.



In terms of diet, most of the captures had an empty stomach (87.5%). Only 6 individuals had stomach contents: 5 had more or less digested remains of the Pitiusas lizard (Podarcis pityusensis), the sixth specimen had 1 field mouse (Apodemus sylvaticus).

stomach contents snakeMoment when "at last" a stomach with contents appears, in this case, the victim was a field mouse.

field mouse horseshoe snakeComplete removal of half-digested remains, note the absence of many of the mouse parts.

As in the third party studies to date, the proportion of empty stomachs was very high. Perhaps this is due to at least two factors. On the one hand, snakes can go for many days without eating; and on the other hand, it is logical to think that the snakes that fall into the traps are those that are attracted to the mouse by actively searching for food, i.e. they are hungry and feel hungry because their stomachs are empty. Therefore, live-bait trapping does not seem to be the best trapping system to analyse stomach contents, although it is the most feasible to date, and therefore the option that remains is to take into account this possible bias in the methodology when interpreting the data.

In any case, we have confirmed the predation of lizards by horseshoe snakes from a very early age, as a juvenile of less than 8 g in weight had lizard remains in its stomach, which, given the size of the remains, must have barely entered its mouth. On the other hand, on two occasions we could practically confirm that the snake had fed on a lizard inside the trap, as apart from the remains of the lizard (regurgitated or in the digestive tract) inside the stomach there were remains of straw exactly the same as the one we use inside the traps for live bait.

snake lizard ibizaHand-caught juvenile in Zone 3, with half-digested lizard remains.

This predation inside the traps has been previously suggested by some experts, it is totally plausible, but it is also plausible that sometimes it is possible to discern the moment of ingestion according to the degree of digestion of the remains in relation to the last visit to the trap. That is to say, there are some cases of predation inside the traps, it is true, but there are also cases of specimens that have hunted outside and have entered the trap without having a completely empty stomach (this can be determined by the degree of digestion of the prey in relation to the date of entry of the snake into the trap). The case of the juvenile in the above photo, with a lizard in its stomach, was captured by hand and therefore pretty much settles the issue: as soon as they hatch, horseshoe snakes can (and do) prey on lizards (this specimen weighed just 8 grams, stomach contents included, and 29.8 cm LHC, it was within the neonate range).

On the other hand, the specimen that had eaten a field mouse came from a road kill. These two specific cases out of the total number of specimens with stomach contents captured with traps raise the possibility that perhaps the proportion of empty stomachs is lower in specimens that have been run over or captured manually than in those captured with traps. This could be an advantage when carrying out an in-depth dietary study, since with a smaller number of snakes we would obtain more information.

sargantanes o serpsHorseshoe whip snake with regurgitated lizard, subarea 2 of Zone A.

regurgitated snakeAnother case of a regurgitated lizard inside the snake compartment, in this case 2 specimens, also in subzone 2 of Zone A (we did not record any cases in Zone B, but we did record a case in a location in Zone C close to Zone A).


Predation pressure

To understand the success of this species in Eivissa and Formentera, it is also important to take into account the scarcity of potential predators. Predation by genets or small birds of prey, the only potential natural predators on the island, has not yet been confirmed (or at most has been merely anecdotal). On the other hand, it is unlikely that domestic animals, feral or not (basically cats), although they presumably have the capacity to hunt them, would be able to control the snake population on the island. Snakes are on the one hand quite aggressive prey, and on the other hand, quite aggressive when they feel threatened. One way of studying the predation pressure on snakes in general is to analyse the wounds on the tail or posterior third of the snakes' bodies, as they indicate that there may have been an attempt at predation and therefore indicate the possible existence of potential predators, even if they did not manage to catch the snake on that occasion. In our case, the data continue to support other studies that affirm that there is hardly any predation in Eivissa, since in only 11 individuals we observed injuries or imperfections in the skin that could be considered indications of predation (which does not imply de facto that there was such an attempt in practice, since the injury could have been caused by other factors that were not strictly an attempt at predation).

Some examples of such injuries or imperfections:

wound 1

wound snake

wound snake 3

wound snake 4

wound snake 5

wound snake 6


This brings the second part to an end. The third part will focus on accidental captures; the necessary participation of the public (volunteering) and the role of public administrations and private companies in the control of the pest; ending with a small anecdotal section on black rats (Rattus rattus) and their effect on snake control, and some brief general conclusions.

To access the first part of this entry:

Notes on snake control in Ibiza and Formentera (1/3).

To access the third part of this entry:

Notes on snake control in Ibiza and Formentera (3/3)


Text: own elaboration.


Photographs: unless otherwise specified, Borja Pérez; and unless otherwise indicated, all photos were taken in Eivissa or Formentera.


Acknowledgements: to our technician and biologist Lara Abascal, for her involvement, beyond the strictly work-related, during the whole process of capturing the snakes and the subsequent processing and writing up of the information collected. Thanks also, to all those who have voluntarily provided us with specimens to be included in the sample of the entry.



See the end of part 3 of this entry.


Find out more:

- Nova Falcons: Snakes in Eivissa and Formentera (Part I).
- Nova Falcons: Snakes in Eivissa and Formentera (Part II).
- Nova Control: snake control in Ibiza and Formentera.
- Instructions for the manufacture of traps for non-native snakes.
- Thesis E.Montes.
- Feasibility of ophidian control in Ibiza and Formentera (Spanish Herpetological Association, 2012).
- The horseshoe snake.
- The ladder snake.


About Nova Falcons:

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-Controls on birds, mammals y reptiles.

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Wildlife control in Ibiza