Lactation Period as a Proxy for Maternal Condition in Hawaiian Monk Seals: Implications For Understanding Population Trends and Environmental Drivers
Johanos TC, Robinson S, Wurth TA, and Littnan CL
The Hawaiian monk seal (Neomonachus schauinslandi occurs throughout the Hawaiian Archipelago, with a total population of about 1,400 seals.
Most endangered Hawaiian monk seals reside in the Northwestern Hawaiian Islands (NWHI), where, until recently, subpopulations declined over many decades. Meanwhile, a small subpopulation in the main Hawaiian Islands (MHI) has grown over the past two decades.
Young Hawaiian monk seals (Neomonachus schauinslandi) encounter multiple threats, including starvation, which has led to poor recruitment.
Juvenile survival is a key limiter to recovery of this endangered species and weaning girth is linked with juvenile survival at several sites.
Females fast during the lactation period and typically go from robust to emaciated condition. Because of this, the duration a female nurses her pup can be used as a measure of maternal condition and foraging success prior to pupping.
Our study utilizes visual observations collected during 1981-2016 to investigate whether lactation periods of individually identified females differ over time or by region, and how differences might relate to population trends and environmental factors.
Introduction
The Hawaiian monk seal (Neomonachus schauinslandi occurs throughout the Hawaiian Archipelago, with a total population of about 1,400 seals.
Most endangered Hawaiian monk seals reside in the Northwestern Hawaiian Islands (NWHI), where, until recently, subpopulations declined over many decades. Meanwhile, a small subpopulation in the main Hawaiian Islands (MHI) has grown over the past two decades.
Young Hawaiian monk seals (Neomonachus schauinslandi) encounter multiple threats, including starvation, which has led to poor recruitment.
Juvenile survival is a key limiter to recovery of this endangered species and weaning girth is linked with juvenile survival at several sites.
Females fast during the lactation period and typically go from robust to emaciated condition. Because of this, the duration a female nurses her pup can be used as a measure of maternal condition and foraging success prior to pupping.
Our study utilizes visual observations collected during 1981-2016 to investigate whether lactation periods of individually identified females differ over time or by region, and how differences might relate to population trends and environmental factors.
This study utilizes data collected at 6 major breeding sites in the NWHI and within the MHI.
The abbreviations we use for various sites and regions are depicted on the map above.
As part of an ongoing study initiated in the eary 1980s, NWHI field camps are established during the breeding season.
Weaned pups are tagged and seal identities are maintained throughout a seal's lifetime.
Births and lactation periods are recorded. We used univariate analysis (lm and glm in R) and multivariate analysis (generalized additive models/GAMS) to determine if lactation periods differ by region or over time, and how any differences might relate to characteristics of the mother, their pup, and environmental factors.
Methods
This study utilizes data collected at 6 major breeding sites in the NWHI and within the MHI.
The abbreviations we use for various sites and regions are depicted on the map above.
As part of an ongoing study initiated in the eary 1980s, NWHI field camps are established during the breeding season.
Weaned pups are tagged and seal identities are maintained throughout a seal's lifetime.
Births and lactation periods are recorded. We used univariate analysis (lm and glm in R) and multivariate analysis (generalized additive models/GAMS) to determine if lactation periods differ by region or over time, and how any differences might relate to characteristics of the mother, their pup, and environmental factors.
Lactation durations are similar among 6 NWHI sites where by-site averages ranged from 36-38 d (overall average 38 d, SD 4.66, n=2,072), whereas females nurse longer in the MHI (average 44 d, SD 4.83, n=139).
Lactation periods in the NWHI tend to track each other, and generally coincide with population trends: decreasing since the late 1980s/early 1990s when there are corresponding decreases in juvenile survival, but showing recent signs of increase.
Meanwhile, lactation durations in the MHI are reliably longer but may have decreased over the past decade, becoming more similar to those in the NWHI, corresponding to a period when population growth in the MHI has slowed.
We examined variables that may impact lactation, pup weaning size, or survival. The environmental variables we chose (Year, El Nino/La Nina conditions, and Winter Sea Surface Temperature) show up as highly significant in many areas.
Findings include:
Across the Hawaiian Archipelago, older moms nurse longer and produce larger pups, and longer lactation periods result in bigger pups.
Bigger pups have higher survival in the NWHI, however this relationship does not hold true in the MHI, where there are other, human-related sources of mortality.
Though males nurse longer, they gain less per day. Weaning size is similar between the sexes at all sites except French Frigate Shoals.
Environment is important, however the interpretation is complex
Year negatively correlates with lactation period in both the NWHI and MHI, and with first year survival in the NWHI
El Nino and La Nina conditions correlate with lactation period and pup size at Laysan and French Frigate Shoals, however the direction of the correlation is not consistent. Winter sea surface temperatures negatively correlate with first year survival at all NWHI sites. Environmental data for the MHI are not included in this analysis.
Results of univariate analysis
Lactation durations are similar among 6 NWHI sites where by-site averages ranged from 36-38 d (overall average 38 d, SD 4.66, n=2,072), whereas females nurse longer in the MHI (average 44 d, SD 4.83, n=139).
Lactation periods in the NWHI tend to track each other, and generally coincide with population trends: decreasing since the late 1980s/early 1990s when there are corresponding decreases in juvenile survival, but showing recent signs of increase.
Meanwhile, lactation durations in the MHI are reliably longer but may have decreased over the past decade, becoming more similar to those in the NWHI, corresponding to a period when population growth in the MHI has slowed.
We examined variables that may impact lactation, pup weaning size, or survival. The environmental variables we chose (Year, El Nino/La Nina conditions, and Winter Sea Surface Temperature) show up as highly significant in many areas.
Findings include:
Across the Hawaiian Archipelago, older moms nurse longer and produce larger pups, and longer lactation periods result in bigger pups.
Bigger pups have higher survival in the NWHI, however this relationship does not hold true in the MHI, where there are other, human-related sources of mortality.
Though males nurse longer, they gain less per day. Weaning size is similar between the sexes at all sites except French Frigate Shoals.
Environment is important, however the interpretation is complex
Year negatively correlates with lactation period in both the NWHI and MHI, and with first year survival in the NWHI
El Nino and La Nina conditions correlate with lactation period and pup size at Laysan and French Frigate Shoals, however the direction of the correlation is not consistent. Winter sea surface temperatures negatively correlate with first year survival at all NWHI sites. Environmental data for the MHI are not included in this analysis.
What are the most important drivers influencing female condition and pup survival?
We examined multivariate and non-linear relationships, constructing separate models for each site because preliminary models showed that site is very influential. This gives the added benefit of seeing which factors are consistent across the range vs only important locally/sporadically.
Mother's age, lactation period, and pup girth are consistently important. Winter Sea Surface Temperature (SST) is important in many of the Survival Models, however the direction of the correlation is not consistent.
Results of multivariate analysis
What are the most important drivers influencing female condition and pup survival?
We examined multivariate and non-linear relationships, constructing separate models for each site because preliminary models showed that site is very influential. This gives the added benefit of seeing which factors are consistent across the range vs only important locally/sporadically.
Mother's age, lactation period, and pup girth are consistently important. Winter Sea Surface Temperature (SST) is important in many of the Survival Models, however the direction of the correlation is not consistent.
