This is the first study reporting IOP and TP in free ranging pygoscelid penguins The present study is the first on the use of the Tonovet® rebound tonometerin penguins living in their natural habitat. In the present study, it was ascertained that the mean STT and IOP values of 129 and 120 adult penguins were 10.16 ± 4.05 mm/min and 38.85 ± 13.19 mmHg, respectively. A statistical difference at the level of p < 0.05 was determined between the islands for the mean IOP values. While no statistically significant difference was detected between the species for the mean IOP values (p = 0.854), the difference observed between the locations was found to be statistically very significant (p < 0.001).
Based on literature review, there is no previous study on ophthalmic findings of penguins living in their natural habitat, and thus, diagnostic ophthalmic reference values have not been established for these animals. As all of the studies conducted to date have been performed under professional care in zoological institutions or wildlife rehabilitation settings in controlled environments, there is need for further research. On the contrary to the expectation of observing differences among species, differences having been observed between the study locations was attributed to the very harsh Antarctic weather conditions characterized by sudden daily changes and to an equal number of samplings not being able to be performed in the investigated penguin species. The multiple comparisons of the study locations for the mean IOP values having demonstrated the presence of differences is in support of this attribution. The mean STT-1 and IOP values determined for each pygoscelid penguin species in this first study on tear production and IOP in wild penguins will serve as reference values not only for future studies to be conducted in the Antarctic Peninsula, but also for captive penguins kept under professional care. Yet there is still need for further research in different locations, time periods, and species, excluding the gentoo penguin.
It has been reported that it may not be possible to develop a standardized tear test for birds, due to anatomical and physiological differences observed between species for tear drainage and lacrimal ducts (Smith et al. 2015). As only very few ophthalmologic studies have been previously conducted in penguins, limited information is available on ocular examination, data interpretation and the difficulty of applying diagnostic tests in these birds during ophthalmologic examination(Martin and Young 1984; Suburo and Scolaro 1990; Pigatto et al. 2005). The establishment of reference ranges for each species is highly important in avoiding erroneous diagnostic interpretations during ophthalmic examination (Stiles et al. 1994; Korbel, L; P. 1998; Harris et al. 2008; Reuter et al. 2011; Labelle et al. 2012).
To date, studies aimed at determining IOP and STT values in penguin species belonging to the order Spheniscus have been conducted in animals kept under professional care in artificial marine and freshwater environments at either wildlife rehabilitation centres or zoos (Table 6) (Swinger et al. 2009; Mercado et al. 2010; Bliss et al. 2015; Gonzalez-Alonso-Alegre et al. 2015; Woodhouse et al. 2016a; Church et al. 2018). In their study on macaroni penguins (Eudyptes chrysolophus) and southern rockhopper penguins (Eudyptes chrysocome) kept at zoos and aquariums in North America, Woodhouse et al. (Woodhouse et al. 2016a) assessed the impact of multiple factors, including husbandry conditions, the presence/absence of cataract and concurrent ocular pathologies, as well as the body position during physical restraint, on IOP values in penguins. Based on literature review, this is the first study on tear production and intraocular pressure in pygoscelid penguins, namely, the Adélie chinstrap and gentoo living in their natural habitat on the southern Antarctic islands. Different from previous controlled studies conducted in a closed environment, this study was carried out in open air, in the natural marine habitat of penguins. Measurements were performed in the animals during their daily routine, such that they were exposed to dust, ocean water spray, strong winds, snow and abrupt weather changes at an average environmental temperature of -4 0C. The objective of the present study was to determine IOP and STT-1 values in clinically normal pygoscelid penguins that would serve as a reference for future research in this field.
When assessing the impact of species-specific anatomical and physiological differences, stress and geographical conditions on parameters such as tear production in raptorial birds, it should be noted that data comparison is able to be made only under optimal conditions (Jeong et al. 2007). In view of differences observed between species for IOP values (Table 7) and in agreement with previous studies (Sheldon et al. 2017), higher IOP values having been detected in pygoscelid penguins was attributed to these species diving up to 30 m beneath the ocean surface, exposing their cornea to high levels of external pressure, and thus, was considered to be an adaptive function related to underwater foraging. The only study previously conducted on these ocular parameters in populations living in their natural habitat was carried out in the Punta San Juan Conservation Area in Peru (Sheldon et al. 2017). This study presents, for the first time, IOP values detected in pygoscelid penguins living in their natural habitat in the Antarctic Peninsula. It was observed that IOP values varied with the age of the penguin, as well as with the year and location. In previous studies conducted by Swinger and Mercado (Swinger et al. 2009; Mercado et al. 2010), IOP values were determined in healthy penguins, but the factors influential on these values were not investigated. Swinger et al. (Swinger et al. 2009) reported to have determined higher IOP values that fell within a larger range, compared to values previously detected in zoo animals. Suggesting that the higher IOP values they had detected were an adaptation of the animals to the higher atmospheric pressure they were exposed to during underwater dives, these researchers also indicated the necessity for further research to confirm their hypothesis (Mercado et al. 2010). In the present study, while no statistically significant difference was determined between the penguin species for the mean IOP values (p = 0.854) (Table 5), the study locations significantly differed for both the STT-1 and IOP values (p < 0.05, p < 0.001) (Table 2, Table 4).
The scarcity on ophthalmic findings in penguins makes it difficult to interpret ocular examination findings and diagnostic test results in these animals. Thus, it is of great importance to establish reference values for routinely used ocular parameters such as IOP and STT in penguins. Several studies have been carried out to determine STT and IOP values in the Humboldt penguin (Spheniscus humboldti) (Swinger et al. 2009; Sheldon et al. 2017), macaroni penguin (Eudyptes chrysolophus) (Bliss et al. 2015; Woodhouse et al. 2016a), southern rockhopper penguin (Eudyptes chrysocome) (Bliss et al. 2015), black-footed penguin (Spheniscus demersus), gentoo penguin (Pygoscelis papua), king penguin (Aptenodytes patagonicus), and chinstrap penguin (Pygoscelis antarctica) (Mercado et al. 2010; Gonzalez-Alonso-Alegre et al. 2015; Church et al. 2018) (Table 6). To date, only Sheldon et al. (Sheldon et al. 2017) have attempted to establish reference values for tear production with the Schirmer tear test and for IOP values with rebound tonometry in wild Humboldt penguins living in their natural habitat. Compared to values previously detected by (Sheldon et al. 2017) in Humboldt penguins living in their natural habitat (Table 6), the present study demonstrated higher values (38.85±13.19 mmHg) falling within a larger range (16–69 mmHg), which were attributed to the harsh Antarctic weather conditions characterized by sudden changes. l.
IOP measurements by rebound tonometry have been previously performed in the Humboldt penguin (Spheniscus humboldti) (Sheldon et al. 2017), macaroni penguin (Eudyptes chrysolophus) (Bliss et al. 2015), southern rockhopper penguin (Eudyptes chrysocome) (Bliss et al. 2015; Woodhouse et al. 2016b), black-footed penguin (Spheniscus demersus) (Mercado et al. 2010; Gonzalez-Alonso-Alegre et al. 2015), gentoo penguin (Pygoscelis papua), king penguin (Aptenodytes patagonicus) and chinstrap penguin (Pygoscelis antarctica) (Church et al. 2018). The mean IOP values of healthy macaroni and southern rockhopper penguins were ascertained as 42.0±9.7 mmHg and 32.9 ± 6.2 mmHg, respectively. No statistically significant difference was detected in these two penguin species for sex or the left/right eye. In previous research on the use of tonometry in penguins, the mean IOP value calculated for a healthy eye was determined to be above 28 mmHg, and thus, was significantly higher than values previously reported in several other avian species (Table 4, Table 5) (Mercado et al. 2010; Bliss et al. 2015; Gonzalez-Alonso-Alegre et al. 2015). A relatively lower mean IOP value of 20.4±4.1 mmHg was reported for the Humboldt penguin (Spheniscus humboldti), but it should be noted that this value was obtained using the applanation tonometry technique, which is known to yield significantly lower IOP values in penguins and other birds, in comparison to rebound tonometry (Swinger et al. 2009). Therefore, it is required to make a comparison of penguin IOP values obtained with the same tonometry technique (Reuter et al. 2010). Although scarce, IOP values obtained with rebound tonometry have been reported for some penguin species (Table 7). The results of the present study are in agreement with those reported in previous studies on the use of TonoVet in penguins. When compared to the IOP ranges previously reported for other avian species, the mean IOP values determined in pygoscelid penguins in the present study were found to be higher (Table 4, Table 5).
Studies available on the use of STTs in penguins are limited to the macaroni penguin (Eudyptes chrysolophus) and rockhopper penguin (Eudyptes chrysocome) (Swinger et al. 2009; Bliss et al. 2015; Woodhouse et al. 2016b; Sheldon et al. 2017), and of these studies, only 2(Swinger et al. 2009; Sheldon et al. 2017) have reported STT-1 values.(Swinger et al. 2009) reported a STT range of 1–12 mm/min and a mean STT value of 6.45 ± 2.9 mm/min for the Humboldt penguin. Different results have been reported for animals rehabilitated in freshwater and marine environments. Accordingly, researchers have reported mean STT values of 4.8 mm/min, and 8.5 mm/min for penguins kept in experimental marine and freshwater environments, respectively, and thus, have demonstrated a significant difference between the two habitats. The mean STT value of freshwater penguins was two-fold of that of marine penguins, and this was attributed to differences between the supraorbital glands of these species (Swinger et al. 2009). In avian species, the Harderian gland, situated in proximity to the base of the nictitating membrane, is the main source of tear fluid (Chieffi et al. 1996; A Bayón et al. 2007). Harris et al.(Harris et al. 2008) suggested that owls and penguins produced a smaller volume of aqueous tear owing to the smaller size or absence of lacrimal glands, when compared to other birds (Korbel et al. 1994). Similarly, Meekins et al.(Meekins et al. 2015) reported that tear production varied greatly among birds of different size and phylogenetic classification. STT-2 values previously reported for macaroni penguins (Eudyptes chrysolophus) and rockhopper penguins (Eudyptes chrysocome) kept at zoos were found to be similar to the STT-1 values detected in pygoscelid penguins in the present study (Table 6).
In their research aimed at establishing STT and IOP ranges for some raptors, Barsotti et al. (Barsotti et al. 2013) determined the presence of significant inter-species differences. When compared to values previously reported for other avian species, the mean IOP and STT-1 values determined in the present study have been observed to be similar to some values, and higher or lower than some other values (Table 7). The differences observed could be related to a marine adaptation serving as an advantage to penguins during underwater diving and foraging.
The number of ophthalmic measurements performed in the different locations vary due to the sudden changes that occurred in the Antarctic weather. While the number of penguins sampled in the Ardley SPA (Ardley III) for STT-1 measurements was 24, IOP values were measured in 33 penguins. Strong winds that blew during the visits, which made it very difficult to place the filter papers in the conjunctival fornix without causing any harm to the penguins, prevented the completion of the measurements in some animals. To avoid any stress to the animals, the restraint of the penguins was not prolonged and the measurements were not repeated. Heavy rain encountered during the visits to the Harmony Point caused the tonometer to display values outside the normal range. As the repetition of tonometer measurements would require the prolonged restraint of the animals, causing increased stress that would prevent the achievement of accurate results, IOP values were not measured. Ophthalmic measurements were not able to be made at Doumer Island/Yelchoo Base due to adverse weather conditions.
In the present study, it took time for the animals to calm down after being captured and physically restrained for clinical tests and observations. In view of the data being collected from the animals under physical restraint and with an aim to prevent any error, data was collected from only one eye in each animal. Apart from two persons being needed to restrain the animals and perform tests on them, another major difficulty encountered was placing the STT test strips in the conjunctival fornix. For the correct placement of the strips, it was required to open the eyelid and at the same time apply the test. Furthermore, given the small size of the eyes of penguins, it should be noted that placing the strips in the conjunctival fornix without touching the cornea is almost impossible, and eventually irritates the eye and causes artifactitious tear production. We consider these aspects to be important for researchers and practitioners, when diagnosing subtle pathological changes in tear production. Moreover, there is a need for further veterinary research on tear production and ocular surface measurements in penguin species other than those investigated in the present study.
In conclusion, this study presents both IOP values measured with a Tonovet® rebound tonometer and STT-1 values detected in clinically healthy pygoscelid penguins, and shows the need for further research to establish reference values for wild animals living in their natural habitat. This study will constitute a reference for future studies to be conducted in different locations and time periods.